DNA synthesis

Question 1

cell cycle stages

Answer 1

G1, S, G2, M

Question 2

what happens during the S phase of the cell cycle?

Answer 2

DNA is replicated, histone and other protein synthesis is greatly increased

Question 3

DNA replication

Answer 3

DNA unzips into two parts and splits with the cell; is semi-conservative

Question 4

what is the purpose of methyl tags in DNA replication?

Answer 4

as methyl copying enzymes lay down methyl tags during replication, they silence the expression of genes in cells where they are not necessary (e.g. albumin in skin cells)

Question 5

how does bacterial replication work?

Answer 5

since bacteria have circular DNA with an origin, replication begins there in both directions

Question 6

how does eukaryotic replication work?

Answer 6

have multiple origins in DNA; replication bubbles arise at points along DNA and synthesis proceeds along a fork in each direction from these points

Question 7

how is helix unwinding accomplished?

Answer 7

the DNA is unwound and opened up by a series of enzymes and single-strand binding proteins to keep DNA from “re-annealing”

Question 8

what enzyme unwinds DNA?

Answer 8

helicase

Question 9

what does topoisomerase do?

Answer 9

it is an enzyme responsible for allowing breaks and rejoining of the DNA to relieve tension from super-coiling

Question 10

what does gyrase do?

Answer 10

it is a prokaryotic enzyme (type of topoisomerase) used to twist the single circular chromosome of prokaryotes upon itself to form supercoils; relieves the stress caused by unwinding DNA

Question 11

what direction do DNA polymerases synthesize?

Answer 11

they synthesize in a 5’ to 3’ direction, but read along the template strand from 3’ to 5’)

Question 12

exonuclease activity

Answer 12

the ability of DNA polymerases to move backwards to cut out incorrect bases from the end of a DNA strand in replication

Question 13

processivity in replication refers to:

Answer 13

how well DNA polymerase stays associated with DNA template during replication

Question 14

RNA primers

Answer 14

a short strand of RNA, made by DNA primase, that is used to elongate a strand of DNA during DNA replication

Question 15

why are RNA primers needed in DNA replication?

Answer 15

DNA polymerase cannot add bases w/o a free 3’ OH group, so RNA primers are synthesized to start replication

Question 16

what is the role of primase?

Answer 16

to add RNA bases in a 5’ to 3’ direction, copying from the template strand (this is followed by DNA polymerase using the 3’ OH on the last base to continue synthesis)

Question 17

leading strand

Answer 17

the new complementary DNA strand synthesized continuously along the template strand toward the replication fork in the mandatory 5’ to 3’ direction

Question 18

lagging strand

Answer 18

discontinuously synthesized DNA strand (stepwise fashion) that elongates by means of Okazaki fragments, each synthesized in a 5’ to 3’ direction away from the replication fork

Question 19

what occurs after Okazaki fragments are synthesized?

Answer 19

RNA primers used for synthesis are removed and are replaced with DNA

Question 20

what is the purpose of ligase?

Answer 20

final binding of nicks in DNA during synthesis and repair

Question 21

telomeres

Answer 21

repeated DNA sequences at the ends of eukaryotic chromosomes; exist so that when chromosomes are replicated and the lagging strand is shorter, replication does not result in shortening of the coding regions
(cells age as telomeres shorten)

Question 22

telomerase

Answer 22

an RNA-dependant DNA polymerase enzyme that catalyzes the lengthening of telomeres; includes a complex of proteins, RNA and enzyme that lengthens ends of chromosomes

Question 23

what does telomerase contain?

Answer 23

reverse transcriptase (similar to polymerase used by HIV)

Question 24

how does reverse transcriptase work?

Answer 24

a single strand of RNA is reverse transcribed to single stranded DNA, (complementary DNA/cDNA)

Question 25

what is the purpose of DNA repair?

Answer 25

prevent the accumulation of mutations (e.g. large side groups causing errors)

Question 26

what are the steps of DNA repair?

Answer 26

1. damaged DNA is recognized
2. enzymes that break down the phosphodiester bond remove the damaged area
3. a DNA polymerase designed to repair rebuilds the damaged area
4. DNA ligase seals the nicks

Question 27

DNA repair mechanisms

Answer 27

any of several processes by which enzymes repair DNA damage

Question 28

nucleotide excision repair (NER)

Answer 28

a DNA repair system in which several nucleotides in the damaged strand are removed from the DNA and the undamaged strand is used as a template to resynthesize a normal strand (this is defective in Xeroderma pigmentosum)