Cytosine is a nucleic base that is a building block of DNA and RNA. It and three other nucleic bases make up the genetic code of every living thing.
What is cytosine?
The exact chemical name of cytosine is 4-amino-1H-pyrimidin-2-one because the amino group of the nucleic base is located at the fourth standard position of a scaffold of pyrimidine. Pyrimidine is a molecule whose structure is formed by a six-membered ring with two nitrogen atoms. An oxygen atom occupies the second position. Pyrimidine is an inorganic particle; it serves as the basis for numerous other constructs such as cytosine, in which it represents an important building block. Cytosine is only partially soluble in ethanol and water. Its normal physical state is solid, with a melting point of 320 to 325 °C. Cytosine is thus one of the more robust chemical compounds. In its pure form, the nucleic base must be labeled “irritant” because it can irritate mucous membranes and cause inflammation. However, cytosine elicits this reaction only in unnatural concentrations, such as do not occur in nature and are produced only by artificial biochemical synthesis.
Function, action, and roles
Cytosine is one of the four nucleic bases that, together with sugar molecules, form DNA (deoxyribonucleic acid) and the similar RNA (ribonucleic acid). Cytosine thus plays an important role in the storage of hereditary information, which is responsible for the control of cells as well as heredity. DNA consists of long chains of so-called nucleotides. In order for the building block cytosine to become a nucleotide, it must combine with other building blocks. The nucleotide consists of a sugar molecule, a phosphate acid and a base. In human DNA, the sugar molecule is a so-called pentose; the name pentose refers to the pentagonal structure of the sugar. Special enzymes read the DNA in the cell nucleus and make a copy, which then passes out of the nucleus. There it is read again by other enzymes and translated into proteins. Biology therefore refers to this process as translation. During translation, the enzyme responsible selects the appropriate protein molecule for each code, which consists of three base pairs, and forms long chains from them. Cytosine forms a so-called base pair together with guanine, another nucleic base. The two DNA building blocks can form a bond with each other like two pieces of a puzzle – but not with the other two nucleic bases, thymine and adenine. Cytosine and guanine combine with the aid of a hydrogen bridge and can dissolve this again by splitting off hydrogen. Thymine is also a pyrimidine base based on the six-membered ring. All living organisms share these basic principles of inheritance and are based on the same basic genetic material. It is only the sequence of base pairs that determines which organism emerges from the genetic code, what shape it takes, and what functions it can perform.
Formation, occurrence and properties
Cytosine is formed from pyrimidine. This substance represents a six-membered ring with two nitrogen atoms. An NH2 group attaches to the pyrimidine with a single bond, as well as a single oxygen molecule with a double bond. In addition, a hydrogen atom attaches to one of the two nitrogen atoms of the pyrimidine ring. The natural sciences distinguish between two forms of cytosine, the tautomers. In addition to the H1 tautomer, another form of cytosine exists, the 3H tautomer. H1 tautomer and H3 tautomer differ from each other in that the various additional groups attach to different positions on the pyrimidine ring. The H1 variant occurs more frequently. There is no optimum value for cytosine; how frequently cytosine occurs in the human body depends on various factors. However, because it is found in the nucleus of every single cell, it occurs very frequently.
Diseases and disorders
Cytosine is a building block of DNA and determines the basic mechanisms of life by its place in the sequence of the genetic code along with three other bases. Accordingly, life without cytosine is not possible.For example, if sufficient basic substances from which cytosine is formed were not available during a very early phase of pregnancy, further development of the embryo would not be possible even at the stage of individual cells. A complete absence of cytosine in a living organism is therefore unthinkable. Cytosine consists of atoms that are very abundant. A deficiency as a result of inadequate nutrition is therefore also extremely unlikely. The body cell can swap cytosine, like all other bases, with another base due to errors in copying the DNA (transcription). This is a mutation, as a result of which the enzymes within the cells form incorrect protein chains. These proteins have no or only limited functionality or achieve a different effect than intended. As a result, they disrupt the regulated processes of the organism. In myoadenylate deaminase deficiency (MAD deficiency or MADD), for example, the genetic code of the AMPD1 gene has an error. At position 34 of exon 2, there should actually be cytosine; however, due to a gene defect, this position is mistakenly occupied by the nucleotide thymine. This small change has drastic effects: The code now signals “stop” at this position, which is why the enzymes synthesize only an incomplete protein during translation. As a result, MADD manifests a metabolic disorder of the skeletal muscles. It causes muscle symptoms such as cramps, weakness, and pain.
