Cell Division and Reproduction: Meiosis, Mitosis, and Gametogenesis
Estrogen
A female hormone that develops secondary sexual characteristics.
Gonads
A gland that produces male or female gametes: testes and ovaries.
Karyotype
The chromosomes of a cell or organism, characterized by the number, size, and configuration of chromosomes.
Cline
A gradient in the genotype or phenotype frequencies along a strip of a territory.
Sperm
Male gamete.
Egg
Female gamete.
Isogamic Reproduction
Type of sexual reproduction in which gametes are equal.
Heterogamic Reproduction
Mating type where gametes are different.
Mitosis
Cell division process by which the nucleus and cytoplasm are divided into two, creating two daughter cells with the same hereditary content as the parent cell. Mitosis is divided into four parts, which are: prophase, metaphase, anaphase, and telophase. Interphase is the period between two successive divisions or mitosis.
Prophase
During prophase, the chromatid network present in the nucleus condenses, shortens, and thickens to form the chromosomes, which at this stage consist of two identical cords or filaments.
Metaphase
a) The chromosomes are oriented through the centromere. b) This period is very short.
Anaphase
The division is completed by the longitudinal splitting of chromosomes, separating the two chromatids, forming two daughter chromosomes. This movement is due to electrostatic forces. Proteins that form the spindle have contractile properties, so the contraction of spindle fibers facilitates the transfer of chromatids toward the cell pole.
Telophase
The chromosomes are gathered around the poles of the centriole in the cell and join to form the network of chromatin. The nuclear membrane, the nucleolus, and the karyolymph reappear. The cytoplasm closes by strangulation. It forms a double membrane called phragmoplast, from which the middle layer forms.
Meiosis
Cell division during which the mother cell (diploid) produces daughter cells or gametes with half the number of chromosomes. This is done by two divisions:
- 1st meiotic division: Is done in phases: prophase I, metaphase I, anaphase I, telophase I. Prophase I is divided into sub-phases: leptotene, zygotene, pachytene, diplotene, diakinesis.
- 2nd meiotic division: Takes place in stages: prophase II, metaphase II, anaphase II, telophase II.
Interphase: Difference Between Mitosis and Meiosis
- a) During meiosis, there are two successive cell divisions, reductional or meiotic, because it reduces to half the number of chromosomes.
- b) Mitosis produces two identical cells to one another and identical to the cell that is formed, in both meiosis, it gives rise to four cells, if not identical, that have only half the number of stem cell chromosomes.
Prophase I: Sub-phases
Leptotene
The chromosomes are thin and thread-like and consist of two chromatids.
Zygotene
The chromosomes unite in pairs (synapses). The homologous chromosomes pair and exchange genetic material.
Pachytene
Supplementing the homologous chromosome pairing and forming a matrix around. It can curl and intertwine, and fragments of chromatids can be exchanged, which is an important event in inheritance.
Diplotene
The chromosomes of each pair are divided lengthwise, four chromatids become visible, although not completely separate, some points are in contact (chiasmata).
Diakinesis
The chromosomes contract and are aimed at ending the equatorial plate prophase I.
Metaphase I
The four pairs of homologous chromosomes are arranged along the equatorial plane of the cell. The spindle is formed, and its fibers are attached to each chromosome counterpart by the centromere. At this stage, the nuclear membrane disintegrates.
Anaphase I
Homologous chromosomes, each with two chromatids, separate and migrate towards the poles of the cell. The centromere does not split.
Telophase I
A haploid number of chromosomes in each pole are arranged in the cell. Each chromosome has two chromatids joined at the centromere. The nuclear membrane forms around each set of chromosomes. This stage is short, and some cells are without it.
Differences Between Mitosis and Meiosis
- Mitosis occurs in all somatic cells; meiosis occurs only in germ cells of tissues.
- In mitosis, the cytoplasm divides a single time; in meiosis, there are two cytoplasmic divisions.
- In mitosis, the chromosomes are duplicated once and do not exchange materials; in meiosis, chromosomes exchange genetic material while remaining paired.
- The prophase of mitosis occurs in one stage; the prophase of meiosis occurs in several subphases.
- Mitosis results in two diploid cells; the cells resulting from meiosis are haploid (gamete).
Gametogenesis
It is the formation of gametes or sex cells and includes spermatogenesis and oogenesis. Spermatogenesis and male gametes from germ cells of the seminiferous epithelium of the ducts. Oogenesis comprises a set of cell divisions that are aimed at egg formation or female gamete.