Biology - Cell Division - Mitosis

MITOSIS IN PLANTS

Interphase (G1, S, G2)

Nuclear material is surrounded by a nuclear envelope. Dark-staining bodies, nucleoli, are visible. Chromosomes appear only as dark granules within the nucleus. Collectively, the chromosome mass is called chromatin. The chromosomes are not individually distinguishable because they are uncoiled into long, thin strands. Chromosomes are replicated during this phase.

Prophase

Chromosomes begin to coil and become distinguishable thin, threadlike structures, widely dispersed in the nucleus during prophase. Although there are no centrioles in plant cells, a spindle begins to form. Nucleoli begin to disappear. The nuclear envelope is still intact.

Prometaphase

By prometaphase, the chromosomes are thick and short. Each chromosome is double-stranded, consisting of two chromatids held together by a centromere. The nuclear membrane breaks down in prometaphase. Chromosomes move toward the equator.

Metaphase

Metaphase begins when the centromeres of the chromosomes lie on the equator of the cell. The arms of the chromatids extend randomly in all directions. A spindle may be apparent. Spindle fibers are attached to centromeres and extend to the poles of the cell. As metaphase ends and anaphase begins, the centromeres split.

Anaphase

The splitting of centromeres marks the beginning of anaphase. Each former chromatid is now a chromosome. Single-stranded chromosomes are drawn apart toward opposite poles of the cell. Anaphase ends when the migrating chromosomes reach their respective poles.

Telophase and Cytokinesis

Chromosomes have now reached the poles. The nuclear envelope reforms around each compact mass of chromosomes. Nucleoli reappear. Chromosomes begin to uncoil and become indistinct. Cytokinesis is accomplished by the formation of a cell plate that begins in the center of the equatorial plane and grows outward to the cell wall.

MITOSIS IN ANIMAL CELLS

(a) Interphase: The G2 stage of inter-phase immediately precedes the beginning of mitosis and follows chromosomal DNA replication during the S phase. The chromosomes, each containing a sister chromatid, are still dispersed and not visible as distinct structures. During interphase, the centrioles also are replicated, forming small daughter centrioles. (b) Early prophase: The centrosomes, each with a daughter centriole, begin moving toward opposite poles of the cell. The chromosomes can be seen as long threads, and the nuclear membrane begins to disaggregate into small vesicles. (c) Middle and late prophase: Chromosome condensation is completed; each visible chromosome structure is composed of two chromatids held together at their centromeres. The microtubular spindle fibers begin to radiate from the regions just adjacent to the centrosomes, which are moving closer to their poles. Some spindle fibers reach from pole to pole; most go to chromatids and attach at kinetochores. (d) Metaphase: The chromosomes move toward the equator of the cell, where they become aligned in the equatorial plane. The sister chromatids have not yet separated. This is the phase in which morphological studies of chromosomes are usually carried out. (e) Anaphase: The two sister chromatids separate into independent chromosomes. Each contains a centromere that is linked by a spindle fiber to one pole, to which it moves. Simultaneously, the cell elongates, as do the pole-to-pole spindles. Cytokinesis begins as the cleavage furrow starts to form. (f) Telophase: New nuclear membranes form around the daughter nuclei; the chromosomes uncoil and become less distinct; and the nucleolus becomes visible again. Cytokinesis is nearly complete, and the spindle disappears as the microtubules and other fibers depolymerize. Throughout mitosis the “daughter” centriole at each pole grows, so that by telophase each of the emerging daughter cells has two full-length centrioles. Upon the completion of cytokinesis, each daughter cell enters the G1 phase of the cell cycle and proceeds again around the cycle.