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genetics > lecture 4 - 07/10/24 > Flashcards

lecture 4 - 07/10/24 Flashcards

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Biology 101 flashcards
1
Q

Who discovered DNA replication and what is it?

A

Meselson and Stahl
1958
half old DNA, half new DNA

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2
Q

Is DNA replicated before or after cell division?

A

before
S -phase

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3
Q

Why was cesium chloride used in the 1958 discovery of semiconservative replication?

A

It has a similar weight as DNA

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4
Q

Describe the semiconservative experiment

A
  • 2 samples of bacteria grown in different N-
    containing (N-14 and N-15) mediums
  • isolate N-DNA and load into centrifuge tube
  • centrifuge at high speed for 48hrs to form a cesium
    chloride density gradient
  • N-14 lighter so closer to top of the tube
  • N-15 heavier so closer to bottom of the tube
  • as gen number increases, [N-14] increases and [N-
    15] decreases in population
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5
Q

What is needed in DNA synthesis?

A
  1. DNA polymerase + Mg2+ (helps maintain the
    mixture)
  2. dNTPs
  3. single stranded template DNA
  4. primer 3’ -OH
  5. 5’ –> 3’ direction
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6
Q

Describe DNA synthesis events

A
  1. complementary base pairing
  2. 5’ –> 3’ direction
  3. requires 3’ -OH residue to extend from
  4. breakage of phosphoanhydride bond (P-O bond
    between phosphate groups) of dNTP
  5. formation of phosphodiester bond
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7
Q

What are telomeres?

A

Areas of highly repetitive DNA that protect chromosome ends from DNA degradation, recombination, and end fusion with other chromosomes

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8
Q

What are centromeres?

A

repetitive DNA which forms the spindle attachment site in mitosis

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9
Q

What is the origin of replication?

A

special sequence where duplication of the DNA begins; each chromosome will have many origins

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10
Q

Describe the movement of direction of DNA replication from the origins

A

bidirectional

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11
Q

Eukaryotic genomes vs prokaryotic genomes

A

eukaryotic prokaryotic

large small
linear chromosomes compact
(usually) circular

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12
Q

How many chromosomes are in humans?

A

23 pairs of chromosomes

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13
Q

How many origins of replication does bacterial DNA have?

A

1

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14
Q

replication forks

A
  • leading strand synthesis is continuous
  • lagging strand synthesis is discontinuous (okasaki
    fragments are joined together)
  • on lagging strand DNA polymerase ‘jumps’ to next
    section and reads short segments 5’ –> 3’
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15
Q

describe lagging strand synthesis

A
  1. new RNA primer synthesised by primase
  2. DNA polymerase adds nucleotides to 3’ end of
    new RNA primer to synthesis okazaki fragment
  3. previous RNA primer removed by nucleases and
    replaced with DNA by repair polymerase
  4. nick sealed by DNA ligase
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16
Q

What enzymes proteins work at the replication fork and what do they do?

A

enzymes act together as a REPLICATION MACHINE

Primase - synthesise RNA primer

sliding clamp - keeps hold of DNA strand and keeps DNA polymerase on DNA as it is in a free flowing cytoplasm

DNA helicase - unwind DNA

single stranded DNA-binding proteins - stabilise DNA

DNA polymerases - synthesis new DNA strands

17
Q

What do topoisomerases do?

A

it untwists the supercoiled DNA (caused by helicase unwinding DNA) ahead of the replication fork by breaking and reforming phosphodiester bonds

18
Q

What is MutS?

A
  • Mismatch repair protein
  • scans along DNA looking for DNA kinks caused by
    mismatched base pairs and recruits DNA repair
    proteins to them to put in the correct base
  • mutations in human Mismatch Repair genes are
    associated with predisposition to some cancers
19
Q

replication errors

A

5’ –> 3’ polymerisation 1 in 10^5
(errors per nucleotide added)

3’ –> 5’ exonucleolytic proofreading 1 in 10^2
(errors not corrected)

strand-directed mismatch repair 1 in 10^3
(errors not corrected)

combined 1 in 10^10

20
Q

Summary of DNA replication

A
  1. helicases unwind the parental double helix
  2. single-stranded binding proteins stabilize the
    unwound parental DNA
  3. leading strand is synthesised continuously 5’ –>
    3’ by DNA polymerase
  4. lagging strand synthesised discontinuously.
    primase synthesises a short RNA primer, which is
    extended by DNA polymerase to form an okasaki
    fragment
  5. after the RNA primer is replaced by DNA, DNA
    ligase joins the okasaki fragment to the growing
    strand

genetics (5 decks)

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