DNA replication

Question 1

What were the 3 possible models of DNA replication?

Answer 1

Conservative - completely new molecule
Semi-conservative - 1 original + 1 new strand
Dispersive - old DNA dispersed throughout chromosome

Question 1

Describe the experiment conducted by Meselson + Stahl in 1958

Answer 1

-grew bacteria in media containing 15N to make heavy DNA
-transferred bacteria to media containing 14N so that the new DNA would be lighter
-separated heavy + light molecules by ultracentrifugation

Question 2

How did Meselson + Stahl separate the heavy + light DNA by ultracentrifugation?

Answer 2

-mix sample with caesium chloride
-spin at high speed for many days
-high density molecules move to bottom + low density move to top

Question 3

What were the results from Meselson + Stahl’s experiment after 1 generation?

Answer 3

-single hybrid band further left than 15N band
-discounts conservative model

Question 4

What were the results of Meselson + Stahl’s experiment after 2 generations?

Answer 4

-1 hybrid band + 1 light band
-discounts dispersive model

Question 5

What were the results of Meselson + Stahl’s experiment after 4 generations?

Answer 5

1 hybrid band + 1 thicker light band

Question 6

In which direction does DNA replication occur?

Answer 6

Bidirectional replication forks - occurs in both directions

Question 6

What is an origin and what is the name of the place where replication takes place?

Answer 6

-an origin is where replication begins
-takes place in replication bubble

Question 7

What enzymes are needed for DNA replication?

Answer 7

-DNA polymerase
-primase
-DNA ligase
-topoisomerase
-helicase
-single stranded binding protein

Question 8

Describe DNA polymerase

Answer 8

-adds nucleotides 1 at a time in a 5’-3’ direction
-uses template strand to form H bonds

Question 9

Describe primase

Answer 9

-generates the primer (usually made of RNA)
-first nucleotide is added to the primer, then more can be added
-primers are removed for ligase

Question 10

Describe DNA ligase

Answer 10

-joins loose ends together into a single strand of DNA
-joins gaps in sugar-phosphate backbone

Question 11

Describe topoisomerase

Answer 11

-relieves pressure from overwinding around the replication bubble
-makes + reseals breaks in DNA

Question 12

Describe helicase

Answer 12

-breaks H bonds between the 2 strands in original DNA molecule
-separates the 2 strands

Question 13

Describe single-stranded binding proteins

Answer 13

Binds to separated strands to prevent them from reannealing (joining back together)

Question 14

Describe the leading strand in DNA replication

Answer 14

-5’-3’ DNA synthesis points towards the replication fork
-can proceed continuously

Question 15

Describe the lagging strand in DNA replication

Answer 15

-5’-3’ DNA synthesis points away from the replication fork
-is discontinuous, so is primed numerous times

Question 16

What is semicontinuous DNA replication?

Answer 16

Continuous replication of leading strand and discontinuous replication of the lagging strand

Question 17

What does the removal of the primer at the end of chromosomes result in and what is it a problem for?

Answer 17

-leaves a gap that can’t be filled as there’s no DNA polymerase to fill it
-on every round of replication, a small piece is lost from end of chromosome
-problem for lagging strand at each end

Question 18

What solves the problem caused by the removal of primers and how?

Answer 18

-telomeres
-short stretches are lost from telomeres at each round of replication
-telomerase replenishes telomeres from an RNA template in some cell types