DNA REPLICATION

DNA replication occurs in a 5'→3' direction.The first stage of DNA replication in prokaryotes is the uncoiling of the DNA double helix by the enzyme helicase. Helicase separates the DNA into two template strands. RNA primase then adds a short sequence of RNA to the template strands. This short sequence of RNA is a primer which allows DNA polymerase III to bind to the strands and start the replication process. Once this is done, DNA polymerase III adds nucleotides to each template strand in a 5'→3' direction. The nucleotides have 3 phosphate groups and are called deoxyribonucleoside triphosphates. Two of these phosphate groups break off during the replication process to release energy. Since the strands are anti-parallel (the two strands have their 5' end and 3' end in opposite sides) and replication can only occur in a 5'→3' direction, one of the strands will be replicated in the same direction as the replication fork and the other will be replicated in the opposite direction of the replication fork. This means that one of the strands is synthesised in a continuous manner (named the leading strand) while the other one is synthesised in fragments (named the lagging strand). The leading strand only needs one primer while the lagging strand needs quite a few as it is formed in fragments. These fragments are called Okazaki fragments. DNA polymerase I will remove the RNA primers and replace these with DNA. The enzyme DNA ligase then joins the Okazaki fragments together to form a continuous strand. 

SUMMARY:

  1. Helicase uncoils the DNA
  2. RNA primase adds short sequences of RNA to both strands (the primer)
  3. The primer allows DNA polymerase III to bind and start replication
  4. DNA polymerase III adds nucleotides to each template strand in a 5'→3' direction
  5. These nucleotides are initially deoxyribonucleoside triphosphates but they lose two phosphate groups during the replication process to release energy
  6. One strand is replicated in a continuous manner in the same direction as the replication fork (leading strand)
  7. The other strand is replicated in fragments (Okazaki fragments) in the opposite direction (lagging strand) 
  8. DNA polymerase I removes the RNA primers and replaces them with DNA
  9. DNA ligase then joins the Okazaki fragments together to form a continuous strand