1. Chapter 12
DNA and RNA

2. 12–1 DNA
Photo credit: Jacob Halaska/Index Stock Imagery, Inc.

3. 12–1 DNA Griffith and Transformation
In 1928, British scientist Fredrick Griffith was trying to learn how certain types of bacteria caused pneumonia.
He isolated two different strains of pneumonia bacteria from mice and grew them in his lab.

4. 12-1 DNA Griffith's Experiments
Griffith set up four individual experiments.
Experiment 1: Mice were injected with the disease-causing strain of bacteria. The mice developed pneumonia and died.
Griffith injected mice with four different samples of bacteria. When injected separately, neither heat-killed, disease-causing bacteria nor live, harmless bacteria killed the mice. The two types injected together, however, caused fatal pneumonia. From this experiment, biologists inferred that genetic information could be transferred from one bacterium to another.

5. Experiment 2: Mice were injected with the harmless strain of bacteria
Experiment 2: Mice were injected with the harmless strain of bacteria. These mice didn’t get sick.
Griffith injected mice with four different samples of bacteria. When injected separately, neither heat-killed, disease-causing bacteria nor live, harmless bacteria killed the mice. The two types injected together, however, caused fatal pneumonia. From this experiment, biologists inferred that genetic information could be transferred from one bacterium to another.

6. Experiment 3: Griffith heated the disease-causing bacteria
Experiment 3: Griffith heated the disease-causing bacteria. He then injected the heat-killed bacteria into the mice. The mice survived.
Heat-killed disease-causing bacteria (smooth colonies)
Griffith injected mice with four different samples of bacteria. When injected separately, neither heat-killed, disease-causing bacteria nor live, harmless bacteria killed the mice. The two types injected together, however, caused fatal pneumonia. From this experiment, biologists inferred that genetic information could be transferred from one bacterium to another.
Lives

7. Heat-killed disease-causing bacteria (smooth colonies)
Experiment 4: Griffith mixed his heat-killed, disease-causing bacteria with live, harmless bacteria and injected the mixture into the mice. The mice developed pneumonia and died.
Harmless bacteria (rough colonies)
Griffith injected mice with four different samples of bacteria. When injected separately, neither heat-killed, disease-causing bacteria nor live, harmless bacteria killed the mice. The two types injected together, however, caused fatal pneumonia. From this experiment, biologists inferred that genetic information could be transferred from one bacterium to another.
Live disease-causing bacteria (smooth colonies)
Dies of pneumonia

8. Heat-killed disease-causing bacteria (smooth colonies)
Griffith concluded that the heat-killed bacteria passed their disease-causing ability to the harmless strain.
Harmless bacteria (rough colonies)
Griffith injected mice with four different samples of bacteria. When injected separately, neither heat-killed, disease-causing bacteria nor live, harmless bacteria killed the mice. The two types injected together, however, caused fatal pneumonia. From this experiment, biologists inferred that genetic information could be transferred from one bacterium to another.
Live disease-causing bacteria (smooth colonies)
Dies of pneumonia

9. 12-1 DNA
Transformation 
Griffith called this process transformation because one strain of bacteria (the harmless strain) had changed permanently into another (the disease-causing strain).
Griffith hypothesized that a factor must contain information that could change harmless bacteria into disease-causing ones.

10. 12-1 DNA
Avery and DNA
Oswald Avery repeated Griffith’s work to determine which molecule was most important for transformation.
Avery and his colleagues made an extract from the heat-killed bacteria that they treated with enzymes.

11. 12-1 DNA
The enzymes destroyed proteins, lipids, carbohydrates, and other molecules, including the nucleic acid RNA.
Transformation still occurred.

12. 12-1 DNA
Avery and other scientists repeated the experiment using enzymes that would break down DNA.
When DNA was destroyed, transformation did not occur. Therefore, they concluded that DNA was the transforming factor.

13. 12-1 DNA
Avery and DNA
Avery and other scientists discovered that the nucleic acid DNA stores and transmits the genetic information from one generation of an organism to the next.

14. 12-1 DNA The Hershey-Chase Experiment
Alfred Hershey and Martha Chase studied viruses—particles smaller than a cell that can infect living organisms.

15. 12-1 DNA
Bacteriophages 
A virus that infects bacteria is known as a bacteriophage.
Bacteriophages are composed of a DNA or RNA core and a protein coat.

16. 12-1 DNA
When a bacteriophage enters a bacterium, the virus attaches to the surface of the cell and injects its genetic information into it.
The viral genes produce many new bacteriophages, which eventually destroy the bacterium.
When the cell splits open, hundreds of new viruses burst out.

