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Chapter 14

Chapter 14 Textbook Notes - DNA and the Gene: Synthesis and Repair

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Department
Biology
Course
BIO152H5
Professor
Fiona Rawle
Semester
Fall

Description
Notes From Reading CHAPTER 14:DNA AND THE G ENE:S YNTHESIS AND REPAIR (PGS.309-329) Chapter 14- DNA and the Gene: Synthesis and Repair Key Concepts - Genes are made of DNA. When DNA is copied, each strand of a DNA double helix serves as the template for the synthesis of a complementary strand. - When a DNA molecule is being replicated, many specialized enzymes are involved in unwinding the double helix, continuously synthesizing the “leading strand” in the 5'  3' direction and synthesizing the “lagging strand” as a series of fragments that are then linked together. - Specialized enzymes repair mistakes in DNA synthesis and damaged DNA. If these repair enzymes are defective, the mutation rate increases. Mutations can lead to cancer 14.1 DNA as the Hereditary Material - The first hint that DNA is the hereditary material came from Griffith's discovery of transformation in Streptococcus pneumonia - He worked with two bacterial strains - populations of genetically identical individuals - One was virulent – ability to cause disease and death, and one was avirulent – strains do not - Medium is a liquid or solid that is suitable for growing cells - Griffith conducted four experimental treatments that he designed - In his first experiment, mice were injected with the virulent smooth strain died o Those injected with the rough nonvirulent strain lived - In the third treatment, Griffith killed cells of the virulent S strain by heating them and then injected them into mice o These mice lived - In the final treatment, Griffith injected mice with heat-killed S cells and live nonvirulent R cells o Unexpectedly these mice died - He called this process transformation – isolate the hereditary material Is DNA the Genetic Material? - Because biologists already knew that chromosomes were a complex of protein and DNA, Griffith's transforming factor had to consist of either protein or DNA. The Avery et al. Experiment - Avery, MacLeod, and McCarty set out to determine whether protein, DNA, or RNA was responsible for the transformation of S. pneumoniae observed by Griffith. - They treated cell extracts from heat-killed virulent S. pneumoniae with enzymes that selectively degraded DNA, RNA, or protein. - Then the researchers tested the extracts to see if they could still transform nonvirulent cells to virulence. - They found that only extracts with intact DNA could transform cells to virulence, thus supporting the hypothesis that DNA is the hereditary material, not RNA or protein. Notes From Reading CHAPTER 14:DNA AND THE G ENE:S YNTHESIS AND REPAIR (PGS.309-329) The Hershey-Chase Experiment - To study whether genes were made of protein or DNA, Hershey and Chase studied how a virus called T2 infects the bacterium Escherichia coli - They knew that T2 infections begin when the virus attaches to the cell wall of E. coli and injects its genes into the cell’s interior - These genes then direct the production of a new generation of virus particles inside the infected cell, which acts as a host for the parasitic virus - During infection, the protein coat of the original parent virus is left behind as a ghost attached to the exterior of the cell. 32 35 - Hershey and Chase radioactively labeled the virus's DNA with P and its protein with S. - The labeled viruses were used to infect E. coli cells. - The radioactive protein was found in the ghosts and the radioactive DNA was found in the cells. - The researchers concluded that this result supports that DNA, not protein, is the genetic material. - After these results were published, proponents of the protein hypothesis had to admit that DNA, not protein, must be the hereditary material - In combination, the evidence from the bacterial transformation experiments and the virus- labelling experiments was convincing Is DNA the Genetic Material? - Two crucial questions were raised by the finding that DNA is the hereditary material: (1) How did the simple primary and secondary structure of DNA hold the information required to make life possible? (2) How is DNA copied so that genetic information is faithfully passed from one cell to another during growth and from parents to offspring during reproduction? 14.2 Testing Early Hypotheses about DNA Synthesis - DNA is a long, linear polymer made up of monomers called deoxyribonucleotides. o Each of these is composed of deoxyribose, a phosphate group, and a nitrogenous base - Deoxyribonucleotides link together into a polymer when a phosphodiester bond forms between a hydroxyl group on the 3’ carbon of deoxyiribose and the phosphate group attached to the 5’ carbon of deoxyribose - The primary structure of a DNA molecule has two major components: o A “backbone” made up of the sugar and phosphate groups of deoxyribonucleotides o A series of nitrogen-containing bases that project from the backbone - DNA has a directionality or polarity: one end has an exposed hydroxyl group on the 3’ carbon of deoxyribose, while the other has an exposed phosphate group on a 5’ carbon  molecule has a 3’ end and a 5’ end Notes From Reading CHAPTER 14:DNA AND THE G ENE:S YNTHESIS ANDR EPAIR(PGS.309-329) - Watson and Crick proposed that two DNA strands line up in the opposite direction to each other, in what is called antiparallel fashion. o Realized their antiparallel strands will twist around each other into a spiral or helix because certain of the nitrogen-containing bases fit together in pairs inside the spiral and form hydrogen bond - The double-stranded molecule that results is called a double helix - This allows certain of the projecting nitrogen-containing bases to fit together in pairs - The structure is stabilized by complementary base pairing o adenine (A) hydrogen bonds with thymine (T) o guanine (G) hydrogen bonds with cytosine (C) - Watson and Crick suggested that existing DNA strands could serve as a template for the production of new strands, with bases being added to the new strands according to complementary base pairing. - Biologists then proposed three alternative hypotheses for how the old and new DNA strands interacted during replication: o semiconservative replication o conservative replication o dispersive replication - Semiconservative replication: Each old DNA strand is copied to generate a new strand. Each new chromosome is composed of one strand of old DNA and one strand of newly synthesized DNA. - Conservative replication: The original chromosome is copied but remains unchanged. One chromosome is composed of old strands and the other of new strands. - Dispersive replication: The replication process generates two new chromosomes, with new and old sections of DNA mixed together randomly. The Meselson-Stahl Experiment - Matthew Meselson and Frank Stahl realized that if they could tag parental and daughter strands of DNA in a way that would make the distinguishable from each other, they could determine whether replication was conservative, semiconservative or dispersive - Meselson and Stahl designed an experiment to provide more information about whether one of these hypotheses was correct. 15 - They grew E. coli in the presence of “heavy” nitrogen ( N) to label the bacteria's DNA - After many generations, they moved the bacteria to a normal N-containing medium and separated the DNA by density - The densities of the resulting DNA samples supported semiconservative DNA replication (in which each old strand is copied to make a new strand) as the mechanism by which the hereditary material is duplicated Notes From Reading CHAPTER 14:DNA AND THE G ENE:S YNTHESIS AND R EPAIR PGS .309-329) 14.3 A Comprehensive Model for DNA Synthesis - The initial breakthrough in research on DNA replication came with the discovery of an enzyme named DNA polymerase as it polymerizes deoxyribonucleotides to DNA - DNA polymerases can add deoxyribonucleotides to only the 3’ end of a growing DNA chaing - As a result, DNA synthesis always proceeds in the 5’ – 3’ direction - The densities of the resulting DNA samples supported semiconservative DNA replication (in which each old strand is copied to make a new strand) as the mechanism by which the hereditary material is duplicated - d
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