DNA: The Molecule of Heredity Chapter 8 DNA: The Molecule of Heredity

8-1 DNA 1928 Frederick Griffith: How does bacteria cause pneumonia? Isolated two strains of pneumonia bacteria in mice; only one of them caused disease Disease strain had smooth colonies; the harmless strain had rough colonies When injected with the disease strain, mice got sick and died; w/ the harmless strain, mice stayed healthy

Griffith cont Next Griffith heat-killed the disease strain and injected it into mice; the mice stayed healthy Next he mixed the heat-killed strain with the harmless strain and injected mice; mice developed pneumonia and died Killed bacteria passed on a GENE to the harmless bacteria which then developed the disease causing capability

Griffith’s experiment

DNA 1944 Oswald Avery repeated Griffith’s experiment and determined that DNA stores and transmits genetic information from one generation to the next

Avery repeated the experiment

Avery’s conclusion

DNA 1952 Alfred Hershey and Martha Chase proved the importance of the chemical nature of DNA Conducted experiments using bacterial viruses Proved that the viral DNA was the part that caused disease

Hershey and Chase’s experiment

8-2 Chargaff’s Rules Erwin Chargaff discovered the percentages of nucleotides in DNA Amounts of guanine = amounts of cytosine Amounts of adenine = amounts of thymine

Rosalind Franklin 1950s used X-ray diffraction to study DNA X-ray created scattered patterns as a result of the reflection of the X-rays

Francis Crick and James Watson 1953 developed the double helix model of DNA Used the X-ray diffraction patterns from Franklin’s studies to help determine the structure

Watson and Crick

12-2 Structure and Function of DNA The components of DNA are deoxyribose , a phosphate group, and a nitrogen base. Nitrogen base- an organic ring structure that contains one or more atoms of nitrogen.

DNA Molecule

Four Possible Nitrogen Bases Adenine (A) Guanine (G) Cytosine (C) Thymine (T) This allows for four nucleotides, each containing one of these four bases These base pairs match as follows: (A) with (T) & (G) with (C)

Chains of Nucleotides Nucleotides do not exist as individual molecules, they combine to form long chains to produce one molecule The two chains are held by hydrogen bonds between the bases This structure resembles a ladder The shape of DNA is known as a double helix because it looks like a twisted ladder

Importance of Nucleotide Sequence The genetic material of living things is made of DNA, they are different because of the order of the nucleotides in the DNA strands of the organism The sequence of nucleotides forms the unique genetic information of an organism The more closely related the more alike the DNA strands are

8-3 Replication of DNA DNA replicates because every time a cell divides, it must make a copy of its chromosomes, so each cell can have a complete set Without replication, species could not survive and individuals could not successfully grow and reproduce

How DNA Replicates During replication, each strand serves as a pattern to make a new DNA molecule It begins as an enzyme breaks the hydrogen bond between nitrogen bases that hold the two strands together The action “unzips” the DNA molecule As the DNA unzips, free nucleotides from the surroundings in the nucleus, bond to the single strands by base pairing

DNA Replication Cont. Another enzyme bonds these new nucleotides into a chain This process continues until the entire molecule has been unzipped and replicated As a result a new strand is formed that is a complement of one of the original, or parent strand The result is the formation of two DNA molecules, each identical to the original DNA molecule

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8-4 DNA to Protein Genetic Code: The sequence of nitrogen base along one of the two strands for the synthesis of proteins There are 20 different amino acids. But DNA only contains 4 bases. A single base can’t represent a single amino acid because that system would code for only 4 different amino acids A sequence of 3 bases provides more than 20 combinations needed to code for all amino acids

Codon: each set of 3 nitrogen bases representing an amino acid The DNA code is often called a triplet code 64 combinations are possible when a sequence of three bases is used, 64 different codons are in the genetic code The order of nitrogen bases in DNA will determine the order of the amino acids in a protein

Codon cont. For any one codon there can only be one amino acid The code is said to be universal because the codons represent the same amino acids in all organisms

