Chapter 9 Cell Division and Mitosis
I. Silver in the Stream of Time
A. The cycle of life includes fertilization of gametes, cell division and growth, production of gametes, and death.
B. All of life depends on the capacity of cells to divide.

II. Dividing Cells: The Bridge Between Generations
A. Overview of Division Mechanisms
1. Reproduction begins with the division of single cells.
2. Each new generation must receive a duplicate of all parental DNA and enough cytoplasmic machinery to start up its own operation.
3. Mitosis and meiosis are eukaryotic nuclear division mechanisms that lead to the distribution of DNA to new nuclei in forthcoming daughter cells.
a. Mitosis is used by multicelled organisms for growth by repeated divisions of somatic cells.
b. Meiosis occurs only in cells that divide to form gametes.
4. Cytokinesis is the actual cytoplasmic division of a parental cell into two daughter cells.
B. Some Key Points About Chromosome Structure
1. Chromosomes are molecules of DNA complexed with proteins.
2. Prior to division, each threadlike chromosome is duplicated; a centromere joins sister chromatids.
3. The centromere is the region where the chromosome will attach to microtubules during nuclear division.
C. Mitosis, Meiosis, and the Chromosome Number
1. The human body is composed of somatic cells (nongametes) and germ cells that produce gametes.
2. In somatic cells, chromosomes exist as pairs (homologous chromosomes).
3. During development, mitosis maintains the chromosome number of the species (example: human somatic cells contain forty-six chromosomes).
4. In meiosis, each diploid cell’s chromosome number is reduced by half, such that haploid gametes result.

III. Mitosis and the Cell Cycle
A. The cell cycle is a recurring sequence of events that extends from the time of a cell’s formation until its division is completed.
B. Most of a cell’s existence (about 90 percent) is spent in interphase; mitosis occupies only a small part.
1. During interphase the cell’s mass increases (G1), the cytoplasmic components approximately double in number (G2), and the DNA is doubled (S).
2. Some cells are arrested in interphase and never divide again (example: brain cells).

IV. Stages of Mitosis
A. The Microtubular Spindle
1. Chromosomes are moved by a spindle apparatus composed of microtubules.
2. Microtubules (components of the cytoskeleton) extend from two “poles” established by the microtubule organizing center (centriole).
B. Prophase: Mitosis Begins
1. Chromosomes become visible as rodlike units, each consisting of two sister chromatids.
2. Spindle forms.
C. Metaphase
1. Nuclear membrane breaks up in transition between pro- and metaphase.
2. Sister chromatids become oriented toward opposite poles.
a. The kinetochore in the centromere region of each chromatid is the place of attachment to several spindle microtubules.
b. Spindle fibers and kinetochores interact to orient chromatids.
3. All chromosomes are aligned at the cell’s equator, halfway between the poles.
D. Anaphase
1. Sister chromatids separate and move toward opposite poles.
a. Microtubules attached to kinetochores shorten as chromosomes approach poles.
b. Other microtubules at the spindle poles elongate to push spindle poles apart.
2. Now each chromatid is an independent chromosome.
E. Telophase
1. Telophase begins when chromosomes arrive at the poles.
2. The nuclear envelope forms from the fusion of small vesicles; mitosis is complete.

V. Cytokinesis
A. In animal cells, contractile microfilaments at the cleavage furrow pull the plasma membrane inward.
B. Plant cells form a cell plate (cellulose) that separates the two new cells.