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gene
Значение термина gene в knolik
gene - gene (hereditary factor)
gene - Unit of the material of inheritance. The material of inheritance. Much of the similarity between organisms related by descent is due to their possessing similar inherited material; i.e., a complex and diverse material that gives rise to accurate copies of itself; units of which are passed from parent to offspring; which is very stable in its properties; and which profoundly influences every aspect of the organism containing it. Each individual starts life with a set of this material received from its parent or parents. During subsequent growth (in a multicellular organism) or reproduction (when the organism is not multicellular) the material is duplicated repeatedly, with great exactness of copying (the process of replication); it influences the characteristics developed by the individual bearing it, so that similarity between related organisms results; and a set is handed on to each of the individual's progeny. One of the great biological discoveries of this century is that the material of inheritance is nucleic acid: a chain molecule, the links of which consist of four different nucleotides, and the complexity of which lies in the arrangement of the sequence of these nucleotides. In the majority of cases it is deoxyribonucleic acid (DNA) but in some viruses ribonucleic acid (RNA). In plants and animals the DNA is situated in the threadlike chromosomes of the nucleus (though it occurs also in some cytoplasmic structures) which contain much protein besides; in bacteria and viruses the chromosomes are simpler, consisting only of a long thread of DNA.
The gene as a unit. The DNA occasionally undergoes a localized change in its sequence of nucleotides, known as a mutation, the changed, mutant, form then being replicated and inherited instead of the original form of the DNA. Such mutant DNA then produces a difference in certain characters of the organisms that inherit it. The results of many different mutations can be found in any large population of individuals of a species. Some of these mutations will differ from each other in that each affects a different set of characters of the individual; and when, as is possible by various methods, the approximate positions (in the set of chromosomes) of the changes in DNA that constitute these different mutations is discovered, each is found to have its own characteristic place. Of the mutations found scattered amongst the individuals of a population there will however also be some that form pairs or larger groups, in that the members of a given pair or group affect the same set of characters, but differ from each other by affecting it in different ways; and within any one of these groups the mutations turn out to have altered the DNA in nearly or quite the same relative place in the chromosomes, the short length of chromosome within which they occur being a gene.
A gene is then a short length of a chromosome, influencing a particular set of characters in a particular way, the set of characters and the length of chromosome constituting a unit because the set of characters is affected as a whole by any mutation occurring anywhere within this length of DNA. (A gene thus defined as part of a chromosome which functions as a unit is often called a cistron.) Gene function. How can there be such units of function within the DNA? Because each gene belonging to the large group of structural genes is responsible, during protein synthesis, for the exact sequence of amino-acids making up one specific kind of polypeptide chain (a protein molecule consists of one or more polypeptide chains). The set of characters influenced by one such gene is that set which is dependent, directly or indirectly, on the presence of its particular protein (most proteins, being enzymes, are likely to have widespread effects). The genes of another large group, the regulative genes, produce their effects by influencing the activities of these structural genes.
The DNA of an organism is then made up of a number of different genes. Each gene during growth is repeatedly exactly replicated. When a gene mutates, its changed structure is thenceforward replicated, and if it is a structural gene this leads to the production of a protein with a different sequence of amino-acids. The mutant gene and the original gene are said to be alleles of (or allelomorphic to) each other. All the different forms that a given gene can be changed to as a result of mutation constitute a series of alleles. Genes and chromosomes. Every cell of each individual of any plant or animal species usually has an identical set of a number of different chromosomes (bearing different genes). Every cell of an individual usually therefore has an identical set of genes (perhaps 10,000 in Drosophila). Individuals of the same species differ from each other in having different members of some of the series of alleles. Each chromosome has its genes arranged in single file ('linear order') along its length. Every specimen of a particular chromosome has its genes arranged in the same characteristic order along its length. The fixed position in the chromosome (relative to other genes) always occupied by one of a series of alleles is a locus. Only one member of a series of alleles can at any one time be present in one locus. Some genes, particularly those concerned with forming the RNA of the ribosomes are, however, repeated (reiterated] many times in identical or similar form within one set of chromosomes. Before nuclear division by mitosis the genes in each chromosome are replicated, and the whole chromosome becomes double. Each of the two cells resulting from mitosis receives one of the replicas and has therefore exactly the same complement of genes and chromosomes.
In the body-cells of animals and higher plants chromosomes occur in pairs, the members of which have identical appearance (except sex-chromosomes). The members of such a pair have the same arrangement of loci; and the genes in the two corresponding loci of a pair of chromosomes may either be identical, or two different members of an alellomorphic series. If identical, the individual is said to be homozygous for that locus; if alleles, heterozygous. Genes and the variety of individuals. While every body-cell of a multi-cellular organism, with rare exceptions, has an identical set of genes, this is not true of the gametes produced by an individual, which are of enormous variety. A gamete, as the result of meiosis, has only one member of each pair of chromosomes. But the two members of any pair of chromosomes usually differ from each other, because some of the loci are heterozygous, so that gametes receiving different members of a pair of chromosomes will be different. Since it is a matter of chance which member of each pair of chromosomes a gamete receives, a gamete may with equal probability have any of the possible combinations of one member of each pair of chromosomes, i.e. any of 2 combinations where x is the number of chromosome pairs. This chance distribution is known as independent assortment. The number of different gametes produced by an individual is actually much further increased, by the phenomenon of crossing-over, the resulting chromosomes being different, in the particular set of alleles they contain, from any possessed by the parent organism. A further, but much rarer, source of diversity in gametes is through mutation during their formation.
Knowing the laws governing the distribution of the chromosomes to the gametes, and assuming that it is a matter of chance which male gamete fertilizes which female gamete, we can calculate the probable distribution of one or more alleles amongst offspring of parents of given genetic constitution. The relative proportions of different kinds of offspring resulting are the Mendelian ratios, whose discovery in actual breeding experiments led Mendel to formulate the basic laws of inheritance.
The result of the variety of gametes produced by an individual is variety of offspring produced by any pair of parents. The chance of two such offspring being genetically alike is very remote, and (barring polyembryony) probably no two individuals of a sexually reproducing population are genetically alike. This means that a great range of variation is always available for evolutionary change by natural selection. This variety depends on the genes behaving as independent units, which segregate in gamete formation. If the genetic material received by an organism from its parents really blended together, then all the gametes produced by that organism would be alike, constituted of the same blend. All the variety within a population does not of course depend upon this diversity of chromosomal genes. Some depends on environmental differences during development of individuals. A small amount may depend on genetic material carried in cytoplasmic structures and not distributed by the segregating mechanism of the chromosomes. Differences between species are probably fundamentally of the same kind as differences within species. For operator gene, regulator gene, structural gene, See also: Operon.
Рядом со словом gene в knolik
| gemmule | В начало буква "" буквосочетание "" | genecology |
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