Messenger, ribosomal, and transfer RNA are the main kinds of RNA in living cells. The cytoplasm contains a further class of RNA, called small cytoplasmic RNA (scRNA), which mainly exists in the form of RNA-protein complexes. Within a special region of the nucleus, called the nucleolus, another class of RNA is found. It is called small nucleolar RNA (SnoRNA) and functions in the manufacture of ribosomal RNA.
The stretch of DNA that codes for one protein is called a gene. In transcription, an mRNA molecule is made from a gene. The bases in the RNA correspond to the bases in the DNA, according to the standard base-pair rules found in DNA.The synthesis of the DNA is catalysed by the enzyme RNA polymerase. What happens next depends on whether the life form concerned is a bacterium or a eukaryote. Most large and familiar forms of life are eukaryotes: human beings, as well as insects, plants, and seaweeds, are eukaryotes. Technically, eukaryotes are creatures whose cells contain a distinct nucleus. Prokaryotic cells, such as bacteria, lack a nucleus.
Ribosomes have a similar general structure in all life forms, but they vary in size between bacteria and eukaryotes. All ribosomes are made up of two subunits, one small and one large. The two units are made up of ribosomal RNA and proteins. Ribosomal RNA has two functions in protein synthesis. It chemically bonds to the mRNA and to the tRNA, physically placing them in the appropriate orientations. Recently, biologists have found that rRNA also catalyses some of the reactions in protein synthesis. Most of the enzymes at work are the proteins that are found in the ribosome, but rRNA itself also acts as an enzyme.
The mRNA message is translated into protein according to the genetic code. The genetic code is the relation between the 64 base triplets, or codons, and the amino acid that each triplet codes for. The transfer RNA...