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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 (see Chapter 11), 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 preceded 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

Bacterial chromosome

Bacterial chromosome

DNA replicates, making a second copy of the chromosome. Origins of replication migrate to ends of cell.

Cell lengthens and new cell wall is laid down Plasma membrane starts to grow inwards

Septum formation is complete and daughter cells separate

Figure 3.19 Binary fission in E. coli. Replication of the single circular chromosome is accompanied by an increase in cell size. The plasma membrane invaginates, and a new cross-wall is synthesised, resulting in two new daughter cells

Figure 3.19 Binary fission in E. coli. Replication of the single circular chromosome is accompanied by an increase in cell size. The plasma membrane invaginates, and a new cross-wall is synthesised, resulting in two new daughter cells 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 (see Chapters 8 & 9). 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

Spindle formation begins

Nuclear membrane is broken down

Chromosomes line up along spindle

Chromosomes line up along spindle

Prophase

Metaphase b) MEIOSIS

Chromosomes move towards opposite poles of cell

Spindle disappears, nuclear membrane reforms

Chromosomes move towards opposite poles of cell

Spindle disappears, nuclear membrane reforms

Telophase

Anaphase

Telophase

Cytokinesis: the two new diploid nuclei separate into two daughter cells

Prophase II

Cytokinesis: the two new diploid nuclei separate into two daughter cells

Prophase II

(Intermediate stages of meiosis II not shown)

Meiosis II, whose steps are similar to those of mitosis, results in four haploid nuclei. Note the recombinant chromosomes arising from crossing over

(Intermediate stages of meiosis II not shown)

Figure 3.20 The main steps of (a) mitosis and (b) meiosis in an organism whose diploid number (2n) =4. Mitosis results in two cells identical to the parent. Meiosis results in a reduction in the chromosome number and introduces genetic variation by means of crossing over. For details see the text

Homologous chromosomes

Homologous chromosomes

Recombinant chromosomes

Figure 3.21 Crossing over leads to recombination of genetic material. During crossing over, portions of homologous chromosomes are exchanged. This forms the basis of genetic recombination in eucaryotes, and ensures that offspring contain new combinations of genetic material 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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