Chapter 10 DNA: The Molecule of Heredity - Structure and Function

Chapter 10 DNA: The Molecule of Heredity - Structure and Function

I. What does it mean when we say DNA is the "molecule of heredity"?
A. DNA—the genetic information of all living organisms B. Genetic information units are called genes 1. Gene—a length of DNA containing information to make a protein

II. How do we know DNA is the genetic material?
A. Discovery of nucleic acids s—Friedrich Miescher B. Popular early theory considered protein to be the genetic material

C. Bacterial transformation —Frederick Griffith —J. L. Alloway D. DNA is the transforming material in bacteria —Oswald Avery, Colin MacLeod and Maclyn McCarty

Biology: Life on Earth (Audesirk)
Figure: 09.1 Title: Transformed bacteria Caption: Griffith’s discovery that bacteria can be transformed from harmless to deadly laid the groundwork for the discovery that DNA contains genes. Chapter 1

E. DNA is the hereditary molecule of bacteriophages —Alfred Hershey and Martha Chase

In 1952, Alfred Hershey and Martha Chase showed that DNA was the genetic material of the phage T2.
The T2 phage, consisting almost entirely of DNA and protein, attacks Escherichia coli (E. coli), a common intestinal bacteria of mammals. This phage can quickly turn an E. coli cell into a T2-producing factory that releases phages when the cell ruptures. Fig. 16.2a Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

The mixtures were spun in a centrifuge which separated the heavier bacterial cells in the pellet from lighter free phages and parts of phage in the liquid supernatant. They then tested the pellet and supernatant of the separate treatments for the presence of radioactivity. Fig. 16.2b Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

III. DNA structure A. Four nucleotide subunits
B. Sugar–phosphate backbone C. Chargaff's rule: A=T, C=G Base Pairing Rule

The phosphate group of one nucleotide is attached to the sugar of the next nucleotide in line. The result is a “backbone” of alternating phosphates and sugars, from which the bases project. Fig. 16.3 Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings Chapter 1

Nitrogenous base Phosphate group DNA Sugar Polynucleotide Nucleotide Figure 10.UN3 Summary: DNA and RNA Structure DNA RNA C G A T C G A U Nitrogenous base Deoxy- ribose Sugar Ribose Number of strands 2 1 Figure 10.UN3 Chapter 1

Figure: 09.3 Title: The Watson-Crick model of DNA structure Caption: (a) Two strands of DNA wind about each other in a double helix, like a twisted ladder. The two DNA strands run in opposite directions. This directionality is especially clear at the ends of the double helix, where the terminal nucleotide of one strand has an unbonded (“free”) sugar and the terminal nucleotide on the other strand has an unbonded (“free”) phosphate. (b) Complementary base pairs (adenine and thymine, guanine and cytosine) hold the two DNA strands together. (c) Hydrogen bonding between specific base pairs in the center of the helix hold the two strands together. Three hydrogen bonds hold guanine to cytosine; two hydrogen bonds hold adenine to thymine. Chapter 1

III. DNA structure D. Helical structure
—Maurice Wilkins and Rosalind Franklin 2. X-ray diffraction E. Double helix/complementary base pairing —James Watson and Francis Crick

Maurice Wilkins and Rosalind Franklin used X-ray crystallography to study the structure of DNA.
In this technique, X-rays are diffracted as they passed through aligned fibers of purified DNA. The diffraction pattern can be used to deduce the three-dimensional shape of molecules. James Watson learned from their research that DNA was helical in shape and he deduced the width of the helix and the spacing of bases. Fig. 16.4 Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

Figure 10.3a Discoverers of the double helix. James Watson (left) and Francis Crick Figure 10.3a Chapter 1

III. DNA structure F. DNA differences
1. Prokaryotes—circular DNA helix 2. Eukaryotes—linear chromosomes

IV. DNA replication A. Necessary for cell division
B. DNA replication requires energy and several enzymes 1. DNA helicase Topoisomerase DNA polymerase 4. DNA ligase

Figure: 09.4 Title: Basic features of DNA replication Caption: During replication, enzymes separate the parental DNA double helix, breaking the hydrogen bonds between complementary bases. Other enzymes select complementary nucleotides and add them to the growing daughter strands. Each parental strand and its new daughter strand form a new double helix. Chapter 1

Figure: 09.5 Title: Replication bubble Caption: DNA replication begins when DNA helicase and related enzymes unwind portions of the parental DNA double helix, creating a replication bubble. In complex cells, DNA replication occurs simultaneously at many locations on the parental DNA double helix, resulting in multiple replication bubbles. DNA replication is completed when all adjacent replication bubbles meet to form two complete and separate DNA double helices. Chapter 1

Figure: 09.6a Title: Details of DNA replication Caption: (a) One DNA strand can be synthesized as a long, continuous strand. The other DNA strand must be synthesized as a series of short segments that are connected by DNA ligase. Chapter 1

IV. DNA replication C. Replication is semiconservative
D. Replication rate: 50–500 nucleotides per second

Figure: 09.UN04 Title: DNA replication Caption: Chapter 1

V. DNA repair A. DNA replication errors
1. 1 mistake per 10,000 base pairs reduced to 1 mistake per billion base pairs B. Several repair enzymes use complementary DNA strand to repair damage

VI. Replication errors can result in mutations
A. Changes in DNA molecules 1. Nucleotide substitution 2. Nucleotide deletion 3. Nucleotide insertion B. Unrepaired replication errors are a source of genetic variability C. Mutations as a result of replication errors can have serious health consequences

mRNA and protein from a normal gene (a) Base substitution Deleted Figure Three types of mutations and their effects. (b) Nucleotide deletion Inserted (c) Nucleotide insertion Figure 10.22 Chapter 1

