(a) Replication of DNA by DNA polymerase and primers.

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

• DNA polymerase needs primers to start replication. 

• 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 adds DNA nucleotides, using complementary base pairing, to the deoxyribose (3’) end of the new DNA strand which is forming.

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

  • 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.

• Fragments of DNA are joined together by ligase.

(b) Polymerase chain reaction (PCR) 

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

  • In PCR, primers are short strands of nucleotides which are complementary to specific target sequences at the two ends of the region of DNA to be amplified.

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

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

• Practical applications of PCR.

  • PCR can amplify DNA to help solve crimes, settle paternity suits, and diagnose genetic disorders.