Mitosis

THE EUKARYOTIC CELL CYCLE

Learning Objectives for this Section

The eukaryotic cell cycle is divided into two major phases: interphase and cell division.

A. Interphase

Ninety percent or more of the cell cycle is spent in interphase. During interphase, cellular organelles double in number, the DNA replicates, and protein synthesis occurs. The chromosomes are not visible and the DNA appears as uncoiled chromatin.

Interphase in a plant cell: see Fig. 1

Interphase in an animal cell: see Fig. 6

Interphase is divided into the following stages: G₁, S, and G₂.

1. G₁ phase

During G₁ phase, the period that immediately follows cell division, the cell grows and differentiates. New organelles are made but the chromosomes have not yet replicated in preparation for cell division.

2. S phase

DNA synthesis occurs during S phase. The chromosomes replicate in preparation for cell division.

3. G₁ phase

During G₂ phase, molecules that will be required for cell replication are synthesized.

B. Cell Division

Cell division consists of nuclear division and cytoplasmic division. Nuclear division is referred to as mitosis while cytoplasmic division is called cytokenesis.

1. Mitosis (nuclear division)

Mitosis is the nuclear division process in eukaryotic cells and ensures that each daughter cell receives the same number of chromosomes as the original parent cell. Mitosis can be divided into the following phases: prophase, metaphase, anaphase, and telophase.

a. Prophase

During prophase, the chromatin condenses and the chromosomes become visible. Also the nucleolus disappears, the nuclear membrane fragments, and the spindle appartus forms and attaches to the centromeres of the chromosomes.

Prophase in a plant cell: see Fig. 2A and Fig. 2B

Prophase in an animal cell: see Fig. 7A and Fig. 7B

b. Metaphase

During metaphase, the nuclear membrane fragmention is complete and the duplicated chromosomes line up along the cell's equator.

Metaphase in a plant cell: see Fig. 3

Metaphase in an animal cell: see Fig. 8

c. Anaphase

During anaphase, diploid sets of daughter chromosomes separate and are pushed and pulled toward opposite poles of the cell. This is accomplished by the polymerization and depolymerization of the microtubules that help to form the spindle apparatus.

Anaphase in a plant cell: see Fig. 4A and Fig. 4B

Anaphase in an animal cell: see Fig. 9

d. Telophase

During telophase, the nuclear membrane and nucleoli reform, cytokinesis is nearly complete, and the chromosomes eventually uncoil to chromatin. Usually cytokinesis occurs during telophase.

Telophase in a plant cell: see Fig. 5A and Fig. 5B

Telophase in an animal cell: see Fig. 10

2. Cytokinesis (cytoplasmic division)

During cytokinesis, the dividing cell separates into two diploid daughter cells. In animal cells, which lack a cell wall and are surrounded only by a cytoplasmic membrane, microfilaments of actin and myosin attached to the membrane form constricting rings around the central portion of the dividing cell and eventually divide the cytoplasm into two daughter cells. In the case of plant cells , which are surrounded by a cell wall in addition to the cytoplasmic membrane, carbohydrate-filled vesicles accumulate and fuse along the equator of the cell forming a cell plate that separates the cytoplasm into two daughter cells.

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