DNA Replication

Watson and Crick…again After releasing their paper on the structure of DNA, they released a followup paper on replication In it, they stated that each chain of the DNA would serve as a template for copying, yielding two identical molecules Through their idea, each daughter stand would contain a chain of the original This is the semiconservative model

However, other models had to be ruled out In a conservative model, the new strand of DNA contains no chain from the parent In this, one is entirely new, other is entirely the original The third possibility is the dispersive model Here, each strand contains part of the original and the copy on each chain

An experiment at Cal Tech was performed to ensure the semiconservative model was correct A culture of bacteria was grown in a medium containing 15N, then transferred into a medium containing only 14N The samples of DNA were then extracted and centrifuged

Replication Specifics Starts at a sequence of nucleotides called origins of replication Length and location varies Proteins attach to this sequence and open the DNA, forming a bubble Replication proceeds in both directions Y-shaped region at either end of bubble is called replication fork At fork, helicase unwinds DNA by breaking the hydrogen bonds

Ahead of the site, topoisomerase breaks and swivels strands, relieving strain When strands are separated, single strand binding proteins (SSBPs) bind to regions of unpaired DNA, stabilizing it Primase enzyme moves in and base-pairs a short (5-10) nucleotide region of RNA This is called the primer DNA polymerase III (DNA pol III) adds nucleotides to existing chain in the 5’->3’ direction (attaches new nucleotides to 3’ end)

The uninterrupted addition of nucleotides on is called the leading strand However, there is strand on the opposite side Must be added in the 5’->3’ direction, which is the opposite direction of what is supposed to happen Is this case, the primase has to move upstream and make another primer DNA pol III then adds DNA nucleotides until it reaches the first primer This partial region is called an Okazaki fragment

DNA pol I then replaces the RNA primer with DNA nucleotides The individual strands are linked by ligase (think of it as a gluing enzyme) DNA Replication DNA Replication Model

If any errors are made, it is corrected by DNA pol II

Telomers When the replication process takes places at the end of DNA, the primer will be removed, but not replaced when going from the 5’ -> 3’ direction This means the daughter strand is going to have a strand with a missing segment The hundreds of the hexanucleotide repeats will be slowly eaten up division after division