17. 12-1 DNA
If Hershey and Chase could determine which part of the virus entered an infected cell, they would learn whether genes were made of protein or DNA.
They grew viruses in cultures containing radioactive isotopes of phosphorus-32 (32P) and sulfur-35 (35S).

18. 12-1 DNA
If 35S was found in the bacteria, it would mean that the viruses’ protein had been injected into the bacteria.
Alfred Hershey and Martha Chase used different radioactive markers to label the DNA and proteins of bacteriophages. The bacteriophages injected only DNA into the bacteria, not proteins. From these results, Hershey and Chase concluded that the genetic material of the bacteriophage was DNA.

19. 12-1 DNA
If 32P was found in the bacteria, then it was the DNA that had been injected.
Alfred Hershey and Martha Chase used different radioactive markers to label the DNA and proteins of bacteriophages. The bacteriophages injected only DNA into the bacteria, not proteins. From these results, Hershey and Chase concluded that the genetic material of the bacteriophage was DNA.

20. 12-1 DNA
Nearly all the radioactivity in the bacteria was from phosphorus (32P).
Hershey and Chase concluded that the genetic material of the bacteriophage was DNA, not protein.

21. 12-1 DNA The Components and Structure of DNA
DNA is made up of nucleotides.
A nucleotide is a monomer of nucleic acids made up of a five-carbon sugar called deoxyribose, a phosphate group, and a nitrogenous base.

22. 12-1 DNA There are four kinds of bases in DNA: adenine guanine
cytosine
thymine
DNA is made up of nucleotides. Each nucleotide has three parts: a deoxyribose molecule, a phosphate group, and a nitrogenous base. There are four different bases in DNA: adenine, guanine, cytosine, and thymine.

23. 12-1 DNA
The backbone of a DNA chain is formed by sugar and phosphate groups of each nucleotide.
The nucleotides can be joined together in any order.

24. 12-1 DNA Chargaff's Rules Erwin Chargaff discovered that:
The percentages of guanine [G] and cytosine [C] bases are almost equal in any sample of DNA.
The percentages of adenine [A] and thymine [T] bases are almost equal in any sample of DNA.

25. 12-1 DNA
X-Ray Evidence 
Rosalind Franklin used X-ray diffraction to get information about the structure of DNA.
She aimed an X-ray beam at concentrated DNA samples and recorded the scattering pattern of the X-rays on film.
This X-ray diffraction photograph of DNA was taken by Rosalind Franklin in the early 1950s. The X-shaped pattern in the center indicates that the structure of DNA is helical.
Photo credit: ©Cold Spring Harbor Laboratory Archives/Peter Arnold, Inc.

26. 12-1 DNA
The Double Helix 
Using clues from Franklin’s pattern, James Watson and Francis Crick built a model that explained how DNA carried information and could be copied.
Watson and Crick's model of DNA was a double helix, in which two strands were wound around each other.

27. 12-1 DNA
DNA Double Helix
DNA is a double helix in which two strands are wound around each other. Each strand is made up of a chain of nucleotides. The two strands are held together by hydrogen bonds between adenine and thymine and between guanine and cytosine.

28. 12-1 DNA
Watson and Crick discovered that hydrogen bonds can form only between certain base pairs—adenine and thymine, and guanine and cytosine.
This principle is called base pairing.

29. 12-2 Chromosomes and DNA Replication
Photo credit: Jacob Halaska/Index Stock Imagery, Inc.

30. 12–2 Chromosomes and DNA Replication
DNA and Chromosomes
In prokaryotic cells, DNA is located in the cytoplasm.
Most prokaryotes have a single DNA molecule containing nearly all of the cell’s genetic information.

31. 12–2 Chromosomes and DNA Replication
Most prokaryotes, such as this E. coli bacterium, have only a single circular chromosome. This chromosome holds most of the organism’s DNA.
E. Coli Bacterium
Bases on the Chromosomes

32. 12–2 Chromosomes and DNA Replication
Many eukaryotes have 1000 times the amount of DNA as prokaryotes.
Eukaryotic DNA is located in the cell nucleus inside chromosomes.
The number of chromosomes varies widely from one species to the next.

33. 12–2 Chromosomes and DNA Replication
Chromosome Structure
Eukaryotic chromosomes contain DNA and protein, tightly packed together to form chromatin.
Chromatin consists of DNA tightly coiled around proteins called histones.
DNA and histone molecules form nucleosomes.
Nucleosomes pack together, forming a thick fiber.

