Sex cells

How are sex cells formed?

Each of the gametes or sex cells (sperm and ovum) is produced by the respective gonads (ovary and testes) through a complex process called gametogenesis.

- Gametogenesis is the process of gamete formation in the female and male gonads. This process occurs by meiosis from germ cells.

- Gametogenesis involves the reduction of the number of chromosomes from 46 to 23 in case of humans, through two successive nuclear divisions (meiosis I and II)

In the case of humans, the gametogenesis process is called differently depending on whether eggs are produced (oogenesis) or whether sperm are produced (spermatogenesis).

Meiosis

Meiosis is a cell division process in which a diploid cell undergoes two successive divisions, with the ability to generate four haploid cells. In organisms with sexual reproduction it is important since it is the mechanism by which the oocytes and sperm (gametes) are produced.

Meiosis is a process that involves two distinct phases: meiosis I and meiosis II. Each of them is made up of various stages:

Meiosis I: First cell division of the diploid (2n), which ends with half of the genetic endowment (n).

Prophase I: The genetic material intersects and a sort of dividing line arises in the cell. The nucleus disappears.
Metaphase I: The chromosomes are located in the center of the cell (equator) and begin to separate. The random genetic distribution has already been carried out.
Anaphase I: Each DNA string tends to one pole of the cell, forming two haploid poles (n).
Telophase I and Cytokinesis I: The plasma membrane separates and gives rise to two haploid cells.

Meiosis I

Meiosis II

Meiosis II: Two individuals are formed by duplicating DNA.

Prophase II: The haploid cells created in meiosis I condense their chromosomes and break the nuclear envelope.
Metaphase II: As before, the chromosomes tend towards the middle of the cell, preparing for a new division.
Anaphase II: The genetic material tends to separate and migrate towards the poles of the cell.
Telophase II and Cytokinesis II: Cell membranes separate again and result in four haploid cells (n), each with a different distribution of the complete genetic code of the individual.

How is a zygot formed?

The zygote is the first stage of life and appears after the union of the ovum and the sperm (fertilization). It consists of only one cell and its size is equal to that of the ovum.

It contains a nucleus and 46 chromosomes, 23 of maternal origin and 23 paternal.