The pentose phosphate pathway (also known as the hexose monophosphate shunt or HMS), mainly serves to produce  for anabolic reduction reactions and ribose-5-phosphate for nucleic acid production.

A nucleoside is composed of both a nitrogenous base as well as a sugar. Cytosine and adenine are just nitrogenous bases. Guanosine monophosphate (or GMP) is also composed of a phosphate group, which designates it as a nucleotide. The only nucleoside is adenosine.

Unmethylated cytosine spontaneously deaminates to uracil. Over time, methylated cytosine is spontaneously deaminated to thymine. Random deamination of methylcytosines causes mutation, creating hot spots. The vast majority of DNA methylations in mammals occurs at CpG (cytosine-phospate-guanine) sites.

A null mutation is one that deactivates a gene entirely. Point mutations are those that occur within a single, small site in a gene. Inversion involves the reversal of orientation of a DNA segment. Deletion occurs when a whole part of a chromosome is removed, joining two ends that were far apart. Translocation involves the exchange of genetic material from two chromosomes that are not homologous.

The silencing of a gene is most often accomplished via methylation of the DNA. The methyl groups are added to the gene's promoter region and thus, the DNA is not read by transcriptional enzymes.

In order to silence a gene by methylation, methyl groups are added to the promoter region of DNA. This area is upstream of the coding sequence and is responsible for initiation of transcription. Thus, methylating the promoter region inhibits further transcription of the gene.

The only two bases that can be methylated are cytosine and adenine.

Alkylating agents and  can also cause cancer, but they lead to methylation and mismatch mutations rather than the formation of pyrimidine dimers.

Reverse transcriptase synthesizes DNA in the 5' to 3' direction, using RNA as a template (hence it is the reverse of transcription). Restriction enzymes act only on DNA, not RNA, and they can cut bacterial as well as viral DNA—indeed, they can provide protection against viruses—and are found in archaea. Restriction enzymes can recognize specific sequences of nucleotides at restriction sites and cut DNA at these sites. Restriction enzymes do not create covalent bonds between adjacent exons after intron excision, rather this is done by tRNA splicing ligase. 

Xanthine oxidase is an enzyme important in purine catabolism. Nucleotides from DNA degradation are metabolized to uric acid by xanthine oxidase.In diseases with high levels of nucleotide production, uric acid levels are also high and produce symptoms of gout (uric acid is deposited abnormally in tissues). Gout is treated with inhibitors of xanthine oxidase such as allopurinol, reducing the levels of uric acid and the symptoms of gout.