MITOSIS (ENGLISH)

CELL DIVISION (MITOSIS)

INTERPHASE

Before a dividing cell enters mitosis, it undergoes a period of growth called interphase. Some 90 percent of a cell's time in the normal cellular cycle may be spent in interphase.

View image of a cell in interphase.

Stages of Interphase

• G1 phase: The period prior to the synthesis of DNA. In this phase, the cell increases in mass in preparation for cell division. Note that the G in G1 represents gap and the 1 represents first, so the G1 phase is the first gap phase.

• S phase: The period during which DNA is synthesized. In most cells, there is a narrow window of time during which DNA is synthesized. Note that the S represents synthesis.

• G2 phase: The period after DNA synthesis has occurred but prior to the start of prophase. The cell synthesizes proteins and continues to increase in size. Note that the G in G2 represents gap and the 2 represents second, so the G2 phase is the second gap phase.

• In the latter part of interphase, the cell still has nucleoli present.

• The nucleus is bounded by a nuclear envelope and the cell's chromosomes have duplicated but are in the form of chromatin.

• In animal cells, two pair of centrioles formed from the replication of one pair are located outside of the nucleus.

PROPHASE

In prophase, the chromatin condenses into discrete chromosomes. The nuclear envelope breaks down and spindles form at opposite "poles" of the cell. Many consider prophase (versus interphase) to be the first true step of the mitotic process.

View image of a cell in prophase.

Changes that occur in a cell during prophase:

• Chromatin fibers become coiled into chromosomes with each chromosome having two chromatids joined at a centromere.

• The mitotic spindle, composed of microtubules and proteins, forms in the cytoplasm.

• In animal cells, the mitotic spindle initially appears as structures called asters which surround each centriole pair.
• The two pair of centrioles (formed from the replication of one pair in Interphase) move away from one another toward opposite ends of the cell due to the lengthening of the microtubules that form between them.

In late prophase:

• The nuclear envelope breaks up.

• Polar fibers, which are microtubules that make up the spindle fibers, reach from each cell pole to the cell's equator.

• Kinetochores, which are specialized regions in the centromeres of chromosomes, attach to a type of microtubule called kinetochore fibers.

• The kinetochore fibers "interact" with the spindle polar fibers connecting the kinetochores to the polar fibers.

• The chromosomes begin to migrate toward the cell center.

METAPHASE

In metaphase, the spindle fully develops and the chromosomes align at the metaphase plate (a plane that is equally distant from the two spindle poles).

View image of a cell in metaphase.

Changes that occur in a cell during metaphase:

• The nuclear membrane disappears completely.

• In animal cells, the two pair of centrioles align at opposite poles of the cell.

• Polar fibers (microtubules that make up the spindle fibers) continue to extend from the poles to the center of the cell.

• Chromosomes move randomly until they attach (at their kinetochores) to polar fibers from both sides of their centromeres.

• Chromosomes align at the metaphase plate at right angles to the spindle poles.

• Chromosomes are held at the metaphase plate by the equal forces of the polar fibers pushing on the centromeres of the chromosomes.

ANAPHASE

In anaphase, the paired chromosomes (sister chromatids) separate and begin moving to opposite ends (poles) of the cell. Spindle fibers not connected to chromatids lengthen and elongate the cell. At the end of anaphase, each pole contains a complete compilation of chromosomes.

View image of a cell in anaphase.

Changes that occur in a cell during anaphase:

• The paired centromeres in each distinct chromosome begin to move apart.

• Once the paired sister chromatids separate from one another, each is considered a "full" chromosome. They are referred to as daughter chromosomes.

• Through the spindle apparatus, the daughter chromosomes move to the poles at opposite ends of the cell.

• The daughter chromosomes migrate centromere first and the kinetochore fibers become shorter as the chromosomes near a pole.

• In preparation for telophase, the two cell poles also move further apart during the course of anaphase. At the end of anaphase, each pole contains a complete compilation of chromosomes.

TELOPHASE

In telophase, the chromosomes are cordoned off into distinct new nuclei in the emerging daughter cells.

View images of a cell in early telophase and late telophase.

Changes that occur in a cell during telophase:

• The polar fibers continue to lengthen.

• Nuclei (plural form of nucleus) begin to form at opposite poles.

• The nuclear envelopes of these nuclei are formed from remnant pieces of the parent cell's nuclear envelope and from pieces of the endomembrane system.

• Nucleoli (plural form of nucleolus) also reappear.

• Chromatin fibers of chromosomes uncoil.

• After these changes, telophase/mitosis is largely complete and the genetic "contents" of one cell have been divided equally into two.

Cytokinesis

Cytokinesis, the division of the original cell's cytoplasm, begins prior to the end of mitosis and completes shortly after telophase/mitosis. At the end of cytokinesis, there are two distinct daughter cells.