What does cytosine mean?
Cytosine is one of the four principal building blocks of nucleic acids — the molecules DNA and RNA that store and transmit genetic information in all living things. Chemically, it belongs to a class of compounds called pyrimidines and is distinguished by its single-ring structure. Within the double helix of DNA, cytosine pairs reliably with guanine through three hydrogen bonds, a precise matching that underlies how genetic code is copied faithfully from one generation of cells to the next. It also appears in RNA, where it performs an equivalent pairing role. Beyond its structural function, cytosine carries chemical modifications such as methylation, which act like molecular switches that turn genes on or off without changing the genetic sequence itself — a key mechanism in epigenetics. Because of this dual role as both a coding element and a regulatory mark, cytosine occupies a central place in genetics, biotechnology, and medicine.
nounCytosine is one of the four nucleobases found in DNA and RNA, a pyrimidine base that pairs specifically with guanine. In DNA it forms three hydrogen bonds with guanine, contributing to the stability of the double helix.
- A pyrimidine nucleobase found in DNA and RNA that pairs with guanine.
- In biochemistry more broadly, any derivative or form of this compound (e.g., methylated cytosines) considered as a class of modified bases.
"The enzyme converted cytosine into uracil, triggering the cell's DNA repair machinery."
"Methylation of cytosine residues in promoter regions can silence gene expression without altering the underlying DNA sequence."
"Ancient DNA studies rely on the fact that cytosine slowly deaminates to uracil over time."
Every letter C in your genome is a molecule that can be chemically tagged — and those tags help decide which of your genes are switched on or off.
Reviewed by Deb Chak, Editor. AI-assisted content curated by RJS Tech Solutions LLP.
Etymology of cytosine
The name 'cytosine' derives ultimately from the Greek word 'kytos', meaning a hollow vessel or cell, reflecting the compound's early association with cellular material. The term was formed by combining this Greek root with the '-ine' suffix commonly used for organic bases. Its cognates include other scientific terms built on the same Greek root, such as 'cytoplasm', 'cytochrome', and 'cytology'.
How cytosine is actually used
Strictly scientific register: used almost exclusively in molecular biology, genetics, biochemistry, and related technical writing. Not used colloquially.
Easily confused with cytosine
Cytosine is a nitrogenous base in nucleic acids, while cytosol is the aqueous fluid inside cells surrounding the organelles.
Thymine is the other major pyrimidine base in DNA; unlike thymine, cytosine pairs with guanine and also occurs in RNA (as does its deamination product, uracil).