Ribonucleic acid (RNA) is a single-stranded polymer of ribonucleotides. RNA is composed of ribose sugar, a phosphate residue, and a purine or pyrimidine base.
Purines in RNA: adenine (A) and guanine (G)
- These are the same as DNA purine bases.
Pyrimidines in RNA: cytosine (C) and uracil (U)
- Uracil is incorporated into RNA rather than thymine, which is one of the two pyrimidine bases that is incorporated into DNA.
Table 2 lists the three primary functional forms of RNA.
Table 2. RNA Functional Forms.| RNA | Function |
| Messenger RNA (mRNA) | mRNA is a single-stranded molecule that is complementary to a copy of a DNA sequence for creating a given protein. The process of making mRNA from DNA is called transcription, which occurs in the nucleus. mRNA directs the synthesis of proteins, which occurs in the cytoplasm. The mRNA formed in the nucleus is transported out of the nucleus and into the cytoplasm where it attaches to the ribosomes. |
| Transfer RNA (tRNA) | tRNA serves as a physical link between mRNA and the synthesis of proteins at the ribosome. The purpose of tRNA is to transfer amino acids to the ribosome for protein production. To ensure that the amino acids are added to the protein in a specific order, tRNA reads the coding sequence contained in the mRNA. The process of synthesizing a protein from mRNA is called translation. tRNA carries amino acids to the ribosome.
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| Ribosomal RNA (rRNA) | rRNA is part of the ribosome that carries out protein synthesis and is responsible for reading the order of amino acids and linking amino acids together to synthesize proteins during the process of translation.
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