6.2: DNA replication

Question 1

Which phase of the cell cycle does DNA replicate?

Answer 1

S phase of interphase

Question 2

What are the three alternative models for DNA replication?

Answer 2

• conservative
• semi conservative
• dispersive

Question 3

What is the conservative model DNA replication? (2)

Answer 3

• parental strands direct synthesis of an entirely new double-stranded molecule
• the parental strands are fully “conserved”

Question 4

What is the semi conservative model of DNA replication? (2)

Answer 4

• The two parental strands each make a copy of itself

- after one round of replication, the two daughter molecules each have one parallel and one new strand

Question 5

What is the dispersive model of DNA replication? (2)

Answer 5

• The material in the two parental strands is dispersed randomly between the two daughter molecules
• after one round of replication, the daughter molecules contain a random mix of parental and new DNA

Question 6

In 1954, Meselson and Stahl performed an experiment using bacteria to determine which replication model was correct. What was the process? (3)

Answer 6

1. Bacteria was cultures with heavy isotope. 15N (diff. Number of neutrons)
2. Bacteria was transferred to a medium with 14N, a light isotope
3. DNA was centrifuged and analyzed after each replication

Question 7

By analyzing samples of DNA after each generation, Meselson and Stahl found that parental strands were following which replication model?

Answer 7

Semi conservative

Question 8

Please see paper on steps of DNA replication

Answer 8

Thanks

Question 9

Since DNAP III can only add nucleotides to the 3’ end, what problem does this cause at the 5’ end? What does this mean?

Answer 9

• there is no way to finish replication on the 5’ end of a lagging strand
• Over many replications, this would mean that the DNA would become shorter and shorter

Question 10

How are the genes on DNA protected from the DNA becoming shorter and shorter due to DNAP III only adding nucleotides to the 3’ end? (4)

Answer 10

• through telomeres
• telomeres are repeating units of short nucleotide sequences that do not code for genes
• form a cap at the end of DNA to help postpone erosion
• the enzyme telomerase adds telomeres to DNA

Question 11

As ___ ________ adds nucleotides to the new DNA strand, it proofreads the bases added. What happens if errors still occur? (2)

Answer 11

- DNA polymerase

• mismatch repair will take place
• enzymes remove and replace the incorrectly paired nucleotide

Question 12

As ___ ________ adds nucleotides to the new DNA strand, it proofreads the bases added. What happens if the segments of DNA are damaged?

Answer 12

• DNA polymerase

- nuclease can remove segments of nucleotides and DNA polymerase and ligase can replace the segments

Question 13

What is the purpose of DNA replication? (3)

Answer 13

• occurs in both mitosis and meiosis
• in order for a cell to divide, it must first replicate its DNA
• thus replication increases DNA for this to occur