What is Cytosine?

Along with adenine, guanine, and thymine, cytosine is one of the four major bases present in DNA and RNA (uracil in RNA). It's a pyrimidine derivative with two substituents and a heterocyclic aromatic ring (an amine group at position 4 and a keto group at position 2). Cytidine is the nucleoside of cytosine. It forms three hydrogen bonds with guanine in Watson-Crick base pairing.


Cytosine Structure

Cytosine is an aminopyrimidine with the amino group at position 4 and is pyrimidin-2-one. It acts as a human metabolite, a metabolite in Escherichia coli, a metabolite in Saccharomyces cerevisiae, and a metabolite in mice. It's a pyrimidine nucleobase, pyrimidone, and aminopyrimidine all rolled into one. The molecule has a planar shape, and in the DNA double helix, cytosine forms three hydrogen bonds with Guanine. In RNA, which is made up of cytosine and ribose, the nucleoside of cytosine is cytidine. It's called deoxycytidine in DNA, and it's made up of cytosine and deoxyribose. The deoxycytidylate nucleotide of cytosine in DNA is made up of cytosine, ribose, and phosphate. A heterocyclic aromatic ring, an amine group at C-4, and a keto group at C-2 make up cytosine.


Cytosine Chemical Formula

C4H5N3O is the molecular formula for cytosine. A heterocyclic aromatic ring, an amine group at C-4, and a keto group at C-2 make up cytosine. Cytosine forms the nucleoside cytidine when it binds to ribose, and deoxyribose forms deoxycytidine when it binds to deoxyribose.


Properties of Cytosine

Cytosine is a pyrimidine derivative with two substituents and a heterocyclic, aromatic ring. Organic compounds (those containing carbon) with a ring structure containing atoms with carbon, such as sulphur, oxygen, or nitrogen, as part of the ring are known as heterocyclic compounds. Aromaticity is a chemical property in which the stability of a conjugated ring of unsaturated bonds, lone pairs, or empty orbitals is greater than can be predicted from conjugation alone. A substituent is an atom or group of atoms that is substituted for a hydrogen atom on the parent chain of a hydrocarbon in organic chemistry.

Cytosine is combined with guanine in DNA and RNA. It is, however, not integrally stable and can degrade into uracil. If the DNA repair enzymes, such as uracil glycosylase, which cleaves a uracil in DNA, do not restore it, a point mutation may occur.

An enzyme called DNA methyltransferase can also methylate cytosine into 5-methylcytosine.


Cytosine Chemical Activity

Guanine and Cytosine bind together by non-covalent hydrogen bonding at three different sites, as seen in the picture. It's worth noting that Watson and Crick first proposed that Guanine and Cytosine bonded by hydrogen bonding at two different sites.

Cytosine is a nucleotide component that can be found in DNA and RNA. Cytidine triphosphate is formed when the nucleoside cytidine binds to three phosphate groups (CTP). This molecule serves as a cofactor for enzymes, assisting in the conversion of phosphate from adenosine diphosphate (ADP) to adenosine triphosphate (ATP) in order to prepare ATP for use in chemical reactions.

Cytosine forms three hydrogen bonds with guanine in DNA and RNA. This device, however, is unstable and can transform into uracil. This is known as spontaneous deamination. If DNA repair enzymes such as uracil glycosylase do not repair the damage by cleaving uracil in DNA, a point mutation may result.


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Tautomerization in Cytosine

Tautomerization occurs when cytosine switches from amino to imino functionality through intermolecular proton transfer.


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History of Cytosine

Albrecht Kossel and Albert Neumann discovered and named cytosine in 1894 when it was hydrolyzed from calf thymus tissues. In 1903, a structure was proposed, and in the same year, it was synthesised (and thus confirmed) in the laboratory.

When Oxford University researchers introduced the Deutsch-Jozsa algorithm on a two-qubit nuclear magnetic resonance quantum computer in 1998, cytosine was used in an early demonstration of quantum information processing (NMRQC).

NASA scientists announced in March 2015 that pyrimidine can produce cytosine, uracil, and thymine under space-like laboratory conditions, which is interesting because pyrimidine has been detected in meteorites but its origin is unknown.


Purines and Pyrimidine Reaction

Purine biosynthesis differs from pyrimidine biosynthesis in that purines are formed first as a nucleotide, whereas pyrimidines are formed first as a free base. Pyrimidines are produced in a variety of tissues in humans, including the spleen, thymus, and gastrointestinal tract.

Cytosine, like other pyrimidines, is made up of several steps, the first of which is the formation of carbamoyl phosphate. A reaction involving bicarbonate, glutamine, ATP, and a water molecule produces carbamoyl phosphate. The enzyme carbamoyl phosphate synthetase catalyses the formation of carbamoyl phosphate.The catalytic activity of aspartate transcarbamylase converts the carbamoyl phosphate to carbamoyl aspartate. Following that, carbamoyl aspartate is converted to dihydroorotate, which is then oxidised to yield orotate. The ribose phosphate 5-phospho—D-ribosyl 1-pyrophosphate (PRPP) reacts with orotate to form orotidine-5-monophosphate (OMP).After that, OMP is converted into other pyrimidines. OMP decarboxylase is an enzyme that aids in the decarboxylation of OMP to produce uridine monophosphate (UMP). Kinases and dephosphorylation of ATPs eventually generate uridine diphosphate (UDP) and uridine triphosphate (UTP) further down the biosynthetic pathway. By modification of UTP with the enzyme CTP synthetase, UTP can be converted to cytidine triphosphate (CTP).

