(a) Structure of DNA

• The base sequence of DNA forms the genetic code.
• DNA is composed of individual units called nucleotides.
• They contain deoxyribose sugar, phosphate and base.
• DNA has a sugar–phosphate backbone.
• Complementary base pairing — adenine with thymine and guanine with cytosine.
• The two DNA strands are held together by hydrogen bonds and have an antiparallel structure, with deoxyribose and phosphate at 3′ and 5′ ends of each strand.

(b) Replication of DNA by DNA polymerase and primers

• Prior to cell division, DNA is replicated by a DNA polymerase.
• DNA is unwound and hydrogen bonds between bases are broken to form two template strands.
• 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.
• This process occurs at several locations on a DNA molecule.
• DNA polymerase adds DNA nucleotides, using complementary base pairing, to the deoxyribose (3′) end of the new DNA strand which is forming.
• Fragments of DNA are joined together by ligase.
• 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.

*You do not need to know about helicase for the Higher Human Biology course.

(c) 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
  • diagnose genetic disorders