Cell Division And Plant Growth

Cell Duplication

Cells duplicate through the process of Mitosis. Mitosis is the process by which a cell duplicates its genetic information (DNA) and in the process generates two, identical daughter cells. Duplication and division of the genetic material in the nucleus is normally followed by cytokinesis. Cytokinesis is the process through which the cytoplasm divides and a new cell wall is formed between the two new daughter protoplasts. The result of the process is two identical daughter cells with more or less an equal distribution of organelles originating from the mother cell. The combined phases of mitosis and cytokinesis define the mitotic phase cell cycles. Somatic cells undergo mitosis, while the sexual reproductive cells are formed by a similar process known as meiosis.

Mitosis is a regulated and sequential process consisting of phases.

These main phases of mitosis are:

• Preprophase
• Prophase
• Rometaphase
• Metaphase
• Anaphase
• Telophase

Preprophase: In plant cells, prophase is preceded by a pre-prophase. The nucleus has to migrate into the centre of the cell before mitosis can begin. A pre-prophase band is formed that is a ring-like band of microtubules just below the plasma membrane that delimits the equatorial plane of the future mitotic spindle of the cell preparing to divide.

Prophase: In a non-dividing cell the genetic material in the nucleus is ordered in a loosely bundled coil called chromatin. When prophase is initiated the chromatin bundle becomes condensed and forms a highly ordered structure called a chromosome. At this stage, the genetic material has already been duplicated and each chromosome has two sister chromatids that are bound together at the so-called centromere. The chromosomes can be seen when using a light microscope at high magnification.

Prometaphase: During prometaphase, the nuclear envelope becomes disassembled, and microtubules invade the nuclear space. The chromosomes now form two kinetochores at the centromere, one attached at each chromatid. The kinetochore is a protein ring that forms a moving device. This motor activity, as well as polymerisation and depolymerisation of microtubules, provides the pulling force necessary to later separate the chromosome's two chromatids. The spindle grows to a sufficient length where the kinetochore microtubules begin searching for kinetochores to attach to.

Metaphase: At the same time, as the microtubules find and attach to kinetochores in prometaphase, the centromeres of the chromosomes arrange themselves along the equatorial plane.

Anaphase: When the kinetochores are all attached to microtubules and the chromosomes have lined up along the metaphase plate, the cell proceeds to anaphase. At this stage the proteins that bind sister chromatids together are cleaved, allowing separation. The sister chromatids are then pulled apart by shortening of the kinetochore microtubules and toward the opposite sides of the cell. The end result of anaphase is the cell that has succeeded in separating identical copies of the genetic material into two distinct populations at opposite sides of the cell.

Telophase: Telophase is in effect a reversal of the events of prophase and prometaphase. The nonkinetochore microtubules lengthen, causing the elongation of the cell. Corresponding sister chromosomes attach at opposite ends of the cell. New nuclear envelopes are formed from fragments of the parent cell nuclear membrane. Both sets of chromosomes, now surrounded by new nuclei, unfold back into chromatin. Mitosis is complete, but cell division is not yet complete.

Cytokinesis: Cytokinesis begins at the same time as telophase. Cell division is driven by vesicles derived from the Golgi apparatus. The vesicles move along the microtubules to the middle of the cell. This structure forms a cell plate at the centre of the nuclei and develops into a cell wall. Each of the daughter cells now has a complete copy of the genome of its parent cell as well as more or less equal parts of the cytoplasm. The importance of mitosis is that it ensures that the chromosomes or genetic information in the nuclei of the two resulting daughter cells will be alike.