DNA Replication

Question 1

Why does DNA replicate?

Answer 1

before a cell divides by mitosis or meiosis all the DNA must replicate to provide a copy for the new cell

Question 2

What type of replication happens in DNA replication?

Answer 2

semi-conservative = one strand form original, one strand from newly synthesised

Question 3

What is the process of DNA replication?

Answer 3

1.) the enzyme DNA helicase breaks the hydrogen bonds between the complementary base pairs between the strands so the helix unwinds so there’s 2 single strands
2.) (ER) each strand kept apart by single strand binding proteins (SSB)
3.) Each original sing strand acts as a template for new strand and free floating nucleotides within the nucleus are attracted to their complementary base pairs on the template strands
4.) the adjacent nucleotides are joined by condensation reaction which is catalysed by the enzyme DNA polymerase forming phosphodiester bonds - DNA polymerase hydrolyses the terminal phosphate to release energy for reaction
→the active site of DNA polymerase is only complementary to the 3’ (prime) end of the newly forming DNA strand so the nucleotides can only be added to the new strand at the 3’ end so the new strand is made in the 5’ to 3’ direction (towards the replication fork) and DNA polymerase moves down template strand in 3’ to 5’ direction ( away from replication fork)
→ DNA strands are anti parallel so it moves in opposite directions (DNA P)

Question 4

What does DNA ligase do?

Answer 4

joins okazaki fragaments together

Question 5

Who discovered that it is semi-conservative replication?

Answer 5

Watson and Crick but many believed it was conservative (new DNA has 2 new strands and original strands stay together)
- validated by Meselson and Stahl

Question 6

What was Meselson and Stahl’s experiment?

Answer 6

1.) two samples of bacteria grown for several generations - one containing light nitrogen, other with heavy nitrogen (isotopes of nitrogen) and as bacteria reproduced it took up nitrogen to help make nucleotides for new DNA (nitrogen gradually became part of the DNA)
2.) a sample was taken form each and spun in centrifuge - DNA from heavy settled lower than light
3.) bacteria grown in heavy put in sample with only light and left for a round of replication and sample was put in centrifuge
→ DNA settled in the middle = showed they had mixed and semi-conservative as they would have one strand of heavy and one of light
→ if replication was conservative the heavy would still be together at the bottom and light would still be at the top