THE FLOW OF GENETIC INFORMATION FROM DNA TO RNA TO PROTEIN
Biology: Life on Earth (Audesirk) THE FLOW OF GENETIC INFORMATION FROM DNA TO RNA TO PROTEIN DNA functions as the inherited directions for a cell or organism. How are these directions carried out? Student Misconceptions and Concerns 1. Less experienced students are often intensely focused on writing detailed notes. The risk is that they miss the overall patterns and the broader significance of the topics discussed. Consider a gradual approach to the subjects of transcription and translation, beginning quite generally and testing comprehension, before venturing into the finer mechanics of each process. 2. Consider placing on the board the basic content from Figure 10.9, noting the sequence, products, and locations of transcription and translation in eukaryotic cells. This reminder can create a quick concept check for students as they learn additional detail. 3. Mutations are often discussed as part of evolution mechanisms. In this sense, mutations may be considered a part of a positive creative process. The dual nature of mutations, potentially deadly yet potentially innovative, should be clarified. Teaching Tips 1. It has been said that, everything about an organism is an interaction between the genome and the environment. You might wish to challenge your students to explain the significance and validity of this statement. 2. The authors note that the sequential information in DNA and RNA is analogous to the sequential information in the letters of a sentence. This analogy is also helpful when explaining the impact of insertion or deletion mutations that cause a shift in the reading frame. 3. The transcription of DNA into RNA is like a reporter who transcribes a political speech. In both situations, the language remains the same, although in the case of the reporter, it changes its form from spoken to written. 4. A parallel can be drawn between the discovery in 1799 of the Rosetta stone, which provided the key that enabled scholars to crack the previously indecipherable hieroglyphic code, and the cracking of the genetic code in Consider challenging your students to explain what part of the genetic code is similar to the Rosetta stone. This could be a short in-class activity for small groups. 5. Another advantage to the use of RNA to direct protein synthesis is that the original code (DNA) remains safely within the nucleus, away from the many potentially damaging chemicals in the cytoplasm. This is like making photocopies of important documents for study, keeping the originals safely stored away. 6. The production of proteins is like a machine requiring fuel. The molecular machinery (ribosomes and tRNA) used in many cellular processes also requires an input of energy in the form of ATP. 7. If you were using a train analogy for the assembly of monomers into polymers, at this point the DNA and RNA trains are traded in 3 for 1 for the polypeptide train. Thus, in general, polypeptides have about 1/3 as many monomers as the mRNA that coded for them. 8. After translation is addressed, consider asking your students (working singly or in small groups) to list all of the places where base pairing is used (in the construction of a DNA molecule during DNA replication, in transcription, and during translation when the tRNA attaches). 9. Students might want to think of the A and P sites as stages in an assembly line. The A site is where a new amino acid is brought in, according to the blueprint of the codon on the mRNA. The P site is where the growing product/polypeptide is anchored as it is being built. To help them better remember details of translation, students might think of the letters for the two sites to mean “A” for addition, where an amino acid is added, and ”P” for polypeptide, where the growing polypeptide is located. 10. A simple way to demonstrate the effect of a reading frame shift is to have students compare the following three sentences. The first is a simple sentence. But look what happens when a letter is added (2) or deleted (3). The reading frame, or triplet groupings, are re-formed into nonsense. (1) The big red pig ate the red rag. (2) The big res dpi gat eth ere dra g. (3) The big rep iga tet her edr ag. 11. The authors have noted elsewhere that a random mutation is like a shot in the dark. It is not likely to improve a genome any more than shooting a bullet through the hood of a car is likely to improve engine performance! © 2010 Pearson Education, Inc. Chapter 1

How an Organism’s Genotype Determines Its Phenotype
Biology: Life on Earth (Audesirk) How an Organism’s Genotype Determines Its Phenotype An organism’s genotype is its genetic makeup, the sequence of nucleotide bases in DNA. The phenotype is the organism’s physical traits, which arise from the actions of a wide variety of proteins. Student Misconceptions and Concerns 1. Less experienced students are often intensely focused on writing detailed notes. The risk is that they miss the overall patterns and the broader significance of the topics discussed. Consider a gradual approach to the subjects of transcription and translation, beginning quite generally and testing comprehension, before venturing into the finer mechanics of each process. 2. Consider placing on the board the basic content from Figure 10.9, noting the sequence, products, and locations of transcription and translation in eukaryotic cells. This reminder can create a quick concept check for students as they learn additional detail. 3. Mutations are often discussed as part of evolution mechanisms. In this sense, mutations may be considered a part of a positive creative process. The dual nature of mutations, potentially deadly yet potentially innovative, should be clarified. Teaching Tips 1. It has been said that, everything about an organism is an interaction between the genome and the environment. You might wish to challenge your students to explain the significance and validity of this statement. 2. The authors note that the sequential information in DNA and RNA is analogous to the sequential information in the letters of a sentence. This analogy is also helpful when explaining the impact of insertion or deletion mutations that cause a shift in the reading frame. 3. The transcription of DNA into RNA is like a reporter who transcribes a political speech. In both situations, the language remains the same, although in the case of the reporter, it changes its form from spoken to written. 4. A parallel can be drawn between the discovery in 1799 of the Rosetta stone, which provided the key that enabled scholars to crack the previously indecipherable hieroglyphic code, and the cracking of the genetic code in Consider challenging your students to explain what part of the genetic code is similar to the Rosetta stone. This could be a short in-class activity for small groups. 5. Another advantage to the use of RNA to direct protein synthesis is that the original code (DNA) remains safely within the nucleus, away from the many potentially damaging chemicals in the cytoplasm. This is like making photocopies of important documents for study, keeping the originals safely stored away. 6. The production of proteins is like a machine requiring fuel. The molecular machinery (ribosomes and tRNA) used in many cellular processes also requires an input of energy in the form of ATP. 7. If you were using a train analogy for the assembly of monomers into polymers, at this point the DNA and RNA trains are traded in 3 for 1 for the polypeptide train. Thus, in general, polypeptides have about 1/3 as many monomers as the mRNA that coded for them. 8. After translation is addressed, consider asking your students (working singly or in small groups) to list all of the places where base pairing is used (in the construction of a DNA molecule during DNA replication, in transcription, and during translation when the tRNA attaches). 9. Students might want to think of the A and P sites as stages in an assembly line. The A site is where a new amino acid is brought in, according to the blueprint of the codon on the mRNA. The P site is where the growing product/polypeptide is anchored as it is being built. To help them better remember details of translation, students might think of the letters for the two sites to mean “A” for addition, where an amino acid is added, and ”P” for polypeptide, where the growing polypeptide is located. 10. A simple way to demonstrate the effect of a reading frame shift is to have students compare the following three sentences. The first is a simple sentence. But look what happens when a letter is added (2) or deleted (3). The reading frame, or triplet groupings, are re-formed into nonsense. (1) The big red pig ate the red rag. (2) The big res dpi gat eth ere dra g. (3) The big rep iga tet her edr ag. 11. The authors have noted elsewhere that a random mutation is like a shot in the dark. It is not likely to improve a genome any more than shooting a bullet through the hood of a car is likely to improve engine performance! © 2010 Pearson Education, Inc. Chapter 1

