DNA Replication Quiz

location of DNA replication in eukaryotic organism

both strands at each end of a replication bubble

origin of DNA replication in eukaryotic organism

many origins of replication within each replicon

how does replication proceed in a eukaryotic genome

• DNA unwinds at each origin, producing a replication bubble

• DNA synthesis occurs on both strands at each end of the bubble + the 2 replication forks spread outward

• forks of adjacent bubbles run into each other + segments of DNA fuse, which produces 2 identical linear synthesized DNA molecules

• bidirectional

• no breakage of nucleotide strand

replication fork

• point where the 2 strands separate from the double-stranded DNA helix

• aka, point of unwinding

replication bubble

loop generated by unwinding of double helix

why are nucleotides brought into the growing strand as triphosphates?

to provide the energy needed for DNA polymerization

what is a primer + why is it necessary in DNA replication?

• primer = RNA nucleotides

• provide 3’-OH group to which DNA polymerase can attach DNA nucleotides

• leading strand only needs primers at 5’ end of new strand

• lagging strand needs new primer at beginning of each Okazaki fragment

DNA helicase function

unwinds DNA at replication fork

DNA primase function

synthesizes a short RNA primer to provide a 3’-OH group for the attachment of DNA nucleotides

single-strand binding proteins (SSBs) function

• attach to exposed single-stranded DNA that has been unwound by helicase

• protect single-stranded nucleotide chains

• prevent formation of secondary structures that would interfere w/replication

DNA polymerase III function

• elongates a new nucleotide strand from the 3’-OH group provided by the primer

• 5’ → 3’ polymerase activity = adds new nucleotides in 5’ → 3’ direction

• 3’ → 5’ exonuclease activity = removes nucleotides in 3’ → 5’ direction (used to correct errors →accurate + efficient DNA synthesis)

topoisomerase function

relaxes supercoiled DNA ahead of the replication fork so replication can proceed

DNA polymerase I function

• has 5’ → 3’ polymerase + 3’ →5’ exonuclease activities

• also has 5’ → 3’ exonuclease activity

  • removes RNA primers + replaces them w/DNA nucleotides

DNA ligase function

joins Okazaki fragments by sealing breaks in the sugar-phospahte backbone of newly synthesized DNA

5’ vs 3’ ends of a molecule

• DNA synthesis is always 5’ → 3’

  • new nucleotides are always added to the 3’ end of the growing nucleotide strand

leading strand in replication fork

• runs in 5’ → 3’ direction in the the replication fork

• grows in direction of replication fork

• undergoes continuous replication

• no fragments formed

• requires one primer

• doesn’t require DNA ligase

lagging strand in replication fork

• opens in 3’ → 5’ direction towards the replication fork

• grows away from replication fork

• undergoes discontinuous replication (forms Okasaki fragments)

• each fragment requires its own primers

• requires DNA ligase

Okasaki fragment

short lengths/fragments of DNA produced by discontinous replication of lagging strand