DNA REPLICATION Flashcards

1
Q

3 steps of prokaryotic replication

A
  1. Initiation
  2. Elongation
  3. Termination
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2
Q

Prokaryotic initation

A

DNA Pol III initiates from a single replication origin called oriC

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3
Q

Structure of DNA Pol III

a) CORE enzyme
b) gamma complex
c) epsilon subunit
d) funny t sub unit
e) beta subunit

A

a) alpha sub unit (5’ - 3’ polymerisation)
sigma sub unit (proof reading exonuclease)
theta core assembly

b) 5 sub units for loading and unloading polymerase onto DNA template via a clamp
c) used as a proof reading subunit - exonuclease
d) connects two polymerase alpha sub units to create dimeric polymerase unit
e) sliding clamp to tether polymerase to the dna

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4
Q

How are RNA primers synthesised?

A

DNA primase

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5
Q

Function of DnaB helicase

A

in prokaryotic initiation it separates the strands

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6
Q

How does prokaryotic DNA replication occur?

A
  • DnaA- ATP proteins recognise oriC and DnaB helicase unwinds strands
  • held apart by single stranded binding proteins
  • RNA primase synthesises primers
  • DNA Pol III polymerises strands
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7
Q

Replication of the leading strand (5’ - 3’) is ______

A

Continuous

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8
Q

Replication of the lagging strand is _____, producing ____ fragments.

A

discontinuous

Okazaki

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9
Q

What slows polymerases

A

Fork traps

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10
Q

Why is lagging strand looped around?

A

Both polymerases need to function in the same direction and simultaneously

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11
Q

What removes RNA primers?

A

DNA pol II using its exonuclease action in a 5’ to 3’ direction

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12
Q

What seals gaps?

A

DNA ligase

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13
Q

What does DNA pol I do?

A

maturation of Okazaki fragments

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14
Q

Why is eukaryotic replication different?

A
  • Humans have 3 billion bp compared to E.coli 30 million
  • therefore humans have multiple origins of replication
  • chromatin disassembly
  • telomeres are used to protect DNA from exonuclease action
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15
Q

Why do telomeres shorten?

A
  • Eukaryotes have difficulty replicating their linear chromosomes completely - ‘the end replication problem’
  • telomerase extends 3’ end of chromosomal DNA by a G-rich repeat, to counteract loss of DNA by incomplete replcation
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16
Q

Somatic cells have ___ telomerase levels. Meaning what?

A

reduced

  • fail to compensate for depletion of DNA at each replication round, leading to telomerase shortening and cell death after 40- 80 cell divisions
17
Q

What is the Hayflick limit?

A

the point at which telomere length is so short that the cells die

18
Q

What can reactivation of telomerase activity in somatic cells lead to?

A

Cancer