Prior to cell division, DNA is replicated by a DNA polymerase.

The following need to be present for DNA replication to take place:

• DNA template

  → original two strands, so that genetic material is exactly copied

• free DNA nucleotide bases

  → A, T, C, + G

• enzymes → DNA polymerase + ligase

• primers

  → needed by DNA polymerase to start replication

• ATP

  → to supply energy needed for the process

DNA replication has the following stages:

1. DNA is unwound and hydrogen bonds between bases are broken to form two template strands.

2. → A primer binds to the 3’ end of the template DNA strand allowing polymerase to add free DNA nucleotides.

   → DNA polymerase adds DNA nucleotides, using complementary base pairing, to the deoxyribose — 3’ — end of the new DNA strand which is forming.

   → DNA polymerase can only add DNA nucleotides in one direction resulting in the leading strand being replicated continuously and the lagging strand replicated in fragments.

3. Fragments of DNA are joined together by ligase.

• DNA polymerase is an enzyme that adds complementary nucleotides to the deoxyribose (3’) end of a newly forming DNA strand.
  • DNA polymerase replicates DNA in the 5’ to 3’ direction, so the new DNA bases are always being added to the 3’ end of the newly forming DNA strand.
• A primer is a short strand of ****nucleotides which binds to the 3’ end of the template DNA strand allowing polymerase to add DNA nucleotides.
  • DNA polymerase needs primers to start replication.
• Ligase is and enzyme that joins fragments of DNA together on the lagging strand.

To replicate a fragment of DNA in a lab, a technique called PCR is used.

Polymerase chain reaction (PCR) amplifies DNA using complementary primers for specific target sequences.

The following are needed for PCR to take place:

• DNA template

• free DNA nucleotides

  → with each base (A, C, G, + T)

• enzyme → heat-tolerant DNA polymerase

• primers

PCR has the following stages:

1. DNA is heated to between 92 and 98°C to separate the strands.
2. It is then cooled to between 50 and 65°C to allow primers to bind to target sequences.
3. It is then heated to between 70 and 80°C for heat-tolerant DNA polymerase to replicate the region of DNA.

→ Repeated cycles of heating and cooling amplify the target region of DNA.

• The cycle above can be repeated (i.e. go from stage 3 to 1 and loop) to double the number of target sequences of DNA each cycle of PCR.

PCR has various practical applications, PCR can amplify DNA to: