Lecture 4 - DNA Replication

Question 1

What was Meselson and Stahl‘s experiment on semiconservative replication?

Answer 1

1. Grew E. coli in both 15 and 14 nitrogen which was used to make bases for DNA in bacteria
2. Isolated the DNA using cesium chloride in a centrifuge finding that the 15 N had a band at the bottom and 14 N had a band at the top
3. E.coli exposed to 14 N and 15 N had a band in the middle meaning they combined.

Question 2

What does DNA synthesis require?

Answer 2

A DNA polymerase, DNTP’s, single stranded DNA template and primer 3’-OH

Question 3

What direction does DNA synthesis occur in?

Answer 3

5’ to 3’

Question 4

What occurs when a Phosphoanhydride bond breaks?

Answer 4

Two phosphate’s are released and a phosphodiester bond is formed

Question 5

What phase of the cell cycle does DNA replication occur in?

Answer 5

S phase

Question 6

What is the structure of a chromosome?

Answer 6

Telomere- highly repetitive, protect ends of DNA from degradation
I
Replication origin- where DNA replication begins
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Centromere- forms spindle attachment in mitosis
I
R
I
T

Question 7

What are replication bubbles?H

Answer 7

Bubbles (circles) in DNA formed by bidirectional replication

Question 8

What are the differences between eukaryotic and prokaryotic genome?

Answer 8

The eukaryotic genome is are larger and arranged as linear chromosomes whereas the prokaryotic genomes are small compact and circular

Question 9

What is the theta structure of prokaryotic DNA?

Answer 9

Theta shape formed as DNA bidirectionally divides from single origin of replication

Question 10

How is the issue of DNA being anti-parallel combat during replication?

Answer 10

Leading strand continues synthesis in 5’ -> 3’ direction
Lagging strand continues synthesis in 3’ -> 5’ direction creating Okazaki fragments

Question 11

How is the lagging strand synthesised?

Answer 11

1. Primase synthesises RNA primer
2. DNA polymerase as nucleotides creating an Okazaki fragment.
3. Primer is removed and gap is sealed by DNA ligase

Question 12

What keeps hold of DNA

Answer 12

Sliding clamps

Question 13

What stabilises single stranded DNA?

Answer 13

Single stranded DNA binding proteins

Question 14

What do topoisomerases do?

Answer 14

Untwist DNA by breaking and reforming phosphodiester bonds after supercoiling occurs

Question 15

What is the expected error per nucleotide added of 5 to 3 polymerisation?

Answer 15

1 in 10^5

Question 16

What is the errors expected after 5 to 3 polymerisation AND 3 to 5 nucleic proofreading by DNA polymerase

Answer 16

1 in 10^2

Question 17

What is the mismatch repair system?

Answer 17

MutS detects incorrect base pairings by looking for kinks where nucleotides don’t bond

Question 18

What can occur if MutS is mutated?

Answer 18

Colon cancer