Uracil

Ball-and-stick Stick Space-filling

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12 atoms, 12 bonds.

IUPAC name: 1H-pyrimidine-2,4-dione
Molar mass: 112.088(13) g/mol
Atoms: 12
SMILES: C1=CNC(=O)NC1=O
InChIKey: ISAKRJDGNUQOIC-UHFFFAOYSA-N
PubChem CID: 1174

Made of

Structural formula of uracil. Each unlabelled corner is a carbon atom, and the two hydrogens on carbon are not drawn. It shows which atoms are joined to which, not the shape. The model above shows the shape.

In the one-letter code used to write RNA, U stands for uracil. Medical Subject Headings (MeSH), the indexing vocabulary of the US National Library of Medicine, counts it among the bases of RNA. The ChEBI database (Chemical Entities of Biological Interest) classes it as a pyrimidine base.

Its ring holds four carbon atoms and two nitrogen atoms, with an oxygen atom double-bonded at positions 2 and 4, which ChEBI calls oxo groups. The systematic name, 1H-pyrimidine-2,4-dione, records the same pair of C=O groups. Four hydrogen atoms complete C₄H₄N₂O₂, and the molar mass is 112.09 g/mol. Haz-Map, a US occupational health database citing the Merck Index, records that it is produced by hydrolysis of nucleic acids, which breaks them down, and goes into biochemical research.

Structure and bonding

The viewer draws uracil as 12 atoms and 12 bonds on one six-membered ring of four carbons and two nitrogens. With Labels on, each nitrogen carries a hydrogen, and the two carbons flanking one N–H each carry an oxygen. The other two carbons share the ring’s only C=C, with one hydrogen apiece. The one next to a C=O carbon is position 5, where fluorouracil has fluorine instead.

Rotated until the ring shows its edge, the model has all 12 atoms, hydrogens included, in one plane. In the Space-filling view the molecule becomes a thin, even tile, nothing rising above or below it. Ball-and-stick shows each carbon with three neighbours and ring angles between 114° and 126°. Chem1 sets such a carbon, with one double bond, at the centre of a plane trigonal arrangement of its three partners.

Stick makes the double bonds easy to count: two C=O bonds pointing out from the ring and one C=C inside it. The remaining ring bonds, four C–N and one C–C, are drawn single. The two N–H and two C=O groups are the kind that form hydrogen bonds.

Where it turns up

ChEBI lists uracil as a metabolite, a compound that cells make or use, in humans, mice, Escherichia coli, Saccharomyces cerevisiae and Daphnia magna. LOTUS, a natural-products database, reports it in Hamigera avellanea and Paraphaeosphaeria minitans.

Uracil is not confined to living things. Before 2022 it was one of three bases of DNA and RNA known from meteorites, with adenine and guanine. That year NASA’s Goddard Space Flight Center reported the other two, cytosine and thymine. Asteroid Bennu’s sample, brought to Earth by OSIRIS-REx in 2023, holds all five, NASA announced in 2025, though the findings show no evidence of life itself.

In 2015 NASA’s Ames Research Center reported making uracil, cytosine and thymine by exposing ice containing pyrimidine to ultraviolet light under space-like conditions. In NASA Goddard’s view, bases formed in space may have helped life begin on the young Earth. They could have arrived in meteorite impacts or as falling dust.

Why it matters

Three bases read together make a codon, the unit of genetic information that encodes a particular amino acid. Of the 64 codons, 61 specify amino acids, and the other three, UAA, UAG and UGA, are stop codons. On a messenger RNA, UAG marks the end of a protein’s production.

Fluorouracil (5-FU) is a cancer drug first approved as an injection in 1962, the US Food and Drug Administration (FDA) records. The National Cancer Institute (NCI) lists breast, colorectal, stomach and pancreatic cancers among its approved uses. Mimicking a natural chemical, it works in several ways. As F-UMP it enters RNA in place of uracil and disrupts RNA processing. As F-dUMP it blocks the enzyme thymidylate synthase, cutting the supply of a key DNA component, so DNA cannot copy itself and rapidly dividing cells die.

DPD (dihydropyrimidine dehydrogenase), the enzyme that starts uracil’s breakdown, also clears 5-FU. Where it or dihydropyrimidinase, the next enzyme, is lacking, the drug can build up and cause severe reactions, according to MedlinePlus Genetics. The FDA approved label changes on the DPD-deficiency risk in 2024.