DNA specifies the synthesis of proteins in two stages: Transcription, the transfer of genetic information from DNA into an RNA molecule Translation, the transfer of information from RNA into a protein Student Misconceptions and Concerns 1. Less experienced students are often intensely focused on writing detailed notes. The risk is that they miss the overall patterns and the broader significance of the topics discussed. Consider a gradual approach to the subjects of transcription and translation, beginning quite generally and testing comprehension, before venturing into the finer mechanics of each process. 2. Consider placing on the board the basic content from Figure 10.9, noting the sequence, products, and locations of transcription and translation in eukaryotic cells. This reminder can create a quick concept check for students as they learn additional detail. 3. Mutations are often discussed as part of evolution mechanisms. In this sense, mutations may be considered a part of a positive creative process. The dual nature of mutations, potentially deadly yet potentially innovative, should be clarified. Teaching Tips 1. It has been said that, everything about an organism is an interaction between the genome and the environment. You might wish to challenge your students to explain the significance and validity of this statement. 2. The authors note that the sequential information in DNA and RNA is analogous to the sequential information in the letters of a sentence. This analogy is also helpful when explaining the impact of insertion or deletion mutations that cause a shift in the reading frame. 3. The transcription of DNA into RNA is like a reporter who transcribes a political speech. In both situations, the language remains the same, although in the case of the reporter, it changes its form from spoken to written. 4. A parallel can be drawn between the discovery in 1799 of the Rosetta stone, which provided the key that enabled scholars to crack the previously indecipherable hieroglyphic code, and the cracking of the genetic code in Consider challenging your students to explain what part of the genetic code is similar to the Rosetta stone. This could be a short in-class activity for small groups. 5. Another advantage to the use of RNA to direct protein synthesis is that the original code (DNA) remains safely within the nucleus, away from the many potentially damaging chemicals in the cytoplasm. This is like making photocopies of important documents for study, keeping the originals safely stored away. 6. The production of proteins is like a machine requiring fuel. The molecular machinery (ribosomes and tRNA) used in many cellular processes also requires an input of energy in the form of ATP. 7. If you were using a train analogy for the assembly of monomers into polymers, at this point the DNA and RNA trains are traded in 3 for 1 for the polypeptide train. Thus, in general, polypeptides have about 1/3 as many monomers as the mRNA that coded for them. 8. After translation is addressed, consider asking your students (working singly or in small groups) to list all of the places where base pairing is used (in the construction of a DNA molecule during DNA replication, in transcription, and during translation when the tRNA attaches). 9. Students might want to think of the A and P sites as stages in an assembly line. The A site is where a new amino acid is brought in, according to the blueprint of the codon on the mRNA. The P site is where the growing product/polypeptide is anchored as it is being built. To help them better remember details of translation, students might think of the letters for the two sites to mean “A” for addition, where an amino acid is added, and ”P” for polypeptide, where the growing polypeptide is located. 10. A simple way to demonstrate the effect of a reading frame shift is to have students compare the following three sentences. The first is a simple sentence. But look what happens when a letter is added (2) or deleted (3). The reading frame, or triplet groupings, are re-formed into nonsense. (1) The big red pig ate the red rag. (2) The big res dpi gat eth ere dra g. (3) The big rep iga tet her edr ag. 11. The authors have noted elsewhere that a random mutation is like a shot in the dark. It is not likely to improve a genome any more than shooting a bullet through the hood of a car is likely to improve engine performance! Chapter 1

Nucleus DNA Figure 10.8 The flow of genetic information in a eukaryotic cell. (Step 1) Cytoplasm Figure Chapter 1

Nucleus DNA TRANSCRIPTION RNA Figure 10.8 The flow of genetic information in a eukaryotic cell. (Step 2) Cytoplasm Figure Chapter 1

Nucleus DNA TRANSCRIPTION RNA Figure 10.8 The flow of genetic information in a eukaryotic cell. (Step 3) TRANSLATION Protein Cytoplasm Figure Chapter 1

RNA polymerase DNA of gene Promoter DNA Initiation Terminator DNA RNA Area shown in part (a) at left Elongation Figure 10.13b Transcription Termination Growing RNA Completed RNA RNA polymerase (b) Transcription of a gene Figure 10.13b Chapter 1

