Biology 1202B Lecture Notes - S Phase, Chromosome, Dna Ligase

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30 Apr 2012
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Biology Lecture Review Notes
Lecture 2
DNA Replication
Semi-conservative:
o The double helix opens, and new strands synthesize to create 2 strands
o The only type that appears to be biologically significant
Conservative:
o The one double helix is completely intact, and the new double helix is completely from
new nucleotides
Dispersive:
o Mixture of both new and old strands
1958 Meselson and Stahl grew bacteria for several generations in 15N (heavy) medium and
then transferred them to 14N (normal/light) medium and allowed them to grow and divide for
several generations
o All DNA is heavy before the transfer (in a test tube they were at the bottom)
o All DNA is light after the transfer
o DNA was extracted from the bacteria cultured in the 15N medium and after each
generation in the 14N medium
o The DNA was centrifuged at a high speed for 48 hours
o Grew them in nitrogen because it is an important part of DNA
o The results completely matched with semiconservative replication
Did not support conservative or dispersive
DNA synthesis occurs in 5’ to 3’ direction!
Alpha, beta, and gamma (phosphate groups):
o Alpha is the one that stays
o Beta and gamma are the ones that are removed (the pyrophosphates)
Eukaryotes are slower at replicating DNA because they are more complex
Replication fork where the antiparallel DNA strands replicate
Leading strand can start replication at the first nucleotide
o Continuously made
Lagging strand has to open up to begin replication (is not able to replicate on the first
nucleotide because of the 5’ to 3’ direction)
o Discontinuously made (Okazaki Fragments)
Helicase a protein that unwinds the DNA helix
There are single stranded binding proteins that bind to DNA and stabilize its single stranded
form
Primase makes the primers (RNA primers) that are used to synthesize the DNA
o Synthesizes and assembles short complementary RNA primers
DNA polymerase adds DNA nucleotides to the 3’ end of RNA primers
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