2.1 DNA replication

0:06     DNA is a molecule made up of two strands

0:09     twisted around each other in a double helix shape.

0:13     Each strand is made up a sequence of four chemical bases

0:17     represented by the letters A, C, G and T.

0:22     The two strands are complementary.

0:25     This means that wherever there's a T in one strand

0:27     there will be in an A in the opposite strand,

0:30     and wherever there's a C

0:32     there will be a G in the other strand.

0:35     Each strand has a 5' end and a 3' end.

0:39     The two strands run in opposite directions.

0:43     This determines how each strand of DNA is replicated.

0:47     The first step in DNA replication is to separate the two strands.

0:52     This unzipping is done by an enzyme called helicase

0:55     and results in the formation of a replication fork.

1:01     The separated strands each provide a template for creating a new strand of DNA.

1:08     An enzyme called primase starts the process.

1:16     This enzyme makes a small piece of RNA

1:18     called a primer.

1:20     This marks the starting point for the construction of the new strand of DNA.

1:25     An enzyme called DNA polymerase binds to the primer

1:29     and will make the new strand of DNA.

1:32     DNA polymerase can only add DNA bases in one direction,

1:36     from the 5' end to the 3' end.

1:40     One of the new strands of DNA, the leading strand,

1:44     is made continuously,

1:46     the DNA polymerase adding bases one by one in the 5' to 3' direction.

1:52     The other strand, the lagging strand, cannot be made in this continuous way

1:57     because it runs in the opposite direction

2:00     the DNA polymerase can therefore only make this strand in a series of small chunks

2:05     called Okazaki fragments.

2:13     Each fragment is started with an RNA primer.

2:20     DNA polymerase then adds a short row of DNA bases

2:23     in the 5' to 3' direction.

2:34     The next primer is then added further down the lagging strand.

2:38     Another Okazaki fragment is then made and the process is repeated again.

2:44     Once the new DNA has been made

2:47     the enzyme exonuclease removes all the RNA primers from both strands of DNA.

2:53     Another DNA polymerase enzyme then fills in the gaps that are left behind

2:57     with DNA.

3:03     Finally the enzyme DNA ligase seals up the fragments of DNA

3:08     in both strands to form a continuous double strand.

3:13     DNA replication is described as semi- conservative

3:17     because each DNA molecule is made up of one old, conserved strand of DNA

3:22     and one new one.