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Kaplan USMLE Review > DNA Replication and Repair > Flashcards

DNA Replication and Repair Flashcards

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USMLE® Step 1 flashcards
1
Q

What is semiconservative replication?

A

it is when each parental strand is used as a template for the replication of a new DNA strand

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2
Q

When are sister chromatids separated in eukaryotes?

A

mitosis

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3
Q

How many origin of replications do you find in prokaryotes when discussing DNA replication?

A

one

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4
Q

How does DNA polymerase initiate strand synthesis?

A

It can’t initiate strand synthesis on it’s own

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5
Q

What allow DNA polymerase to to prevent errors during DNA replication?

A

It has 3’ - 5’ exonuclease activity

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6
Q

What is the specific action of helicase?

A

It breaks the H-bonds holding the base pairs together

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7
Q

What is the reason for single strand binding proteins?(2 points)

A

they prevent reassociation of DNA and destruction of DNA from nucleases

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8
Q

What is the main DNA polymerase in the DNA replication process in prokaryotes

A

DNA polymerase III

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9
Q

What is the leading strand?

A

the newly synthesize strand during DNA replication. It is the strand which is usually synthesized towards the replication fork.

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10
Q

What is the lagging strand?

A

a strand of DNA that is synthesized discontinuously during DNA replication forming Okazaki fragments. These strands are synthesized away from the replication fork.

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11
Q

What is the direction of synthesis by DNA polymerase?

A

5’-3’

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12
Q

What removes the RNA primers during DNA replication in eukaryotes?

A

RNAaseH

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13
Q

What fills in the gaps during DNA replication?

A

DNA polymerase

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14
Q

What is the action of DNA polymerase I?(2 points)

A

It removes the primers having 5’ exonuclease activity and synthesizes at 5’ Okazaki fragments

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15
Q

What is the action of DNA ligase?

A

it seals the nick at Okazaki fragments

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16
Q

What is the action of DNA gyrase?

A

it introduces negative supercoils by nicking both strands of DNA to overcome the formation of the positive supercoils

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17
Q

In eukaryotes, what is the action of DNA polymerase α and δ?(2 points)

A

they work together synthesizing the leading and the lagging strand

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18
Q

What is the action of DNA polymerase γ in eukaryotes??

A

they synthesize mitochondrial DNA

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19
Q

What is the action of DNA polymerase β and ε in eukaryotes?

A

they participate primarily in DNA repair

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20
Q

What DNA polymerase can substitute for DNA polymerase δ in certain cases?

A

DNA ε

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21
Q

What is the action of DNA polymerase δ in eukaryotes?

A

works together with DNA polymerase α to synthesize both the leading and the lagging strands of DNA

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22
Q

What are receptive sequences at the end of liner DNA molecule?

A

telomeres

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23
Q

What DNA polymerase in eukaryotes can synthesize at the 5’ end?

A

none of them

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24
Q

How does replication of telomeres take place?

A

Telomerase

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25
Q

What special type of activity does telomerase has?

A

hTRT or reverse transcriptase activity

26
Q

Levofloxacin, Ciprofloxacin, Moxifloxacin are the most active against what specific type of bacteria?(2 points)

A

aerobic gram negative bacteria

27
Q

What is the action of levoflxcin, ciprofloxacin and moxifloxacin?

A

they block the activity of topoisomerase II in prokaryotes or DNA gyrase in eukaryotes

28
Q

How is AZT a good treatment for patients with HIV?

A

It impedes the activity of the reverse transcriptase found in HIV

29
Q

What specific type of enzyme is reverse transcriptase?

A

It is an RNA dependent DNA polymerase

30
Q

What direction is the movement of DNA polymerase α and δ during DNA synthesis?

A

towards the replication fork

31
Q

What is the direction of lagging strand synthesis in DNA replication?

A

away from the replication fork

32
Q

What gene encodes a protein that prevents a cell with damaged DNA from entering the S phase?

A

p53 gene

33
Q

Inactivation or deletion of the p53 gene is associated with what syndrome?

