DNA replication mechanism

Question 1

STEP 1-4

Answer 1

DNA helicase catalyses the unwinding and separating the double helix strand of DNA . This separate the two parental strand of DNA double helix . This unwinding causes tighter twisting and strain ahead of the replication fork . DNA gyrase catalyses the relieve of this strain , preventing the overwinding .

Question 2

STEP 5-8

Answer 2

Single-strand binding protein (SSBP) bind to unpaired DNA strands . This stabilizes the structure of DNA (preventing from re-pairing) until strands are copied . Primase initiates the synthesis of polynucleotide by adding RNA nucleotides to form a short RNA primer . DNA polymerase III adds free DNA nucleotides , complementary to the bases of the template forming the new DNA strand .

Question 3

STEP 9-12

Answer 3

Formation of new DNA strand must only be in the direction of 5’ to 3’ . Along one template strand , a new strand is continuously synthesized toward the replication fork in the 5’ to 3’ direction . This new strand is the leading strand . Along the other template strand , a new strand is synthesized away from the replication fork

Question 4

13-17

Answer 4

This new strand is the lagging strand , which is synthesized discontinuously . These pieces are called Okazaki fragments . RNA primer will be replaced with DNA nucleotides by DNA Polymerase I . Okazaki fragments will be joined by DNA ligase . After DNA replication process settled , two identical copies of the DNA double helix are produced .