1. DNA Replication

2. 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

3. 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

5. 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

7. 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

8. 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)

10. 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

11. 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

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

15. 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