Nucleotides are ring-shaped molecules composed of nitrogen, carbon, oxygen, and hydrogen. There are two kinds: Purines such as adenine and guanine that are composed of two rings, and pyrimidines such as thymidine and cytosine (uracil substitutes for thymidine in RNA). The chemistry of these molecules is such that cytosine and guanine form base pairs via hydrogen bonding and adenine forms base pairs with either thymidine or uracil. To make a nucleic acid, a nucleotide is affixed to a ringed sugar molecule (ribose or deoxyribose). Nucleic acids can be "strung" together like beads by means of phosphate molecules that link the sugar moieties to each other.
Deoxyribonucleic acid (DNA) is double-stranded. The two strands are complimentary to each other in sequence but reversed in polarity. Only one of the strands is the reading (information-bearing) strand. Ribonucleic acid (RNA), on the other hand, is single-stranded, slightly different in chemical composition from DNA, and it exists in three forms in the cell. Messenger RNA is the transcription product that forms when the DNA sequence is "read." Ribosomal RNA is found in the subcellular structure (organelle) called the ribosome, a two-part body that acts as the template upon which the message encoded in mRNA is "translated" into protein. Transfer RNA is a folded molecule that is attached at one pole to an amino acid and contains an "anticodon" that complimentarily matches the tripartate codon that specifies that particular amino acid.
Trust me, we're going somewhere with all this. There might be a quiz...
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