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Cytosine is one of the bases that spell out genes in your DNA.

It is so versatile that it’s been called the ‘wild card’ of nucleic acids. Learn about the structure and functions of the letter ‘C’ in DNA’s alphabet!

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What Is Cytosine?

Cytosine is an important part of DNA and RNA, where it is one of the nitrogenous bases coding the genetic information these molecules carry. Cytosine can even be modified into different bases to carry epigenetic information. Cytosine has other roles in the cell, too, as the energy carrier and cofactor CTP.

Structure

As a nitrogenous base, cytosine is full of nitrogen atoms (it has three). It also has one ring of carbon, which makes it a pyrimidine.

A purine, on the other hand, has two rings of carbon. There are two pyrimidines, cytosine and thymine, and two purines, adenine and guanine, in DNA. RNA also has two pyrimidines, cytosine and uracil, and two purines, adenine and guanine.

In DNA, adenine and thymine are present in the same percentages and always pair with each other. That leaves cytosine to pair with its double-ringed buddy, guanine. Cytosine also pairs with guanine in RNA.

Function in Nucleic Acids

Cytosine can be part of a nucleotide, a molecule that includes a nitrogenous base along with a sugar and one or more phosphates. When nucleotides join together, they can form the nucleic acids DNA and RNA.When cytosine is on one strand of a nucleic acid, the other strand will contain a guanine to match.

These two are friends because they fit together perfectly with three hydrogen bonds. Cytosine can be easily converted into other bases, and so it’s been called the wild card base. In fact, sometimes it accidentally loses a few atoms and becomes uracil, one of the bases in RNA.

But don’t worry, our cells can find and fix this error.Cytosine can also be methylated, which is a change that can cause genes to be silenced. That’s an example of epigenetics, information that’s not part of the main DNA sequence, but is carried by DNA anyway. Think of it like a sticky note in a book.

Other Functions

When it’s not part of DNA, cytosine floats around the cell as a nucleotide with three phosphates. In that form, we call it cytidine triphosphate, or CTP.

CTP can carry energy like its adenosine-containing cousin ATP. It’s also a cofactor, or helper, for enzymes that add sugars to proteins and enzymes that make glycerophospholipids, a component of cell membranes.

Lesson Summary

Cytosine is one of the nitrogenous bases in DNA and RNA.

It has one ring, so it’s a pyrimidine, and it makes three hydrogen bonds, making it the perfect partner for guanine. It can be modified, on purpose or by accident, making it the wild card of bases, and an important player in epigenetics. Outside of nucleic acids, cytosine is part of CTP, an energy carrier and cofactor that can help enzymes do their jobs.

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