a
nucleic acid that is present in all living cells and controls cellular protein synthesis; it replaces
deoxyribonucleic acid (DNA) as a carrier of genetic codes in some viruses. RNA is similar in composition to DNA with two exceptions: the sugar in RNA is
ribose, as opposed to the
deoxyribose of DNA, and in RNA the pyrimidine
uracil is found instead of the
thymine of DNA. The structure of RNA varies from helical to uncoiled strands of varying lengths, depending on the number of nucleotide units forming the strand. This variance in structure is evident in the different types of RNA. For example, transfer RNA (tRNA) contains only about 75 nucleotide units, while other types may contain thousands of units.
Messenger RNA (mRNA) receives its name from its function of carrying the genetic code from the nucleus of the cell to the cytoplasm, where most cellular functions take place. The transfer of the genetic code from DNA to mRNA is called transcription. Molecules of mRNA migrate to the ribosomes, where the manufacture of proteins occurs. The strands of RNA contain codons, some of which signal when formation of a particular protein should stop and the formation of another should start.
Transfer RNA (tRNA) brings about the transfer of specific amino acid molecules to protein molecules during the synthesis of proteins. Each of the 20 common amino acids found in protein molecules has a corresponding type of transfer RNA. Thus, a specific tRNA carries the appropriate amino acid to its appropriate place in the chain of the protein molecule being synthesized.
Ribosomal RNA (rRNA) is so called because it is found in the
ribosomes. It is the most abundant type of RNA and in some way affects the linking of amino acids into protein molecules.