A

Li Fraumen syndrome

34
Q

What gene encodes a kinase essential for p53 activity?

A

ATM gene

35
Q

How is ATM gene affected in ataxia telangiectasia?

A

It is inactivated

36
Q

The inactivation of the ATM gene in ataxia telangiectasia has what characteristic features when it comes to the presentation of the patient?(2 points)

A

hypersensitivity to x-rays and predisposition to lymphomas

37
Q

The mutation of the BRCA-1 gene is affected in what cancers?(3)

A

breast, prostate and ovarian cancer

38
Q

The mutation of the BRCA-2 gene is affected in what cancer?

A

breast cancer

39
Q

What is the role of the Rb gene?(3 points)

A

a negative regulator of the cell by its ability to bind to the EF transcription which prevents the transcription of genes needed for the S phase

40
Q

What phase in the life cycle of a cell does most DNA repair take place?

A

G1 phase

41
Q

What phase does mismatch repair take place?

A

G2 phase

42
Q

UV radiation damage causing thymine dimers is repaired during what phase of the cell cycle most of the time?

A

G1

43
Q

UV radiation causing thymine dimers is repaired by what repair enzymes using what activity?(3 points)

A

Excision endonuclease, DNA polymerase and DNA ligase

44
Q

Mismatched base pairs during DNA replication are usually repaired in what phase of the cell cycle?

A

G2

45
Q

Mismatched base pairs during DNA replication are usually repaired by what enzymes?(2 points)

A

DNA ligase and DNA polymerase

46
Q

What is deficient in patients with Xeroderma pigmentosum?

A

excision endonuclease activity

47
Q

What is deficient in patients who don’t seem to have the ability to repair DNA mismatches?

A

A defective hMSH2 of hMSH1 gene

48
Q

What is HNPCC?

A

Hereditary nonpolyposis colorectal cancer

49
Q

Cytosine deamination as a result of spontaneous heat introduction is repaired during what phase of the cell cycle?

A

G1

50
Q

Cytosine deamination as a result of spontaneous event/heat introduction is repaired by what enzymes using what type or recognition/excision enzyme?(3 points)

A

Repaired by DNA ligase and DNA polymerase. Uracil glyclylase AP endonuclease

51
Q

UV light induces what type of DNA damage?

A

thymine dimers

52
Q

How does the excision endonuclease remove the dimer?

A

It makes nicks in the phosphodiester backbone of the damaged strand on both sides of the thymine dimer and removes the defective oligonucleotide

53
Q

Cytosine deamination due to a spontaneous event/heat introduction, is repaired during the G1 phase. What enzyme removes the uracil that forms after the removal of the amino group?

A

Uracil glycolyase enzyme removes the uracil

54
Q

What enzyme subsequently removes the damaged sequences from the DNA after the uracil glycolyase removes the uracil group?

A

AP endonuclease

55
Q

What enzymes synthesize the missing DNA stands after any DNA repair has taken place and seals the nick in the repaired strand?

A

DNA ligase and DNA polymerase

56
Q

What important gene controls entry into the S-phase?(2 points)

A

p53 and Rb

57
Q

Xeroderma pigmentosum patients are deficient in what enzyme?

A

exicision endonuclease enzyme

58
Q

What type of genetic inheritance for Xeroderma pigmentosum patients?

A

autosomal recessive

59
Q

How can Xeroderma pigmentosum be diagnosed?

A

measurement of the relevant excision endonucease enzyme in white blood cells

60
Q

What genes are defected in patients with Hereditary Nonpolyposis Colorectal Cancer?

A

hMSH1 and hMSH2 gene are defective

61
Q

HNPCC patients are known to have what characteristic feature doctors use a diagnostic tool when looking at the DNA base sequences in certain somatic cell expressing the defective gene?

A

microsatellite instability

62
Q

What is micro satellite instability?

A

Short tandem repeats of sequences, di, tri and tetra nucleotide repeats dispersed throughout the DNA usually in noncoding regions.

Kaplan USMLE Review (5 decks)

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