DNA Replication Flashcards
Why does a cell require DNA replication?
DNA copies itself before cell division so that each new cell has the full amount of DNA after divisions.
Why is replication known as semi-conservative?
Because half of the strands in each new DNA molecule are from the original DNA molecule.
Why does DNA replication ensure genetic continuity?
As the cells produced by cell division inherit their genes from their parent cells.
Who came up with the THEORY of semi-conservative DNA replication?
Watson and Crick THEORISED the semi-conservative replication model.
What is the function of DNA helicase?
It breaks the hydrogen bonds between bases on two polynucleotide DNA strands, unwinding the two strands.
What is the first step in semi-conservative replication?
DNA helicase unwinds the two strands.
After replication, each new strand contains what?
One strand from the original DNA molecule and one newly formed strand.
Explain the process of semi-conservative replication:
1) DNA helicase breaks hydrogen bonds to unwind the two strands.
2) Each new strand acts as a template and complementary base pairing attracts free nucleotides to the exposed bases.
3) Condensation reactions join the new nucleotide strands, catalysed by DNA polymerase enzyme.
4) Each new strand contains one strand from the original DNA molecule and one new strand.
What is the role of DNA polymerase?
It catalyses the condensation reactions that join the nucleotides of the new strands together (phosphodiester bonds).
What are the names of the two ‘ends’ of the DNA?
3’ (three prime) and 5’ (five prime).
Explain why DNA polymerase molecules act in opposite directions:
In the DNA helix, the strands run antiparallel, the active site of DNA polymerase is complementary only to the 3’ end. This means the new strand is only made in the 5’ to 3’ direction.
Who was the first to validate the semi-conservative replication method?
Meselson and Stahl.
What did Meselson and Stahl experimented DNA replication?
They used two isotopes of nitrogen - heavy (N15) and light (N14).
Explain how Meselson and Stahl validated the semi-conservative method of DNA replication:
1) Two samples of bacteria were grown, one in N14 and one in N15.
2) A sample of DNA was taken from each batch of bacteria and spun in a centrifuge.
3) The N15 DNA settled lower down than the N14 DNA as it was more dense.
4) Then N15 bacteria was left to replicate once in N14 solution and was then centrifuged.
5) This bacteria lay between the 14N and 15N lines showing there was one 15N and one 14N strand in the ‘new’ strand.
What happened once Mehelson and Stahl proved that bacteria used semi-conservative replication?
Other scientists carried out experiments to find that semi-conservative replication was universal in organisms.
