DNA Replication

Question 1

conservative replication

Answer 1

original molecule completely conserved and new molecule completely new

Question 2

semi-conservative replication

Answer 2

• both original strands act as a template
• each newly formed double-helix has 1 original polynucleotide strand and 1 new polynucleotide strand

Question 3

dispersive replication

Answer 3

the 2 resulting molecules contain both old and new double helix DNA

Question 4

what did Meselson and Stahl’s experiment do?

Answer 4

provided evidence to reject conservative and dispersive replication

Question 5

why did Meselson and Stahl use E. coli

Answer 5

• in optimum conditions, they have a short generation time of ~ 50 minute
• DNA easier to obtain as free in cytoplasm and not surrounded by a nuclear envelope

Question 6

1st step of Meselson and Stahl experiment

Answer 6

• 2 populations of E. coli grown on separate agar plates, one with 14N (light isotope) and one with 15N (heavy isotope) nitrogen
• take up N for nitrogenous bases, so have 14N or 15N DNA
• sample of DNA removed and centrifuged, with 14N settling at top and 15N at bottom of test tube

Question 7

why was nitrogen used in Meselson and Stahl’s experiment?

Answer 7

nitrogen taken up by bacteria and used to form nitrogenous bases of their DNA

Question 8

2nd step of Meselson and Stahl

Answer 8

• 15N DNA washed and transferred to 14N DNA and allowed to divide once
• centrifuged and settle in middle
• rules out conservative as all DNA contains both 14N and 15N

Question 9

why is the 15N DNA washed before dded to 14N agar

Answer 9

to remove 15N agar so only 14N can be used in division

Question 10

3rd step of Meselson and Stahl

Answer 10

• DNA allwed to divide again
• centrifuged
• half settles at the top and half at the middle
• rules out dispersive as would all settle in middle if dispersive

Question 11

4th step of Meselston and Stahl

Answer 11

with each further division, amount of 14N DNA increases and amount of mixed DNA stays the same

Question 12

7 steps of DNA replication

Answer 12

1. DNA helicase unwinds and unzips double helix, breaking H-bonds between strands
2. both the 2 polynucleotides have exposed bases which act as templates
3. free DNA nucleotides in the nucleoplasm align themselves against ech template strand
4. H-bonds form between complementary base pairs, joining the aligned nucleotides to their exposed base pairs on the template (A-T and C-G)
5. DNA polymerase catalyses the formation of the new polynucleotide strands in a condensation reaction forming phosphodiester bonds
6. Enzymes rewind the DNA molecule back into a double helix
7. As both original strands acted as a template, each newly formed DNA molecule has 1 original and 1 newly formed polynucleotide strand