Summary

Concept 7.1: Different Life Cycles Use Different Modes of Cell Reproduction

• Cell division is necessary for the reproduction, growth, and repair of organisms. Review Figure 7.1
• Asexual reproduction produces clones, new organisms that are virtually identical genetically to the parent. Any genetic variation is the result of mutations.
• In sexual reproduction, two haploid gametes—usually one from each parent—unite in fertilization to form a genetically unique diploid zygote. There are many different sexual life cycles that can be haplontic, diplontic, or involve alternation of generations. Review Figure 7.3 and ACTIVITY 7.1
• Diploid cells contain homologous pairs of chromosomes. In sexually reproducing organisms, certain cells undergo meiosis, a process of cell division in which the chromosome number is halved. Each of the haploid daughter cells contains one member of each homologous pair of chromosomes.

Concept 7.2: Both Binary Fission and Mitosis Produce Genetically Identical Cells

• Cell division must be initiated by a reproductive signal. Before a cell can divide, the genetic material (DNA) must undergo replication and segregation 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 7.4
• During most of the eukaryotic cell cycle, the cell is in interphase, which is divided into three subphases: G1, S, and G2. DNA is replicated during S phase. Mitosis (M phase) and cytokinesis follow. Review Figure 7.5 and ANIMATED TUTORIAL 7.1
• In mitosis, a single nucleus gives rise to two nuclei that are genetically identical to each other and to the parent nucleus.
• At mitosis, the replicated chromosomes, called sister chromatids, are held together at the centromere. Each chromatid contains one double-stranded DNA molecule. During mitosis, sister chromatids line up at the equatorial plate and attach to the spindle. Review ACTIVITY 7.2
• Mitosis can be divided into several phases called prophase, prometaphase, metaphase, anaphase, and telophase. Review Figure 7.6 and ACTIVITY 7.3
• Nuclear division is usually followed by cytokinesis. Animal cell cytoplasms divide via 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 7.7

Concept 7.3: Cell Reproduction Is Under Precise Control

• Interactions between cyclins and CDKs regulate the passage of cells through checkpoints in the cell cycle. External controls such as growth factors can stimulate the cell to begin a division cycle. Review Figure 7.10

Concept 7.4: Meiosis Halves the Nuclear Chromosome Content and Generates Diversity

• Meiosis consists of two nuclear divisions, meiosis I and meiosis II, which collectively reduce the chromosome number from diploid to haploid. Meiosis results in four genetically diverse haploid cells, often gametes. Review ANIMATED TUTORIAL 7.2
• In meiosis I, entire chromosomes, each with two chromatids, migrate to the poles. In meiosis II, the sister chromatids separate. Review Figures 7.11 and 7.12 and ACTIVITY 7.4
• 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 Figure 7.13
• Both crossing over during prophase I and independent assortment of the homologs as they separate during anaphase I ensure that gametes are genetically diverse.
• Meiotic errors can result in abnormal numbers of chromosomes in the resulting gametes and offspring. Review Figure 7.14

Concept 7.5: Programmed Cell Death Is a Necessary Process in Living Organisms

• 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 both external and internal signals. These signals result in activation of a class of enzymes called caspases that hydrolyze proteins in the cell. Review Figure 7.15