structure and replication of DNA

• The deoxyribose sugar contains 5 carbons.
• DNA is a double stranded helix and consists of 2 long chains of subunits called nucleotides
• A nucleotide consists of three main components:

  • Deoxyribose sugar
  • Phosphate
  • base
• The deoxyribose sugar contains 5 carbons
• The phosphate group bonds to the 5’ carbon of the sugar. The 3’ carbon is exposed on the bottom of the pentagon.
• The components of a nucleotide that make up the backbone of DNA are the sugar-phosphate groups. The backbone is known as the sugar-phosphate backbone.
• Hydrogen bonds form between complementary base pairs and these are weak bonds.
• One strand runs in a 5’ to 3’ direction, whilst the opposite strand runs in 3’ to 5’ direction. This is an antiparallel structure forming the double helix.
• The sequence of bases on DNA forms the genetic code.
• DNA replication requires the use of ATP, free DNA nucleotides and other enzymes throughout the process.
• DNA polymerase is the enzyme that replicates DNA.

  DNA polymerase requires a primer to start DNA replication.
• A primer is a short strand of nucleotides which binds to the 3’ end of the template DNA strand. This allows DNA polymerase to add new nucleotides.
• The process of DNA replication:

  1. DNA is unwound and hydrogen bonds between bases are broken – forming two template strands.
  2. Primer attaches to a short sequence on the DNA allowing the DNA polymerase to bind.
  3. DNA polymerase will add nucleotides using the complementary base pairing rule to the deoxyribose (3’) end of the new strand which is forming.
• DNA polymerase can only add nucleotides to the 3’end of the growing strand.
• one strand is replicated continuously, and we call this the leading strand.
• the opposite strand has to be replicated in fragments. This is known as the lagging strand. This requires the use of many primers and the fragments produced are joined together by the enzyme ligase.
• The polymerase chain reaction amplifies DNA using complementary primers for specific target sequences.
• The polymerase chain reaction consists of repeated cycles of heating and cooling to amplify the target DNA.
• 1. DNA is heated between 92°C and 98°C to break the hydrogen bonds between bases and separate the two strands.
  2. The DNA is then cooled to between 50°C and 65°C to allow primers to bind to target sequences.
  3. It is then heated to between 70°C and 80°C for heat tolerant DNA polymerase (Taq polymerase) to replicate the region of DNA.
• Examples of the practical applications of PCR are to help solve crimes, settle paternity suits and diagnose genetic disorders.
• 1. tRNA attaches to a specific amino acid.
  2. Anticodon (on tRNA) joins with codon (on mRNA).
  3. This ensures that the amino acids are in the correct order.