Uracil is a nitrogenous base found in nucleotides that make up DNA and RNA. It belongs to the pyrimidine group of bases, which also includes cytosine and thymine. Adenine and guanine belong to the purine group. Base pairing is specific, with adenine pairing with thymine or uracil, and cytosine pairing with guanine. Thymine is only found in DNA, while uracil is only found in RNA. Other differences between DNA and RNA include their structure and pentose sugar. Thymine has a methyl group that uracil lacks, allowing enzymes to distinguish between DNA and RNA molecules.
Uracil is one of five nitrogenous bases that attach to nucleotides found inside cells. A nucleotide consists of a five-carbon sugar, pentose sugar, with a phosphate group and a base attached. Nucleotides are the building blocks of two important nucleic acids: deoxyribonucleic acid, DNA, and ribonucleic acid, RNA. The other four bases are cytosine, guanine, adenine and thymine.
The five bases are divided into two groups according to their structure. Pyrimidine bases consist of a single ring of atoms and include cytosine, thymine, and uracil. Purine bases have a double ring of atoms and include adenine and guanine. They are called nitrogenous bases as they contain nitrogen and carbon atoms in the rings.
Inside the cell, bases are paired to form DNA molecules and also during certain processes, such as DNA replication and protein synthesis. Base pairing is very specific and each base pairs only with each other. Adenine pairs with thymine or uracil and cytosine pairs with guanine. Complementary base pairs always consist of a purine base and a pyrimidine base to ensure that the distance between the paired strands is uniform and stable.
Three of the bases – adenine, cytosine and guanine – are attached to the nucleotides that make up both DNA and RNA molecules. Depending on whether the nucleic acid is an RNA molecule or a DNA molecule, it will determine whether the base used is thymine or uracil. Thymine is only used to make DNA molecules, while uracil is found only in RNA molecules. This is one of the main ways that RNA and DNA molecules differ.
DNA and RNA molecules have three distinct differences. The first difference, as stated above, was whether uracil or thymine was the base used. The second difference is that DNA is double stranded while RNA is single stranded. Finally, the five carbon sugars are different for RNA nucleotides than DNA nucleotides. The pentose sugar in RNA is ribose and has an oxygen atom not found in the DNA sugar, deoxyribose.
The structure of uracil and thymine also differ slightly. The only difference is that uracil lacks a methyl group, a carbon atom with three hydrogen atoms attached, that thymine has. This slight difference is enough for enzymes to distinguish between DNA and RNA molecules. Enzymes have a very specific shape in the active site where they attach to other molecules. The methyl group gives thymine a different shape from uracil, which ensures that the correct enzymes interact with the correct molecules.
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