When DNA is transcribed, the result is an RNA molecule. RNA is then translated into a sequence of amino acids in a polypeptide. Student Misconceptions and Concerns 1. Less experienced students are often intensely focused on writing detailed notes. The risk is that they miss the overall patterns and the broader significance of the topics discussed. Consider a gradual approach to the subjects of transcription and translation, beginning quite generally and testing comprehension, before venturing into the finer mechanics of each process. 2. Consider placing on the board the basic content from Figure 10.9, noting the sequence, products, and locations of transcription and translation in eukaryotic cells. This reminder can create a quick concept check for students as they learn additional detail. 3. Mutations are often discussed as part of evolution mechanisms. In this sense, mutations may be considered a part of a positive creative process. The dual nature of mutations, potentially deadly yet potentially innovative, should be clarified. Teaching Tips 1. It has been said that, everything about an organism is an interaction between the genome and the environment. You might wish to challenge your students to explain the significance and validity of this statement. 2. The authors note that the sequential information in DNA and RNA is analogous to the sequential information in the letters of a sentence. This analogy is also helpful when explaining the impact of insertion or deletion mutations that cause a shift in the reading frame. 3. The transcription of DNA into RNA is like a reporter who transcribes a political speech. In both situations, the language remains the same, although in the case of the reporter, it changes its form from spoken to written. 4. A parallel can be drawn between the discovery in 1799 of the Rosetta stone, which provided the key that enabled scholars to crack the previously indecipherable hieroglyphic code, and the cracking of the genetic code in Consider challenging your students to explain what part of the genetic code is similar to the Rosetta stone. This could be a short in-class activity for small groups. 5. Another advantage to the use of RNA to direct protein synthesis is that the original code (DNA) remains safely within the nucleus, away from the many potentially damaging chemicals in the cytoplasm. This is like making photocopies of important documents for study, keeping the originals safely stored away. 6. The production of proteins is like a machine requiring fuel. The molecular machinery (ribosomes and tRNA) used in many cellular processes also requires an input of energy in the form of ATP. 7. If you were using a train analogy for the assembly of monomers into polymers, at this point the DNA and RNA trains are traded in 3 for 1 for the polypeptide train. Thus, in general, polypeptides have about 1/3 as many monomers as the mRNA that coded for them. 8. After translation is addressed, consider asking your students (working singly or in small groups) to list all of the places where base pairing is used (in the construction of a DNA molecule during DNA replication, in transcription, and during translation when the tRNA attaches). 9. Students might want to think of the A and P sites as stages in an assembly line. The A site is where a new amino acid is brought in, according to the blueprint of the codon on the mRNA. The P site is where the growing product/polypeptide is anchored as it is being built. To help them better remember details of translation, students might think of the letters for the two sites to mean “A” for addition, where an amino acid is added, and ”P” for polypeptide, where the growing polypeptide is located. 10. A simple way to demonstrate the effect of a reading frame shift is to have students compare the following three sentences. The first is a simple sentence. But look what happens when a letter is added (2) or deleted (3). The reading frame, or triplet groupings, are re-formed into nonsense. (1) The big red pig ate the red rag. (2) The big res dpi gat eth ere dra g. (3) The big rep iga tet her edr ag. 11. The authors have noted elsewhere that a random mutation is like a shot in the dark. It is not likely to improve a genome any more than shooting a bullet through the hood of a car is likely to improve engine performance! Chapter 1

RNA nucleotides RNA polymerase Figure 10.13a Transcription Newly made RNA Direction of transcription Template strand of DNA (a) A close-up view of transcription Figure 10.13a Chapter 1

RNA polymerase DNA of gene Promoter DNA Initiation Terminator DNA RNA Area shown in part (a) at left Elongation RNA nucleotides RNA polymerase Termination Figure Transcription Growing RNA Newly made RNA Completed RNA Direction of transcription Template strand of DNA RNA polymerase (a) A close-up view of transcription (b) Transcription of a gene Figure 10.13 Chapter 1

What are the rules for translating the RNA message into a polypeptide? A codon is a triplet of bases, which codes for one amino acid. Student Misconceptions and Concerns 1. Less experienced students are often intensely focused on writing detailed notes. The risk is that they miss the overall patterns and the broader significance of the topics discussed. Consider a gradual approach to the subjects of transcription and translation, beginning quite generally and testing comprehension, before venturing into the finer mechanics of each process. 2. Consider placing on the board the basic content from Figure 10.9, noting the sequence, products, and locations of transcription and translation in eukaryotic cells. This reminder can create a quick concept check for students as they learn additional detail. 3. Mutations are often discussed as part of evolution mechanisms. In this sense, mutations may be considered a part of a positive creative process. The dual nature of mutations, potentially deadly yet potentially innovative, should be clarified. Teaching Tips 1. It has been said that, everything about an organism is an interaction between the genome and the environment. You might wish to challenge your students to explain the significance and validity of this statement. 2. The authors note that the sequential information in DNA and RNA is analogous to the sequential information in the letters of a sentence. This analogy is also helpful when explaining the impact of insertion or deletion mutations that cause a shift in the reading frame. 3. The transcription of DNA into RNA is like a reporter who transcribes a political speech. In both situations, the language remains the same, although in the case of the reporter, it changes its form from spoken to written. 4. A parallel can be drawn between the discovery in 1799 of the Rosetta stone, which provided the key that enabled scholars to crack the previously indecipherable hieroglyphic code, and the cracking of the genetic code in Consider challenging your students to explain what part of the genetic code is similar to the Rosetta stone. This could be a short in-class activity for small groups. 5. Another advantage to the use of RNA to direct protein synthesis is that the original code (DNA) remains safely within the nucleus, away from the many potentially damaging chemicals in the cytoplasm. This is like making photocopies of important documents for study, keeping the originals safely stored away. 6. The production of proteins is like a machine requiring fuel. The molecular machinery (ribosomes and tRNA) used in many cellular processes also requires an input of energy in the form of ATP. 7. If you were using a train analogy for the assembly of monomers into polymers, at this point the DNA and RNA trains are traded in 3 for 1 for the polypeptide train. Thus, in general, polypeptides have about 1/3 as many monomers as the mRNA that coded for them. 8. After translation is addressed, consider asking your students (working singly or in small groups) to list all of the places where base pairing is used (in the construction of a DNA molecule during DNA replication, in transcription, and during translation when the tRNA attaches). 9. Students might want to think of the A and P sites as stages in an assembly line. The A site is where a new amino acid is brought in, according to the blueprint of the codon on the mRNA. The P site is where the growing product/polypeptide is anchored as it is being built. To help them better remember details of translation, students might think of the letters for the two sites to mean “A” for addition, where an amino acid is added, and ”P” for polypeptide, where the growing polypeptide is located. 10. A simple way to demonstrate the effect of a reading frame shift is to have students compare the following three sentences. The first is a simple sentence. But look what happens when a letter is added (2) or deleted (3). The reading frame, or triplet groupings, are re-formed into nonsense. (1) The big red pig ate the red rag. (2) The big res dpi gat eth ere dra g. (3) The big rep iga tet her edr ag. 11. The authors have noted elsewhere that a random mutation is like a shot in the dark. It is not likely to improve a genome any more than shooting a bullet through the hood of a car is likely to improve engine performance! Chapter 1

