CB6 Replication

Question 1

Define the semi-conservative model of DNA replication

Answer 1

• Use the original parent DNA molecule as a template to make new strands of DNA which have one of the parent copy & one of the new copy
• both DNA strands act as templates to create 2 new identical DNA molecules
• each of the daughter DNA molecules has one original strand and one new strand

Question 2

Define the leading & lagging strands in DNA replication

Answer 2

Leading strand: template DNA runs 3’ to 5’, very easy for cell to replicate

Lagging strand: template DNA runs 5’ to 3’, difficult to replicate, generates short fragments called Okazaki fragments

Question 3

Define the three essential properties common to all DNA polymerases

Answer 3

• unable to separate 2 DNA strands
• can only elongate a pre-existing dna/rna strand
• can only grow 5’ to 3’

Question 4

Describe initiation, elongation & termination in DNA replication

Answer 4

Initiation: the process of DNA synthesis is begun by helicase which binds to specific DNA sequences called replication origins - here helicase unwinds the DNA strands forming a replication fork

Elongation: DNA polymerase catalyses the addition of nucleotides to the 3’end of a growing DNA strand, using parental strand as a template

Termination: process of expanding DNA strands continues until either there is no more DNA template strand left to replicate or two replication forks meet. On lagging strand, DNA lipase joins Okazaki fragments together

Question 5

Define the role of DNA primase

Answer 5

Synthesises RNA primer

Question 6

Define the role of DNA helicase

Answer 6

Unwinds DNA to allow access to DNA parent strands

Question 7

Define the role of topoisomerase

Answer 7

Relieves supercooling caused by helices to prevent damage to DNA

Question 8

Define the role of DNA polymerase Ill

Answer 8

Synthesises DNA on the leading strand
5’ to 3’
Proof-reading ability

Question 9

Define the role of DNA polymerase I

Answer 9

Synthesises DNA on lagging strand
Builds DNA 5’ to 3’
Removes Okazaki fragments RNA primers; replaces with DNA

Question 10

Define the role of DNA ligase

Answer 10

Joins Okazaki fragments together
Forms phosphodiester bonds

Question 11

Define the role of Okazaki fragments

Answer 11

DNA can only be synthesised 5’ to 3’
Lagging strand made discontinuously in successive small pieces of DNA called Okazaki fragment - which are later joined by lipase to form a continuous strand

Question 12

Define the role of single strand binding proteins

Answer 12

Binds to exposed bases & protects them, holds backbone steady

Question 13

Explain proof reading and editing by DNA polymerases

Answer 13

DNA polymerase checks whether the previously added nucleotide is correctly base paired to the template strand

If so, the polymerase adds the next nucleotide

If not, the polymerase clips off the mis-paired nucleotide & tries again

Question 14

Outline the functions of the telomere & telomerase

Answer 14

Telomere = ends of chromosome

Telomerase = catalyses DNA synthesis from an RNA template which it provides

Question 15

Describe the levels of packing (condensation) of DNA

Answer 15

1. Nucleosides - DNA wrapped around histones 7 times
2. 30nm fibre - wrap up histones on a coil (requires histone 1)
3. Scaffold associated - attachment of 30nm fibres into protein scaffold
4. Higher levels of condensation - chromosomes