It is the sequence of nucleotides that encodes genetic information by specifying the amino acid sequence of proteins manufactured by the cell. DNA molecules are composed of two nucleotide chains oriented in opposite directions and held together by hydrogen bonds between adjacent bases. This double chain is coiled into a double helix, a form that imposes constraints on which bases can bond with each other. Thus, only two types of pairings are possible: adenine to thymine and guanine to cytosine. Either order of each pairing is possible, so, DNA can have four possible cross-rungs (adenine-thymine, thymine-adenine, guanine-cytosine, and cytosine-guanine).
There are three types of ribonucleic acid (RNA):
(1) messenger RNA (mRNA) carries instructions from the DNA in the nucleus to the sites of protein synthesis in the cytoplasm.
(2) ribosomal RNA (rRNA) is a structural component of these protein synthesis sites, called ribosomes.
(3) transfer RNA (tRNA) brings amino acids into the ribosomes so that they can be assembled into proteins according to the set of blueprints provided by the mRNA.
All three types of RNA differ from DNA in several ways: RNA has ribose as its sugar; RNA does not contain thymine but, instead, contains a related pyrimidine base, called uracil; and RNA is single-stranded.