The Genetic Code The genetic code is: The set of rules relating nucleotide sequence to amino acid sequence Shared by all organisms Student Misconceptions and Concerns 1. Less experienced students are often intensely focused on writing detailed notes. The risk is that they miss the overall patterns and the broader significance of the topics discussed. Consider a gradual approach to the subjects of transcription and translation, beginning quite generally and testing comprehension, before venturing into the finer mechanics of each process. 2. Consider placing on the board the basic content from Figure 10.9, noting the sequence, products, and locations of transcription and translation in eukaryotic cells. This reminder can create a quick concept check for students as they learn additional detail. 3. Mutations are often discussed as part of evolution mechanisms. In this sense, mutations may be considered a part of a positive creative process. The dual nature of mutations, potentially deadly yet potentially innovative, should be clarified. Teaching Tips 1. It has been said that, everything about an organism is an interaction between the genome and the environment. You might wish to challenge your students to explain the significance and validity of this statement. 2. The authors note that the sequential information in DNA and RNA is analogous to the sequential information in the letters of a sentence. This analogy is also helpful when explaining the impact of insertion or deletion mutations that cause a shift in the reading frame. 3. The transcription of DNA into RNA is like a reporter who transcribes a political speech. In both situations, the language remains the same, although in the case of the reporter, it changes its form from spoken to written. 4. A parallel can be drawn between the discovery in 1799 of the Rosetta stone, which provided the key that enabled scholars to crack the previously indecipherable hieroglyphic code, and the cracking of the genetic code in Consider challenging your students to explain what part of the genetic code is similar to the Rosetta stone. This could be a short in-class activity for small groups. 5. Another advantage to the use of RNA to direct protein synthesis is that the original code (DNA) remains safely within the nucleus, away from the many potentially damaging chemicals in the cytoplasm. This is like making photocopies of important documents for study, keeping the originals safely stored away. 6. The production of proteins is like a machine requiring fuel. The molecular machinery (ribosomes and tRNA) used in many cellular processes also requires an input of energy in the form of ATP. 7. If you were using a train analogy for the assembly of monomers into polymers, at this point the DNA and RNA trains are traded in 3 for 1 for the polypeptide train. Thus, in general, polypeptides have about 1/3 as many monomers as the mRNA that coded for them. 8. After translation is addressed, consider asking your students (working singly or in small groups) to list all of the places where base pairing is used (in the construction of a DNA molecule during DNA replication, in transcription, and during translation when the tRNA attaches). 9. Students might want to think of the A and P sites as stages in an assembly line. The A site is where a new amino acid is brought in, according to the blueprint of the codon on the mRNA. The P site is where the growing product/polypeptide is anchored as it is being built. To help them better remember details of translation, students might think of the letters for the two sites to mean “A” for addition, where an amino acid is added, and ”P” for polypeptide, where the growing polypeptide is located. 10. A simple way to demonstrate the effect of a reading frame shift is to have students compare the following three sentences. The first is a simple sentence. But look what happens when a letter is added (2) or deleted (3). The reading frame, or triplet groupings, are re-formed into nonsense. (1) The big red pig ate the red rag. (2) The big res dpi gat eth ere dra g. (3) The big rep iga tet her edr ag. 11. The authors have noted elsewhere that a random mutation is like a shot in the dark. It is not likely to improve a genome any more than shooting a bullet through the hood of a car is likely to improve engine performance! Chapter 1

Second base of RNA codon Phenylalanine (Phe) Tyrosine (Tyr) Cysteine (Cys) Serine (Ser) Stop Stop Leucine (Leu) Stop Tryptophan (Trp) Histidine (His) Leucine (Leu) Proline (Pro) Arginine (Arg) Glutamine (Gln) First base of RNA codon Third base of RNA codon Asparagine (Asn) Serine (Ser) Figure The dictionary of the genetic code, listed by RNA codons. Isoleucine (Ile) Threonine (Thr) Lysine (Lys) Arginine (Arg) Met or start Aspartic acid (Asp) Valine (Val) Alanine (Ala) Glycine (Gly) Glutamic acid (Glu) Figure 10.11 Chapter 1

Gene 1 DNA molecule Gene 2 Gene 3 DNA strand TRANSCRIPTION Figure Transcription of DNA and translation of codons. RNA TRANSLATION Codon Polypeptide Amino acid Figure 10.10 Chapter 1

Next amino acid to be added to polypeptide tRNA binding sites Growing polypeptide P site A site Ribosome tRNA Large subunit mRNA binding site mRNA Figure The ribosome. Small subunit Codons (a) A simplified diagram of a ribosome (b) The “players” of translation Figure 10.16 Chapter 1

Next amino acid to be added to polypeptide Growing polypeptide tRNA Figure 10.16b The ribosome. mRNA Codons (b) The “players” of translation Figure 10.16b Chapter 1

