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BIOLOGY 1A03 (168)
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Chapter 14

Textbook and Class Notes Collaborated - Unit 3 - Chapter 14 Bio 1A03

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McMaster University
Xudong Zhu

Bio 1A03 Unit Three: Gene Structure and Expression Chapter 14: DNA and the Gene: Synthesis and Repair  Genes are made up 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 o If these repair enzymes are defective, the mutation rate increases – mutation can lead to cancer 14.1 DNA as the Hereditary Material  The Griffith experiment in 1928  The Avery-MacLeod-McCarty experiment in 1944  The Hershey-Chase experiment in 1952 The Griffith Experiment  The first hint that DNA is the hereditary material came from Griffith’s discovery of transformation in Streptococcus pneumoniae  Worked with two bacterial strains (populations of genetically identical individuals) o One was virulent – caused disease o One was avirulent – did not cause disease  Injected mice with bacterial strains o Mice injected with the virulent smooth strain died  Polysaccharide coat prevents detection by hosts immune system o Those injected with the rough nonvirulent strain lived  Lacking protective coat, it is recognized and destroyed by hosts immune system  Found that heat-killed virulent S. pneumoniae cells could transform a nonvirulent rough strain into a smooth virulent strain  Transformation Bio 1A03 Is DNA the Genetic Material?  Because biologists already knew that chromosomes were a complex of proteins and DNA, Griffith’s transforming factor had to consist of either protein or DNA 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  Treated cell extracts from heat-killed virulent S. pneumoniae with enzymes that selectively degraded DNA, RNA or protein  Researchers tested the extracts to see if they could still transform nonvirulent cells to virulence  Found that only extracts with intact DNA could transform cells to virulence –supports the hypothesis that DNA is the hereditary material, not RNA or protein 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  T2 infection of E. coli begins when the virus attaches to the cell and injects its genes into the cell – genes then direct production of new virus particles  During infection, the protein coat of the original parent virus is left behind as a ghost attached to the exterior of the cell  Radioactively labeled the virus’s DNA with P and its protein with S5  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  Concluded – this result supports that DNA, not protein, is the genetic material Bio 1A03 14.2 Testing Early Hypotheses About DNA Synthesis DNA is the Genetic Material  Two crucial questions were raised by the finding that DNA is the hereditary material 1. What is the primary and secondary structure of DNA that holds 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 Primary Structure of DNA  DNA is a long linear polymer made up of monomers called deoxyribonucleotides o Each is composed of a deoxyribose, a phosphate group and a nitrogenous base o Deoxyribonucleotides are joined at the phosphate group attached to the 5’ carbon of deoxyribose and the hydroxyl group attached to the 3’ carbon of deoxyribose  Two major components o Backbone made up of the sugar and phosphate groups of deoxyribonucleotides o A series o nitrogen-containing bases that project from the backbone  DNA has directionality o One end has an exposed hydroxyl group on the 3’ carbon of deoxyribose o One end has an exposed phosphate group on a 5’ carbon Secondary Structure of DNA  Watson and Crick proposed that two DNA strands line up in the opposite direction to each other – antiparallel o Allows projecting nitrogen-containing bases to fit together in pairs  Structure is stabilized by complementary base pairing o Adenine (A) hydrogen bonds with Thymine (T) o Guanine (G) hydrogen bonds with cytosine (C)  Antiparallel strands twist to form a double helix Hypothesis of DNA Replication  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  Tree alternative hypotheses for how the old and new DNA strands interacted during replication o 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 o Conservative replication  The original chromosome is copied but remains unchanged  One chromosome is composed of old strands and the other of new strands Bio 1A03 o Dispersive replication  The replication process generates two new chromosomes, with new and old sections of DNA mixed together randomly Meselson-Stahl Experiment  Experiment to provide more information about whether one of these hypotheses was correct  Grew E. coli in the presence of “heavy” nitrogen ( N) to label the bacteria’s DNA 14  After many generations, they moved the bacteria to a normal N- containing medium and separated the DNA by density  If conservative – daughter cells should have both double-stranded DNA with either N or N, but not both o Result in two bands, one of high density and one of low density  If dispersive – daughter cells should have double-stranded DNA with an equal mix of N and N for every generation o Result in one intermediate band for each trial  If semiconservative – daughter cell should have double stranded DNA with an equal mix of N and14 1N for the first generation, but then have half of the daughter cells contain only N o Result in one intermediate band for the first trial, then an intermediate band and a lower density band for the next trial  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 14. 3 A Comprehensive Model for DNA Synthesis  Deoxyribonucleoside triphosphates (dNTPs) – monomers that make up a DNA strand o N stands for the nitrogenous base (adenine, thymine, guanine or cytosine) o Has high potential energy because of the three close phosphate groups – make the formation of phosphodiester bonds exergonic  The enzyme DNA polymerase adds dNTPs to a DNA strand o Several different types of DNA polymerases o DNA polymerase can add dNTPs only to the 3’ end of a strand o DNA synthesis always proceeds in the 5’3’ direction How Does Replication Get Started  A “bubble” forms in a chromosome that is actively being replicated  Replication bubbles grow as replication proceeds, as synthesis is bidirectional  In bacterial chromosomes, the replication process begins at a single location, the origin of replication  Eukaryotes also have bidirectional replication but they have multiple origins of replication and thus have multiple replication bubbles Bio 1A03  A replication fork is the Y-shaped region where the DNA is split into two separate strands for copying How Is The Hel
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