Lecture 3,5,6 - DNA replication Flashcards
What are the three types of replication?
Semi-conservative
Conservative
Dispersive
What is semi-conservative replication?
Where the parent strand is used as a template and part of the parent strand remains in the new strand
What is conservative replication?
Where both parent strands act as a template for two new strands to be made, and afterwards the two parent strands re pair up and the two new strands pair up
It may occur through detachment or reassociation
What is dispersive replication?
Template switching occurs (the template strand is switched back and fourth between the two strands) and repeats along the strand until you’re left with two hybrids
What did scientists use to think about DNA replication?
They thought it occurred through dispersive replication
This is because DNA is plectonemic and circular so cannot be unwinded
What was the aim of the Meselson-Stahl experiment
To distinguish between the three modes of DNA replication and find out what occurs in E.coli
What was the process of the Meselson-Stahl experiment?
A culture of bacteria are grown in a medium with heavy nitrogen
The culture medium had heavy ammonium chloride (14NH4Cl)
All of the bacteria’s DNA became labelled with 15N
15 and 14 can be distinguished by density gradient centrifugation
What were the results of the Meselson-Stahl experiment?
After one round of cell division the band showed mixes of 14N and 15N
After 2 cell divisions 2 bands were shown with 14N-14N and 14N-15N
What was the conclusion of the Meselson-Stahl experiment?
DNA is replicated by semi-conservative replication
The strands are separated without unwinding with DNA topoisomerases
What are topoisomerases?
Enzymes that prevent supercoils from forming and separate DNA strands without unwinding them
What are type 1 topoisomerases and how do they work?
They nick one DNA strand
This allows the other strand to pass through and removes the double helix structure
What are type 2 topoisomerases?
They bind to DNA and cut both strands
This allows another part of the DNA to pass through the cut and remove the double helix
What is Telomerase?
Prevents ends of chromosomes getting shortened in replication
Extends DNA by adding sequence TTAGGG multiple times as a buffer
Allows final Okazaki fragment to be primed
Where in the cell cycle does replication happen?
In the S phase
We synchronise cells so they’re all in the S phase
