DNA replication Flashcards
When does DNA replication occur?
During interphase
Why does DNA replicate itself by semi conservative replication?
Because half of the strands in each new DNA molecule from the original DNA molecule. this means that there is genetic continuity between generations of cells (the cells produced by cell division inherit their genes from their parent cells)
What are the 4 requirements for semi conservative replication?
- 4 types of nucleotides present AGCT
- 2 strands of DNA to act as a template
- Enzyme DNA helicase and DNA polymerase
- source of chemical energy (ATP)
Step by step of semi conservative replication
Know diagram
1) DNA helicase breaks the hydrogen bonds between the base pairs
2) The double helix separates into two strands and Unwinds
3) Each exposed strand acts as a template for free complimentary nucleotides to bind
4) New nucleotides are joined in a condensation reaction using DNA polymerase to form a new strand. (hydrogen bonds formed between the bases on the original and new strands)
5) Each new DNA molecule contains one original strand and one newly built strand
Which way does DNA polymerase move?
DNA polymerase moves in opposite ways along antiparallel DNA strands.
The active site of DNA polymerase is only contemporary to the 3’ end of the newly forming DNA strand so the enzyme can only add nucleotides to the new strand at the 3’ end
Who came up with the theory of semi conservative replication?
Crick and Watson
However, it wasn’t until Meselson and Stahl’s experiment a few years later that the theory was validated.
How did Meselson and Stahl show that DNA is replicated using the semi conservative method?
Their experiment used 2 isotopes of nitrogen.
Isotopes are different forms of the same element.
How did Meselson and Stahl’s experiment work?
2 samples of bacteria were grown~ sample of DNA containing heavy and sample containing light nitrogen after being soaked in nitrogen from broth (as bacteria reproduced they took up the nitrogen)
The heavy DNA sample containing heavy nitrogen settled out at the bottom of the centrifuge tube and the light DNA at the top.
Bacteria grown in heavy nitrogen broth were taken out and put into light nitrogen broth. Bacteria was left for one one round of replication and another DNA sample was taken out and and spun in the centrifuge.
If replication was conservative-heavy DNA would settle at bottom and light DNA would settle at top
If replication was semi conservative-would contain one strand of old heavy and one strand of new light, so the DNA would settle in the middle of where the light and heavy was showing it was a mixture of the heavy and light. So the bacterial DNA had replicated semi conservatively in the light nitrogen.
