Volume 2

Gene expression / Benjamin Lewin.

  • Lewin, Benjamin.
Date:
1974-1977
Catalogue details

Licence: Attribution-NonCommercial 4.0 International (CC BY-NC 4.0)

Credit: Gene expression / Benjamin Lewin. Source: Wellcome Collection.

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    plement (n). Union of the gametes reestablishes the diploid complement, so that each zygote obtains one copy of each chromosome from each parent. The diploid complement thus comprises one haploid set of paternal origin and one of maternal origin. Meiosis is responsible for the segregation of paternal and maternal alleles into different germ cells and for the indepen dent inheritance of genes located on different chromosomes predicted by Mendel's laws. Genes located on the same chromosome can be separated only by a recom bination event in which there is physical exchange of material between homologues. The frequency of recombination is taken to represent the dis tance apart of linked genes. However, the size of the eucaryotic chromosome means that genetic exchange occurs often enough between genes located some distance apart to render their inheritance effectively independent. But by measuring their linkage to intervening markers, it is possible to show that both reside on one chromosome. This allows each gene to be placed on a linkage group. In cases in which it has been possible to map enough genes, the genetic map corresponds well with the visible features of the chromo somes: the number and the relative sizes of the linkage groups are the same as those of the chromosomes of the haploid set. The eucaryotic chromosome therefore comprises a discrete genetic entity, a unique set of genes organized into a characteristic structure and in addition to the genetic material itself, each chromosome possesses the features neces sary to fulfill its role as a unit of inheritance in cell division. Cytological studies of chromosomes have been confined largely to observa tions of their behavior during cell division. Although interphase, to use the cytological description of the period between mitoses, is a misleading term in the sense that most of the synthetic activities of the cell take place during this period, an interphase cell appears quiescent compared with the spec tacular changes in cell morphology seen at mitosis. An interphase nucleus is bounded by the nuclear membrane and the twisted filaments of interphase chromatin are distributed throughout the nucleus. Individual chromosomes cannot be distinguished, although chromocenters may be seen where the chromatin appears more condensed. Nucleoli often are visible and tend to be associated with the chromocenter(s). The start of mitosis is defined when individual chromosomes can first be distinguished as they appear as extended threads. At this point, each chromosome appears to consist of two coiled threads, the chromatids. Prior to the start of visible division, the chromosomes have been duplicated and the chromatids represent the two daughter copies of each chromosome. Since each homologue consists of two chromatids, the cell has four copies of the haploid set. As mitosis continues, the nuclear membrane dissolves and the former cell compartments are replaced by the spindle. The chromosomes become aligned on the equator in a highly compact form; then the sister chromatids of each homologue separate and move to opposite poles of the cell to form two dip-