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How Cells Divide

11.1 Prokaryotes divide far more simply than do eukaryotes.
Cell Division in Prokaryotes
• Most prokaryotes have a genome made up of a single, circular DNA molecule, and replicate via binary fusion. (p. 208)
• Binary fusion begins with DNA replication, which starts at the origin site and proceeds bidirectionally around the circular DNA to a specific site of termination. (p. 208)
• The evolution of eukaryotic cells led to much more complex genomes and, thus, new and different ways to replicate and segregate the genome during cell division. (p. 209)

11.2 The chromosomes of eukaryotes are highly ordered structures.
Discovery of Chromosomes
• Chromosomes were first discovered in 1882 by Walther Fleming. (p. 210)
• The number of chromosomes varies from one species to another. Humans have 23 nearly identical pairs for a total of 46 chromosomes. (p. 210)
The Structure of Eukaryotic Chromosomes
• The DNA is a very long, double-stranded fiber extending unbroken through the entire length of the chromosome. A typical human chromosome contains about 140 million nucleotides. (p. 211)
• Every 200 nucleotides, the DNA duplex is coiled around a core of eight histone proteins, forming a nucleosome. (p. 211)
• The particular array of chromosomes an individual possesses is its karyotype. (p. 212)
• The number of different chromosomes a species contains is known as its haploid (n) number, and is considered one complete set of chromosomes. (p. 212)
• Humans are diploid, with homologues coming from both the maternal and paternal lineages. (p. 212)

11.3 Mitosis is a key phase of the cell cycle.
The Cell Cycle
• The typical cell cycle consists of five phases: G1 is the primary growth phase; S is the replication phase; and G2 is the second growth phase. (G1, S, and G2 combined constitute interphase.) M (mitosis) is the phase during which the microtubular apparatus separates sister chromatids, and C (cytokinesis) is the phase during which the cytoplasm divides, creating two daughter cells. (p. 213)
• The duration of the cell cycle varies from species to species, and can range from about 8 minutes to over a year. (p. 213)
Interphase: Preparing for Mitosis
• The cell grows throughout interphase. The G1 and G2 phases are periods of protein synthesis and organelle production, while the S phase is when DNA replication occurs. (p. 214)
• Chromatin condensation continues into prophase. The spindle apparatus is assembled, and sister chromatids are linked to opposite poles of the cell by microtubules. The nuclear envelope breaks down. (p. 215)
• During metaphase, chromosomes align in the center of the cell along the metaphase plate. (p. 215)
• Anaphase begins when centromeres divide, freeing the two sister chromatids from each other. Sister chromatids are pulled to opposite poles as the attached microtubules shorten. (pp. 216—217)
• In telophase, the spindle apparatus disassembles, and the nuclear membrane begins to re-form. (p. 217)
• Cytokinesis is the phase of the cell cycle when the cell actually divides. Cytokinesis generally involves the cleavage of the cell into roughly equal halves, forming two daughter cells. (p. 218)

11.4 The cell cycle is carefully controlled.
General Strategies of Cell Cycle Control
• A cell uses three main checkpoints to both assess the internal state of the cell and integrate external signals. The G1/S checkpoint is the primary point at which the cell decides to divide; the G2/M checkpoint represents a commitment to mitosis; and the spindle checkpoint ensures that all chromosomes are attached to the spindle in preparation for anaphase. (p. 219)
Molecular Mechanisms of Cell Cycle Control
• Two groups of proteins, cyclins and Cdk's, interact and regulate the cell cycle. (p. 220)
• Cells also receive protein signals (growth factors) that affect cell division. (p. 222)
Cancer and the Control of Cell Proliferation
• Cancer is failure of cell division control. (p. 223)
• It is believed that a malfunction in the p53 gene may allow cells to go through repeated cell division without being stopped at the appropriate checkpoints. (p. 223)
• Proto-oncogenes are normal cellular genes that become oncogenes when mutated. Proto-oncogenes can encode growth factors, protein relay switches, and kinase enzyme. (p. 224)
• Tumor-suppressor genes can also lead to cancer when they are mutated. (p. 224)

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