BIO152H5 Chapter Notes - Chapter 14: Chromosome, Mutation Rate, Thymine

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31 Jan 2013
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CHAPTER 14: DNA AND THE GENE: SYNTHESIS 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.
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Notes From Reading
CHAPTER 14: DNA AND THE GENE: SYNTHESIS 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.
- Hershey and Chase radioactively labeled the virus's DNA with 32P and its protein with 35S.
- 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
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