genetics 2: dna replication

3’ to 5’ exonuclease activity

enables some dna polymerases to proofread + fix mistakes

most organizsms hae dna repair proteins that can replace an incorrectly placed nucleotide

proposed models of dna replication

-conservative: og double strand serves a s a template for new molecule

-dispersive: both strands break down into fragments and serve as templates

-semi-conservative: strand unwinds and used as template

meselson & stahl

1st: grow bacteria in heavy 15N for many gens

2nd: took sample and switch to 14N. new dna has lighter 14N 

3rd: centrifuged to distinguish by weight

after 2 rounds of replication found semiconservative 

several methods of dna replication

theta rep

rolling circle rep

linear euk rep

theta rep

1st: double strand begins to unwinds producing single strands which function as a template for new strand (replication bubble)

2nd: unwinding countinues + bubble gets larger. replication fork: where unwinding happens 

3rd: dna synthesis happens in 5’ to 3’ direction 

rolling circle

1st: single strand break creating a 3’ oh and 5’ p group

2nd: new nucleotides added at 3’ oh brake using inner strand as template

3rd: as new strand gets elongated, old strand gets displaced, eventually rolling off entire plasmid. can countinue for several rounds 

4th: linear fragment that rolls off can be used as a template for new strand

stages of dna replication

initiation

unwinding

elongation

termination 

prokaryotic dna replication

-initiation: dnaA protein beinds to oriC (opens small site for helicase and single-stranded binding protein to enter & being unwinding the 2 strands)

-unwinding:

dna helicase breaks h-bonds between bases

s-s binding proteins: bind to the s-s breaks once they form to stabalize the structure

dna gyrase/topoisomerase: prevents supercoiling and upstream torsional strain that builds up during unwinding

-elongation: dna poly III adds nucleotides after the primer (runs in 5’ to 3’ direction but if mistake made it can go back and fix it

poly I; removes rna primers and replaces with dna

prokaryotic