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Cell and Molecular Biology > DNA Replication > Flashcards

DNA Replication Flashcards

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1
Q

What direction is DNA replicated in?

A

5’ to 3’ end
5’ = phosphate group
3’ = OH group

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2
Q

Why is DNA synthesis an irreversible reaction?

A

It is coupled to a breakdown of PPi by 2Pi

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3
Q

Where is pyrophosphatase found?

A

It is ubiquitous to all cells

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4
Q

DNA synthesis is exothermic or endothermic?

A

Exothermic - -7kcal/mole

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5
Q

dNTP + (dNMP)n =

A

(dNMP)n+1 +2Pi

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6
Q

The energy required for DNA synthesis comes from what?

A

The breakage of 2 high energy phosphate bonds

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7
Q

Where is DNA synthesis initiated?

A

At the replication fork

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8
Q

Which enzyme separated paired DNA strands?

A

DNA Helicase

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9
Q

The leading strand runs in which direction?

A

The 5’ end to the 3’ end

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10
Q

What does the lagging strand form?

A

Okazaki fragments

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11
Q

Which enzyme synthesises the primer for DNA Synthesis?

A

DNA primase - synthesises a short RNA

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12
Q

How long are primers usually?

A

About 10 nucleotides long

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13
Q

How does DNA polymerase differe from DNA primase?

A

Unlike DNA polymerase, DNA primase can start a new polynucleotide chain by joining two nucleotide triphosphates together

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14
Q

What four enzymes are required for lagging strand synthesis?

A

DNA Primase
DNA Polymerase
Ribonuclease H
DNA Ligase

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15
Q

What is the role of ribonuclease H?

A

Removes the RNA Primer

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16
Q

What are the steps of DNA replication in the lagging strand?

A

1) New RNA primer synthesis by DNA primase
2) DNA polymerase adds to RNA primer to make a new okazaki fragment
3) DNA polymerase finishes the DNA fragment
4) The old RNA primer erased and replaced by DNA
5) Nick sealing by DNA ligase joins new okazaki fragment to the growing chain

17
Q

How does DNA ligate newly synthesised okazaki fragments?

A

DNA ligase seals a broken phosphdiester bond
DNA ligase uses a molecule of ATP to activate the 5’ end at the nick before forming a new bond:
- ATP + 5’p = p-p + 5’-p-AMP
- p-p = 2Pi + free energy

18
Q

Describe the structure of DNA Helicase

A

Has a symetrical hexometric structure

6 subunits

19
Q

What is the rate at which DNA Helicase pulls apart the helix?

A

1000 nucleotide pairs per second

20
Q

What are single strand binding proteins? SSBs

A

Bind tightly and cooperatively to exposed single stranded DNA without covering the bases
They aid DNA helicase by stabilising unwound DNA
They straighten out the lagging strand template to prevent hairpin helices forming

21
Q

Why are hairpin helices indesirable?

A

They can impede DNA synthesis by DNA polymerase

22
Q

What is Werner Syndrome?

A

Progeria (premature aging)

Mutations are autosomal recessive, occuring in RECQ helicase gene WRN

23
Q

What causes blood syndrome?

A

Loss of function mutation in a Recq- family DNA helicases which maintain genome integrity

24
Q

What is the role of the sliding clamp?

A

Encircles DNA like a nut on a bolt and helps to move DNA polymerase forward along the DNA

25
Q

What is the role of DNA topoisomerases?

A

These prevent DNA from becoming tangled during replication
Unwinding of DNA at the replication fork introduces superhelical tension into the DNA helix
Tension is relaxed by topisomerases which nick and reseal the back bone of the parental helix

26
Q

What is the specific role of type 1 topoisomerases?

A

They nick and reseal one of the 2 DNA strands, no ATP required

27
Q

What is the specific role of type 2 topoisomerases?

A

They nick and reseal both DNA strands, ATP is required

28
Q

When does replication selection occur on eukaryotes?

A

During G1

29
Q

What occurs in origin activation?

A

Unwinding of DNA and recruitment of DNA polymerase

Occurs in S phase

30
Q

How long does DNA replication take in mammals?

A

Approx 8 hours

31
Q

Temporal separation of growth and synthesis ensures what?

A

That each origin is used and that each chromosome is replicated exactly once in each cell cycle

32
Q

How is the pre-replicative complex formed?

A

The origin recognition complex binds to a replicator sequence
Helicase loading proteins Cdc6 and Cdt1 bind to the ORC
The Helicase Mcm2-7 binds to complete the formation of pre-RC

33
Q

What happens if Cdk activity levels are low in S phase?

A

pre-RC formation is allowed

But pre-RCs are not activated

34
Q

What happens if Cdk activity levels are high?

A

New pre-RC formation inhibition

Existing pre-RC activation is allowe

35
Q

In what phases are Cdk levels low?

A

G1

36
Q

In what phases are Cdk levels high?

A

S, G2 and M phases

37
Q

What is the problem with RNA primer removal?

A

It causes progressive shortening of linear DNA sequences

38
Q

Telomerase extends which end of DNA?

A

The 3’ end

Cell and Molecular Biology (17 decks)

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