twisted around each other in a double helix shape.

Each strand is made up a sequence of four chemical bases

represented by the letters A, C, G and T.

The two strands are complimentary.

This means that wherever there's a T in one strand

there will be in an A in the opposite strand,

and wherever there's a C

there will be a G in the other strand.

Each strand has a 5' end and a 3' end.

The two strands run in opposite directions.

This determines how each strand of DNA is replicated.

The first step in DNA replication is to separate the two strands.

This unzipping is done by an enzyme called helicase

and results in the formation of a replication fork.

The separated strands each provide a template for creating a new strand of DNA.

An enzyme called primase starts the process.

This enzyme makes a small piece of RNA

called a primer.

This marks the starting point for the construction of the new strand of DNA.

An enzyme called DNA polymerase binds to the primer

and will make the new strand of DNA.

DNA polymerase can only add DNA bases in one direction,

from the 5' end to the 3' end.

One of the new strands of DNA, the leading strand,

is made continuously,

the DNA polymerase adding bases one by one in the 5' to 3' direction.

The other strand, the lagging strand, cannot be made in this continuous way

because it runs in the opposite direction

the DNA polymerase can therefore only make this strand in a series of small chunks

called Okazaki fragments.

Each fragment is started with an RNA primer.

DNA polymerase then adds a short row of DNA bases

in the 5' to 3' direction.

The next primer is then added further down the lagging strand.

Another Okazaki fragment is then made and the process is repeated again.

Once the new DNA has been made

the enzyme exonuclease removes all the RNA primers from both strands of DNA.

Another DNA polymerase enzyme then fills in the gaps that are left behind

with DNA.

Finally the enzyme DNA ligase seals up the fragments of DNA

in both strands to form a continuous double strand.

DNA replication is described as semi- conservative

because each DNA molecule is made up of one old, conserved strand of DNA

and one new one.