DNA replication

Question 1

what is semi conservative replication

Answer 1

-A,G,C,T must be present
-both strands of DNA nucleotides must act as a template
-requires energy
- designed by watson and crick

Question 2

describe how the watson and cricks theory of semi-conservative replication of DNA was proved

Answer 2

-Meselson and Stahl’s Experiment
-Bacteria are grown in a culture of the heavy (15N) nitrogen isotope
-DNA contains nitrogen in its bases
-As the bacteria replicated, they used nitrogen from the culture to make new DNA nucleotides
-the culture of bacteria had DNA containing only heavy (15N) nitrogen
-A sample of DNA from the 15N culture of bacteria was extracted and spun in a centrifuge
-This showed that the DNA containing the heavy nitrogen settled near the bottom of the centrifuge tube
-The bacteria containing only 15N DNA was then taken out of the 15N culture and added to a culture containing only the lighter 14N nitrogen. The bacteria were left for enough time for one round of DNA replication to occur before their DNA was extracted and spun in a centrifuge
-If conservative DNA replication had occurred, the original template DNA molecules would only contain the heavier nitrogen and would settle at the bottom of the tube, whilst the new DNA molecules would only contain the lighter nitrogen and would settle at the top of the tube
-If semi-conservative replication had occurred, all the DNA molecules would now contain both the heavy 15N and light 14N nitrogen and would therefore settle in the middle of the tube (one strand of each DNA molecule would be from the original DNA containing the heavier nitrogen and the other (new) strand would be made using only the lighter nitrogen)
-Meselson and Stahl confirmed that the bacterial DNA had undergone semi-conservative replication.
-The DNA from this second round of centrifugation settled in the middle of the tube, showing that each DNA molecule contained a mixture of the heavier and lighter nitrogen isotopes
-If more rounds of replication were allowed to take place, the ratio of 15N:14N would go from 1:1 after the first round of replication, to 3:1 after the second and 7:1 after the third

Question 3

how do DNA replicate

Answer 3

• DNA helicase catalyses the breaking of H bonds between complimentary bases in polynucleotides
  -unwinding the double helix
  -each free nucleotide acts as a template to which complimentary free nucleotides bind by specific base pairings
  DNA polymerase catalyses the formation of phosphodiester bonds in condensation reactions between adjacent nucleotides
  -DNA polymerase active site specific to only the 5’ end of the nucleotide
  -DNA ligase seals the DNA nucleotides together