RNA

Ribonucleic acid, a nucleic acid structurally distinguished from DNA by the presence of an additional oxygen atom attached to each pentose ring, and functionally distinguished by its multiple roles in the intracellular transmission of genetic information from the site of transcription (from DNA) to the site of translation (into protein).

RNA has four different bases; adenine?, guanine, cytosine, and uracil?. The first three bases are the same as those found in DNA, but uracil replaces thymine? as the complementary base to adenine. There are three main varieties of RNA found in all living cells:

• Messenger RNA, abbreviated mRNA, is transcribed directly from a genetic DNA substrate and is used to encode proteins. mRNA synthesized by eukaryote cells often undergoes extensive post-transcriptional modification before it exits the cell nucleus, including the excision of non-coding regions called exons (the regions of mRNA that remain are called introns). This allows for more complex regulation of gene expression, including alternate splicing patterns that can result in multiple different proteins being produced by a single DNA coding region. Most RNA splicing is performed by enzymes, but some RNA molecules are also capable of catalyzing their own splicing (see ribozymes?). Prokaryotes do not extensively modify their mRNA, and there are only a handful of known prokaryotic genes with exons.
• Transfer RNA, abbreviated tRNA, are short RNA strands that are used to transport individual amino acids to ribosomes and match them up with the three-base codons that describe the sequence of the protein encoded by an mRNA molecule. There is a variety of tRNA for each codon in the genetic code, and each variety of tRNA binds to a single specific amino acid.
• Ribosomal RNA, abbreviated rRNA, is the primary constituent of ribosomes. Ribosomes are the protein-manufacturing organelles of living cells, and exist in the cell's cytoplasm. rRNA is transcribed from DNA like all RNA, and in eukaryotes it is processed in the nucleolus before being transported through the [nuclear membrane]?.

It is thought that the first life on Earth may have been RNA-based, due to RNA's ability to both carry genetic information like DNA and also to catalyze useful biochemical reactions like enzymes. This possiblity is termed the RNA world hypothesis. Even today some viruses, such as retroviruses, use RNA as their sole genetic material. RNA is more unstable than DNA is, however, and is also a less efficient catalyst than an enzyme, and so has fallen out of favour among complex organisms as the preferred genetic material.

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mRNA runs through several steps during its usually brief existence: During

1. transcription, an enzyme called [RNA polymerase]? makes a copy of
2. a gene from the DNA to mRNA on demand. In eukaryotes, this copy (called primary transcript or precursor mRNA) is
3. spliced to remove introns (certain genetic sequences unwanted in protein biosynthesis). In prokaryotes, mRNA is generally not spliced. Also only in eukaryotes, the mRNA is then
4. exported from the nucleus through special structures in the nuclear membrane known as nuclear pores.
5. Ribosomes read the mRNA and produce the according polypeptide (or protein) in a process called translation. Finally,
6. the mRNA is disassembled into its nucleotides by RNAses.

Double-stranded mRNA

Double-stranded mRNA, abbreviated dsRNA, inhibits translation of genes in many eukaryotes for the matching single-stranded mRNA only, if the dsRNA has the same sequence as the mRNA of the gene. That means a gene will never be expressed as protein if a matching double-stranded mRNA is present in the cell. This is speculated to be a defense mechanism of the cell against retroposons (transposons that use dsRNA as an intermediate state) or viruses, since both of them can use double-stranded mRNA as an intermediate. In biochemical laboratories, this effect has been used to study gene functions, with simply shutting down the studied gene by adding its double-stranded mRNA transcript. Such studies have been done in the worm [C. elegans]?.

See also : genetics