DNA polymerases are the enzymes that replicate DNA in living cells. They do this by adding individual nucleotides to the 3-prime hydroxl group of a strand of DNA. The process uses a complementary, single strand of DNA as a template.
The energy required to drive the reaction comes from cutting high energy phosphate bonds on the nucleotide-triphosphate's used as the source of the nucleotides needed in the reaction.
The illustration above highlights important aspects of the reaction.
DNA polymerases can not create new strands of DNA. They only synthesis double stranded DNA from single stranded DNA. The starting point is a a stretch of single stranded DNA which is double stranded for at least part of its length. In the polymerase chain reaction the double stranded stretch is created by attaching short DNA primers. In living cells, RNA primers are used.
DNA polymerase uses the bases of the longer strand as a template. During strand elongation, two phosphates are cleaved from the incoming nucleotide triphosphate and the resulting nucleotide monophosphate is added to the DNA strand. This results in the:
- Formation of a phosphodiester bond between the phosphate attached to the 5' carbon of the incoming nucleotide and the hydroxyl group on the trailing 3' carbon
- Release of a pyrophosphate molecule
- Extension of the DNA polymer by one nucleotide
Removing two phosphates from the incoming nucleotide and bonding the remaining phosphate to the oxygen on the 3' carbon of the existing strand maintains the repeating sugar-phosphate-sugar-phosphate pattern that makes up the backbone of each DNA polymer.
Orientation of the strand is important. Dependence on energy from the phosphates linked to the 5-prime carbon of the incoming nucleotides means that DNA polymerase can only extend DNA strands by adding nucleotides to the 3-prime end of a DNA strand.
Test your understanding of the concepts covered by this illustration with the DNA Polymerase concept questions.