What’s Cytosine?

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Cytosine is a nitrogenous base that, along with adenine, guanine, thymine, and uracil, makes up nucleotides in DNA and RNA molecules. Bases are divided into two groups, purine and pyrimidine, and specific base pairing keeps the molecule uniform and stable. The sequence of bases determines the genetic code for a cell’s proteins and function.

Cytosine is one of five nitrogenous bases that are attached to a five-carbon sugar, pentose, and a phosphate group to produce nucleotides. Nucleotides are the units that join together to form DNA and RNA molecules. The other bases, in addition to cytosine, that make up a DNA molecule are adenine, guanine and thymine. In one RNA molecule, uracil replaces thymine.

The bases are divided into two different groups. Adenine and guanine are purine bases and cytosine, thymine and uracil are pyrimidine bases. The two groups differ in their basic structure. Purine bases consist of two rings of atoms, while pyrimidine bases consist of only one ring. The bases are called nitrogenous bases as the rings contain nitrogen and carbon atoms.

Bases always mate with only one other base. Purine bases bind only to pyrimidine bases. Purine bases never bind with other purine bases and pyrimidine bases never bind with other pyrimidine bases. In particular, cytosine always pairs with guanine and adenine always pairs with thymine or uracil, depending on whether it is a DNA or RNA molecule. This mate is referred to as a specific base mate.

Specific base pairing keeps the molecule much more uniform and stable. By having purine bases bonded only to pyrimidine bases, the distance between the two strands of a DNA molecule will be uniform, a double loop and a single loop. If a purine base were to bond with another purine base, there would be a double ring bonded to a double ring. If a pyrimidine base were to bond with another pyrimidine base, a single ring would be bonded to a single ring. If that were the case, the structure of the DNA molecule would not be uniform, it would bend in and out depending on the paired bases.

Finally, the specific pairing is determined by the structure of each base. Structure affects how bases bond together and how many hydrogen bonds are formed. When cytosine binds to guanine, three hydrogen bonds are formed between the two bases. When adenine bonds with thymine or uracil, only two hydrogen bonds are formed. Only these base pairs are capable of forming the necessary hydrogen bonds in a DNA molecule.

The sequence of bases along a DNA molecule forms the code to instruct a cell to make particular proteins or genes. Base triplets code for specific amino acids, the building blocks of proteins. The sequence determines which amino acids should be joined and in what order. The proteins in a cell determine the structure and function of a cell, so the nitrogenous bases carry the genetic code for a cell.

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