BIOL239 Lecture Notes - Lecture 11: Matthew Meselson, Semiconservative Replication, Franklin Stahl
14 May 2018
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Set 11: DNA replication
●Watson-Crick model of DNA replication → suspect that one strand serves as template
but unsure how replication is done
1. Strands separate
2. Complementary bases align opposite templates
3. Enzymes link sugar-phosphates of aligned nucleotides into a continuous new
strand
oThis model proposed a mechanism whereby DNA replication is
semiconservative – in daughter molecules, one strand is conserved from the
parental molecule and the other is newly synthesized
●Experimental proof of semiconservative replication
o1958 Matthew Meselson and Franklin Stahl – studies with controlled isotopic
composition of nucleotides incorporated into daughter DNA strands
▪track nitrogen type
▪grow one group of e.coli in N14 and the other in N15 for some generations
(so that their DNA contain the isotope N14 or N15); N14 is light and N15
is heavy, therefore simple centrifuge technique can be used to distinguish
them
▪mix DNAs containing N14 and N15
▪after one generation, they see a hybrid band ⇒ ruled out conservative
▪2nd generation → half hybrid and half light
▪3rd gen → hybrid band stays the same but light band continues to rep.
oRuled out conservative replication after first generation, ruled out dispersive after
second
●The molecular mechanism of DNA replication
oComplex process; occurs at a precise moment in the cell cycle (S phase in
eukaryotes)
oTwo steps: initiation and elongation
oE. coli
DNA replication as a model:
▪Circular chromosome (unlike linear chr in eukaryote)
●Initiation (at a precise point)
oOrigin of DNA replication, OriC
– AT rich (big strings of As and Ts; easy to pull
apart bc only two hydrogen bonds between A and T)
oDnaA protein binds to the four 9-bp repeats in oriC
(one of which is AT rich,
other region signals DnaA)
ocomplex of initiator protein bind with oriC and open up the helix (replication
bubble); so that enzymes can get in and start replication
oDNA helicase catalyzes the unwinding of the parental double helix (moves in
both directions – two helicases, one moving on each end of replication bubble;
synthesis occurs in both directions); SSB proteins (single-stranded binding
proteins) keep strands separated as helicase unwinds them
●Elongation
