Figure 2.17 Mitosis is the process of nuclear division in which the chromosomes are distributed to two daughter nuclei. Together with cytokinesis, it produces two identical daughter cells. Interphase Early Prophase Late Prophase Centrosomes (each- has 2 centriolos) Early mitotic spindle Spindle pole Nonkinetochore microtubule -Plasma membrane Fragments of nuclear envelope Aster Nucleolus - Chromatin - Centromere Chromosome Nuclear envelope consisting of two sister chromatids Kinetochore - Kinetochore microtubule Interphase Prophase-first phase of mitosis Interphase Interphase is the period of a coll's life when it Early Prophase • The chromatin coils and condenses, forming barlike chromosomes. Late Prophase • The nuclear envelope breaks up, allowing the spindle to interact with the chromosomes. • Some of the growing spindle microtubules attach to kinetochores (ki-ne' to-korz), special protein structures at each chromosome's centromere. Such microtubules are called kinetochore microtubules. carries out its normal metabolic activities and grows. Interphase is not part of mitosis. • Each duplicated chromosome consists of two identical threads, called sister chromatids, held together at the centromere. (Later when the chromatids separate, each will be a new chromosome.) • As the chromosomes appear, the nucleoli disappear, and the two centrosomes separate from one another. • During interphase, the DNA-containing material is in the form of chromatin. The nuclear envelope and one or more nucleoli are intact and visible. • The remaining (unattached) spindle microtubules are called nonkinetochore microtubules. The microtubules slide past each other, forcing the poles apart. • The kinetochore microtubules pull on each chromosome from both poles in a tug-of-war that ultimately draws the chromosomes to the center, or equator, of the cell. • There are three distinct periods of interphase: G1, S, and G2. • The centrosomes act as focal points for growth of a microtubule assembly called the mitotic spindle. As the microtubules lengthen, they propel the centrosomes toward opposite ends (poles) of the cell. • Microtubule arrays called asters ("stars") extend from the centrosome matrix. The light micrographs show dividing lung cells from a newt. Fluorescent markers color cell structures. The chromosomes appear blue and the microtubules green. (The red fibers are intermediate filaments.) The schematic drawings show details not visible in the micrographs. For simplicity, only four chromosomes are drawn. Metaphase Anaphase Telophase Cytokinesis Nuclear- Nucleolus forming Contractile envelope forming ring at cleavage furrow Spindle Metaphase plate Daughter chromosomes Metaphase second phase of mitosis Anaphase-third phase of mitosis Telophase-final phase of mitosis • The two centrosomes aro at opposite poles of the cell. • The chromosomes cluster at the midline of the cell, with their centromeres precisely aligned at the spindle equator. This imaginary plane midway between the poles is called the metaphase plate. • At the end of metaphase, enzymes that will act to separate the chromatids from each other are triggerod. The shortest phase of mitosis, anaphase begins abruptly as the centromeres of the chromosomes split simultaneously. Each chromatid now becomes a chromosome in its own right. • The kinetochore microtubules, moved along by motor proteins in the kinetochores, gradually pull each chromosome toward the pole it faces. • At the same time, the nonkinetochore microtubules slide past each other, lengthen, and push the two poles of the cell apart. • The moving chromosomes look V shaped. The centromeres lead the way, and the chromosomal "arms" dangle behind them. • Moving and separating the chromosomes is helped by the fact that the chromosomes are short, compact bodies. Diffuse threads of chromatin would trail, tangle, and break, resulting in imprecise "parceling out" to the daughter cells. Telophase Telophase begins as soon as chromosomal movement stops. This final phase is like prophase in reverse. • The identical sets of chromosomes at the opposite poles of the cell begin to uncoil and resume their threadlike chromatin form. •A new nuclear envelope forms around each chromatin mass, nucleoli reappear within the nuclei, and the spindle breaks down and disappears. • Mitosis is now ended. The cell, for just a brief period, is binucleate (has two nuclei) and each new nucleus is identical to the original mother nucleus. Cytokinesis-division of cytoplasm Cytokinesis begins during late anaphase and continues through and boyond telophase. A contractile ring of actin microfilaments forms the cleavage furrow and pinches the cell apart.