3.3 - DNA replication Flashcards

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1
Q

DNA replication

A
  • synthesis of new DNA
  • occurs during S-phase of the cell cycle
  • semi-conservative
  • 2 strands of parental double helix unwind, and each specifies a new daughter strand by base-pairing rules
  • 3 Steps: Initiation, elongation (of the new strand), termination
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2
Q

Initiation in DNA Replication

A
  • Begins at an origin of replication (OriC)
  • involves: initiator proteins, helicases, primase, polymerase III
  • at the end of initiation: helicases are ready to unwind, RNA primers are in place, polymerases are ready for elongation
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3
Q

Initiation Components: Initiator proteins

A

-separate the strands on OriC

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4
Q

Initiation Components: Helicases

A
  • loaded only

- 1 at each fork

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5
Q

Initiation Components: Primase

A

-synthesizes 2 RNA Primers

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6
Q

Initiation Components: Primer

A
  • synthesized by primase
  • short (4-6 nt) RNA
  • complementary to DNA template
  • antiparallel
  • (attracts DNA polymerase to build)
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7
Q

Initiation Components: Polymerase III

A
  • loaded only

- 2 at each fork

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8
Q

Elongation in DNA Replication

A
  • Helicases unwind duplex bidirectionally
  • DNA polymerase adds DNA nucleotides to the 3′ end
  • involves: helicases, DNA polymerase III dimers, dNTPs, primases (RNA primers), DNA polymerae I, DNA Ligase
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9
Q

Elongation Components: Helicases

A
  • unwind duplex bidirectionally

- 2 forks move away from the origin

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10
Q

Elongation Components: DNA polymerase III

A
  • adds single nucleotides to 3’ OH of pre-existing polynucleotide basepaired to a DNA template
  • 5’ > 3’ polymerase activity
  • will also “proofread” 3’ - 5’
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11
Q

Elongation Components: dNTPs

A

deoxynucleoside triphosphate precursors

  • dATP – deoxyadenosine triphosphate
  • dGTP – deoxyguanosine triphosphate
  • dCTP – deoxycytidine triphosphate
  • dTTP – deoxythymidine triphosphate
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12
Q

Lagging strand

A
  • lagging strand synthesis is discontinuous (okazaki fragments)
  • Parent template is 5’ > 3’
  • Contain primase that synthesizes RNA primer
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13
Q

Elongation Components: DNA polymerase I

A
  • Primer removal and gap filling in the completion of an Okazaki fragment
  • at 3’OH of one Okazaki fragment (so can only go 5’ to 3’ direction)
    1) breaks phosphodiester “forward” (5’ > 3’)
    2) adds deoxynucleotide to 3’OH (5’ > 3’)
  • these two activities alternate until primer is removed
  • will also “proofread” 3’ - 5’
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14
Q

Elongation Components: DNA Ligase

A
  • nicks still exist between okazaki fragment, therefore…..
  • forms the final phosphodiester linkage on lagging strand between 3’OH of one okazaki fragment and 5’PO4 of next okazaki fragment
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15
Q

Mismatched base-pairs (proof-reading)

A
  • mismatched bps create distortion in diameter detected by trailing portion of polymerase
  • polymerizing activity stops
  • 3’ to 5’ exonuclease activity breaks 3-4 phospodiester linkages including mismatched nucleotides
  • resumes 5’ to 3’ polymerase activity
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16
Q

Termination in DNA Replication

A
  • stops DNA replication by addition of telomere sequence at end
  • includes in circular chromosome: topoisomerase II
  • includes in linear chromosome: telomerase, DNA polymerase I, DNA ligase
17
Q

Telomere Synthesis

A

-does not occur after chromosome replication in all cells

1) after telomerase extension of 3’ end
2) primase-primed DNA polymerase synthesis of other strand
3) ligase forms final phosphodiester linkage
4) repair polymerase removes primer

18
Q

Leading strand parent template

A
  • parent runs runs 3’ > 5’

- therefore, leading strand synthesizes 5’ > 3’ w/o problems