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I.Properties of genetic material
What was known about the genetic material prior to
the 1940s?
Chapter 10
DNA: The Chemical Nature of the Gene
A. Replication—be able to give exact copy to
daughter cells
B. Mutability—be able to change
C. Location—must be part of chromosomes
D. Affect the phenotype is specific ways
E. Why most scientist in the early 40s thought
proteins were the genetic material:
DNA has equal ratios of purines: pyrimidines
suggested DNA has simple structure
F. Evidence of nucleic acids as genetic material
1. Griffith (1925) laid the groundwork for later discoveries
by demonstrating transformation in vivo.
Proteins vary in composition and are
complex – 20 amino acids
20 building blocks vs 4
F. Evidence of nucleic acids as genetic material
2. Avery, McLeod and McCarty (1944) demonstrated in
vitro that the transforming factor was DNA.
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F. Evidence of nucleic acids as genetic material
3. Hershey and Chase (1953) used T2 phage to demonstrate
that DNA was the genetic material.
Label proteins with S35.
No label is recovered in progeny phage.
Label DNA with P32.
Label is recovered in progeny phage.
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F. Evidence of nucleic acids as genetic material
4. Fraenkel-Conrat and Williams (1955) demonstrated that
RNA can also be genetic material
II.
Chemistry of nucleic acids
A.
B.
1.
2.
Nucleoside—C5 sugar + base at 1′ C
Nucleotide—nucleoside + phosphate at 5′ C
Purines—adenine, guanine
Pyrimidines—thymine (in DNA), cytosine,
uracil (in RNA)
C. Phosphodiester bond hooks nucleotides
together
D. Structure—double-stranded helix (Watson-Crick)
1. X-ray data (Franklin and Wilkins)
—DNA is highly ordered and has multiple chains.
—Repeating structure every 3.4 angstroms.
—Helical.
2. Chargaff's rule:
A=T
G=C
purines = pyrimidines
3. Other information
—DNA is made of nucleotides with 3' to 5'
linkage.
—DNA can be denatured w/out breaking covalent
bonds: ➔ DNA is stabilized by H-bonds.
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Watson-Crick model
a. Strands held together by H-bonds
b. A opposite T; G opposite C
c. Strands antiparallel
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F. Properties
1. Complexity—different sequences give potential for
different proteins
2. Replication—model proposes strands separate
“It has not escaped our notice that the specific
pairing we have postulated immediately suggests a
possible copying mechanism for the genetic
material.”
3. Location—nucleus or chromosome
The Central Dogma: Flow of information in the cell
The Central Dogma: modifications
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G. DNA can take up different structures
-- condition dependent
-- sequence dependent
B-DNA right-hand helix most common
A-DNA right-hand helix has less H2O; shorter,
wider
Z-DNA has left-hand helix
-- zigzag
-- role in gene regulation
Special structures in DNA and RNA
Methylation
DNA methylation in eukaryotes
frequently occurs at CpG sites
Prokaryotic – distinguish ‘self’ from ‘other’
Eukaryotic – gene regulation
C methylation in animals 5%
in plants 50%
in yeast none
in flies
very very low
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