Theme 1c Flashcards

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1
Q

How was semiconservative DNA discovered

A

Meselson and Stahl experiment
-track parental and newly synthesized DNA strands over several generations with nitrogen isotopes
-nitrogen isotopes are incorporated into the DNA molecules via nitrogenous bases

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2
Q

What does the banding pattern look like for semiconservative DNA and for conservative or dispersive DNA (original DNA is grey and replicated DNA is red)

A

-Semiconservative: after 2 rounds of replication there are 4 strands, 2 that are fully red and 2 that are half grey and half red
-Conservative: 3/4 strands are red and 1 is fully grey
-Dispersive: all strands are both red and grey

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3
Q

Where on the DNA molecule are nucleotides added

A

To the 3’ OH end so synthesis occurs in the 5’ to 3’ direction

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4
Q

Where does the energy for the formation of new phosphodiester bonds come from

A

Hydrolysis of pyrophosphate

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5
Q

What is DNA polymerase and it’s properties

A

-synthesizes new DNA strand only in the 5’ to 3’ direction
-travels/reads the template strand in the 5’ to 3’ direction
-has a single active site that can catalyze 4 different reactions (dATP, dCTP, dGTP and dTTP)

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6
Q

What is the order of the 8 DNA replication enzymes and what do they do

A

1) helicase: unwinds double helix by breaking h-bonds
2) primase: synthesizes RNA primers fro DNA polymerase
3) single stranded binding proteins: stabilizes ssDNA before replication by preventing reannealing so that the strands can serve as a template
4) DNA topoisomerase/gyrase: removes super coils that form ahead of the replication fork
5) DNA polymerase lll: synthesizes DNA by adding nucleotides to the new DNA strand
6) DNA polymerase l: removes RNA primer and fills the gaps with DNA
7) sliding clamp: attaches DNA pol lll to DNA template, replication is more efficient
8) DNA ligase: joins the ends of DNA segments by forming phosphodiester bonds

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7
Q

Continuous vs discontinuous DNA replication

A

Leading strand is synthesized continuously
Lagging strand is synthesized discontinuously

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8
Q

Describe DNA replication in bacterial chromosomes (3 steps)

A

1) initiation: unwinding and separation of two template strands at the origin of replication site forming a replication bubble with 2 forks
2) elongation: simultaneous synthesis of the two new DNA strands from the template strands for the template strands from DNA polymerase
3) termination: DNA replication stops when a termination site is reached

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9
Q

Does DNA have more A-T bonds or more C-G bonds and why

A

A-T bonds because there are less h-bonds to break

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10
Q

What is the polymerase chain reaction (PCR)

A

DNA replication and amplification in a test tube

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11
Q

What are the 4 steps for PCR

A

1) start with template DNA and denature at 95C
2) anneal the primer using bacterial polymerase to withstand high temp (50-60C)
3) extend the strand at 72C (now have 2 DNA copies)
4) repeat for 30 cycles (have 2^30 molecules)

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12
Q

What are telomeres for

A

-solution to end replication problem
- telomeres add a non coding single-stranded DNA added to the 3’ end of chromosomes
-telomeres will be worn away after each replication instead of chromosomes
-when the telomere region is gone, the cell stops dividing

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13
Q

What is the telomere hypothesis

A

-since many cancers involve mutations that activate the telomere gene to negate limitations of rapid cell division, if we could come up with a vaccine that kills cancer cells that express the telomerase gene it could stop cancer cells from replicating
-telomerase is an enzyme that restores shortened telomere, present in gametes and stem cells (babies, older people have shorter telomeres)

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14
Q

What is the DNA replication error rate

A

1/10,000,000,000 nucleotides because of DNA repair mechanisms

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15
Q

How does DNA polymerase proofread DNA

A

DNA pol lll can detect mistakes and can remove and replace the incorrect nucleotides
Reduces error rate to 1/10,000,000

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16
Q

What is the DNA replication error with out proofreading

A

1/10,000

17
Q

What is DNA mismatch repair (MMR)

A

Covers for replication errors not corrected by proofreading

18
Q

What do MutS, MutL, MutH, Exo1, DNA pol lll and DNA ligase do

A

MutS and MutL: recognizes mismatch damage of nucleotides
MutH: nicks daughter strand several nucleotides away from the mismatch
Exo1: 5’ - 3’ exonuclease excises region of daughter strand surrounding the mismatch
DNA pol lll: fills the gap and repairs the mismatch
DNA ligase: seals the nick left after the gap