Properties

Properties of uracil Molar mass summed from IUPAC/CIAAW 2024 standard atomic weights; formula, atom and bond counts and PubChem's molecular weight from PubChem CID 1174.
Property	Value	Unit	Source
Molecular formula (Hill order)	C₄H₄N₂O₂
Molar mass	112.088(13) g/mol	g/mol	IUPAC/CIAAW 2024
Carbon atoms	4
Hydrogen atoms	4
Nitrogen atoms	2
Oxygen atoms	2
Bonds	12
Single bonds	9
Double bonds	3
PubChem molecular weight	112.09 g/mol	g/mol	PubChem

Safety

Uracil powder meets none of the hazard criteria of the Globally Harmonized System (GHS) in 14 of 17 company reports to the European Chemicals Agency. It is classed as causing skin irritation, serious eye irritation and possible respiratory irritation in 11.8% of reports each, under the signal word Warning. The PubChem Laboratory Chemical Safety Summary for uracil gathers these classifications for anyone working with the powder.

Questions

What does the U stand for in RNA?

U is uracil, one of the bases from which RNA is built. Messenger RNA (mRNA) is made from a DNA template in the process called transcription, and its bases are then read in sets of three, the codons. Certain viruses use RNA, not DNA, as their genetic material, which puts uracil in their genomes as well.

How does the body break down uracil?

Dihydropyrimidine dehydrogenase (DPD) takes the first step, turning uracil into 5,6-dihydrouracil. A second enzyme, dihydropyrimidinase, then opens that ring, and the three-step pyrimidine pathway finishes with beta-alanine and beta-aminoisobutyric acid. Mutations in DPYD, the gene for DPD, interfere with this breakdown, and uracil then collects in excess in the blood and urine, according to MedlinePlus Genetics. How that excess relates to the neurological problems seen in some patients is unclear.

What is tegafur-uracil?

It is a combination of uracil with tegafur, a prodrug of the cancer drug fluorouracil (5-FU): a compound the body converts into 5-FU, and one that is absorbed when swallowed. The uracil is present in high concentration to block the enzyme dihydropyrimidine dehydrogenase (DPD), reversibly, according to the NCI Drug Dictionary. With that enzyme held back, the 5-FU escapes breakdown by DPD in the liver on its first pass after absorption.

Made of the same elements

C₄H₅N₃O Cytosine 13 atoms, 13 bonds.
C₅H₆N₂O₂ Thymine 15 atoms, 15 bonds.
C₂H₅NO₂ Glycine 10 atoms, 9 bonds.
CO(NH₂)₂ Urea 8 atoms, 7 bonds.
C₅H₄N₄O₃ Uric acid 16 atoms, 17 bonds.
C₃H₇NO₂ Alanine 13 atoms, 12 bonds.
C₅H₅N₅O Guanine 16 atoms, 17 bonds.
C₅H₉NO₄ Glutamic acid 19 atoms, 18 bonds.

All 25 compounds made of carbon, hydrogen, nitrogen and oxygen

C₈H₉NO₂ Acetaminophen 20 atoms, 20 bonds.
C₈H₁₀N₄O₂ Caffeine 24 atoms, 25 bonds.
C₈H₁₁NO₂ Dopamine 22 atoms, 22 bonds.
C₆H₁₃NO₂ Isoleucine 22 atoms, 21 bonds.
C₆H₁₃NO₂ Leucine 22 atoms, 21 bonds.
C₉H₁₁NO₂ Phenylalanine 23 atoms, 23 bonds.
C₉H₁₁NO₃ Tyrosine 24 atoms, 24 bonds.
C₁₀H₁₂N₂O Serotonin 25 atoms, 26 bonds.
C₁₁H₁₂N₂O₂ Tryptophan 27 atoms, 28 bonds.
C₉H₁₃NO₃ Epinephrine 26 atoms, 26 bonds.
C₁₃H₁₆N₂O₂ Melatonin 33 atoms, 34 bonds.
C₁₂H₁₇NO DEET 31 atoms, 31 bonds.
C₁₄H₁₈N₂O₅ Aspartame 39 atoms, 39 bonds.
C₁₃H₂₁NO₃ Albuterol 38 atoms, 38 bonds.
C₁₄H₂₂N₂O Lidocaine 39 atoms, 39 bonds.
C₁₇H₂₁NO Diphenhydramine 40 atoms, 41 bonds.
C₂₀H₂₄N₂O₂ Quinine 48 atoms, 51 bonds.