DNA replication

Question 1

what is the function of DNA helicase?

Answer 1

unwinds DNA

Question 2

what is the function of DNA polymerase?

Answer 2

synthesis of DNA in 5’-3’ direction

Question 3

what is the function of DNA topoisomerase?

Answer 3

relieves the tension in DNA

Question 4

What is the function of DNA primase?

Answer 4

Synthesises RNA primers

Question 5

What is the function of ribonuclease?

Answer 5

Degrades RNA primers

Question 6

What is the function of DNA ligase?

Answer 6

Joins DNA fragments

Question 7

How does DNA unwind?

Answer 7

• DNA helicase unwinds the DNA and uses ATP to proper itself along the DNA
• Single-strand DNA binding protein (SSB) binds and keeps the strands apart
• DNA topoisomerase relieves the tension

Question 8

what is a primer?

Answer 8

short segment of RNA complementary to the template

Question 9

what does DNA polymerase require?

Answer 9

template and primer

Question 10

what does DNA synthesis take place in?

Answer 10

replication fork

Question 11

What is the difference between replication on the leading and lagging strands?

Answer 11

Replication progresses 5’-3’ so it is continuous on the leading strand but it cannot progress in the opposite direction so replication is discontinuous on the lagging strand

Question 12

What are the short DNA sequences synthesised on the lagging strand called?

Answer 12

Okazaki fragments

Question 13

Explain the mechanism of the sliding clamp

Answer 13

• the DNA polymerase remains attached to the DNA template by interaction with a protein called sliding clamp
• a new clamp has to be loaded on the lagging strand as each Okazaki fragment is synthesised
• only one clamp is required on the leading strand
• clamp attached to replication fork

Question 14

describe replication on the lagging strand

Answer 14

DNA primase attaches RNA to template

DNA polymerase III adds nucleotides until it reaches the previous primer

Ribonuclease H digests the RNA primer, leaving a gap

DNA polymerase I fills the gap

DNA ligase joins fragments together

Question 15

what are telomeres?

Answer 15

Repetitive regions at the ends of chromosomes

G-rich series of repeat bases (TTAGGG repeated hundreds/thousands times in mammals)

Question 16

What is the function of a telomere?

Answer 16

They act as caps to protect internal regions of chromosomes and are worn slightly each replication

Okazaki fragments can’t cover end of chromosome as primer would fall off end so no way of starting

Question 17

explain the role of telomerase

Answer 17

• Telomerase are an RNA- dependent DNA polymerase, meaning an enzyme that can make DNA using RNA as a template
• the enzyme binds to a special RNA molecule that contains the sequence complementary to the telomeric repeat (AAUUCCC-TTAGGG)
• Telomerase recognises tip of existing repeat sequence, uses RNA template within enzyme to add additional repeats to the telomere DNA
• when the overhang is long enough, a matching strand can be made by DNA polymerase a, which has its own primase subunit

Question 18

How often does DNA polymerase make a mistake?

Answer 18

Once every 10 000 000 pairs

Question 19

What is the function of exonuclease?

Answer 19

Cutting out the wrong bases during proof-reading

Question 20

Name 4 factors that can damage DNA

Answer 20

Ionising radiation exposure

UV light

Toxic chemical agents

Reactive oxygen species

Question 21

What is depurination?

Answer 21

Removal of purine bases (guanine/adenine) leaving sugar-phosphate group. No DNA breaks

Question 22

What is deamination?

Answer 22

No DNA breaks and results in C to U transition

Question 23

What is a thymine dimer?

How do they arise?

What can they result in?

Answer 23

2 adjacent thymine bases become covalently attached to each other

Arise from UV light exposure and leads to stalling of replication machinery

Failure to repair thymine dimer is problem in xeroderma pigmentosum

Question 24

what are the 3 types of mutations?

Answer 24

Point mutations

Insertion mutations

Deletion mutations

Question 25

explain the 3 types of point mutation

Answer 25

A silent mutation results in a codon that still encodes same amino acid so the protein is unaffected and the organism's phenotype is not significantly altered A missense mutation results in a codon that encodes a different amino acid so the primary protein sequence is altered. can be conservative or radical: Conservative when the new amino acid has similar function to the original e.g. similar R group size/charge and similar protein shape/function Radical when new amino acid functions differently to original as the R group is different in charge/size so the protein may have altered secondary/tertiary structure and function is affected A nonsense mutation results in a stop codon so the protein is truncated and may not function properly or at all

Question 26

Explain insertion mutations

Answer 26

It changes the number of nucleotides so each one is shifted alone by one Every codon from the point of insertion is different Different amino acids are added to the protein and it is unlikely to function properly if at all ORF has been moved, known as a FRAMESHIFT mutation

Question 27

Explain deletion mutations

Answer 27

It removes a piece of DNA so each nucleotide is shifted alone by one Every codon from the point of deletion is different Different amino acids are added to the protein and it is unlikely to function properly if at all ORF has been moved, known as a FRAMESHIFT mutation

Question 28

List the 3 factors affecting tolerance of insertion/deletion mutations

Answer 28

Size: effect on protein structure Location: introns/exons, coding potential and gene regulation Open reading frame

Question 29

what are the 2 classes of mutation?

Answer 29

gain of function loss of function

Question 30

what happens with a gain of function mutation? Give an example

Answer 30

DNA sequence changes that leads to an increase or alternative activity e.g overactivity of a gene oviduct overrides existing control mechanism leading to cancerous cell

Question 31

what happens with loss of function mutations. Give an example

Answer 31

a DNA sequence change that leads to decreased activity e.g the nucleotide change leads to a loss of expression of the protein (null mutation)

Question 32

what is the mechanisms for DNA repair

Answer 32

1. Excision: recognition and removal of damage using exonuclease 2. Repair Re- synthesis of missing DNA using DNA polymerase 3. Joining Sealing the nick using DNA ligase

Question 33

How are thymidine dimers repaired?

Answer 33

Bubble formed, mistake cut out and gap repaired

Question 34

Explain non-homologous end joining to repair double-stranded breaks

Answer 34

End 'polished' to produce blunt ends Ends joined by DNA ligase Results in loss of nucleotides and consequence depends on location Very quick - useful for rapidly dividing cells

Question 35

Explain homologous end joining to repair double-stranded breaks

Answer 35

End 'polished' to produce blunt ends Complete repair so no sequence lost Recombination between 2 corresponding regions of alleles Other allele used as template Slower - less useful for rapidly dividing cells but more accurate Potential for introducing recessive traits in heterozygous individuals

Question 36

mismatch repair reduces DNA replication mistakes to what?

Answer 36

1 in 10^9