BIOLOGY Lecture Notes - Golden Pages 2003

3. CELL DIVISION

Mitosis

Mitosis (from the Greek "mitos" meaning thread) is the primary method of Cell Division. Karyokinesis refers to indirect nuclear division (karyon = nucleus, kinesis = movement), occurring through distinct stages.

Mitosis is part of the mitotic cycle (mitosis + interphase).

Class="center">Phases of Mitosis

Prophase. Consists of several stages:

1. Thickening and shortening of Chromatin.

2. Superspiralization.

3. Condensation.

4. Breakdown of the nuclear envelope.

5. Disintegration of the nucleolus, dispersing into the material.

6. Migration of centrioles and formation of poles (microtubules grow outward from the poles).

7. Engagement of Chromosomes in saltatory (jerky) movement driven by microtubules and the nuclear matrix.

8. Formation of achromatic fibers extending from the centrioles.

9. Formation of the mitotic spindle.

Metaphase — chromosomes align along the equator of the spindle, and their jerky movements cease.

Each chromosome is split longitudinally into two chromatids; the chromatids are held together at the centromere (kinetochore), and achromatic fibers are attached to the centromeres.

The metaphase spindle resembles an American football or rugby ball with chromosomes positioned along the equator.

The spindle is formed by Two Types of fibers: those connecting the chromosomes to the poles and those stretching from pole to pole. These are bundles of microtubules of varying sizes.

Anaphase — chromatids separate into two daughter chromosomes that migrate toward opposite poles.

The spindle fiber connecting the kinetochore to the respective pole exerts a poleward force on the chromosome. The magnitude of this force is proportional to the length of the fiber. Microtubules act as the "motor" regulating the speed of chromosome movement. Elongation of the spindle during metaphase requires ATP (dynein ATPase). Chromosome movement toward the poles does not require ATP (driven by the mutual repulsion of the poles). The assembly of MTs is regulated by the centrosome, kinetochore, Ca²⁺ (tubulin concentration, etc.).

Telophase — chromosomes despiralize, the site of the new nuclear envelope begins to form, and the nucleolus reappears. Two new daughter Cells are produced, identical to the parent cell but half its size.

Once they reach the size of the parent cell, they may enter division.

Cytokinesis — the division of the Cytoplasm of the parent cell into two daughter cells at the end of mitosis or Meiosis. It typically follows (or coincides with) telophase and leads into the postmitotic (presynthetic) period of interphase.

Significance of mitosis:

1) precise transmission of Genetic information to daughter cells;

2) increase in the number of cells in the Organism, i.e., one of the primary Mechanisms of Growth;

3) a mode of asexual reproduction in organisms and cellular regeneration.

Amitosis (from the Greek "a" — a negative particle, and "mitosis") is direct division of an interphase nucleus by constriction without chromosome formation outside the mitotic cycle. Amitosis may be accompanied by cell division, or it can be limited to nuclear division without cytokinesis, leading to The formation of bi- and multinucleated cells. Amitosis occurs in various Tissues within specialized cells destined to die, particularly in the cells of mammalian fetal membranes. A cell undergoing amitosis is subsequently incapable of entering a normal mitotic cycle.

Meiosis (from the Greek "meiosis" — reduction) is a type of cell division in diploid cells that halves the chromosome number, making it haploid; therefore, meiosis is also referred to reductional division. This process yields four daughter cells from a single parent cell.

Another key feature of meiosis is the exchange of chromosomal segments, and consequently DNA, between the chromatids of homologous chromosomes before they segregate into daughter cells. Meiosis consists of two successive nuclear divisions with a short interphase between them. The First Division is the most complex and critical stage, subdivided into prophase I, metaphase I, anaphase I, and telophase I. In prophase I, homologous chromosomes of the diploid parent cell pair up, cross over (Crossing Over), form bridges (chiasmata), and then exchange segments—a process that enables Gene recombination—after which the chromosomes separate. In metaphase I, these homologous chromosomes align along The Cell's equator, and spindle fibers attach to each of them: one chromosome connected to one pole, and the second to the opposite pole. In anaphase I, duplicated (two-chromatid) chromosomes move toward opposite poles of the cell: one from each pair goes to one pole, and the other to the opposite pole. As a result, the number of chromosomes at the poles becomes half that of the parent cell, though they remain two-chromatid chromosomes. This is followed by telophase I and interphase (which may be omitted). During the interphase between the First and Second meiotic divisions, the synthesis (S) phase is absent because the chromosomes remained duplicated after the first division (DNA molecules are also duplicated). The Second Division of meiosis differs from mitosis only in that it occurs in cells with a haploid (single) set of chromosomes. Prophase II is sometimes skipped. In metaphase II, the two-chromatid chromosomes align at the equator, with division taking place simultaneously in two daughter cells. In anaphase II, single-chromatid chromosomes migrate to the poles. In telophase II, nuclei form in the four daughter cells, followed by Cell wall/membrane formation between them. Thus, meiosis results in four haploid cells with single-chromatid chromosomes (1n1c), which serve either as Gametes in animals and humans or as spores in plants.

Significance of meiosis:

✵ generation of a haploid set of chromosomes;

✵ creation of genetic Variability through Crossing over and the independent assortment of chromosomes.