Cytosine: Definition, Structure, and Examples

Cytosine Definition

Cytosine is an organic pyrimidine base that has the formula of C4H5N3O and it pairs complementary with guanine in nuclei acids like DNA and RNA. It has a molecular mass of 111.10 g/mol.

What is Cytosine?

Monomeric nucleotides comprises polymers of nuclei acid. A nitrogenous nucleobase (purine or pyramiding), 5-C sugar and a phosphoric acid comprise a nucleotide. The 5 fundamental nucleobases of the genetic code include guanine, adenine, thymine, uracil and cytosine.

Cytosine, Cytosine Definition, Cytosine Examples

Cytosine Properties

Cytosines like other pyrimidine is aromatic and heterocyclic with a single pyrimidine ring which has nitrogen and carbon atoms present in an alternating fashion. Cytosine forms a component of nuclei acids in form of nucleoside or nucleotide and has a melting point of 320-325 °C.

Cytosine base bonds with guanine via 3 hydrogen bonds in nuclei acids. It is readily deaminated into uracil through spontaneous reactions as it is unstable. With the help of enzyme uracil glycosylase such defects in DNA can be repaired. If not repaired, this may lead to a point mutation.

Adenine vs Thymine or Uracil

Pyrimidine nucleobases include uracil, cytosine and thymine. Cytosine can be distinguished with the presence of an amine group at 4th position and a keto group at the 2nd position in the aromatic ring. It has a chemical formula of C4H5N3O. Unlike uracil and thymine that pairs with adenine, cytosine pairs with guanine.

Cytosine Synthesis

Unlike purines, pyrimidines are biosynthesized as a free base initially. Their synthesis occurs in tissues like the thymus, spleen, and gastrointestinal tract. Cytosine synthesis starts with carbamoyl phosphate and progresses in a series of steps similar to other pyrimidines.

Bicarbonate, ATP, water molecule, and glutamine are required for the production of and this reaction is catalyzed by carbamoyl phosphate synthetase. Enzyme aspartate transcarbamylase catalysis conversion of carbamoyl phosphate into carbamoyl aspartate that then is converted into dihydroorotate.

A series of intermediate reactions lead to the formation of ultimately the orotidine-5-monophosphate (OMP) that is converted into pyrimidines. The decarboxylation reaction of OMP catalyzed by OMP decarboxylase yields uridine monophosphate (UMP). Dephosphorylation of ATPs and the action of kinases yields UDP and UTP. The enzyme CTP synthetase catalyzes the amination of UTP to form CTP or cytidine triphosphate.

Nucleosides of cytosine include deoxycytidine and cytidine. On phosphorylation with 3 phosphoric acid groups they yield dCTP and CTP nucleotides that play a role in the building of nucleic acids like DNA and RNA.

The degradation pathway of cytosine or CMP leads to the production of CO2, ammonia, and β-alanine and involves the formation of intermediate compounds like uracil and N-carbamoyl- β-alanine. Utilizing the salvage pathway cytosine can be re-utilized or by the process of deamination can be converted to uracil. In a reaction catalyzed by uridine phosphorylase, on reaction with ribose-1-phosphate uridine is formed from uracil. Eventually, UMP is formed from uridine and the reaction is catalyzed by enzyme nucleoside kinase.

A mutation is characterized by an alteration in the sequence of the nucleotides of a gene. Point mutation results from a single base change in the nucleic acid. Cytosine can relatively undergo deamination to form uracil. Under such circumstances, DNA repair mechanisms are initiated. For example, in base excision repair the uracil is cleaved and removed by uracil glycosylase from the DNA.

The mutated region is degraded and using complementary strand is replaced with the correct sequence. Point mutations on heterochromatic regions or non-coding sequences are not readily exhibited. While in the case of coding regions it can disrupt protein translation leading to even malfunctions in protein structure and function if left unrepaired.

Biological Function of Cytosine

The 5 canonical bases comprise thymine, guanine, uracil, and adenine that constitute the genetic code in nucleic acids where their sequence dictates a specific protein. They perform essential functions in inheritance, metabolism, and other cellular functions.

Cytosine also plays an important role as co-factors in the form of CTP may to enzymes. It converts adenosine diphosphate to ATP with the transfer of phosphate.

Cytosine Trivia

A German biochemist named Albrecht Kossel along with his students discovered the primary nucleobases.

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