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Structure + Function 1 > DNA replication > Flashcards

DNA replication Flashcards

1
Q

what is the function of DNA helicase?

A

unwinds DNA

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2
Q

what is the function of DNA polymerase?

A

synthesis of DNA in 5’-3’ direction

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3
Q

what is the function of DNA topoisomerase?

A

relieves the tension in DNA

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4
Q

What is the function of DNA primase?

A

Synthesises RNA primers

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5
Q

What is the function of ribonuclease?

A

Degrades RNA primers

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6
Q

What is the function of DNA ligase?

A

Joins DNA fragments

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7
Q

How does DNA unwind?

A
  • 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
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8
Q

what is a primer?

A

short segment of RNA complementary to the template

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9
Q

what does DNA polymerase require?

A

template and primer

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10
Q

what does DNA synthesis take place in?

A

replication fork

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11
Q

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

A

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

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12
Q

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

A

Okazaki fragments

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13
Q

Explain the mechanism of the sliding clamp

A
  • 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
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14
Q

describe replication on the lagging strand

A

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

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15
Q

what are telomeres?

A

Repetitive regions at the ends of chromosomes

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

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16
Q

What is the function of a telomere?

A

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

17
Q

explain the role of telomerase

A
  • 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
18
Q

How often does DNA polymerase make a mistake?

A

Once every 10 000 000 pairs

19
Q

What is the function of exonuclease?

A

Cutting out the wrong bases during proof-reading

20
Q

Name 4 factors that can damage DNA

A

Ionising radiation exposure

UV light

Toxic chemical agents

Reactive oxygen species

21
Q

What is depurination?

A

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

22
Q

What is deamination?

A

No DNA breaks and results in C to U transition

23
Q

What is a thymine dimer?

How do they arise?

What can they result in?

A

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

24
Q

what are the 3 types of mutations?

A

Point mutations

Insertion mutations

Deletion mutations

25
Q

explain the 3 types of point mutation

A

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

26
Q

Explain insertion mutations

A

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

27
Q

Explain deletion mutations

A

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

28
Q

List the 3 factors affecting tolerance of insertion/deletion mutations

A

Size: effect on protein structure

Location: introns/exons, coding potential and gene regulation

Open reading frame

29
Q

what are the 2 classes of mutation?

A

gain of function

loss of function

30
Q

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

A

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

31
Q

what happens with loss of function mutations. Give an example

A

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)

32
Q

what is the mechanisms for DNA repair

A
  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
33
Q

How are thymidine dimers repaired?

A

Bubble formed, mistake cut out and gap repaired

34
Q

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

A

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

35
Q

Explain homologous end joining to repair double-stranded breaks

A

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

36
Q

mismatch repair reduces DNA replication mistakes to what?

A

1 in 10^9

Structure + Function 1 (18 decks)

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