Cell division

Dividing plant cells, like animal cells, are involved in a cell cycle. Cell division is similar in these organisms, al-though showing considerable differences as well. These are the consequences of the presence of the plant cell wall, the absence of the centrioles in higher plants, or are connected to other properties. The two principal forms of the cell division are the mitosis and meiosis, maintaining or halving chromosome number, respectively. In plants only the spores are generated by meiosis, while the cells of the sporophyte and the reduced gametophyte undergo mitosis and this holds true also for the formation of the gametes. Before both kinds of division the interphase involves G₁, S, and G₂phases. During the S phase (synthesis) the DNA is doubled, so in a diploid cell its quantity increasesfrom 2C to 4C before division.

Division may be equal or unequal. This does not concern the distribution of the DNA – it is always equal – rather the distribution of the cytoplasmic constituents is concerned. Unequal division yields in two cells of different composition and frequently of different size. These cells follow altered differentiation pathways, which may include in an extreme case the quick degeneration of one of them.

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It may occur that during division only the nuclei divide (karyokinesis), without the division of the cells (cytokin-esis). In this case dikaryotic, or further multikaryotic cells (syntitia) may develop. It is common that later such syntitia are fragmented into cells simultaneously. In angiosperms one type of the endosperm develops this way.

A special case of the division is the endomitosis, when chromosomes segregate within the nucleus. By this mech-anism haploid cells may develop into autodiploid homozygous cells.

3.5.1. Mitosis

It occurs in both diploid and haploid cells, partitioning the genetic material and the cytoplasm, and leading to the separation of the daughter cells. The process can be divided into phases.

MitosisP= prophase, M= metaphase, A= anaphase, T= telophase, C= cytokinesis

In theprophasethe doubled DNA strands are condensing. Before condensation, in interphase, they are 1-2 m in length. During transcription the DNA must be decondensed, to be accessible for the enzymes. In this state the DNA is less stainable, and forms the euchromatic region of the nucleus. The inactive, more condensed DNA is staining more intensively, and forms the heterochromatic region. During prophase the DNA has to be packed into chromo-somes, which makes possible their transport to the poles. The process can be monitored easily by light microscopy, due to the staining of the DNA. At the beginning of the phase the thin chromosomes seem to form a reel, then they are shortened and thickened all the more the DNA is wound up. Before prophase the microtubules form a ring in the equatorial plane of the cell (preprophase band), which disappears by the end of the phase. This is characteristic of the plant cells, but its function is not exactly known. Even if higher plants lack centrioles, they have cytocenters, which organize microtubules at the poles of the cell. The cytoplasmic organelles are distributed into the two halves of the cell.

Between prophase and metaphase (in theprometaphase) the nuclear envelope is fragmented, and the forming vesicles are dispersed in the cytoplasm. The mitotic spindle consisting of microtubules is formed from the two cytocenters; the elongating microtubules grow toward the chromosomes or toward the antipodal cytocenters. The former ones move chromosomes into the equatorial plane by joining to the kinetochore in the centromere region of the chromosome.

Duringmetaphasethe chromosomes are aligned by the kinetochore fibers such a way that each chromosome is connected to both cytocenters, one kinetochore being connected to one of the centers. The disjunction of chromo-somes is halted until these conditions are fulfilled. The chromochromo-somes can be investigated at the very most in this

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phase. Each chromosome consists of two sister chromatids, which are equal due to the doubling of the DNA in the S phase of the cell cycle.

In theanaphasethe sister chromatids segregate, and move toward the poles. This is the result of their interaction with the kinetochore fibers.

In the telophasethe nuclear envelope is formed again around the decondensing chromosomes at the poles, partly from the fragments of the former envelope, and partly from the rough endoplasmic reticulum. The mitotic spindle is disassembled, although a part of it remains in the mid-zone. These fibers together with the joined actin filaments will move the pectin containing vesicles from the Golgi apparatus to the equatorial plane. This cytoskeletal-vesi-cular complex is termed phragmoplast.

Duringcytokinesisthe vesicles coalesce, their pectinic content will form the cell plate, while their membrane will yield the new cell membranes of the daughter cells. At certain points the fusion is not complete; here develop the plasmodesmata connecting the cells.

3.5.2. Meiosis

During meiosis four haploid cells originate from one diploid cell. The process involves two consecutive divisions, the second one being mitosis. The essential events belong to the first stage. Compared this to the mitosis, the im-portant difference is that the paternal and maternal chromosomes are paired, and these will segregate, rather than the chromatids, during division. This way one daughter cell will receive either the paternal or the maternal variant of the chromosome. Before disjunction certain alleles are exchanged between the paired chromosomes by crossing over, which yields in recombination i.e. a limited mixing of the maternal and paternal properties. Apart from this the phases of the process are similar to those of mitosis, except prophase, which is longer and more complex (in-volving also pairing of chromosomes); therefore it is divided into further phases.

Meiosis I

The first stage ofprophase Iisleptotene, when the partially condensed chromosomes seem as thin threads with two chromatids. Paternal and maternal chromosomes are paired in thezygotenestage. Bivalents consisting of four chromatids are formed and are interconnected by special proteins, thesynaptonemal complex. Chromosomes are further shortened and thickened in thepachytenestage. In the synaptonemal complex nodules are visible, which are probably enzyme complexes active in recombination. In thediplotenestage the connection between the paired chromosomes is loosened, although they remain coupled at several sites. These points are the chiasmata, the morphological appearance ofcrossing over. In thediakinesisstage the chiasmata are sliding toward the terminal parts of the chromosomes.

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MeiosisP= prophase, M= metaphase, A= anaphase, T= telophase, I.= meiosis I., II.= meiosis II.

About the end of prophase I the nuclear envelope is disintegrated, which corresponds to theprometaphase. In metaphase Ithe bivalents are arranged into the equatorial plane, but the chromosomal fibers join either to paternal or maternal chromosomes, as these have one common kinetochore, rather than one per each chromatid. Consequently inanaphase Ithe parental chromosomes with two chromatids migrate toward the poles. Such a way the nuclei forming intelophasecontain half as many chromosomes as the nucleus of the mother cell had. The two cells do not always separate, nuclei may undergo the second division without cytokinesis, and the partitioning may happen at the end. In case of the tetrasporic embryo sac development, cellularization commences after further mitotic di-visions.

Meiosis II

During the second main stage of meiosis, the haploid daughter cells divide mitotically, i.e. the chromatids will segregate. Such a way four haploid cells will be formed, two-two of them being equal.

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