DNA Replication

DNA Replication

Watson and Crick noticed the huge benefit of double strands

Each strand can serve as a template for making for making the other

DNA Replication

•Prior to mitosis and meiosis DNA must be copied•Like cells, all DNA comes from previously existing DNA

Semiconservative Model Each strand serves

as a template for the creation of a new strand of DNA

2 DNA molecules are created, each containing 1 strand of the original DNA

Semiconservative Replication

The Beginning

•Topoisomerase unwinds the helix•Helicase separates the strands•Single stranded binding proteins (SSBs) keep the strands separated

Topoisomerase, Helicase and Single-Strand Binding Protein

Priming DNA

A short RNA primer is added first by primase

DNA can only be added to an existing primer

DNA Polymerases

Each nucleotide is added one by one by DNA polymerases

A added to T

G added to C etc.

Antiparallel DNA StrandsThe two strands are arranged in opposite directions

Each strand has an “end”

The Leading Strand

Built continuously as the replication fork opens in front of it

The Lagging Strand

•Built in pieces the opposite direction of replication•Built in fragments, called Okazaki fragments

Review Helicase separates strands, SSBs help keep

strands apart DNA polymerase adds nucleotides DNA ligase fuses Okazaki fragments of lagging

strands together

Topoisomerase

Helicase Single Stranded Binding Proteins

Primase

3’

5’

DNA Polymerase

Leading Strand

Lagging StrandOkazaki Fragments

DNA Ligase