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Cell division in procaryotes and eucaryotes
In, unicellular procaryotes, cell division by binary fission leads to the creation of a new individual. Growth occurs in individual cells until a maximum size is achieved and a cross-wall forms. Before cell division takes place, the genetic material must replicate itself, and one copy pass to each new daughter cell (Figure 3.19).
Cell division in eucaryotes also results in two identical daughter cells. In the case of unicellular eucaryotes, this results in two individual organisms (asexual reproduction), while in multicellular forms there is an increase in overall size. Cell division is pre-ceded by a process of nuclear division called mitosis, which ensures that both daughter cells receive a full complement of chromosomes. The principal phases of mitosis are summarised in Figure 3.20(a). In interphase, the chromosomes are not clearly visible under the microscope; DNA replication takes place during this period. The duplicated chromosomes, held together as sister chromatids by the centromere, move towards the centre of the cell during prophase. A series of microtubules form a spindle between
the centrioles, and the chromosomes line up along this during metaphase. Also, during this phase the nuclear membrane breaks down, and each centromere duplicates. One chromosome from each pair then migrates away from the centre to opposite ends of the spindle. This stage is called anaphase. Finally, in telophase, new nuclear membranes surround the two sets of chromosomes, to form two nuclei. Mitosis is followed by cell division. Overall, the process of mitosis results in two identical nuclei containing the original (diploid) chromosome number.
At various stages of eucaryotic life cycles, a process of meiosis may occur, which halves the total number of chromosomes, so that each nucleus only contains one copy of each. In sexual reproduction, the haploid gametes are formed in this way, and the diploid condition is restored when two different gametes fuse. In some eucaryotes, not just the gametes but a substantial part of the life cycle may occur in the haploid form. Meiosis (Figure 3.20b) comprises two nuclear divisions, the second of which is very similar to the process of mitosis just described. In the first meiotic division, homologous chromosomes (i.e. the two members of a pair) line up on the spindle together and eventually migrate to opposite poles. While they are together, it
is possible for crossing over to occur, a process by which the two chromosomes swap homologous stretches of DNA (Figure 3.21). Since these may not be identical, crossing over serves to introduce genetic variation into the daughter nuclei. In the second meiotic division, sister chromatids separate as before, resulting in four haploid nuclei.
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