The Mechanics of Cell Division

Mitosis is unique to eukaryotes and very similar in all of them. It is reliable showing only one error per 100,000 cell divisions.

A. Stages of Mitotic Cell Division

G2 of Interphase

-> nucleus well defined

-> one or more nucleoli present

-> two pairs of centrioles adjacent to nucleus

-> chromosomes have already duplicated (in S phase) but not distinguishable

Prophase

-> nucleoli disappear

-> chromatin coils & folds into observable chromosomes

-> each chromosome consists of two identical chromatids joined at centromere

-> spindle forms consisting of microtubules

-> centrosomes move apart

Prometaphase

-> nuclear membrane fragments

-> microtubules interact with chromosomes

-> spindle fibers extend from each pole

-> bundles of microtubules (kinetochore microtubules) attach & put chromosomes in motion

Metaphase

-> chromosomes move to metaphase plate

-> identical chromatids are attached to kinetochore fibers radiating from opposite ends of the parent cell

Anaphase

-> sister chromatids separate and are considered chromosomes

-> spindle apparatus starts moving separate chromosomes to opposite poles

-> poles of cell move farther apart slightly elongating the cell

Telophase

-> daughter nuclei begin to form at two poles

-> nuclear envelopes form

-> nucleoli reappear

-> chromatin uncoils and chromosomes become less distinct

By the end of telophase mitosis (equal division of one nucleus into two genetically identical nuclei) is complete. Cytokinesis has begun and the appearance of two separate daughter cells occurs shortly after mitosis is completed.

B. Structure and Function of the Mitotic Spindle

The mitotic spindle is important to mitosis. It forms in the cytoplasm during prophase and is composed of fibers formed from microtubules and associated proteins.

-> aggregates of two proteins, alpha & beta tubulin

-> elongation of spindle microtubules occurs by addition of protein subunits at one end

-> microtubules are polar, with distinct + and - ends

-> length changes by addition or removal of tubulin units away from centrosome, the + end

-> kinetochore microtubules shorten during anaphase by depolymerizing at their kinetochore or + end, pulls chromosomes poleward.

-> ATP provides the necessary energy

C. Mechanism of Cytokinesis

Animals

-> cleavage furrow forms as a shallow groove in cell surface near old metaphase plate

-> contractile ring forms on cytoplasmic side of furrow

-> microfilaments of ring contract, reducing diameter of ring

-> cell pinched in two

Plants

-> cell plate forms across midline of old cell (interior)

-> plate results from fusion of vesicles of Golgi

-> vesicle fusion forms two membranes that grow laterally and unite with existing membrane, thus forming 2 cells each with their own cell membrane

-> new cell wall forms between membranes of cell plate