Mitosis

Mitosis is the process by which all cells divide. Many cells have a limited life span, and mitosis allows them to be renewed on a regular basis. Mitosis is also responsible for generating the many millions of cells that are needed for an embryo to develop into a fetus, an infant, and finally an adult.

Most human cells continually undergo a cycle of different phases. The phases have distinct names but flow smoothly into one another. The mitotic (M) phase is the phase in which the cell's genetic material is split in two. Once the phase is completed, the cell is physically divided into two daughter cells, in a process called cytokinesis.

Before entering the M phase, cells are in interphase, the phase between two cell divisions. Interphase is itself divided into three phases: G₁, S, and G₂, where G stands for gap or growth, and S for synthesis. During the G₁ Various stages of mitosis are evident in this magnification of an onion root tip. The chromosomes are stained red. phase, daughter cells formed in the previous M phase undergo active cell growth. During the S phase, the genetic material (DNA) contained in the chromosomes is duplicated so that both of the future daughter cells receive the same set of chromosomes. This ensures that they will be genetically identical to each other and to the cell from which they originated.

In human somatic cells, each of the forty-six chromosomes replicates to produce two daughter copies that are called sister chromatids. These two copies remain attached to each other at a single point, the centromere, which is a DNA sequence of about 220 nucleotides. The centromere has a disk-shaped protein molecule, called a kinetochore, attached to it. In interphase, the chromosomes are not visible as discrete entities under the light microscope. Interphase chromosomes are uncoiled threads composed of DNA and protein molecules. This noncondensed form of chromosomes is also called chromatin.

During the G₂ phase of the cell cycle, the chromatin fibers start to condense, eventually turning into tightly coiled, compact bodies, visible as chromosomes. The cell also begins to manufacture protein fibers called microtubules, which will be used later to move the chromosomes to opposite poles of the cell, so two new daughter cells can form. Chromosome condensation and microtubule formation both begin in the G₂ phase but occur mostly during the first stage of the M phase, which is called prophase. The microtubules are organized into a three-dimensional spindle apparatus, where each fiber of the spindle apparatus connects one cell pole to the other like a bridge.

During the next stage of mitosis, called prometaphase, the envelope surrounding the cell nucleus breaks down so that the chromosomes are free to migrate to the central plane of the spindle apparatus. A second group of microtubules grows out, to connect the two opposite sides of the kineto-chore to the two poles of the spindle. This arrangement is crucial for ensuring that the two sister chromatids end up in two separate daughter cells rather than being pulled into the same cell.

In the next stage of mitosis, metaphase, the chromosomes become maximally condensed and line up in an imaginary plane, called the metaphase plate, in the center of the cell and perpendicular to the spindle apparatus. All the centromeres are neatly arranged in a circle, about halfway between the two cell poles.

In human cells, at this point the twenty-three pairs of chromosomes, each made up of two condensed sister chromatids held together by a centromere, are visible under the microscope. Unlike in meiosis, the paternal and maternal copies in each pair of chromosomes align independently in the metaphase plate and are not associated with each other. At the end of metaphase, the centromeres that hold the two sister chromatids together all divide simultaneously.

During the next stage, anaphase, microtubules that are attached to the sister chromatids' kinetochores draw the chromatids quite rapidly to opposite poles of the spindle. The separation of sister chromatids completes the partitioning of the replicated genetic material.

The only task remaining during the final phase of mitosis, telophase, is to disassemble the spindle apparatus and re-form the nuclear envelope around each set of sister chromatids. The chromatids can be called chromosomes again, because they each have their own centromere. The chromosomes begin to uncoil, and the genes they carry begin to be expressed again. This is the end of mitosis.

The cell cycle is completed by cytokinesis, the physical division of the cytoplasm into two daughter cells. By the time cytokinesis occurs, other cytoplasmic organelles, such as mitochondria, already have been replicated during the S or G₂ phases, and they have also been directed to the areas around the cell poles that will become the daughter cells. Cytokinesis is followed by the G₁ phase, with active cell growth occurring in each of the two daughter cells.