DNA Replication

Replication

• Occurs during cell division• Must be accurate

Semiconservative Replication

• Each strand serves as a template for new strand

• Each new molecule has one parent strand and one new strand

Replication Process

• Replication begins when proteins bind to replication origin– Specific genomic sequence

• DNA Helicase unwinds double helix• Strands kept apart by single stranded

binding proteins• Replication goes in both directions from

origin

Details

• DNA is always synthesized 5’ to 3’– Nucleotides added to 3’ end by DNA

polymerase III• Primase makes RNA primer allowing

DNA polymerase to start elongation– Polymerase III can’t start a new strand

itself

Synthesis

• Because DNA is made 5’ to 3’, only one strand is synthesized continuously– Leading strand

• Other strand is built discontinuously– Lagging strand– Built in small fragments called Okazaki

fragments

Finishing Touches

• DNA polymerase I removes RNA primers– Replaces them

with nucleotides• DNA ligase joins

Okazaki fragments

Repair

• Polymerase I and polymerase III act as quality control– Proofread sequence against template

• These enzymes can act as exonucleases– Backtrack and remove incorrect nucleotide,

replacing it with correct one