Ribonucleic acid. Linear, generally single-stranded polymer, composed of ribose nucleotides, that is synthesized by transcription of DNA or by copying of RNA. The three types of cellular RNA – mRNA, rRNA, and tRNA – play different roles in protein synthesis.
In eukaryotes, the structural unit of the genetic material consisting of a single, linear double-stranded DNA molecule and associated proteins wrapped into a high order structure. During mitosis, chromosomes condense into compact structures visible in the light microscope. In prokaryotes, a single circular double-stranded DNA molecule constitutes the bulk of the genetic material.