A linear segment of DEOXYRIBONUCLEIC ACID (DNA) containing a unique series of NUCLEOTIDES, which codes for the sequences of AMINO ACIDS in the PROTEINS required for CELL function. Genes are inherited by successive generations of cells and organisms, and each cell contains a full complement of an organism’s genes. The gene has both coding regions, which translate directly into amino acids, and non-coding regions, including sequences used for promoting and enhancing GENE TRANSCRIPTION. When a protein such as an ENZYME is required by the cell, the gene is transcribed in order to produce a messenger ribonucleic acid (MESSENGER RNA) copy, which will be used as a template for building the protein from individual amino acids.
Although each cell in a multicellular organism contains the same genes, these cells use selective sets of genes in order to produce their PHENOTYPE, and therefore control of GENE EXPRESSION is vital to appropriate cellular function. Which genes are expressed as proteins is determined by a series of gene regulatory proteins that bind to specific sequences in the DNA called regulatory regions. These influence the binding of transcription factors and enzymes to a region of the gene called the PROMOTER, and thus influence the initiation of transcription. In a similar manner, gene expression may be up-regulated or down-regulated, depending on the events which occur in the cell, and the consequent requirement for a gene product by the cell at a given time. Variations in sequences close to the gene promoter determine the affinity of transcription factors, and therefore the rate of transcription, providing another degree of control over the rate of gene expression. In every cell, some genes known as HOUSEKEEPING GENES are required to be activated at all times in order to make proteins essential for cell survival.
These have highly conserved sequences known as CONSENSUS SEQUENCES, close to the promoter, which greatly enhance the binding of transcription factors, thus ensuring gene expression. Gene expression may be further controlled at the level of mRNA, by a process called RNA EDITING, or ALTERNATIVE SPLICING. For example, during development, different portions of the gene for the NMDA RECEPTOR may be transcribed to form slightly different mRNA molecules, known as SPLICE VARIANTS, each of which produces an NMDA receptor with a different characteristic, such as permeability to CALCIUM ions. Such control of gene expression may underlie many alterations in gene regulation in the developing organism.
While most genes for a specific protein are found in all members of a species, and indeed, many important proteins show a great degree of homology across many species, small variations occur in the precise sequences between individuals. Any different variation of the same gene is called an ALLELE. Such variants are often brought about by random MUTATION involving substitution, loss or addition of one or more nucleotides during DNA replication or repair, and may affect the function of the gene to a greater or lesser extent. Different alleles of the gene for eye colour, for example, may be of little significance to the survival of the species. However, some mutations in structure of a protein or enzyme may alter its function, or prevent it altogether, and this may be highly detrimental to the function of the cell, resulting in disease and possibly death of the individual. Several diseases have been shown to possess a genetic background, including HUNTINGTON’S CHOREA, and familial Alzheimer’s disease. The ability to isolate genetic mutations has provided the potential for screening genetic diseases, leading to early detection, and possibly treatment. Strategies such as administration of viral vectors, which aim to replace defective genes, may become viable methods of treating genetic disease (see GENE THERAPY).
FIONA M.INGLIS
This is the complete article, containing 607 words
(approx. 2 pages at 300 words per page).
