BSC-2010 Lecture Notes - Lecture 2: Primase, Dna Replication, Helicase

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1 Nov 2016
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DNA Replication
- Cell division
- DNA contains more information than RNA or protein
- For each gene, a RNA molecule is created by copying one strand of DNA from that
gene
- RNA translates to a protein
- Replication, transcription, and translation are three separate processes
Semi-conservative replication
- Every new strand is half original, half new
- Matthew Meselson and Franklin Stahl supported the semiconservative model and
created an experiment to see if it was true
They labeled nucleotides of the old strands with a heavy isotope of nitrogen
(N^15), while any new nucleotides were labeled with a lighter one, N^14
1. Bacteria was cultured in n^15
2. Bacteria transferred to medium n^14
3. DNA sample was centrifuged after first replication
4. Centrifuged after second replication
Control of DNA replication
- Each cell has a specific amount of DNA
- Diploid Cells: 2 copies of each chromosome
- Cells do not tolerate more or less DNA
- Cells must replicate their DNA in preparation for cell division
Initiation happens at origins of replication
- Recruit proteins that are necessary for replication, which act as starting points for
replication
Helicase
Single strand binding protein
Primase
Topoisomerase
Polymerase
Ligase
- Helicase unwinds DNA double strand by breaking hydrogen bonds
- Single strand binding protein binds to single strand DNA to keep strands from
rejoining
- Primase adds an RNA primer to single stranded DNA which is usually 10 to 20
nucleotides long
- Topoisomerase travels along DNA, ahead of the replication fork. It cuts the sugar
phosphate backbone, unwinds the DNA one turn, and then reconnects the sugar
phosphate backbone
New nuleotides an only e added to the 3’ end of a growing strand
Supercoils are bad for replication
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DNA polymerases catalyze the elongation of new DNA at a replication fork
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