18. Genes and chromosomes

Last updated on November 19, 2018 at 17:16


  • DNA is comprised of combinations of G≡C and A=T
  • Several unwanted, non-enzymatic processes can occur in DNA, that damages it. These are deamination, depurination, pyrimidine dimerization
  • Alkylating agents cause DNA damage by methylating DNA bases


Cytosine (C) and thymine (T) are pyrimidine bases, while adenine (A) and guanine (G) are purine bases. C binds with G with three strong bonds, while A and T are bound with just two.

If a part of DNA contains many alternating T and C residues, one of the strands can coil back on the two other strands, creating a short triple helix.

To the right you can see what bases looks like before and after deamination.

DNA damage

Deamination, where a part of a base loses an amino group, happens spontaneously at a rate of approx. 100 times each day in a cell. Because an amino group is the only the only difference between some bases, deamination can turn a base into a whole different one. Deamination of cytosine yields uracil (see the figure above). If this happens in RNA, some information is lost, as a base is switched out with a new one. In DNA however, which doesn’t use uracil (but uses thymine instead), the DNA strand can break. You can read more about how these damages are fixed in a later chapter.

Depurination is a similar process, where a nucleotide in DNA can spontaneously lose its base, to yield an apurinic residue. This happens more often than deamination, approx. 5000 times each day in each cell.

To the right you can see what happens during depurination.

UV light can induce formation of pyrimidine dimers. If two pyrimidine bases are next to each other in DNA, and they are exposed to UV-light, the two bases can form a chemical bond with each other, creating a dimer. Such dimers introduce kinks in the DNA strand, which is obviously not good.

DNA bases can be methylated by so-called alkylating agents. Guanine can be methylated to O6-methylguanine, for example. This is repaired by an enzyme called O6-alkylguanine-DNA alkyltransferase (AGT). Examples of alkylating agents are NaNO3, adoMet and dimethylsulfate.

Previous page:
17. Nucleotide synthesis and degradation

Next page:
19. DNA replication

Leave a Reply

Only the "Comment" field must be filled in. It is not compulsory to fill out your name; you can remain anonymous. Do not fill out e-mail or website; if you do, your comment will not be published.