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 Synthesis1. Proteins & enzymes bind at replication origin. Helicase, an enzyme,

unwinds/unzips the DNA molecule.

Steps of DNA Synthesis2. Another enzyme, RNA primase, lays down an RNA

primer so that the next enzyme knows where to begin DNA synthesis.

Steps of DNA Synthesis3. The enzyme DNA polymerase adds nucleotides to the pre-

existing DNA strand by matching the correct base pairs.

Steps of DNA Synthesis4. 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 ReplicationWHY 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