Ribonucleic acid (RNA): A nucleic acid molecule similar to DNA but containing ribose rather than deoxyribose. RNA is formed upon a DNA template. There are several classes of RNA molecules.
They play crucial roles in protein synthesis and other cell activities:
- Messenger RNA (mRNA) is a type of RNA that reflects the exact nucleoside sequence of the genetically active DNA. mRNA carries the "message" of the DNA to the cytoplasm of cells where protein is made in amino acid sequences specified by the mRNA.
- Transfer RNA (tRNA) is a short-chain type of RNA present in cells. There are 20 varieties of tRNA. Each variety combines with a specific amino acid and carries it along (transfers it), leading to the formation of protein with a specific amino acid arrangement dictated by DNA.
- Ribosomal RNA (rRNA) is a component of ribosomes. Ribosomal RNA functions as a nonspecific site for making polypeptides.