Transfer RNA (tRNA) Transfer RNA (tRNA): Acts as a molecular interpreter Carries amino acids Matches amino acids with codons in mRNA using anticodons Student Misconceptions and Concerns 1. Less experienced students are often intensely focused on writing detailed notes. The risk is that they miss the overall patterns and the broader significance of the topics discussed. Consider a gradual approach to the subjects of transcription and translation, beginning quite generally and testing comprehension, before venturing into the finer mechanics of each process. 2. Consider placing on the board the basic content from Figure 10.9, noting the sequence, products, and locations of transcription and translation in eukaryotic cells. This reminder can create a quick concept check for students as they learn additional detail. 3. Mutations are often discussed as part of evolution mechanisms. In this sense, mutations may be considered a part of a positive creative process. The dual nature of mutations, potentially deadly yet potentially innovative, should be clarified. Teaching Tips 1. It has been said that, everything about an organism is an interaction between the genome and the environment. You might wish to challenge your students to explain the significance and validity of this statement. 2. The authors note that the sequential information in DNA and RNA is analogous to the sequential information in the letters of a sentence. This analogy is also helpful when explaining the impact of insertion or deletion mutations that cause a shift in the reading frame. 3. The transcription of DNA into RNA is like a reporter who transcribes a political speech. In both situations, the language remains the same, although in the case of the reporter, it changes its form from spoken to written. 4. A parallel can be drawn between the discovery in 1799 of the Rosetta stone, which provided the key that enabled scholars to crack the previously indecipherable hieroglyphic code, and the cracking of the genetic code in Consider challenging your students to explain what part of the genetic code is similar to the Rosetta stone. This could be a short in-class activity for small groups. 5. Another advantage to the use of RNA to direct protein synthesis is that the original code (DNA) remains safely within the nucleus, away from the many potentially damaging chemicals in the cytoplasm. This is like making photocopies of important documents for study, keeping the originals safely stored away. 6. The production of proteins is like a machine requiring fuel. The molecular machinery (ribosomes and tRNA) used in many cellular processes also requires an input of energy in the form of ATP. 7. If you were using a train analogy for the assembly of monomers into polymers, at this point the DNA and RNA trains are traded in 3 for 1 for the polypeptide train. Thus, in general, polypeptides have about 1/3 as many monomers as the mRNA that coded for them. 8. After translation is addressed, consider asking your students (working singly or in small groups) to list all of the places where base pairing is used (in the construction of a DNA molecule during DNA replication, in transcription, and during translation when the tRNA attaches). 9. Students might want to think of the A and P sites as stages in an assembly line. The A site is where a new amino acid is brought in, according to the blueprint of the codon on the mRNA. The P site is where the growing product/polypeptide is anchored as it is being built. To help them better remember details of translation, students might think of the letters for the two sites to mean “A” for addition, where an amino acid is added, and ”P” for polypeptide, where the growing polypeptide is located. 10. A simple way to demonstrate the effect of a reading frame shift is to have students compare the following three sentences. The first is a simple sentence. But look what happens when a letter is added (2) or deleted (3). The reading frame, or triplet groupings, are re-formed into nonsense. (1) The big red pig ate the red rag. (2) The big res dpi gat eth ere dra g. (3) The big rep iga tet her edr ag. 11. The authors have noted elsewhere that a random mutation is like a shot in the dark. It is not likely to improve a genome any more than shooting a bullet through the hood of a car is likely to improve engine performance! Chapter 1

Amino acid attachment site Hydrogen bond RNA polynucleotide chain Figure The structure of tRNA Anticodon tRNA (simplified representation) tRNA polynucleotide (ribbon model) Figure 10.15 Chapter 1

Initiation occurs in two steps: First, an mRNA molecule binds to a small ribosomal subunit, then an initiator tRNA binds to the start codon. Second, a large ribosomal subunit binds, creating a functional ribosome. Student Misconceptions and Concerns 1. Less experienced students are often intensely focused on writing detailed notes. The risk is that they miss the overall patterns and the broader significance of the topics discussed. Consider a gradual approach to the subjects of transcription and translation, beginning quite generally and testing comprehension, before venturing into the finer mechanics of each process. 2. Consider placing on the board the basic content from Figure 10.9, noting the sequence, products, and locations of transcription and translation in eukaryotic cells. This reminder can create a quick concept check for students as they learn additional detail. 3. Mutations are often discussed as part of evolution mechanisms. In this sense, mutations may be considered a part of a positive creative process. The dual nature of mutations, potentially deadly yet potentially innovative, should be clarified. Teaching Tips 1. It has been said that, everything about an organism is an interaction between the genome and the environment. You might wish to challenge your students to explain the significance and validity of this statement. 2. The authors note that the sequential information in DNA and RNA is analogous to the sequential information in the letters of a sentence. This analogy is also helpful when explaining the impact of insertion or deletion mutations that cause a shift in the reading frame. 3. The transcription of DNA into RNA is like a reporter who transcribes a political speech. In both situations, the language remains the same, although in the case of the reporter, it changes its form from spoken to written. 4. A parallel can be drawn between the discovery in 1799 of the Rosetta stone, which provided the key that enabled scholars to crack the previously indecipherable hieroglyphic code, and the cracking of the genetic code in Consider challenging your students to explain what part of the genetic code is similar to the Rosetta stone. This could be a short in-class activity for small groups. 5. Another advantage to the use of RNA to direct protein synthesis is that the original code (DNA) remains safely within the nucleus, away from the many potentially damaging chemicals in the cytoplasm. This is like making photocopies of important documents for study, keeping the originals safely stored away. 6. The production of proteins is like a machine requiring fuel. The molecular machinery (ribosomes and tRNA) used in many cellular processes also requires an input of energy in the form of ATP. 7. If you were using a train analogy for the assembly of monomers into polymers, at this point the DNA and RNA trains are traded in 3 for 1 for the polypeptide train. Thus, in general, polypeptides have about 1/3 as many monomers as the mRNA that coded for them. 8. After translation is addressed, consider asking your students (working singly or in small groups) to list all of the places where base pairing is used (in the construction of a DNA molecule during DNA replication, in transcription, and during translation when the tRNA attaches). 9. Students might want to think of the A and P sites as stages in an assembly line. The A site is where a new amino acid is brought in, according to the blueprint of the codon on the mRNA. The P site is where the growing product/polypeptide is anchored as it is being built. To help them better remember details of translation, students might think of the letters for the two sites to mean “A” for addition, where an amino acid is added, and ”P” for polypeptide, where the growing polypeptide is located. 10. A simple way to demonstrate the effect of a reading frame shift is to have students compare the following three sentences. The first is a simple sentence. But look what happens when a letter is added (2) or deleted (3). The reading frame, or triplet groupings, are re-formed into nonsense. (1) The big red pig ate the red rag. (2) The big res dpi gat eth ere dra g. (3) The big rep iga tet her edr ag. 11. The authors have noted elsewhere that a random mutation is like a shot in the dark. It is not likely to improve a genome any more than shooting a bullet through the hood of a car is likely to improve engine performance! Chapter 1

Met Large ribosomal subunit Initiator tRNA P site A site mRNA Figure The initiation of translation Start codon Small ribosomal subunit Figure 10.18 Chapter 1

Amino acid Polypeptide P site mRNA Anticodon A site Codons Codon recognition ELONGATION Figure The elongation of a polypeptide. (Step 3) Peptide bond formation New peptide bond mRNA movement Translocation Figure Chapter 1