Transcription-from DNA to RNA RNA structure: RNA is a nucleic acid Differs from DNA structure in 3 ways: 1. RNA is usually composed of a single strand rather than a double strand 2. RNA also contains a different type of sugar molecule, ribose instead of deoxyribose 3. RNA also contains four nitrogen bases, but rather than thymine RNA contains Uracil (U)

Making RNA Transcription: Transcription: the process by which enzymes make an RNA copy of a DNA strand This process is similar to DNA replication except the main difference is that the process results in the formation of a single-stranded RNA molecule This RNA copy carries information from the DNA out into the cytoplasm of the cell

RNA by Transcription This is called messenger RNA (mRNA) mRNA carries the information for making a protein chain Some portions of DNA code for the RNA that makes up ribosomes, where proteins are synthesized This type of RNA is called ribosomal RNA (rRNA) rRNA helps to produce enzymes needed to bond amino acids together during protein synthesis

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8-5 Translation –RNA to Protein Translation: the process of converting information in a sequence of nitrogen bases in mRNA into a sequence if amino acids that make up protein This occurs on ribosomes, and involves a third kind of RNA Transfer RNA (tRNA): brings amino acids to the ribosomes so they can be assembled into proteins

Translating the mRNA code: Correct translation of the code depends upon the joining of each mRNA codon with the anticodons of the proper tRNA molecules The end result of translation is the formation of the large variety of proteins that make up the structure of organisms and help them function

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8-7 Genetic Changes Mutation – A Change in DNA Mutation – any mistake or change in the DNA sequence Point Mutation – a change in a single base pair in DNA (EX) The dog chased the car. Changing a single letter in this sentence changes the entire meaning, a change in a single nitrogen base can change the entire structure of the protein.

11.3 Section Summary 6.3 – pages 296 - 301 The effects of point mutations mRNA Normal Protein Stop Replace G with A Point mutation mRNA Protein Stop 11.3 Section Summary 6.3 – pages 296 - 301

Mutations cont. Frameshift mutation: a mutation when a single base is added or deleted from DNA EX: a mRNA strand has a new amino acid added to the protein for every codon on the mRNA strand; now a single base is deleted from that strand; this new sequence is now translated into mRNA, but it is out of position by one base; this causes every codon to be out of position by one base; this would cause every amino acid to be changed

11.3 Section Summary 6.3 – pages 296 - 301 Frameshift mutations Deletion of U Frameshift mutation mRNA Protein 11.3 Section Summary 6.3 – pages 296 - 301

Chromosomal Mutations Mutation affecting gene distribution to gametes during meiosis, most commonly by deletions, insertions, inversions, or translocations Some ways they are described as mutating include: parts of the chromosomes are broken off or lost during mitosis or meiosis, chromosomes break and then rejoin incorrectly, or the parts join backwards or even to the wrong chromosome

Effects of chromosomal mutation Occur in living organisms, but they are especially common in plants Affect the distribution of genes to gametes during meiosis Gametes that should have a complete set of genes may end up with extra copies of some genes or a complete lack of certain genes Few chromosomes mutations are passed on to the next generation because the zygote usually dies

Types of chromosomal mutation Deletions: occur when part of a chromosome is left out Insertions: occur when a part of a chromatid breaks off and attaches to its sister chromatid. The result of genes on the same chromosome. Inversions: occur when part of a chromosome breaks out and is reinserted backwards Translocation: occur when part of one chromosome breaks off and is added to a different chromosome

Errors in Disjunction Many chromosomal mutations result from the failure of chromosomes to separate properly during meiosis Nondisjunction: the failure of homologous chromosomes to separate properly during meiosis

In one form of nondisjunction, 2 kinds of gametes result: Trisomy: the presence of an extra chromosome Triploidy: involves a total lack of separation of homologous chromosomes (this condition is rare in animals, but frequent in plants) Monosomy: absence of a chromosome (these usually do not survive)

Causes of Mutation Spontaneous mutation: mutations that occur at random Environmental agents also cause mutations, such as exposure to x-rays, ultraviolet light, radioactive substances, or certain chemicals Mutations often result in sterility or the lack of normal development in an organism In human gametes mutations may cause birth defects or in body cells it may cause cancer