Haploidy Diploidy and the Eukaryote Cell Cycle

Eukaryotes are normally regarded as diploid, having two copies of each gene carried on pairs of homologous chromosomes. While this is true of the majority of multicellu-lar animals and many single-celled eukaryotes, there are significant exceptions. Many plants are polyploid, especially angiosperms (flowering plants). About half of the present-day angiosperms are thought to be polyploid, especially tetraploid or hexa-ploid. For example, coffee (ancestral haploid number = 11) exists as variants with 22, 44, 66, or 88 chromosomes (i.e. 2n, 4n, 6n and 8n). Polyploid plants have larger cells and the plants themselves are often larger. In particular, polyploids have often been selected among domesticated crop plants, since they tend to give bigger plants with higher yields.

Polyploidy is unusual in animals, being found in occasional insects and reptiles. So far the only polyploid mammal known is a rat from Argentina that was discovered to be tetraploid in 1999. It actually has only 102 chromosomes, having lost several from the original tetraploid set of 4n = 112. The tetraploid rat has larger cells than its diploid relatives. The only haploid animal known is an arthropod, a mite, Brevipalpus phoeni-cis, which was discovered in 2001. Infection of these mites by an endosymbiotic bacterium causes feminization of the males. The genetic females of this species reproduce by parthenogenesis (i.e. development of unfertilized eggs into new individuals).

In most animals, only the gametes, the egg and sperm cells, are haploid. After mating two haploid gametes fuse to give a diploid zygote that develops into a new animal. However, in plants and fungi, haploid cells often grow and divide for several generations before producing the actual gametes. It seems likely that in the ancestral eukaryote a phase consisting of haploid cells alternated with a diploid phase. In yeasts, both haploid and diploid cells may be found and both types grow and divide in essentially the same manner (see above). In lower plants, such as mosses and liverworts, the haploid phase, or gametophyte, may even form a distinct multicellular plant body.

During animal development, there is an early division into germline and somatic cells. Only cells from the germline can form gametes and contribute to the next generation of animals. Somatic cells have no long-term future but grow and divide only as long as the individual animal continues to live. Hence, genetic defects arising in somatic cells cannot be passed on through the gametes to the next generation of animals. However, they may be passed on to other somatic cells. Such somatic inheritance is of great importance as it provides the mechanism for cancer. In plants and fungi there is no rigid division into germline and somatic cells. The cells of many higher plants are totipotent. In other words, a single cell from any part of the plant has the potential to develop into a complete new plant, which can develop reproductive tissues gametophyte Haploid phase of a plant, especially of lower plants such as mosses and liverworts, where it forms a distinct multicellular body germline cell Cell capable of forming gametes and so contributing to the next generation of animals somatic cell Cell making up the body, as opposed to the germline totipotent Capable of giving rise to a complete multicellular organism

Protein coat or capsid

FIGURE 2.28 Structural Components of a Virus

A virus is composed of a protein coat and nucleic acid. Note that there are no ribosomes or phospholipid membranes and only one type of nucleic acid is present.

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