The Specific Phases Of The Cell Cycle

Centrosomes (with centriole pairs)

Early mitotic spindle

Pair of centrioles

Centrosomes (with centriole pairs)

Early mitotic spindle

Pair of centrioles

New daughter nucleus

New nuclear membrane

METAPHASE

ANAPHASE

TELOPHASE AND CYTOKINESIS

Fig. 5.6 The Cell Cycle and its stages.

New daughter nucleus

New nuclear membrane

METAPHASE

ANAPHASE

TELOPHASE AND CYTOKINESIS

Fig. 5.6 The Cell Cycle and its stages.

Not being held in by the nuclear membrane anymore, the sister chromatids are contacted by microtubules, which move them towards the middle of the cell.

  1. Metaphase. As its name indicates, metaphase (MET-ah-fayz) is the ''phase'' occurring ''after'' (meta-) prophase. By this stage, the mitotic spindle is completely formed, with a pair of centrioles at either pole or end. The sister chromatids are pushed into a single horizontal row along the equator or middle of the cell.
  2. Anaphase. Up to this time, each of the 46 duplicated chromosome pairs or sister chromatids have all stayed attached together. In anaphase (AN-uh-fayz), however, the centromeres split, and each of the duplicated chromatid sisters finally move ''apart'' (ana-) from one another. The 92 separated, new chromosomes (often called the daughter chromosomes) are now pulled towards opposite poles of the cell, by their attached microtubules.
  3. Telophase and cytokinesis. The ''phase'' that formally ''ends'' (telo-) mitosis is technically called telophase (TELL-uh-fayz). In telophase, many of the events that happened during prophase are exactly reversed! For example, remember that in prophase the nuclear membrane disappeared. But during telophase, two new daughter nuclei, each with its own individual nuclear membrane, start to appear, one at either end of the dividing cell. Further, the mitotic spindle (which first arose during prophase) eventually breaks apart into tiny fragments.

All of the four main phases of mitosis (prophase, metaphase, anaphase, and telophase) essentially describe a division of the original cell's nucleus and its contents. But just duplicating chromosomes and splitting the nucleus into new daughter nuclei really isn't enough to end the Cell Cycle, is it? [Study suggestion: What would happen if, say, the original parent cell still remaining at telophase was left intact within our body tissues? Would such a cell still be considered normal? Why, or why not?]

Enter cytokinesis (sigh-toh-kih-NEE-sis), which is a ''division or movement'' (kines) of the ''cytoplasm'' (cyto-) into two parts. Cytokinesis occurs at the same time that telophase is proceeding. But instead of focusing upon the nucleus and its chromosomes, cytokinesis involves creation of a cleavage furrow, and what happens to the cytoplasm after it appears. The cleavage furrow is basically a furrow or indentation that starts at the equator on either side of the cell, and just keeps going deeper and deeper into the cytoplasm from each side. Eventually, the cleavage furrow completely pinches the original parent cell into two new daughter cells.

Telophase (and the rest of mitosis) have already finished by this complete division of the original cell's cytoplasm. Therefore, the two new daughter cells pinched off from the original parent by cytokinesis have a full package of all 46 chromosomes, contained within their own nucleus and nuclear membrane. The two daughter cells each go into an interphase of their own, and the seemingly endless magic of the Cell Cycle begins anew.

To capsulize:

INTERPHASE MITOSIS

Cell 2

Cell 2

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