DNA strands separate and then these strands are used as a template ; idea suggested by Watson and Crick called the semi-conservative model

The semi-conservative model was proven by Meselson and Stahl

Enzymes used in DNA replication

Gyrase - relieves tension of the unwinding of the double helix

Helicase -break H bonds of complementary strands

Primase - Make RNA primer ( ~10 nucleotides long in eukaryotes )

DNA Polymerases - Add DNA nucleotides to new strand ( I, II,II, in prokaryotes 5 diff types in eukaryotes)

Ligases - attach Okazaki fragments on the lagging strand

Rate of nucleotides added

The rate of nucleotides added at a rate of 50/s in eukaryotes

The rate of nucleotides added at a rate of 500/s in prokaryotes

The source of Energy for DNA replication is Nucleoside triphosphate

In eukaryotes DNA replication occurs at more than one site at a time, resulting in hundreds of replication forks across a DNA strand.

Eventually the replication bubbles become continuous and the two new double stranded daughter molecules are completely formed

DNA polymerase III adds complementary nucleotides in the 5’ to 3’ direction using RNA primers as starting points

DNA polymerase II adds deoxyribonucleotides from primer to primer, forming Okazaki fragments

DNA polymerase I replaces RNA primers with appropriate deoxyribonucleotides. A gap is left between fragments

DNA ligase joins fragments by creating phosphodiester bonds

DNA is replicated 5’ to 3’

replication bubble

A summary of DNA replication

1. Helicase unwind the parental double helix

2. single strand binding proteins stabilize the unwound parental DNA

3. the leading strand is synthesized continuously

4. the lagging strand is synthesized discontinuously Primase synthesizes a short RNA primer, which is extend by DNA polymerase to form an Okazaki fragment

   1. After the RNA primer is replaced by DNA ( by another DNA polymerase, not shown) DNA ligase joins the Okazaki fragments to the growing strand

      

DNA proofreading

DNA polymerase proofreads (I and III)

• check each nucleotide against the template as it is added

• if there is a mistake, it backs up and removes incorrect nucleotide and repplaces it before continuing ( exonuclease)

DNA repair