Unequal cell division

A third justifreation for studying the eell eycle of this yeast is that it afiords a convenient system in which to study cell polarity. Together with asymmetric cell division (irrherent in the S. cerevisiae cell cyele with the unequal sized mothers and daughters), the development of polarity is emeial in many aspects of development and differentiation. Furthermore, as explained in more detail later in this chapter, the correct... 

The first cell division of meiosis occurs in the primary oocyte but the process is arrested during prophase and remains so until puberty. Just before ovulation, meiosis, which has been arrested since before birth, resumes. The first division halves the number of chromosomes to produce the haploid secondary oocyte. The process is the same as that in spermatozoa (Chapter 20 see Figure 20.29 ) except that the two resulting haploid daughter cells are unequal in size. One is the large functional secondary oocyte whereas the other is much smaller and is known as the first polar body. The second meiotic division is arrested at metaphase. It is completed only at fertilisation. Once again, the division is unequal. One cell is large, the secondary oocyte. The other is small, a second polar body, which is discarded. 

The unequal division of chromosomes during mitosis may sometimes occur (aneuploidization). This may be the result of exposure of the cell to agents which damage or disturb the spindle fibres or interfere in some way with the process of cell division. The result is a cell with more or less chromosomes than normal, which may or may not be viable. 

During embryogenesis, the earliest stage In animal development, asymmetric cell division often creates the Initial diversity that ultimately culminates In formation of specific differentiated cell types. Even in bacteria, cell division may yield unequal daughter cells, for example, one that remains attached to a stalk and one that develops flagella used for swimming. Essential to asymmetric cell division is polarization of the parental cell and then differential incorporation... 

A FIGURE 22-20 General features of asymmetric cell division. Various mechanisms can lead to asymmetric distribution of cytoplasmic components, such as particular proteins or mRNAs (red dots) to form a polarized parental cell. Division of a polarized cell will be asymmetric if the mitotic spindle is oriented so that the localized cytoplasmic components are distributed unequally to the two daughter cells, as shown here. However, if the spindle Is positioned differently relative to the localized cytoplasmic components, division of a polarized cell may produce equivalent daughter cells. 

Asymmetric cell division requires polarization of the dividing cell, which usually entails localization of some cytoplasmic components, and then the unequal distribution of these components to the daughter cells (see Figure 22-20). 

Pathway 11 An asymmetric initial division produces two cells of unequal size. The ball of cells may originate from the continued division of the larger vegetative cell while the smaller generative cell may divide a few times before degenerating. 

Amino Acids. — Alkaloids. — Porphyrins. — Cell Division. — Differential Gene Activity as Principle of Differentiation. — Regulation. — Polarity and Unequal Cell Division as Fundamentals of Differentiation. — Cell Elongation. 

Polarity And Unequal Cell Division As Fundamentals Of Differentiation... 

Fig. 187. Unequal cell division in the development of the stomatal apparatus and short cells in monocotyledons.

Let us now consider a few examples of unequal cell division, which illustrate the principle put forward by Running in particular. We shall see from the nature of the examples that the occurrence of unequal cell division is in no way limited to apical meristems. 

In some species every or almost every cell of the rhizodermis can develop into a root hair. In many cases, however, root hairs are formed only from quite special cells of the young rhizodermis, the trichoblasts. These trichoblasts arise by unequal cell division a cell of the young rhizoepidermis divides into a larger and a smaller, plasma-rich cell. The fate of the larger cell is varied. It can still divide several times but it is also... 

Fig. 188. Unequal cell division in root hair formation of Phleum (modified from Torrey 1968).

Fig. 189. Unequal cell division in the first pollen mitosis of lilies and further pollen development, g = generative, v = vegetative cell. The generative cell undergoes a second mitosis to give two sperm cells or nuclei (sp) which first occurs during outgrowth of the pollen tube (from Walter 1962).

Examples of unequal cell division can easily be multiplied. It must suffice simply to mention the formation of sieve tubes and accompanying cells in angiosperms, of elaters and spore mother cells in liverworts, and of hyalin and chlorophyll cells in peat mosses. After this demonstration of conspicuous cases of unequal cell division, let us return to the problem we raised at the beginning of the chapter, to cell divisions which are unequal in the sense that one of the two daughter cells can continue to divide whereas the other begins to differentiate. The first manifestation of differentiation is usually elongation of the cell in question. 

In normal gametogenesis, the diploid microspwrocytes or pwllen mother cells in the anther undergo meiotic division to yield haploid microspores. Each uni-nulceate microspore then divides mitotically to produce a cell with two haploid nuclei, which is the bi-nudeate state of pollen Cytokinesis that follows produces two cells that are of unequal size. The larger cell... 

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