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BIOL243 > Theme 1C > Flashcards

Theme 1C Flashcards

DNA Replication & Repair

1
Q

Semiconservative replication

A

each daughter cell remains paired with its complementary parental strand

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2
Q

Conservative replication

A

after replication, both daughter strands pair up

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3
Q

Dispersive replication

A

daughter strands will have a mixture of parental and newly-synthesized DNA

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4
Q

Semiconservative DNA Replication: Meselson and Stahl (3rd Classical Experiment)

A

track parental and newly-synthesized DNA strands over several generations with nitrogen isotopes

nitrogen isotopes are incorporated into DNA molecules via nitrogenous bases

Concluded that DNA replication is semiconservative

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5
Q

DNA synthesis reaction

A

nucleotides can only be added to the new strand at the 3’-OH end

hydrolysis of pyrophosphate provides energy for the formation of new phosphodiester bond

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6
Q

DNA synthesis occurs in the

what direction?

A

5’-3’ direction

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7
Q

DNA polymerases

A

synthesizes new strand only in the 5’-3’ direction

travels and reads the template strand.

cannot synthesize a new strand de novo - requires RNA primer with a 3’-OH for synthesis

has single active site that can catalyze four different reactions (incorporation of dATP, dCTP, dGTP, dTTP)

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8
Q

Replisome

A

molecular machine of enzymes that replicate DNA

  • helicase
  • primase
  • single-strand binding protein
  • DNA topoisomerase/gyrase
  • DNA polymerase III
  • DNA polymerase I
  • sliding clamp
  • DNA ligase
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9
Q

Helicase

A

unwinds the double helix by breaking hydrogen bonds

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10
Q

Primase

A

synthesizes RNA primers for DNA polymerase

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11
Q

Single-strand binding protein

A

stabilizes ssDNA before replication by preventing reannealing so that the strands can serve as template

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12
Q

DNA topoisomerase/gyrase

A

removes super coils that form ahead of the replication form, relives torque of mainly circular DNA

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13
Q

DNA polymerase I

A

removes RNA primer and fills gaps with DNA

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14
Q

DNA polymerase III

A

synthesizes DNA by adding nucleotides to the new DNA strand

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15
Q

Sliding clamp

A

attaches to DNA Pol III to DNA template, makes replication more efficient

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16
Q

DNA ligase

A

joins the ends of the DNA segments by forming phosphodiester bonds

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17
Q

Replication forks

A

synthesis of two new strands from the template strand occurs concurrently at the forks

18
Q

Because DNA polymerase can only synthesize in the 5’-3’ direction:

A
  • one new strand is synthesized continuously (leading strand)
  • other new strand is synthesized discontinuously (lagging strand) in small fragments
19
Q

Okazaki fragments

A

small DNA fragments of lagging strand

20
Q

How does DNA replication work?

A
  • DNA helicase unwinds double helix
  • RNA primase lays down RNA primer
  • Topoisomerase prevents twisting ahead of replication fork during unwinding
  • as helicase unwinds, DNA Pol III synthesizes leading and lagging strand
  • DNA Pol I removes RNA primer of Okazaki fragments and fills in gaps with dNTPs - but it can’t fill in all nicks
  • DNA ligase seals the nicks by reforming the phosphodiester bond
21
Q

What direction is the template strand read by DNA polymerase during DNA replication

A

3’-5’

22
Q

Initiation

A

unwinding and separation of two template DNA strands at the origin of replication site (oriC)

23
Q

Elongation

A

simultaneous synthesis of the two new DNA strands from the template by DNA polymerase

24
Q

Termination

A

DNA replication stops when it reaches a termination site (circular DNA) or at the end of a chromosome (linear DNA)

25
Q

Review slide 10

A
26
Q

Polymerase chain reaction

A

slide 11

27
Q

End replication problem

A

requirement for RNA primer to initiate all new DNA synthesis presents problem, as no DNA polymerase can fill the gaps at the chromosomal ends

  • therefore, there will be additive loss at the chromosomal ends for every round of DNA replication/cell division
28
Q

Solution to end replication problem

A

telomeres

  • non-coding single-stranded DNA are added to the 3’ end of the chromosomes by telomeres
  • usually repeats of 5-8 G’s and T’s
  • telomeres can be worn away after each DNA replication as they do not have genes in them
  • exist to ensure ends of linear chromosomes are fully replicated
  • when all telomere region is done, the cell stops dividing
    – leading cause of natural death
    – telomeres in older individuals are shorter
29
Q

Telomerase

A

enzyme that restores shortened telomeres
- not present in most eukayotic cells
- present in gametes and stem cells (which keep dividing and dividing)

30
Q

Human telomerase (hTERT) mutations can be used as a biomarker in cancer because many cancers aquire _______________ to negate the limitations of rapid cell division

A

mutations that activate the telomere gene

31
Q

How do cancer cells keep dividing?

A

they have telomeres and cells somehow reactivate telomerase genes that allows for rapid division

  • vaccine would attack telomere genes
32
Q

High fidelity of DNA replication

A

DNA must fully replicate devoid of all errors

  • telomeres exist to ensure ends of linear chromosomes are fully replicated
  • failure to maintain this causes defected genomes possibly resulting in disease (cancer) and death of organism
  • DNA repair mechanisms mediated by enzyme complexes ensure that replication error rate is low
33
Q

Proofreading activities of DNA polymerase

A
  • first way to catch and fix mistake
  • DNA Pol III synthesizes new strand

(optimum conformation of active site and incoming nucleotide allows catalysis of the correct base pair but mistakes can still happen)

  • DNA pol III detects mistake and uses 3’-5’ exonuclease activitiy to remove most recent mismatched nucleotides
  • it replaces correct nucleotide and resumes synthesis of the new DNA strand
34
Q

DNA mismatch repair (MMR)

A

MMR covers for replication errors not corrected by proofreading
- recognition of mismatch damage by DNA binding protein MutS and MutL

35
Q

MutH

A

endonuclease daughter strand several nucleotides away from mismatch

36
Q

Exo1

A

5’-3’ exonuclease excises region of daughter strand surrounding mismatch

37
Q

DNA Pol III (MMR)

A

fills gaps and repairs mismatch

38
Q

The nick left after gap is sealed in MMR is sealed by

A

DNA ligase

39
Q

In DNA replication, DNA polymerase catalyzes new synthesis in the ____ direction

A

5’-3’

40
Q

The leading strand is synthesized continuously in the _____ direction

A

5’-3’

41
Q

Telomerase enzyme is needed because

A

RNA primers are removed (After replication)

DNA pol can only add nucleotides to an existing 3’ end of a nucleic acid

Linear chromosomes are shortened with each cell division

BIOL243 (7 decks)

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