Structure and Replication of DNA

• A strong chemical bond forms between the sugar at the 3' end of one nucleotide and the phosphate at the 5' end of the next. This makes a sugar-phosphate backbone.
• The two polynucleotide strands stand alongside each other. The strands are antiparallel, i.e. they run in opposite directions.
• One end of a DNA strand is named 5' and the other is named 3'. The 3' end can only grow by adding nucleotides to its 3' end, and the reverse is true for the complimentary strand on the right.
• DNA requires the enzyme DNA polymerase in order to replicate.
• DNA replication is described as semi-conservative. This is because each daughter molecule produced is made up of one new strand and one of the original parent strands.
• 1 - Parent DNA strand unwinds and hydrogen bonds between the bases break.

  
  

  2 - Each parent strand acts as a template and individual nucleotides (present in the nucleus) align with the nucleotides on the template according to the base pairing rule. New hydrogen bonds form.

  
  

  3 - The sugar-phosphate backbone forms on the new strand.

  
  

  4 - Each new DNA molecule is identical to the original parent.
• For DNA replication to take place, the nucleus must contain:

  • DNA (to act as a template)
  • Primers
  • A supply of the 4 types of DNA nucleotide
  • Enzymes (DNA polymerase and ligase)
• Stage 1

  • A starting point on the DNA molecule is recognised, the DNA unwinds and the weak hydrogen bonds between the base pairs break allowing the strands to separate (unzip)
  • A primer is a short sequence of bases needed to start DNA replication.
  • DNA polymerase NEEDS primers to start replication.
  • A primer can only start at a 3' end of the template DNA strand.
  • Therefore, the process of DNA replication is slightly different on each strand.
  • The strand with the free 3' end is called the leading strand.
  • The strand with the free 5' end is called the lagging strand.
• Stage 2 - The leading strand

  • The enzyme that controls the sugar-phosphate bonding between individual nucleotides into the new DNA strand is called DNA polymerase.
  • For DNA polymerase to work it must add on new nucleotides to the pre-existing chain and this is why the primer is important.
  • DNA polymerase can only add on new nucleotides to a 3' end of a template DNA strand. Replication of the first strand is continuous.
  • Each strand of DNA in the double helix runs in opposite directions, i.e. the strands are anti-parallel.
• Stage 2 - The lagging strand

  • Since DNA polymerase is only able to add nucleotides to the free 3' end of a template strand, the template strand that has to be replicated in fragments each starting at the 3' end of a primer.
  • DNA polymerase then binds sections of individual nucleotides together.
  • Then the fragments are joined together with the help of an enzyme called ligase.
  • The strand is called the lagging strand and is discontinuous.
• The replication of the leading DNA strand is done continuously and DNA polymerase is required.
• The lagging DNA strand is replicated in fragments away from the replication fork.
• The phosphate group is exposed on the 5' end of the DNA strand.
• DNA ligase joins fragments of DNA together.
• Hydrogen bonds link primers to DNA strands.
• The deoxyribose sugar contains 5 carbons, this is why it is represented as a pentagon.

  The carbons are numbered 1’-5’ (the small dash represents prime). The phosphate group bonds to the 5’ (5 prime) carbon of the sugar. The 3’ carbon is exposed on the bottom of the pentagon.
• Different proteins can be expressed from one gene as a result of alternative RNA splicing. Different mature mRNA transcripts are produced from the same primary transcript depending on which exons are retained.