Reproduction requires life to replicate its DNA. DNA replicationDNA replication is the process in which one DNA strand becomes two.

This means that both sides of the DNA ladder and all of the intervening steps must be replicated.

The two DNA strands that emerge from this process receive one side one side of the original DNA of the original DNA ladderladder. The other side is synthesised during synthesised during replicationreplication.

Original strand

Newly synthesised stand

Semi-conservative model

This process is controlled by a protein called DNA polymeraseDNA polymerase. This protein untwists the ladder, and then it uses each side as a template.

New sides are added to this template. DNA polymerase moves along the entire chromosome until every step has been replicated.

DNA polymerase has proof-readingproof-reading mechanisms built into it.

If it adds the wrong step by mistake, it can remove it, and replace it with the correct step.

As a result, this process is very accurate. DNA DNA polymerasepolymerase rarely makes a mistake. Before a cell can divide it must replicate all of its genes.

Each chromosome must be duplicated. Thus, DNA replication always precedes both mitosis mitosis and meiosismeiosis.

DNA ReplicationDNA Replication

A portion of the doublehelix is unwound by theenzyme helicase.

The Steps:The Steps:

A molecule of DNA DNA polymerasepolymerase binds to one strand of the DNA and begins moving along it in the 3' to 5' 3' to 5' directiondirection, using it as a template for assembling a leading strand of nucleotides and reforming a double helix.

Because DNA synthesis can not occur in the 5' 5' to 3’ directionto 3’ direction, a second DNA polymerase molecule is used to bind to the other template strand as the double helix opens.

This molecule must synthesise discontinuous segments of polynucleotides (called Okazaki fragmentsOkazaki fragments).

Another enzyme, DNA DNA ligaseligase then stitches these together into the lagging strand.

DNA Replication

There are several enzymes that are involved in replication

The DNA is always replicated from the 5'5' end to the 3'3' end.

This is based on the numbering of the carbon atoms in the deoxyribose molecule.

The phosphate attaches to the number 1 Carbon atom.

The nucleotide base attaches to the number 5 Carbon atom.

Because the two strands are antiparallelantiparallel, this means there's a "leading leading strandstrand", which just keeps on going, and a "lagging strandlagging strand" which consists of small fragments that must later be linked together (by DNA Ligase DNA Ligase enzymeenzyme).

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