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Chapter 19

BIOL 110 Chapter Notes - Chapter 19: Semiconservative Replication, Sister Chromatids, Nucleic Acid Double Helix

Course Code
BIOL 110
Denise Woodward

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Tutorial 19
Thursday, October 20, 2016
11:32 AM
DNA Replication
DNA replication
o S-phase
o Chromosome comprised of two identical sister chromatids
o Double helix unwinds and nitrogenous bases are added to each strand of the parent
molecule, two identical copies of the parent strand
o Semiconservative replication - each newly formed DNA has one stand conserved from the
parent molecule and one synthesized strand
o Origins of replication
o Enzymes
Helicase - unwinds the DNA double helix
Single strand binding proteins - keep the strands separate
Primases - initiate replication
DNA polymerase - adds nucleotides to the unwound parent molecule
o The strands have opposite chemical polarities
H-bonding between bases is caused by the opposite polarities
Antiparallel - one is the 3'-to-5' strand and the other the 5'-to-3' strand
o DNA polymerase
Limitation; it can only add nucleotides to the 3 end of the newly synthesized strand of
Nucleotide addition is smooth and continous on the leading strand
It is discontinous on the lagging strand
DNA polymerase can only work by starting at from the replication fork (where
the DNA is unwinding) and progressing outward
Lagging strand
Primase has the ability to synthesize a short primer made of RNA nucleotides
Okazaki fragments
Short stretches of DNA
Ligase seals these fragments to make them continuous
Errors During DNA replication result in mutations
o Mistakes occur about once in every 10000 base pairs
o Observed error rate is very low
o Mismatch repair occurs when DNA polymerase and other proofreading enzymes remove
incorrectly paired nucleotides
o Excision repair involves the removal of damaged nucleotides from a DNA molecule
o Mutation is a permanent change in an organism's DNA
If it occurs in a reproductive cell, the mutation can be passed on to future generations
and possible be established in a population
Harmful - sickle cell, CF
Beneficial - antibiotic resistance
Neutral - DNA sequence changes but it has no effect on the phenotype
There are enzymes that defend against mutations
XP (Xeroderma pigmentosum)
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