Question

As a reminder, PCR is the technique used to replicate DNA in the lab. PCR is based on the way DNA is replicated in cells.

During PCR template DNA plus other ingredients are placed in a test tube and the solution goes through repeated cycles of heating and cooling.

The 'ingredients' of a basic PCR reaction are: DNA template, primer, nucleotides, DNA polymerase, and any salts required for polymerase function (buffer).

No ligase is required in PCR but it is a necessary for DNA replication within the cell.

If DNA ligase is essential for DNA replication in a cell why isn't it necessary in PCR?

Hint: Think about what the role of ligase is normally in replication.

Answer 1

For PCR -

The origin of replication and RNA primase are not necessary as we add a sequence-specific pair of DNA primers to the reaction.

The equivalent of DNA polymerase I and DNA ligase  are unnecessary due to the absence of RNA primers and Okazaki fragments during the process of PCR . Thus there is no need of ligase as here there is no requirement of joining the fragments those are formed in the machine. PCR requires very high temperatures, thus a typical DNA  polymerase cannot be used since it will be denatured by the intense heat.

In a normal cell -

DNA ligase is an enzyme which repairs irregularities or breaks in the backbone of double-stranded DNA molecules.It has three basic functions: It seals and repairs the DNA, it seals recombination fragments, and it connects Okazaki fragments of DNA. Without DNA ligase , Okazaki fragments on the lagging strand cannot  be joined together; leading strand synthesis would be unaffected. Okazaki fragments are found on the strand that replicates discontinuously.