The nucleosides of cytosine are cytidine and deoxycytidine. They become cytidine triphosphate (CTP) and deoxycytidine triphosphate (dCTP), which are nucleotides that make up RNA and DNA molecules, respectively, when phosphorylated with three phosphoric acid groups.

When the pyrimidine nucleotide cytidine monophosphate (CMP) or cytosine is catabolized, the by-products -alanine, ammonia, and carbon dioxide are created. The following is the general degradation pathway: cytosine » uracil » N-carbamoyl-alanine » -alanine, carbon dioxide, and ammonia. Cytosine, on the other hand, can be recycled via the salvage pathway. Deamination, for example, can transform cytosine to uracil. Uridine phosphorylase reacts with ribose-1-phosphate to convert uracil to uridine. Uridine is converted to uridine monophosphate by the enzyme nucleoside kinase (UMP).


Mutation in Nitrogenous Bases

A mutation occurs when the nucleotide sequence of a gene or chromosome changes. A point mutation is a small-scale mutation in which only one nucleotide base in the DNA or RNA molecule changes. Cytosine is a relatively volatile substance. It has the potential to deaminate spontaneously into uracil. When this occurs in DNA, the DNA sequence is corrected by repair mechanisms. For example, uracil glycosylase is extracted from DNA by cleaving the cytosine-turned-uracil. The affected area is then removed and substituted using the other strand as a reference (i.e. the complementary strand).Base excision repair is the term for this form of procedure. In non-coding sequences, point mutations often have no discernible consequences. When it comes to coding sequences, a single nucleotide change can result in incorrect decoding during protein translation, particularly if the mutation is left uncorrected. A protein's altered structure may cause it to be unstable or non-functional, resulting in its impairment.


Biological Function of Cytosine

The other four primary (or canonical) nucleobases are thymine, uracil, guanine, and adenine. Cytosine is one of the five primary (or canonical) nucleobases. The genetic code is made up of these basic nucleobases. The genetic code for a specific protein is contained in nucleic acids such as DNA and RNA molecules, which is dependent on the sequence of nucleobases.. Nucleic acids play a crucial role in cellular functions, heredity, and organism survival. Cytosine, in the form of cytidine triphosphate (CTP), may be used as an enzyme co-factor. It can convert adenosine diphosphate (ADP) to ATP by transferring a phosphate. ATP is a high-energy molecule that is involved in a variety of cellular functions and essential biological reactions.


Difference Between Purine and Pyrimidine

The most critical distinction to understand between purines and pyrimidines is their structural differences.

Purines (adenine and guanine) have a two-ringed structure, as seen in the two diagrams below, consisting of a nine-membered molecule with four nitrogen atoms.

Purines are clearly larger than pyrimidines since they are pyrimidines fused with a second ring. Part of the explanation for complementary pairing is the size gap. Purines in DNA strands will be so large if they are bound to each other instead of the pyrimidines that the pyrimidines will be unable to enter other pyrimidines or purines on the other hand! The gap between them would be so wide that the DNA strand would be unable to hold itself together. Similarly, if the pyrimidines in DNA bonded together, the purines would run out of space.


Important Points

IUPAC name or chemical name of cytosine is 4-aminopyridine-2(1H)-one.

The chemical cytosine formula is C4H5N3O.

The molecular mass of cytosine is 111.104 g/mol.


Did You Know that?

  • Cytosine is one of the 5 main nucleobases used in storing and transporting genetic information within a cell in the nucleic acids DNA and RNA. 

  • Cytosine is combined with guanine in DNA and RNA. It is, however, unstable and can transform into uracil (spontaneous deamination). If not repaired by DNA repair enzymes like uracil glycosylase, which cleaves a uracil in DNA, this may result in a point mutation.

FAQs (Frequently Asked Questions)

1. Explain Cytosine Structure.

Answer: It's a pyrimidine derivative with two substituents and a heterocyclic aromatic ring. The cytosine formula is C4H5N3O, this means that the compound is composed of four carbon atoms, five hydrogen atoms, three nitrogen atoms, and one oxygen atom. The heterocyclic ring of cytosine contains an amine group at position 4 and a keto group at position 2. Cytidine is the nucleoside of cytosine.

2. Give one Cytosine Function.

Answer: Cytosine is a nitrogenous base derived from pyrimidine that is found in nucleic acids. Cytosine plays major role in:

  • Regulating the components of all living cells and heredity.

  • Act as a controlling agent in certain coenzymes. 

  • Helps the substances that work in tandem with enzymes to carry out chemical reactions in the body.