DNA proofreading and repair
- Cells have a variety of mechanisms to prevent mutations, or permanent changes in DNA sequence.
- During DNA synthesis, most DNA polymerases "check their work," fixing the majority of mispaired bases in a process called proofreading.
- Immediately after DNA synthesis, any remaining mispaired bases can be detected and replaced in a process called mismatch repair.
- If DNA gets damaged, it can be repaired by various mechanisms, including chemical reversal, excision repair, and double-stranded break repair.
- Proofreading, which corrects errors during DNA replication
- Mismatch repair, which fixes mispaired bases right after DNA replication
- DNA damage repair pathways, which detect and correct damage throughout the cell cycle
- DNA polymerase adds a new base to the 3' end of the growing, new strand. (The template has a G, and the polymerase incorrectly adds a T rather than a C to the new strand.)
- Polymerase detects that the bases are mispaired.
- Polymerase uses 3' to 5' exonuclease activity to remove the incorrect T from the 3' end of the new strand.
- A mismatch is detected in newly synthesized DNA. There is a G in the new strand paired with a T in the template (old) strand.
- The new DNA strand is cut, and a patch of DNA containing the mispaired nucleotide and its neighbors is removed.
- The missing patch is replaced with correct nucleotides by a DNA polymerase.
- A DNA ligase seals the remaining gap in the DNA backbone.
DNA damage repair mechanisms
- Direct reversal: Some DNA-damaging chemical reactions can be directly "undone" by enzymes in the cell.
- Excision repair: Damage to one or a few bases of DNA is often fixed by removal (excision) and replacement of the damaged region. In base excision repair, just the damaged base is removed. In nucleotide excision repair, as in the mismatch repair we saw above, a patch of nucleotides is removed.
- Double-stranded break repair: Two major pathways, non-homologous end joining and homologous recombination, are used to repair double-stranded breaks in DNA (that is, when an entire chromosome splits into two pieces).
Reversal of damage
Base excision repair
- Deamination converts a cytosine base into a uracil. This results in a double helix in which a G in one strand is paired with a U in the other. The U was formerly a C, but was converted to U via deamination.
- The uracil is detected and removed, leaving a base-less nucleotide.
- The base-less nucleotide is removed, leaving a 1-nucleotide hole in the DNA backbone.
- The hole is filled with the right base by a DNA polymerase, and the gap is sealed by a ligase.
Nucleotide excision repair
- UV radiation produces a thymine dimer. In a thymine dimer, two Ts that are next to each other in the same strand link up via a chemical reaction between the bases. This creates a distortion in the shape of the double helix.
- Once the dimer has been detected, the surrounding DNA is opened to form a bubble.
- Enzymes cut the damaged region (thymine dimer plus neighboring regions of same strand) out of the bubble.
- A DNA polymerase replaces the excised (cut-out) DNA, and a ligase seals the backbone.
Double-stranded break repair
DNA proofreading and repair in human disease
- Hereditary nonpolyposis colorectal cancer (also called Lynch syndrome) is caused by mutations in genes encoding certain mismatch repair proteins. Since mismatched bases are not repaired in the cells of people with this syndrome, mutations accumulate much more rapidly than in the cells of an unaffected person. This can lead to the development of tumors in the colon.
- People with xeroderma pigmentosum are extremely sensitive to UV light. This condition is caused by mutations affecting the nucleotide excision repair pathway. When this pathway doesn't work, thymine dimers and other forms of UV damage can't be repaired. People with xeroderma pigmentosum develop severe sunburns from just a few minutes in the sun, and about half will get skin cancer by the age of unless they avoid the sun.