9.1 How do prokaryotic and eukaryotic cells divide?

Cell division is necessary for the reproduction, growth, and repair of organisms.

Cell division must be initiated by a reproductive signal. Before a cell can divide, the genetic material (DNA) must be replicated and segregated to separate portions of the cell. Cytokinesis then divides the cytoplasm into two cells.

In prokaryotes, most cellular DNA is a single molecule, usually in the form of a circular chromosome. Prokaryotes reproduce by binary fission. Review Figure 9.2

In eukaryotes, cells divide by either mitosis or meiosis. Eukaryotic cell division follows the same general pattern as binary fission, but with significant differences in detail. For example, eukaryotic cells have a distinct nucleus (not just a chromosome) that must be replicated. Replicated chromosomes, called sister chromatids, must be separated by mitosis.

Cells that produce gametes undergo a special kind of nuclear division called meiosis; the two nuclei produced by meiosis are not genetically identical.

9.2 How is eukaryotic cell division controlled?

The eukaryotic cell cycle has two main phases: interphase (during which cells are not dividing) and mitosis or M phase (when cells are dividing).

During most of the cell cycle, the cell is in interphase, which is divided into three subphases: S, G1, and G2. DNA is replicated during the S phase. Mitosis and cytokinesis take place during the M phase. Review Figure 9.3

Cyclin/Cdk complexes regulate the passage of cells through checkpoints in the cell cycle. The suppressor protein RB inhibits the cell cycle. Cdk4 and Cdk2 act in concert to inactivate RB and allow the cell cycle to progress beyond the restriction point. Review Figure 9.6

Controls external to the cell, such as growth factors and hormones, can also stimulate the cell to begin a division cycle.

9.3 What happens during mitosis?

See Web/CD Tutorial 9.1

In mitosis, a single nucleus gives rise to two nuclei that are genetically identical to each other and to the parent nucleus.

DNA is wrapped around proteins called histones, forming beadlike units called nucleosomes. A eukaryotic chromosome contains strings of nucleosomes bound to proteins in a complex called chromatin. Review Figure 9.8

At mitosis, the replicated chromatids are held together at the centromere. Each chromatid consists of one double-stranded DNA molecule. Review Figure 9.9, Web/CD Activity 9.1

Mitosis can be divided into several phases, called prophase, prometaphase, metaphase, anaphase, and telophase.

During mitosis, sister chromatids, attached by cohesin protein, line up at the equatorial plate, and attach to the spindle. The chromatids separate (becoming daughter chromosomes) and migrate to opposite ends of the cell. Review Figure 9.10, Web/CD Activity 9.2

Nuclear division is usually followed by cytokinesis. Animal cell cytoplasm usually divides by a contractile ring made up of actin microfilaments. In plant cells, cytokinesis is accomplished by vesicles that fuse to form a cell plate. Review Figure 9.12

9.4 What is the role of cell division in sexual life cycles?

Asexual reproduction produces a clone, a new organism that is genetically identical to the parent. Any genetic variation is the result of mutations.

In sexual reproduction, two haploid gametes—one from each parent—unite in fertilization to form a genetically unique, diploid zygote. There are several patterns of sexual life cycles: haplontic life cycles, alternation of generations, and diplontic life cycles. Review Figure 9.14, Web/CD Activity 9.3

In sexually reproducing organisms, certain cells in the adult undergo meiosis, a process by which a diploid cell produces haploid gametes. Each gamete contains a random selection of one of each pair of homologous chromosomes from the parent.

The numbers, shapes, and sizes of the chromosomes constitute the karyotype of an organism.

9.5 What happens when a cell undergoes meiosis?

See Web/CD Tutorial 9.2

Meiosis consists of two nuclear divisions, meiosis I and meiosis II, that collectively reduce the chromosome number from diploid to haploid. It ensures that each haploid cell contains one member of each chromosome pair, and results in four genetically diverse haploid cells, usually gametes. Review Figure 9.16, Web/CD Activity 9.4

In anaphase I, entire chromosomes, each with two chromatids, migrate to the poles. By the end of meiosis I, there are two nuclei, each with the haploid number of chromosomes.

In meiosis II, the sister chromatids separate. No DNA replication precedes this division, which in other aspects is similar to mitosis.

During prophase I of the first meiotic division, homologous chromosomes undergo synapsis to form pairs in a tetrad. Chromatids can form junctions called chiasmata and genetic material may be exchanged between the two homologs by crossing over. Review Figure 9.18

Both crossing over during prophase I and the independent assortment of the homologs that migrate to each pole during anaphase I ensure that the genetic composition of each haploid gamete is different from that of the parent cell and from that of the other gametes.

In nondisjunction, one member of a homologous pair of chromosomes fails to separate from the other, and both go to the same pole. Pairs of homologous chromosomes may also fail to stick together when they should. Both problems can lead to one gamete having an extra chromosome and another lacking that chromosome. Review Figure 9.20

The union of a gamete with an abnormal chromosome number with
a normal haploid gamete at fertilization results in aneuploidy. Such genetic abnormalities are invariably harmful or lethal to the organism.

9.6 How do cells die?

Cells may die by necrosis, or they may self-destruct by apoptosis, a genetically programmed series of events that includes the detachment of the cell from its neighbors and the fragmentation of its nuclear DNA. Review Table 9.2

Apoptosis is regulated by external and internal signals. These signals result in activation of a class of enzymes called caspases that hydrolyze proteins of the cell, leading to its destruction. Review Figure 9.21