1B - DNA replication Flashcards
Through what process does DNA replicate itself?
Semi-conservation replication.
What is semi-conservative replication?
The process in which DNA replicates.
Why is the process by which DNA replicates itself called semi-conservative replication?
Because half of the strands in each new DNA molecule are from the original DNA molecule.
What does semi-conservative replication mean happens through generations?
There’s genetic continuity between generations of cells. (Genes from the parent cell are inherited by the cells produced by the cell division).
Describe the process of semi-conservative replication:
The enzyme DNA helicase breaks the hydrogen bonds between bases on the 2 polynucleotide DNA strands. This makes the helix unwind to form 2 single strands.
Each original single strand acts as a template for a new strand. Complementary base pairing means that free-floating DNA nucleotides are attracted to their complementary exposed bases on each original template strand - A with T and C with G.
Condensation reactions join the nucleotides of the new strands together - catalysed by the enzyme DNA polymerase. Hydrogen bonds form between the bases on the original and new strands.
Each new DNA molecule contains one strand from the original DNA molecule and one new strand.
Explain how DNA polymerase adds nucleotides to a new strand to DNA
The active site of DNA polymerase is only complementary to the 3’ end of the newly forming DNA strand so the enzyme can only add nucleotides to this end.
What is special about the 2 strands that make up the DNA double helix?
They are antiparallel with each 3’ and 5’ end at opposite ends.
Who came up with the theory of semi-conservative replication?
Watson and Crick.
Who validated Watson and crick’s theory of semi-conservative replication?
Meselson and Stahl.
What would have happened if DNA replication was conservative rather than semi-conservative?
The original DNA strands would stay together and the new DNA molecules would contain two new strands.
How did Meselson and Stahl show that DNA replication was semi-conservative?
Describe their method
1) 2 samples of bacteria grown - one in a nutrient broth containing light nitrogen (14N) and one in a nutrient broth containing heavy nitrogen (15N). As the bacteria reproduced it took up the nitrogen to help make nucleotides for new DNA so the nitrogen gradually became part of the bacteria’s DNA.
2) Sample of DNA taken from each batch of bacteria and spun in a centrifuge. The DNA from the heavy nitrogen bacteria settled lower down the centrifuge tube because it is heavier.
3) Bacteria grown in heavy brith taken out and put in light nitrogen broth. They were left for 1 round of DNA replication and then another sample was removed and spun in a centrifuge.
4) If conservative replication - original heavy DNA would settle at bottom and light at top.
If semi-conservative - there would be a new and old strand so a heavy and light strand so the DNA would settle out between where the light would settle and where the heavy would settle.
5) It settled in the middle.
How did Meselson and Stahl’s results show that DNA replicates semi-conservatively?
If conservative replication - original heavy DNA would settle at bottom and light at top.
If semi-conservative - there would be a new and old strand so a heavy and light strand so the DNA would settle out between where the light would settle and where the heavy would settle.
It settled in the middle.
