Chapter Summary

• 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, a chromosome. Prokaryotes reproduce by binary fission. Review Figure 11.2

• In eukaryotes, cells divide by either mitosis or meiosis. In contrast to prokaryotes, eukaryotic cells have a distinct nucleus whose chromosomes are replicated prior to separating into two daughter cells.

• The eukaryotic cell cycle has two main phases: interphase, during which cells are not dividing and the DNA is replicating, and mitosis or M phase, when the cells are dividing.

• During most of the eukaryotic cell cycle, the cell is in interphase, which is divided into three subphases: S, G1, and G2. DNA is replicated during S phase. Mitosis (M phase) and cytokinesis follow. Review Focus: Key Figure 11.3

• Cyclin–Cdk complexes regulate the passage of cells through checkpoints in the cell cycle. Retinoblastoma protein (RB) inhibits the cell cycle at the restriction (R) point. One cyclin–Cdk functions by inactivating RB and allows the cell cycle to progress. Review Figures 11.4, 11.5

• External controls such as growth factors can stimulate the cell to begin a division cycle.

• 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 11.8

• At mitosis, the replicated chromosomes (sister chromatids) are held together at the centromere. Each chromatid consists of one double-stranded DNA molecule. During mitosis sister chromatids, attached by cohesin, line up at the equatorial plate and attach to the spindle apparatus. The chromatids separate (becoming daughter chromosomes) and migrate to opposite ends of the cell. Review Figures 11.9, 11.10, Activities 11.1, 11.2

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

• Nuclear division is usually followed by cytokinesis. Animal cell cytoplasms divide via a contractile ring made up of actin microfilaments and myosin. In plant cells, cytokinesis is accomplished by vesicles that fuse to form a cell plate. Review Figure 11.12

• Asexual reproduction produces clones, new organisms that are genetically identical to the parent. Any genetic variation is the result of changes in genes.

• In sexual reproduction, two haploid gametes—one from each parent—unite in fertilization to form a genetically unique, diploid zygote. Sexual life cycles can be haplontic, diplontic, or involve alternation of generations. Review Figure 11.14, Activity 11.3

• In non-haplontic sexually producing organisms, certain cells in the adult undergo meiosis to produce gametes, each of which contains one copy of each of the homologous chromosome pairs of the organism.

• Meiosis consists of two nuclear divisions, meiosis I and meiosis II, that reduce the chromosome number from diploid to haploid. Meiosis ensures that each haploid cell contains one member of each chromosome pair, and results in four genetically diverse haploid cells, usually gametes. Review Figure 11.15, Activity 11.5

• In meiosis I, entire chromosomes, each with two chromatids, migrate to the poles. In meiosis II, the sister chromatids separate.

• During prophase I, 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 Figures 11.16, 11.17

• Both crossing over during prophase I and independent assortment of the homologs as they separate during anaphase I ensure that the gametes are genetically diverse.

• In nondisjunction, two members of a homologous pair of chromosomes go to the same pole during meiosis I, or two chromatids go to the same pole during meiosis II or mitosis. This leads to one gamete having an extra chromosome and another lacking that chromosome. Review Figure 11.19

• The union between a gamete with an abnormal chromosome number and a normal haploid gamete results in aneuploidy. Such genetic abnormalities can be harmful or lethal to the organism.

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

• Polyploids have more than two sets of haploid chromosomes. Sometimes these sets come from different species.

• A cell may die by necrosis, or it may self-destruct by apoptosis, a genetically programmed series of events that includes the fragmentation of the cell’s nuclear DNA.

• Apoptosis is regulated by external and internal signals. These signals result in activation of a class of enzymes called caspases that hydrolyze proteins in the cell. Review Figure 11.21

• Cancer cells divide more rapidly than normal cells and can be metastatic, spreading to distant organs in the body.

• Cancer can result from changes in either of two types of proteins that regulate the cell cycle. Oncogene proteins stimulate cell division and are activated in cancer. Tumor suppressor proteins normally inhibit the cell cycle, but in cancer they are inactive. Review Figure 11.23

• Multiple genetic events are necessary to form a malignant cancer cell. Review Figure 11.24

• Cancer treatment often targets the cell cycle in tumor cells. Review Figure 11.25