DNA Replication
• When/why do cells need to replicate (copy) DNA?
• Growth
• Reproduction
• Repair
• Where do we start?
• Replication origin – a specific sequence of DNA (or region on a chromosome)
at which DNA synthesis, or replication begins
Prokaryotes VS Eukaryotes - Replication
• Prokaryotes – only 1 replication origin
• Circular DNA, called a plasmid
• Eukaryotes – many replication origins (because they contain so much
more DNA; would take too long to replicate)
Replication enzymes:
At the replication origin:
• Helicase = enzyme that unwinds & unzips DNA
• RNA primase = produces an RNA primer
• DNA polymerase = enzyme that makes new DNA
• Ligase = joins separate DNA fragments
• This whole combination of the enzymes, proteins, & DNA = replisome
Steps of DNA Synthesis
1. Proteins & enzymes bind at replication origin. Helicase, an enzyme,
unwinds/unzips the DNA molecule.
Steps of DNA Synthesis
2. Another enzyme, RNA primase, lays down an RNA
primer so that the next enzyme knows where to begin
DNA synthesis.
Steps of DNA Synthesis
3. The enzyme DNA polymerase adds nucleotides to the preexisting DNA strand by matching the correct base pairs.
Steps of DNA Synthesis
4. DNA polymerase replaces the RNA primers with DNA and replication
continues until the entire chromosome has been replicated.
PROBLEM!
• DNA Polymerase is very stubborn… it only likes to work in one
direction.
• What do we know about the direction of DNA’s 2 strands?
Because DNA is antiparallel, we call one strand the leading strand (5’
→ 3’) and the other the lagging strand (3’ → 5’).
• Leading strand = continuous DNA synthesis
• Lagging strand = discontinuous DNA synthesis
Lagging Strand Replication
WHY is the lagging discontinuous???
• DNA polymerase can only work in one direction (3’ → 5’), so in
lagging strand – DNA synthesis occurs in short, unconnected
segments (called Okazaki fragments) that get joined by another
enzyme, ligase.
End result of DNA replication
• End result = 2 identical double helices, each with one original strand
& one newly synthesized strand
• Called Semi-conservative DNA synthesis b/c each helix has an
original & a new strand
Amoeba sisters- DNA Replication
• https://www.youtube.com/watch?v=5qSrmeiWsuc