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DNA Structure and Repair
Chapter 13 (pgs. 251-258)
Mutations in the DNA Sequence can also
lead to diseases…
Germ-line cell: cells
that become egg or
sperm…passed on
from one generation
to the next generation
Ex. Huntington disease
Somatic cell: cells in
the body that are not
egg or sperm
Ex. Skin cancer
Environmental factors:
Cigarette smoke, ultraviolet light from the
sun, etc..
Breaks DNA
Links adjacent Tbases together
If the cell can not
repair the damage,
then the cell has a
permanent change in
its base sequence….
continued cell growth
25 million cells dividing each second!
Surprisingly, very few mistakes happening….
When a cell becomes damaged, we have
enzymes that can repair the damage
The nucleus contains long
strands of DNA, which
holds all the information to
make and control every cell
1951 Watson hears a talk by Rosalind Franklin
DNA X-ray crystallographs
Rosalind Franklin
Gave a talk on her DNA
photographs
Not yet suggesting a
structure for DNA
Proposed a triple helix with bases on the outside of
the molecule
Sugar-phosphate backbone ran down the center
Phosphate groups of each chain was bound to one
another by magnesium
Invited Franklin to their laboratory
Franklin pointed out that their three-chain molecule
could not exist because any magnesium would be
bound to water
Franklin stated the correct amount water in DNA in
her talk, but Watson had forgotten
Rival Linus Pauling
Discovers alpha-helix
In 1953 announces that he
discovered the structure of
DNA
3 chain helix with a sugarphosphate backbone at the
center
Watson and Crick examined Pauling’s model
Relieved that he couldn’t be right
Watson put together 2-chain molecules
Put sugar-phosphate backbone in the center
Sugar-phosphate backbone on the outside
5’ phosphate
Phosphate group
Sugar group
Nucleotide base:
T= thymine
A= adenine
G= guanine
C= cytosine
3’ hydroxyl
Hydrogen bonds
5'
3'
3'
5'
DNA Replication
Each single strand can
serve as a template for a
new strand
Enzyme Helicases cause
the two strands to separate
New nucleotides get added
on at the 3‘ end
DNA Replication
Each single strand can
serve as a template for a
new strand
Enzyme Helicases cause
the two strands to separate
New nucleotides get added
on at the 3‘ end
Joined together by an
enzyme known as DNA
polymerase
DNA polymerase:
DNA editing functions
Removes misplaced
nucleotides and
replaces it with the
proper nucleotide
Tri-nucleotide Repeat Diseases
Affect the CNS
Loss of motor neurons
Inherited diseases
Repeated CAG or CTG
DNA polymerase is
unable to correct the
mistake
Some mutations may be beneficial
from an evolutionary standpoint