DNA Replication

Question 1

RNA primase

Answer 1

synthesizes a short RNA primer in 5’ to 3’ direction, providing a 3’ -OH for DNA polymerase

Question 2

DNA polymerase III

Answer 2

prokaryotic DNA polymerase that elongates the chain (5’ to 3’ synthesis)
*has 3’ to 5’ exonuclease activity for proofreading

note - DNA pol I also has 5’ to 3’ exonuclease activity because it removes the RNA primer; DNA pol III does NOT have this

Question 3

DNA polymerase I

Answer 3

prokaryotic DNA polymerase
-5’ to 3’ exonuclease activity: removes the RNA primer from Okazaki fragments
-5’ to 3’ polymerase activity: synthesizes DNA to fill the gap
-3’ to 5’ exonuclease activity: proofreading

DNA pol I has 5’ to 3’ AND 3’ to 5’ exonuclease activity because it removes the RNA primers (DNA pol III only has 3’ to 5’)

Question 4

DNA ligase

Answer 4

joins Okazaki fragments with ATP-dependent formation of a phosphodiester bond between the 5’ phosphate and 3’ OH on DNA chain

Question 5

components of the replisome

Answer 5

-topoisomerases
-DNA helicases
-single-stranded binding proteins

Question 6

topoisomerases

Answer 6

relieve coiling of double-stranded DNA

Question 7

DNA helicase

Answer 7

unwinds and opens DNA; requires ATP

Question 8

single-stranded binding proteins (SSBs)

Answer 8

bind to DNA strands to keep them apart and protect them from degradation by nucleases

Question 9

DNA polymerase delta

Answer 9

eukaryotic DNA polymerase responsible for chain elongation
-3’ to 5’ exonuclease activity for proofreading

Question 10

prokaryotic vs. eukaryotic DNA replication

Answer 10

-multiple sites of origin in eukaryotes vs. one in prokaryotes
-eukaryotes must displace nucleosomes in order to synthesize DNA
-telomeres in eukaryotes

Question 11

telomeres

Answer 11

short, G-rich DNA sequences at the ends of the chromosomes that protect fusion of chromosome ends and prevent degradation
*in humans, (TTAGGG)n with ~1000 copies per telomere

Question 12

telomerase functions

Answer 12

an enzyme that reforms the telomeres after DNA replication
-most somatic cells do not express telomerase
*in cancers, somatic telomerase is re-activated, allowing indefinite cell division and cancer progression
*telomerase inhibitors can limit cancer cell proliferation

Question 13

telomerase - components

Answer 13

*a ribonucleoprotein complex (RNA + protein)
*reverse transcriptase (synthesizes DNA using its RNA as a template)
*adds short DNA repeats to 3’-ends of chromosomes
*allows a primer to bind and initiate lagging-strand synthesis on the other strand