cytosine
Meaning of cytosine: Written as C in a sequence, it always sits opposite G, and it carries the chemical tag that can quiet a gene.
Definition of cytosine
Cytosine is a nitrogen-containing compound that serves as one of the chemical bases of DNA and RNA, the molecules that carry genetic information in cells. It belongs to the pyrimidines, bases built on a single ring, and it is written as the letter C in a genetic sequence. In both DNA and RNA it pairs with guanine.
How does cytosine pair with guanine?
The two strands of DNA are held together by bonds between bases facing each other across the helix. Cytosine binds guanine through three hydrogen bonds, while adenine binds thymine through two. Because the pairing is fixed, each strand carries the full information needed to rebuild the other, which is what makes accurate copying possible when a cell divides. The three-bond pairing is also slightly stronger than the two-bond one, so stretches of DNA rich in cytosine and guanine hold together more tightly and need more heat to separate.
How do the bases divide into classes?
| Class | Structure | Members | Pairs with |
|---|---|---|---|
| Pyrimidines | One ring | Cytosine, thymine in DNA, uracil in RNA | A purine |
| Purines | Two fused rings | Adenine, guanine | A pyrimidine |
Pairing a one-ring base with a two-ring base keeps the width of the double helix constant along its length. Cytosine appears in both DNA and RNA, unlike thymine, which is found in DNA, and uracil, which replaces it in RNA.
Why is cytosine singled out in the study of gene control?
Cells can attach a small chemical tag, a methyl group, to cytosine. Where a cytosine sits directly before a guanine in the sequence, clusters of such sites often mark the start of a gene, and heavy methylation of those clusters is generally associated with the gene being switched off. The underlying sequence is unchanged, so this is a layer of control laid on top of the code rather than a rewrite of it. Methylated cytosine is also chemically fragile: it can lose an amino group and turn into thymine, which is one reason certain sites in the genome accumulate changes more readily than others. Changes involving cytosine, whether a swapped base or an altered pattern of methylation, can therefore affect how a gene is read.
Also known as
Used in a sentence
The genetic sequencing report described a substitution involving a cytosine base at the site of interest.
Good to know
Cytosine pairs with guanine rather than with adenine or thymine, a fixed rule that underlies both the shape of the double helix and the accuracy with which DNA is copied.
Word origin
Where “cytosine” comes from
Formed from cyto-, from Greek kytos, meaning a hollow vessel and by extension a cell, with the chemical suffix -ine, reflecting its isolation from cell nuclei.