Amino acid Polypeptide P site mRNA Anticodon A site Codons Codon recognition ELONGATION Figure The elongation of a polypeptide. (Step 4) Stop codon Peptide bond formation New peptide bond mRNA movement Translocation Figure Chapter 1

CHAPTER 10 THE GENETICS OF VIRUSES The Genetics of Viruses
1. Researchers discovered viruses by studying a plant disease 2. A virus is a genome enclosed in a protective coat 3. Viruses can only reproduce within a host cell: an overview 4. Phages reproduce using lytic or lysogenic cycles 5. Animal viruses are diverse in their modes of infection and replication 6. Plant viruses are serious agricultural pests 7. Viroids and prions are infectious agents even simpler than viruses. 8. Viruses may have evolved from other mobile genetic elements Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

CHAPTER 10 THE GENETICS OF VIRUSES
Viruses and bacteria are the simplest biological systems - microbial models where scientists find life’s fundamental molecular mechanisms in their most basic, accessible forms. Microbiologists provided most of the evidence that genes are made of DNA, and they worked out most of the major steps in DNA replication, transcription, and translation. Viruses and bacteria also have interesting, unique genetic features with implications for understanding diseases that they cause.

Bacteria are prokaryotic organisms.
Their cells are much smaller and more simply organized that those of eukaryotes, such as plants and animals. Viruses are smaller and simpler still, lacking the structure and most meta- bolic machinery in cells. Most viruses are little more than aggregates of nucleic acids and protein - genes in a protein coat. Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

I. Researchers discovered viruses by studying a plant disease
The story of how viruses were discovered begins in 1883 with research on the cause of tobacco mosaic disease by Adolf Mayer. This disease stunts the growth and mottles plant leaves. Mayer concluded that the disease was infectious when he found that he could transmit the disease by spraying sap from diseased leaves onto healthy plants. He concluded that the disease must be caused by an extremely small bacterium, but Dimitri Ivanovsky demonstrated that the sap was still infectious even after passing through a filter designed to remove bacteria.

In 1897 Martinus Beijerinck ruled out the possibility that the disease was due to a filterable toxin produced by a bacterium and demonstrated that the infectious agent could reproduce. The sap from one generation of infected plants could be used to infect a second generation of plants which could infect subsequent generations. Bierjink also determined that the pathogen could reproduce only within the host, could not be cultivated on nutrient media, and was not inactivated by alcohol, generally lethal to bacteria. In 1935, Wendell Stanley crystallized the pathogen, the tobacco mosaic virus (TMV).

II. A virus is a genome enclosed in a protective coat
Stanley’s discovery that some viruses could be crystallized was puzzling because not even the simplest cells can aggregate into regular crystals. However, viruses are not cells. They are infectious particles consisting of nucleic acid encased in a protein coat, and, in some cases, a membranous envelope. Viruses range in size from only 20nm in diameter to that barely resolvable with a light microscope. Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

The genome of viruses includes other options than the double-stranded DNA that we have studied.
Viral genomes may consist of double-stranded DNA, single-stranded DNA, double-stranded RNA, or single-stranded RNA, depending on the specific type of virus. The viral genome is usually organized as a single linear or circular molecule of nucleic acid. The smallest viruses have only four genes, while the largest have several hundred. Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

The capsid is a protein shell enclosing the viral genome.
Capsids are built of a large number of protein subunits called capsomeres, but with limited diversity. The capsid of the tobacco mosaic virus has over 1,000 copies of the same protein. Adenoviruses have 252 identical proteins arranged into a polyhedral capsid - as an icosahedron. Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

Some viruses have viral envelopes, membranes cloaking their capsids.
These envelopes are derived from the membrane of the host cell. They also have some viral proteins and glycoproteins. Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

The most complex capsids are found in viruses that infect bacteria, called bacteriophages or phages.
The T-even phages that infect Escherichia coli have a 20-sided capsid head that encloses their DNA and protein tail piece that attaches the phage to the host and injects the phage DNA inside. Fig. 18.2d Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

III. Viruses can reproduce only within a host cell: an overview
Viruses are obligate intracellular parasites. They can reproduce only within a host cell. An isolated virus is unable to reproduce - or do anything else, except infect an appropriate host. Viruses lack the enzymes for metabolism or ribosomes for protein synthesis. An isolated virus is merely a packaged set of genes in transit from one host cell to another. Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

Most viruses of eukaryotes attack specific tissues.
Each type of virus can infect and parasitize only a limited range of host cells, called its host range. Viruses identify host cells by a “lock-and-key” fit between proteins on the outside of virus and specific receptor molecules on the host’s surface. Some viruses (like the rabies virus) have a broad enough host range to infect several species, while others infect only a single species. Most viruses of eukaryotes attack specific tissues. Human cold viruses infect only the cells lining the upper respiratory tract. The AIDS virus binds only to certain white blood cells. Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

A viral infection begins when the genome of the virus enters the host cell.
Once inside, the viral genome commandeers its host, reprogramming the cell to copy viral nucleic acid and manufacture proteins from the viral genome. The nucleic acid molecules and capsomeres then self-assemble into viral particles and exit the cell. Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

IV. Phages reproduce using lytic or lysogenic cycles
While phages are the best understood of all viruses, some of them are also among the most complex. Research on phages led to the discovery that some double-stranded DNA viruses can reproduce by two alternative mechanisms: the lytic cycle and the lysogenic cycle. Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

Virulent phages reproduce only by a lytic cycle.
In the lytic cycle, the phage reproductive cycle culminates in the death of the host. In the last stage, the bacterium lyses (breaks open) and releases the phages produced within the cell to infect others. Virulent phages reproduce only by a lytic cycle. Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

But, natural selection favors resistant phage mutants.
While phages have the potential to wipe out a bacterial colony in just hours, bacteria have defenses against phages. Natural selection favors bacterial mutants with receptors sites that are no longer recognized by a particular type of phage. Bacteria produce restriction endonucleases that recognize and cut up foreign DNA, including certain phage DNA. Modifications to the bacteria’s own DNA prevent its destruction by restriction nucleases. But, natural selection favors resistant phage mutants. Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

