DNA damage and repair

• All biological polymers (starch, proteins, nucleic acids) can be damaged by the cell's own heat and the watery environment.
• DNA is no exception.

• It is estimated that about 5,000 purine bases (A and G) are lost from the DNA of each human cell every day because heat breaks the covalent bonds joining them to deoxyribose.
• Chemicals and radiation also damage DNA.
• Any damage can change the information and produce harmful changes in the cell's proteins.

• Although thousands of changes occur daily, only two or three permanent changes accumulate in a cell's DNA per year.
• Nearly all damage is removed by about 20 kinds of DNA repair enzymes.
• Working together, the enzymes recognise and cut out a damaged region and replace it with nucleotides complementary to those on the opposite (undamaged) strand, then ligase seals the gap.

Repair depends on having two copies of the information, one in each strand.

• While one strand is undamaged it serves as a template to repair its partner.
• Damage is only permanent if both strands are damaged at the same place at the same time.
• By contrast, single-stranded RNA genomes (some viruses) have no second copy and cannot be repaired; such viruses show high rates of genetic change.
• So the double helix is vital for genetic stability.