14.2

Question 1

Explain how the Meselson-Stahl experiment contributed to our understanding of DNA replication and describe this process.

Answer 1

o They saw DNA replicated semi-conservatively, meaning one strand is conserved while the other one is synthesized.
o Used heavy isotope of nitrogen

Question 2

Define the semiconservative model of DNA replication.

Answer 2

One old strand and one new strand

Question 3

Define origin of replication and replication fork.

Answer 3

o Origin of rep.-where replication begins
o Rep. Fork-Y-shaped region where new DNA strands are elongating

Question 4

Describe the functions of E. coli DNA polymerases.

Answer 4

o Synthesize DNA
o polymerase I: acts on lagging strand to remove RNA primers & replace them with DNA (has 5’ to 3’ exonuclease activity)
o polymerase II: (important/involved in DNA repair processes)
o polymerase III: (main replication enzyme. synthesizes new DNA by bringing in nucleotides.)
all 3 polymerases have 3’-to-5’ exonuclease activity and have proofreading abilities

Question 5

Identify the function of the replicative enzymes/proteins: helicase

Answer 5

catalyze the disruption of the hydrogen bonds that hold the two strands of double-stranded DNA together

Question 6

Identify the function of the replicative enzymes/proteins: single-strand binding proteins

Answer 6

bind to parental DNA immediately after the helicase, preventing the two single strands from joining and re-forming a double helix.

Question 7

Identify the function of the replicative enzymes/proteins: topoisomerase

Answer 7

to cleave DNA strands

Question 8

Identify the function of the replicative enzymes/proteins: primase

Answer 8

uses the original DNA sequence as a template to synthesize a short RNA primer

Question 9

Identify the function of the replicative enzymes/proteins: ligase

Answer 9

seals together okazaki fragments

Question 10

Compare and contrast leading and lagging strand synthesis.

Answer 10

lagging strand replicates discontinuously forming short fragments, whereas the leading strand replicates continuously.

Question 11

Explain how the lagging strand is discontinuously synthesized as Okasaki fragments.

Answer 11

DNA polymerase is not able to synthesize the chain in 3’ to 5’ direction, on the lagging strand, the synthesis of the chain is discontinuous in the 5’ to 3’ direction. The discontinuous replication results in several short segments which are called Okazaki fragments