DNA Replication Flashcards
RNA primase
synthesizes a short RNA primer in 5’ to 3’ direction, providing a 3’ -OH for DNA polymerase
DNA polymerase III
prokaryotic DNA polymerase that elongates the chain
*has 3’ to 5’ exonuclease activity for proofreading
DNA polymerase I
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 ligase
joins Okazaki fragments with ATP-dependent formation of a phosphodiester bond between the 5’ phosphate and 3’ OH on DNA chain
components of the replisome
-topoisomerases
-DNA helicases
-single-stranded binding proteins
topoisomerases
relieve coiling of double-stranded DNA
DNA helicase
unwinds and opens DNA; requires ATP
single-stranded binding proteins (SSBs)
bind to DNA strands to keep them apart and protect them from degradation by nucleases
DNA polymerase delta
eukaryotic DNA polymerase responsible for chain elongation
-3’ to 5’ exonuclease activity for proofreading
prokaryotic vs. eukaryotic DNA replication
-multiple sites of origin in eukaryotes vs. one in prokaryotes
-eukaryotes must displace nucleosomes in order to synthesize DNA
-telomeres in eukaryotes
telomeres
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
telomerase functions
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
telomerase - components
*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
