DNA Replication

Question 1

The 3 theories of DNA replication

Conservative, Dispersive, Semi-Conservative

Answer 1

Conservative replication theory:
-The parental DNA helix is conserved and a whole new DNA helix is made

Dispersive replication theory:
-There are two new DNA helices created. Each containing fragments of the old DNA and the new DNA

Semi-Conservative replication theory:

• The parental DNA separates into two strands
• Each strand acts as a template for the synthesis of new strand

Question 2

Semi-Conservative replication Process

Answer 2

1) DNA Helicase breaks bonds between complementary base pairs of the double helix (unzips)
2) The double helix unwinds and separates into two strands
3) Bases are now exposed and each polynucleotide strand acts as a template to which free nucleotides (bases) in the cytoplasm are attracted to
4) Primase makes a small RNA primer to indicate the starting point of replication
5) DNA Polymerase binds to the primer and adds DNA bases in one direction continuously from the 5’ end to the 3’ end on the leading strand. Forming phosphodiester bonds between adjacent nucleotides.
6) On the lagging strand only chunks of DNA can be made at a time, called Okazaki fragments
7) RNA primer added by Primase and bases are added between fragments by Polymerase, in 5’ to 3’ direction
8) Once new DNA is made the enzyme Exonuclease removes the primers, and Polymerase fills the gaps with bases
9) DNA Ligase seals up the fragments of DNA in both strands to form a continuous double strand

Question 3

Semi-Conservative replication Process (Short)

Answer 3

1) Helicase breaks hydrogen bonds between complementary base pairs, helix unwinds and bases are exposed
2) Primase creates RNA primer to indicate start point
3) DNA Polymerase binds to primer and adds complementary bases continuously in 5’ to 3’ direction on the leading strand.
4) On lagging strand Okazaki fragments are formed in sections of RNA primers and bases, in 5’ to 3’ direction the opposite way
5) DNA Ligase seals up the fragments of DNA in both strands to form a continuous double strand

Question 4

Meselson-Stahl Experiment method

Answer 4

1) Bacteria cultured in amino acid medium made with heavy nitrogen (Nitrogen-15)
2) Bacteria washed and transferred to light nitrogen (Nitrogen-14) medium. Washing prevents carrying over of N-15 medium
3) DNA extracted and spun in ultracentrifuge in caesium chloride solution
4) DNA bands form due to heavier DNA molecules settling further down the tube than lighter, less dense molecules.

Question 5

Meselson-Stahl Results

Answer 5

Generation 0:
One single heavy band of N-15

Generation 1:

• One single intermediate band of N-15 and N-14
• Conservative replication can be ruled out as there would have been two bands (One light and one heavy)

Generation 2:

• Two bands of equal widths, one light and one intermediate
• Intermediate contains N-15 and N-14
• Light contains N-14 only
• Dispersive replication can be ruled out as there should be only one intermediate band if the DNA always mixes