RNA

RNA (RiboNucleic Acid), like DNA, is a form of nucleic acid found in cells. RNA, like DNA, is a polymer composed of nucleoside monophosphates. The nucleoside monophosphates in RNA are called ribonucleoside monophosphates, however, because they contain the sugar ribose instead of 2-deoxyribose, as is found in DNA.

The bases in RNA are adenine (A), guanine (G), cytosine (C), and uracil (U). These are the same bases as DNA except that the base uracil is used in place of thymine (T). Unlike DNA, RNA is rarely composed of two strands base paired with each other. Instead, RNA exists as a single-stranded entity, though extensive regions of many RNAs may form double helices within themselves by the base pairing rules.

The three predominant forms of RNA are all involved in translating the genetic information in the sequence of bases in DNA to a sequence of amino acids in proteins. They are called messenger RNA (mRNA), transfer RNA (tRNA), and ribosomal RNA (rRNA).

> mRNA is made directly from DNA, so mRNA carries the genetic information in the DNA sequence from the cell nucleus to the ribosomes where proteins are made. Information is organized in DNA (and mRNA) in a sequence of three nucleotides called a codon. One codon specifies the incorporation of a specific amino acid into a protein.

> tRNAs translate the genetic code. One end of the tRNA contains a three nucleotide sequence called the anticodon loop that is complementary to the codon of the mRNA. The other end of the tRNA is covalently attached to a specific amino acid. Because the amino acid carried by a tRNA is specific for each anticodon and each anticodon is complementary to the codons in mRNA, the tRNA provide the link between nucleic acid sequence and amino acid sequence for a protein during translation. This process, which occurs on ribosomes, sequentially incorporates amino acids corresponding to the order of codons in the mRNA. tRNAs contain numerous chemical modifications to the bases within them. Examples include pseudouridine, ribothymidine, and dihydrouridine (See Figure 27.7)

> rRNA is a component of the ribosomes where translation (protein synthesis) is occurring.

Another type of RNA in eukaryotic cells, called snRNA (for small nuclear RNA) helps process some RNAs after they are made (see here). Many eukaryotic RNAs have portions of them removed by a process called splicing. It is this process in which the snRNAs participate.

See also: Transcription, Ribozymes

INTERNET LINKS:

> 1. The RNA World

> 2. RNA Secondary Structures