In the lysogenic cycle, the phage genome replicates without destroying the host cell.
Temperate phages, like phage lambda, use both lytic and lysogenic cycles. Within the host, the virus’ circular DNA engages in either the lytic or lysogenic cycle. During a lytic cycle, the viral genes immediately turn the host cell into a virus-producing factory, and the cell soon lyses and releases its viral products. Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

The viral DNA molecule, during the lysogenic cycle, is incorporated by genetic recombination into a specific site on the host cell’s chromosome. In this prophage stage, one of its genes codes for a protein that represses most other prophage genes. Every time the host divides, it also copies the viral DNA and passes the copies to daughter cells. Occasionally, the viral genome exits the bacterial chromosome and initiates a lytic cycle. This switch from lysogenic to lytic may be initiated by an environmental trigger.(Such as UV with Herpes simplex I) Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

V. Animal viruses are diverse in their modes of infection and replication
Many variations on the basic scheme of viral infection and reproductions are represented among animal viruses. One key variable is the type of nucleic acid that serves as a virus’ genetic material. Another variable is the presence or absence of a membranous envelope. (Classes of viruses are shown here) Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

Viruses equipped with an outer envelope use the envelope to enter the host cell.
Glycoproteins on the envelope bind to specific receptors on the host’s membrane. The envelope fuses with the host’s membrane, transporting the capsid and viral genome inside. The viral genome duplicates and directs the host’s protein synthesis machinery to synthesize capsomeres with free ribosomes and glycoproteins with bound ribosomes. After the capsid and viral genome self-assemble, they bud from the host cell covered with an envelope derived from the host’s plasma membrane, including viral glycoproteins. Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

Some viruses have envelopes that are not derived from plasma membrane.
The envelope of the herpes virus is derived from the nuclear envelope of the host. These double-stranded DNA viruses reproduce within the cell nucleus using viral and cellular enzymes to replicate and transcribe their DNA. Herpes virus DNA may become integrated into the cell’s genome as a provirus. The provirus remains latent within the nucleus until triggered by physical or emotional stress to leave the genome and initiate active viral production. Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

The viruses that use RNA as the genetic material are quite diverse, especially those that infect animals. In some with single-stranded RNA (class IV), the genome acts as mRNA and is translated directly. In others (class V), the RNA genome serves as a template for mRNA and for a complementary RNA. This complementary strand is the template for the synthesis of additional copies of genome RNA. All viruses that require RNA -> RNA synthesis to make mRNA use a viral enzyme (RNA transcriptase) that is packaged with the genome inside the capsid. Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

Retroviruses (class VI) have the most complicated life cycles.
These carry an enzyme, reverse transcriptase, which transcribes DNA from an RNA template. The newly made DNA is inserted as a provirus into a chromosome in the animal cell. The host’s RNA polymerase transcribes the viral DNA into more RNA molecules. These can function both as mRNA for the synthesis of viral proteins and as genomes for new virus particles released from the cell. Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

Human immunodeficiency virus (HIV), the virus that causes AIDS (acquired immunodeficiency syndrome) is a retrovirus. The viral particle includes an envelope with glycoproteins for binding to specific types of white blood cells, a capsid containing two identical RNA strands as its genome and two copies of reverse transcriptase. Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

The reproductive cycle of HIV illustrates the pattern of infection and replication in a retrovirus.
After HIV enters the host cell, reverse transcriptase synthesizes double stranded DNA from the viral RNA. Transcription produces more copies of the viral RNA that are translated into viral proteins, which self-assemble into a virus particle and leave the host. Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

The link between viral infection and the symptoms it produces is often obscure.
Some viruses damage or kill cells by triggering the release of hydrolytic enzymes from lysosomes. Some viruses cause the infected cell to produce toxins that lead to disease symptoms. Other have molecular components, such as envelope proteins, that are toxic. In some cases, viral damage is easily repaired (respiratory epithelium after a cold), but in others, infection causes permanent damage (nerve cells after polio). Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

Many of the temporary symptoms associated with a viral infection results from the body’s own efforts at defending itself against infection. The immune system is a complex and critical part of the body’s natural defense mechanism against viral and other infections. Modern medicine has developed vaccines, harmless variants or derivatives of pathogenic microbes, that stimulate the immune system to mount defenses against the actual pathogen. Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

When exposed to smallpox, the boy resisted the disease.
The first vaccine was developed in the late 1700s by Edward Jenner to fight smallpox. Jenner learned from his patients that milkmaids who had contracted cowpox, a milder disease that usually infects cows, were resistant to smallpox. In his famous experiment in 1796, Jenner infected a farmboy with cowpox, acquired from the sore of a milkmaid with the disease. When exposed to smallpox, the boy resisted the disease. Because of their similarities, vaccination with the cowpox virus sensitizes the immune system to react vigorously if exposed to actual smallpox virus. Effective vaccines against many other viruses exist. Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

All tumor viruses transform cells into cancer cells after integration of viral nucleic acid into host DNA. Viruses may carry oncogenes that trigger cancerous characteristics in cells. These oncogenes are often versions of proto-oncogenes that influence the cell cycle in normal cells. Proto-oncogenes generally code for growth factors or proteins involved in growth factor function. In other cases, a tumor virus transforms a cell by turning on or increasing the expression of proto-oncogenes. It is likely that most tumor viruses cause cancer only in combination with other mutagenic events. Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

VII. Viroids and prions are infectious agents even simpler than viruses
Viroids, smaller and simpler than even viruses, consist of tiny molecules of naked circular RNA that infect plants. Their several hundred nucleotides do not encode for proteins but can be replicated by the host’s cellular enzymes. These RNA molecules can disrupt plant metabolism and stunt plant growth, perhaps by causing errors in the regulatory systems that control plant growth. “Cadang-Cadang” disease of coconut palms (10x106 killed) - causes errors in regulatory systems that control genes. Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

Prions are infectious proteins that spread a disease.
They appear to cause several degenerative brain diseases including scrapie in sheep, “mad cow disease”, and Creutzfeldt-Jacob disease in humans. According to the leading hypothesis, a prion is a misfolded form of a normal brain protein. It can then convert a normal protein into the prion version, creating a chain reaction that increases their numbers. Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

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Chapter 10 DNA: The Molecule of Heredity - Structure and Function

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Chapter 10 DNA: The Molecule of Heredity - Structure and Function

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