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Cytosine – Nucleobase in DNA and RNA

Cytosine is a nitrogen-containing base and a key building block of DNA and RNA. It belongs to the pyrimidine group and plays a central role in storing and transmitting genetic information.

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Things worth knowing about "Cytosine"

Cytosine is a nitrogen-containing base and a key building block of DNA and RNA. It belongs to the pyrimidine group and plays a central role in storing and transmitting genetic information.

What is Cytosine?

Cytosine is an organic compound belonging to the group of pyrimidine bases and is one of the five nucleobases found in nucleic acids. It is an essential building block of both deoxyribonucleic acid (DNA) and ribonucleic acid (RNA). In DNA, cytosine forms a complementary base pair with guanine, connected by three hydrogen bonds.

Chemical Properties

Cytosine has the molecular formula C₄H₅N₃O and a molecular weight of approximately 111.1 g/mol. It is a heterocyclic aromatic ring containing an amino group (-NH₂) and a keto group (=O). Within cells, cytosine commonly exists as the nucleoside cytidine (in RNA) or deoxycytidine (in DNA) when bonded to a sugar molecule.

Biological Function

Cytosine performs several fundamental roles in living organisms:

Genetic coding: As a component of codons in mRNA, cytosine contributes to the encoding of amino acids and thereby to protein synthesis.
DNA methylation: Cytosine can be methylated at the 5-position by the enzyme DNA methyltransferase, converting it to 5-methylcytosine. This epigenetic modification regulates gene expression without altering the DNA sequence itself.
Repair and replication: During DNA replication, cytosine is specifically paired with guanine, ensuring the accuracy of genome duplication.

Cytosine in Epigenetics

A particularly important aspect of cytosine is its role in epigenetics. The methylation of cytosine -- especially at CpG dinucleotides (regions where cytosine is followed by guanine) -- is a central mechanism for regulating gene activity. Hypermethylation can silence genes, while hypomethylation is associated with increased gene activity. Alterations in these methylation patterns are linked to the development of cancer, neurodegenerative diseases, and developmental disorders.

Mutations and Disease Relevance

Cytosine is one of the most reactive nucleobases and is susceptible to spontaneous or mutagen-induced deamination, which converts cytosine into uracil. If this change is not recognized and corrected by cellular repair systems, it can lead to a point mutation (C to T transition). Such mutations are common and can contribute to the development of genetic diseases or tumors.

Medical and Pharmaceutical Significance

Understanding the structure and function of cytosine has contributed to the development of important medications:

Cytosine arabinoside (Cytarabine): A cytostatic drug used in the treatment of leukemias. It inhibits DNA synthesis in cancer cells.
5-Azacytidine (Azacitidine): A DNA methylation inhibitor used in the treatment of myelodysplastic syndromes and certain forms of leukemia. It inhibits methyltransferase and reactivates silenced tumor suppressor genes.
Decitabine: Another demethylating agent also developed on the basis of cytosine analogues.

References

Alberts B. et al. - Molecular Biology of the Cell, 6th Edition, Garland Science, 2014.
Lodish H. et al. - Molecular Cell Biology, 8th Edition, W.H. Freeman and Company, 2016.
Jones P.A. - Functions of DNA methylation: islands, start sites, gene bodies and beyond. Nature Reviews Genetics, 2012. PubMed PMID: 22641018.

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