(ribonucleic acid) A class of molecules used in translation of genetic information contained in DEOXYRIBONUCLEIC ACID (DNA) into PROTEINS. RNA differs from DNA as follows: RNA is normally single-stranded, it contains a ribose sugar backbone, and the base thymidine is substituted by uracil.
In EUKARYOTE organisms, RNA synthesis, or DNATRANSCRIPTION, occurs in the NUCLEUS of the CELL, and is initiated by the binding of the enzyme RNA polymerase to a region of DNA called the promoter, upstream from the GENE. During transcription the DNA helix is unwound and RNA polymerase catalyses the addition of complementary ribonucleotides in a FIVE-PRIME-THREE-PRIME (5′-3′) manner. As the RNA molecule grows the DNA helix reforms, causing release of the RNA. Several types of RNA are formed by transcription: MESSENGER RNA (mRNA), TRANSFER RNA (tRNA) and RIBOSOMAL RNA (rRNA). mRNA is a copy of a gene which will be translated into protein. Its synthesis is initiated by RNA polymerase II, which transcribes DNA to form a primary transcript of heterogeneous nuclear RNA. This molecule contains regions called EXONS (see EXON/INTRON) which code directiy for AMINO ACIDS used in proteins and non-coding regions called INTRONS (see EXON/INTRON), which are removed to form mRNA. The 5′ end of mRNA is capped with methy-lated guanine and a poly-adenine tail is added to the 3′ end. These additions function to protect mRNA from breakdown and facilitate its transport to the cytoplasm and recognition by the RIBOSOMES.
The ribosome is a small structure in the cell cytoplasm which contains proteins and ribosomal RNA (rRNA). rRNA comprises two subunits, transcribed by RNA polymerase I and III. Ribosomes bind mRNA and catalyse translation of its sequence into protein. Each amino acid in a protein is encoded by a specific series of three adjacent NUCLEOTIDES, called a CODON, within mRNA. Translation of each codon into protein involves transfer RNA (tRNA), synthesized by RNA polymerase III. tRNA has a characteristic ‘clover-leaf’ structure, containing four loops and a stem, caused by the molecule folding and forming covalent bonds between apposing nucleotides. The 3′ end of tRNA, located in the stem, binds a specific amino acid, while the loop opposite contains the ANTICODON, a complementary sequence to the codon for the amino acid. The anticodon of tRNA binds to the mRNA codon and the amino acid is released from tRNA and attached to a growing polypeptide chain. When a STOP CODON is reached, protein synthesis is terminated and the finished protein released.
Of these molecules, mRNA is short-lived, and its production controlled. The activity of RNA polymerase II is regulated by a large number of proteins called TRANSCRIPTION FACTORS, which recognize promoter and enhancer sites within DNA. Through these mechanisms regulation of mRNA synthesis permits control of gene expression within the cell.
FIONA M.INGLIS
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