DNA polymerase can make mistakes while adding nucleotides. It edits the DNA by proofreading every newly added base. Incorrect bases are removed and replaced by the correct base, and then a new base is added. Most mistakes are corrected during replication, although when this does not happen, the mismatch repair mechanism is employed. Mismatch repair enzymes recognize the wrongly incorporated base and excise it from the DNA, replacing it with the correct base. In yet another type of repair, nucleotide excision repair, the incorrect base is removed along with a few bases on the 5′ and 3′ end, and these are replaced by copying the template with the help of DNA polymerase. The ends of the newly synthesized fragment are attached to the rest of the DNA using DNA ligase, which creates a phosphodiester bond.
Most mistakes are corrected, and if they are not, they may result in a mutation defined as a permanent change in the DNA sequence. Mutations can be of many types, such as substitution, deletion, insertion, and translocation. Mutations in repair genes may lead to serious consequences such as cancer. Mutations can be induced or may occur spontaneously.
mutation that results from exposure to chemicals or environmental agents
variation in the nucleotide sequence of a genome
type of repair mechanism in which mismatched bases are removed after replication
nucleotide excision repair
type of DNA repair mechanism in which the wrong base, along with a few nucleotides upstream or downstream, are removed
function of DNA pol in which it reads the newly added base before adding the next one
mutation that affects a single base
mutation that is not expressed
when a purine is replaced with a purine or a pyrimidine is replaced with another pyrimidine
when a purine is replaced by a pyrimidine or a pyrimidine is replaced by a purine