34. 12–2 Chromosomes and DNA Replication
Eukaryotic Chromosome Structure
Chromosome
Nucleosome
DNA double helix
Coils
Supercoils
Eukaryotic chromosomes contain DNA wrapped around proteins called histones. The strands of nucleosomes are tightly coiled and supercoiled to form chromosomes.
Histones

35. 12–2 Chromosomes and DNA Replication
Each strand of the DNA double helix has all the information needed to reconstruct the other half by the mechanism of base pairing.
In most prokaryotes, DNA replication begins at a single point and continues in two directions.

36. 12–2 Chromosomes and DNA Replication
In eukaryotic chromosomes, DNA replication occurs at hundreds of places. Replication proceeds in both directions until each chromosome is completely copied.
The sites where separation and replication occur are called replication forks.

37. 12–2 Chromosomes and DNA Replication
Duplicating DNA 
Before a cell divides, it duplicates its DNA in a process called replication.
Replication ensures that each resulting cell will have a complete set of DNA.

38. 12–2 Chromosomes and DNA Replication
During DNA replication, the DNA molecule separates into two strands,
then produces two new complementary strands following the rules of base pairing.
Each strand of the double helix of DNA serves as a template for the new strand.

39. 12–2 Chromosomes and DNA Replication
New Strand
Original strand
Nitrogen Bases
Growth
Growth
During DNA replication, the DNA molecule produces two new complementary strands. Each strand of the double helix of DNA serves as a template for the new strand.
Replication Fork
Replication Fork
DNA Polymerase

40. 12–2 Chromosomes and DNA Replication
How Replication Occurs
DNA replication is carried out by enzymes that “unzip” a molecule of DNA.
Hydrogen bonds between base pairs are broken and the two strands of DNA unwind.

41. 12–2 Chromosomes and DNA Replication
The principal enzyme involved in DNA replication is DNA polymerase.
DNA polymerase joins individual nucleotides to produce a DNA molecule and then “proofreads” each new DNA strand.

42. 12–3 RNA and Protein Synthesis

43. 12–3 RNA and Protein Synthesis
Genes are coded DNA instructions that control the production of proteins.
Genetic messages can be decoded by copying part of the nucleotide sequence from DNA into RNA.
RNA contains coded information for making proteins.

44. 12–3 RNA and Protein Synthesis
The Structure of RNA
RNA consists of a long chain of nucleotides.
Each nucleotide is made up of a 5-carbon sugar (ribose), a phosphate group, and a nitrogenous base.

45. 12–3 RNA and Protein Synthesis
There are three main differences between RNA and DNA:
The sugar in RNA is ribose instead of deoxyribose.
RNA is generally single-stranded.
RNA contains uracil in place of thymine.

46. 12–3 RNA and Protein Synthesis
Types of RNA
There are three main types of RNA:
messenger RNA
ribosomal RNA
transfer RNA

47. 12–3 RNA and Protein Synthesis
The three main types of RNA are messenger RNA, ribosomal RNA, and transfer RNA.
Messenger RNA (mRNA) carries copies of instructions for assembling amino acids into proteins.

48. 12–3 RNA and Protein Synthesis
Ribosome
Ribosomal RNA
The three main types of RNA are messenger RNA, ribosomal RNA, and transfer RNA. Ribosomal RNA is combined with proteins to form ribosomes.
Ribosomes are made up of proteins and ribosomal RNA (rRNA).

49. 12–3 RNA and Protein Synthesis
Amino acid
The three main types of RNA are messenger RNA, ribosomal RNA, and transfer RNA.
Transfer RNA
During protein construction, transfer RNA (tRNA) transfers each amino acid to the ribosome.

50. 12–3 RNA and Protein Synthesis
Transcription
RNA molecules are produced by copying part of a nucleotide sequence of DNA into a complementary sequence in RNA. This process is called transcription.
Transcription requires the enzyme RNA polymerase.

51. 12–3 RNA and Protein Synthesis
During transcription, RNA polymerase binds to DNA and separates the DNA strands.
RNA polymerase then uses one strand of DNA as a template from which nucleotides are assembled into a strand of RNA.

52. 12–3 RNA and Protein Synthesis
RNA polymerase binds only to regions of DNA known as promoters.
Promoters are signals in DNA that indicate to the enzyme where to bind to make RNA.

53. 12–3 RNA and Protein Synthesis
RNA polymerase
DNA
During transcription, RNA polymerase uses one strand of DNA as a template to assemble nucleotides into a strand of RNA.

