DNA replication

Question 1

why does DNA replicate

Answer 1

DNA copies itself before cell division, so that each new cell has the full amount of DNA. The method is called semi-conservative replication because half of the strands in each new DNA molecule are from the original DNA molecule. This means that there’s genetic continuity between generations of cells.

Question 2

how is DNA replicated?

Answer 2

has a paired base structure,makes it easy to copy itself
1. the enzyme DNA helicase breaks the hydrogen bonds between bases on the two polynucleotide DNA strands, this makes the helix unwind to form two single strands
2. 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
3. condensation reactions join the nucleotides of the new strand 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 molecules and one new strand

Question 3

The action of DNA polymerase

Answer 3

each end of a DNA strand is slightly different in its structure, one end is called the 3’ end and one end is called the 5’ end. During DNA replication to the active site of DNA polymerase is only complimentary to the 3’ end of the newly forming DNA strand - so the enzyme can only add nucleotides to the new strand. This means that the new strand is made in a 5’ to 3’ direction and that DNA polymerase moves down the template strand in a 3’ to 5’ direction. Because the strands in the double helix are antiparallel, the DNA polymerase working on one of the template strands moves in the opposite direction to the DNA polymerase working on the other template strand

Question 4

evidence for semi conservative replication - Meselson and Stahl’s experiment

Answer 4

Mehelson and Stahl showed DNA is replicated using the semi-conservative method. experiment used two isotopes of nitrogen - heavy nitrogen and light nitrogen
1. two samples of bacteria were grown for many generations - one in a nutrient broth containing light nitrogen, and one in a broth with heavy nitrogen. As the bacteria reproduced, they took up nitrogen from the broth to help make nucleotides for new DNA. so the nitrogen gradually became part of the bacteria’s DNA
2. a sample of DNA was taken from each batch of bacteria and spun in a centrifuge. The DNA from the heavy nitrogen bacteria settled lower down the centrifuge tube than night nitrogen bacteria
3. then the bacteria grown in heavy nitrogen broth were taken out and put in a broth containing only light nitrogen. the bacteria were left for one round of DNA replication, and then another DNA sample was taken out and spun in the centrifuge

Question 5

how did the experiment prove that replication was semi-conservative?

Answer 5

if replication was conservative the original heavy DNA which would still be together would settle at the bottom and the new light DNA would settle at the top
If replication was semi-conservative the new bacterial DNA molecules would contain one strand of the old DNA containing heavy nitrogen and one strand of new DNA containing light nitrogen. So DNA would settle out between where the light nitrogen DNA settled out and where heavy nitrogen DNA settled out.
DNA settled out in the middle showing that the DNA molecules contained a mixture of heavy and light nitrogen. the bacterial DNA had replicated semi-conservatively in the light nitrogen