Guidelines for the testing of chemicals for mutagenicity / Committee on Mutagenicity of Chemicals in Food, Consumer Products, and the Environment.

  • Great Britain. Committee on Mutagenicity of Chemicals in Food, Consumer Products, and the Environment.
Date:
1981
Catalogue details

Licence: Open Government Licence

Credit: Guidelines for the testing of chemicals for mutagenicity / Committee on Mutagenicity of Chemicals in Food, Consumer Products, and the Environment. Source: Wellcome Collection.

    19/108 (page 11)
    gamete possesses a haploid set of chromosomes, and the special process of production of haploid gametes from diploid germ cells is called meiosis. The ultimate fusion of the female haploid gamete with the male haploid gamete produces the zygote, which possesses a diploid set of chromosomes and which by subsequent mitotic cell divisions produces the individual of the next generation. 3.16 During transcription into mRNA the nuclear chromosomal material in a eukaryotic cell is dispersed in its filamentous form throughout the nucleus and is not visible by conventional optical microscopical techniques. Prior to mitosis, the production of mRNA ceases and the process of DNA replication takes place. In the early stages of mitosis, ie prophase, the chromatin complex can be seen via optical microscopy to condense and form long and thin thread-like structures. These contract further into discrete elements which, because of their ease of staining and particulate nature, are called chromosomes (‘“‘stained bod- ies’). Each chromosome at this stage is in fact a pair of identical double helices of DNA compacted and condensed to form a pair of distinct chromatids linked together at a point (primary constriction) where a small body known as the centromere (spindle attachment) is organized. During the next stage, metaphase, the nuclear membrane disappears and the chromosomes move to the equatorial part of the cell, and a spindle apparatus develops between the centromeres and the polar regions of the cell. Colchicine inhibits the formation of the spindle and thus prevents the organization of the chromosomes in the equatorial region between the centromeres. They now become widely dispersed in the cell and thus individually more recognizable. It was by the use of techniques such as this that the characteristic number and shape of the chromosomal pattern for the cells of different animals was established. Man has 46 chromosomes (2n = 46), namely 22 pairs of homologous chromosomes, called autosomes, plus 2 sex chromosomes, X and Y in the male or XX in the female. The Chinese hamster, for example, has only 22 (2n = 22) and the cow 60 (2n = 60). At the next stage, anaphase, the centromere of each chromatid pair splits and the individual chromatids separate and move to opposite poles of the cell as new chromosomes. In the final phase, a partition is developed in the equatorial region which divides the cell into 2 halves, the chromosomes become diffuse, and 2 new nuclei are developed. 3.17 The process of meiosis differs from that of mitosis. At its beginning homologous chromosomes pair with each other and having exchanged genetic material (see para 3.21), separate from each other in the first meiotic division. Thus, in contrast to mitosis, the chromosome number of the daughter cells is half that of the mother cell and each chromosome still consists of 2 chromatids. Because of the exchange of genetic material in the first meiotic division, the 2 chromatids of a chromosome are usually no longer identical. The second meiotic divison resembles a mitotic one in that it splits the centromeres, separates the chromatids and does not change the number of chromosomes. Thus, in meiosis each diploid germ cell produces 4 haploid cells. In the male, these 4 cells will mature into 4 spermatozoa; in the female they will yield one ovum and the polar bodies.