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BMS2002 > L19 > Flashcards

L19 Flashcards

1
Q

only some Some cells in adult humans are actively proliferating give examples for those

A

intestinal epithelial stem cells

lymphocytes during immune responses

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2
Q

what type of cells are neurons in terms of proliferation

A

terminally differentiated

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3
Q

How do mitogens drive progression into S phase?

A

Mitogen binds to cell surface receptor tyrosine kinase

Ras-Raf-MAPK kinase signalling pathway is triggered

“Immediate early” genes including Myc are expressed

Myc is a transcription factor which upregulates genes including Cyclin D

Cyclin D, together with Cdk4 or 6, forms G1-Cdk

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4
Q

How does G1-Cdk drive progression into S?

A

A key target of G1-Cdk is Retinoblastoma protein, Rb

In an early G1 cell, Rb binds to and inactivates the transcription factor E2F

Phosphorylation of Rb by G1-Cdk inactivates Rb

E2F is now free to upregulate expression of genes including Cyclin E and Cyclin A

Cyclins E and A associate with Cdk2 to form G1/S-Cdk and S-Cdk

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5
Q

what can prevent G1-Cdk driven progression into S

A

p53 is a central regulator of checkpoint responses to stress (Lecture 17)

p53 is normally maintained at low levels in cells by Mdm2-mediated degradation

DNA damage activates kinase signalling through ATM/ATR then Chk1/Chk2

Phosphorylation of p53 displaces Mdm2 and p53 is stabilised

p53 acts as a transcription factor to turn on expression of CKIs such as p21 which inhibits G1/S-Cdk activity

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6
Q

what is the main task of S-phase

A

replication of genomic DNA

Duplication of the centrosomes

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7
Q

What are the major concerns when we think about replicating DNA?

A

produce exactly one additional copy of each chromosome

be high fidelity
(almost mutation free; 1 nucleotide change per 10^10 nucleotides per cell division)

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8
Q

what are the features of semi conservative replication in prokaryotes

A

Requires an origin of replication: initiation

Involves replication forks: elongation

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9
Q

describe initiation of bacterial DNA replication

A

Circular bacterial chromosomes have a single origin of replication defined by DNA sequence

Specific proteins form an Origin of Replication Complex (ORC) to initiate DNA synthesis

Allows regulated initiation to ensure one round of replication (in about 30 min)

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10
Q

how long would human chromosomes take if they were replicated like bacterial DNA

A

a month

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11
Q

Eukaryotes with larger genomes have 1 origin of replication

A

False

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12
Q

How does the cell ensure that the DNA is only copied once?

A

through Licencing

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13
Q

describe what happens in Licencing

A

origins of replication can only recruit pre-replicative proteins to origins in G1
Origins can only “fire” DNA replication in S phase, and then are deactivated

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14
Q

what are the components of the DNA replication machine

A

Helicase
- to separate the DNA double helix

Single-strand binding protein
- to maintain separation of single strands

DNA primase
- to initiate DNA polymerization

Two DNA polymerases
- to synthesise the two new strands of DNA

A sliding clamp (PCNA)
- to keep polymerase on DNA

Topoisomerases
- nick or cut and reseal DNA ahead of the replication fork to remove supercoils and tangles

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15
Q

what is the difference between the leading strand and the lagging strand

A

DNA strands are anti-parallel

The leading strand can be synthesised continuously

The lagging strand must be synthesised non-continuously as Okazaki fragments

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16
Q

how does proofreading work

A

The wrong nucleotide can sometimes be added, but “proofreading” activity removes the wrong nucleotide through exonuclease editing

17
Q

explain the way primers are involved in DNA replication

A

Proofreading DNA polymerases need a perfectly base-paired nucleotide to add new nucleotides

So, these polymerases cannot initiate new DNA synthesis

They need “primers” from which to extend the new strand

DNA primase creates these primers using RNA

Because the primers are RNA, they can be distinguished from DNA and removed later
DNA ligase then joins the Okazaki fragment

18
Q

what problem does the need for primers create

A

how to replicate the end of linear DNA?

The answer is to have a special structure at the chromosome ends: telomeres

In humans, these are repeating units of GGGTTA (approx 1000 repeats)

These repeating units are produced by an enzyme called telomerase

19
Q

what does telomerase do

A

Telomerase extends the 3’ end of the chromosome so it can be back filled by lagging strand synthesis without losing genetic information

Telomere binding proteins plus a T-loop structure protect the free end

20
Q

what pathways repair DNA damage

A
  1. Strand-directed mismatch repair
  2. Base and nucleotide excision repair
  3. DNA break repair
21
Q

what does the cell use to repair DNA

A

Where possible, the cell makes use of the information from the undamaged DNA strand to carry out the repair

In some cases, the cell can even use the information in a sister chromosome to correct damage

22
Q

describe what happens in Strand-directed mismatch repair

A

Mis-incorporations during DNA synthesis are not always corrected by DNA polymerase proofreading

Scanning proteins (MutS and MutL) can detect mismatched nucleotides

But, has to be a mechanism to correct only the new strand

In eukaryotes, this happens because only the new nicked strands are repaired

23
Q

describe what happens in Base and nucleotide excision repair

A

(A) A number of enzymes (e.g. Uracil DNA glycosylase) recognize different types of damaged base

AP endonuclease and phosphodiesterase remove sugar phosphate

DNA polymerase adds new nucleotide and DNA ligase seals nick

(B) Other enzymes can recognize distortions in the double helix

e.g. excision nuclease recognizes pyrimidine dimers and excises the DNA bit containing it

DNA helicase cuts the excised DNA section

DNA polymerase and DNA ligase make up the new DNA segment

24
Q

cytosine can deaminate to uracil

A

True

25
Q

how does DNA break repair work

A

Double stranded breaks (DSBs) are a potentially catastrophic form of DNA damage

Can be repaired fairly simply by recognizing and re-ligating the free ends together (NHEJ) but this is error-prone

If cell is in S/G2, then homologous recombination (HR) can use the information in a sister chromosome to accurately repair the damage

26
Q

how does G2 checkpoint arrest the cell from Mitosis

A
  1. Inhibiting M-Cdk through p53 and p21, as described earlier for the G1 checkpoint
  2. By inhibiting Cdc25, the phosphatase needed to activate M-Cdk

BMS2002 (21 decks)

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