54. 12–3 RNA and Protein Synthesis
RNA Editing
The DNA of eukaryotic genes contains sequences of nucleotides, called introns, that are not involved in coding for proteins.
The DNA sequences that code for proteins are called exons.
When RNA molecules are formed, introns and exons are copied from DNA.

55. 12–3 RNA and Protein Synthesis
Exon
Intron
DNA
The introns are cut out of RNA molecules.
The exons are the spliced together to form mRNA.
Pre-mRNA
mRNA
Many RNA molecules have sections, called introns, edited out of them before they become functional. The remaining pieces, called exons, are spliced together. Then, a cap and tail are added to form the final RNA molecule.
Cap
Tail

56. 12–3 RNA and Protein Synthesis
The Genetic Code
The genetic code is the “language” of mRNA instructions.
The code is written using four “letters” (the bases: A, U, C, and G).

57. 12–3 RNA and Protein Synthesis
A codon consists of three consecutive nucleotides on mRNA that specify a particular amino acid.
A codon is a group of three nucleotides on messenger RNA that specify a particular amino acid.

58. 12–3 RNA and Protein Synthesis
Each codon specifies a particular amino acid that is to be placed on the polypeptide chain.
Some amino acids can be specified by more than one codon.

59. 12–3 RNA and Protein Synthesis
There is one codon AUG that can either specify the amino acid methionine or serve as a “start” codon for protein synthesis.
There are three “stop” codons that do not code for any amino acid. These “stop” codons signify the end of a polypeptide.

60. 12–3 RNA and Protein Synthesis
Translation
Translation is the decoding of an mRNA message into a polypeptide chain (protein).
Translation takes place on ribosomes.
During translation, the cell uses information from messenger RNA to produce proteins.

61. 12–3 RNA and Protein Synthesis
Messenger RNA is transcribed in the nucleus, and then enters the cytoplasm where it attaches to a ribosome.
Nucleus
During translation, or protein synthesis, the cell uses information from messenger RNA to produce proteins. The cell uses all three main forms of RNA during this process.
mRNA

62. 12–3 RNA and Protein Synthesis
Translation begins when an mRNA molecule attaches to a ribosome.
As each codon of the mRNA molecule moves through the ribosome, the proper amino acid is brought into the ribosome by tRNA.
In the ribosome, the amino acid is transferred to the growing protein chain.

63. 12–3 RNA and Protein Synthesis
Each tRNA molecule carries only one kind of amino acid.
In addition to an amino acid, each tRNA molecule has three unpaired bases.
These bases, called the anticodon, are complementary to one mRNA codon.

64. 12–3 RNA and Protein Synthesis
The ribosome binds new tRNA molecules and amino acids as it moves along the mRNA.
Lysine
Phenylalanine
tRNA
Methionine
Ribosome
During translation, or protein synthesis, the cell uses information from messenger RNA to produce proteins. The cell uses all three main forms of RNA during this process.
mRNA
Start codon

65. 12–3 RNA and Protein Synthesis
Lysine
tRNA
During translation, or protein synthesis, the cell uses information from messenger RNA to produce proteins. The cell uses all three main forms of RNA during this process.
mRNA
Translation direction
Ribosome

66. 12–3 RNA and Protein Synthesis
The process continues until the ribosome reaches a stop codon.
Polypeptide
Ribosome
tRNA
During translation, or protein synthesis, the cell uses information from messenger RNA to produce proteins. The cell uses all three main forms of RNA during this process.
mRNA

67. 12–3 RNA and Protein Synthesis
The Roles of RNA and DNA
The cell uses the DNA “master plan” to prepare RNA “blueprints.” The DNA stays in the nucleus.
The RNA molecules go to the protein building sites in the cytoplasm—the ribosomes.

68. 12–3 RNA and Protein Synthesis
Genes and Proteins
Genes contain instructions for assembling proteins.
Many proteins are enzymes, which catalyze and regulate chemical reactions.
Proteins are each specifically designed to build or operate a component of a living cell.

69. Amino acids within a polypeptide
Codon
Codon
Codon
The sequence of bases in DNA is used as a template for mRNA.
The codons of mRNA specify the sequence of amino acids in a protein.
Single strand of DNA
Codon
Codon
Codon
mRNA
This diagram illustrates how information for specifying the traits of an organism is carried in DNA. The sequence of bases in DNA is used as a template for mRNA. The codons of mRNA specify the sequence of amino acids in a protein, and proteins play a key role in producing an organism’s traits.
Alanine
Arginine
Leucine
Amino acids within a polypeptide