The sex cells, Also called gametic cells or simply gametes, they are the cells that multicellular organisms, be they animals or plants, use to reproduce. Generally, the female sex cells are known as ovules and the male ones as sperm..
Among the most relevant characteristics of sex cells, the fact that they are produced by meiosis stands out, which is why they are usually haploid cells, that is, they contain half the number of chromosomes of the individual that gave rise to them..
In a diploid organism (which has 2 copies of each chromosome), sex cells are produced by the meiotic division of precursor cells that are also diploid (2n).
These cells go through one round of DNA duplication and then two nuclear division events, resulting in 4 haploid cells (n) for every cell that divides. The process of production or formation of sex cells is also known as gametogenesis.
During sexual reproduction, two individuals, usually one male and the other female, "bring" their sex cells together through copulation (in animals) or pollination (in plants) in order for fertilization to occur, that is: fusion of an egg and a sperm to form a zygote.
The zygote, then, is the cell resulting from the fusion of two gametes, and in it the genetic load of the species is restored, that is, an organism is obtained with a set of chromosomes from one of the parental individuals and another chromosomal set of the other.
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Male gametes are almost always much smaller compared to female gametes and, in many cases, are mobile cells (flagellates).
However, in flowering plants (angiosperms) the male gametes are immobile, which means that they are not flagellate cells, but are transported by pollinating agents over great distances in the structure that we call pollen grain..
Mammals and other animals produce their gametes in organs known as gonads, which are part of the reproductive system. Not only does meiotic cell division occur in the gonads, but also the maturation of sex cells.
The male gonads of humans and other animals are called testicles and its size, shape and anatomical location vary depending on the species.
In flowering plants the pollen grains are produced in a structure known as the androecium, which actually consists of a set of what we more commonly call stamens.
Male sex cells are very similar in many different organisms. In humans they are small cells that have a prolongation that gives them mobility.
Its body is divided into the "head" region, the middle region and the "tail". In the head, human sperm have a kind of cap known as an acrosome, which helps in the fusion process between the egg and the sperm.
The sperm cell nucleus is in the head region and the middle region is the one that contains the mitochondria that are responsible for producing the energy necessary for the flagellum or tail to move (swim).
In plants, as we have already mentioned, the sex cells are contained in the pollen grains produced in the androecium of flowering plants. Each pollen grain is a tiny structure, whose shape, size and structure is considered a taxonomic character (they are very different between species).
Pollen grains are dispersed by pollinating agents, whether they are animals or wind or water currents.
A pollen grain consists of one or more vegetative cells and a reproductive cell that is actually the male gamete..
When a pollen grain comes into contact with the female portion of a flower, it "germinates", which means that the vegetative cells form a kind of tube (the pollen tube) through which the reproductive cell can travel and reach the ovum. to fertilize it.
Therefore, male sex cells in plants are quite different from those in animals, especially with regard to their ability to move or swim..
In the other "lower" plant groups, plant sperm are very similar to those of many other organisms. In algae, ferns, bryophytes and other plant organisms there are sex cells with very different shapes and sizes, almost all mobile.
Compared to a sperm, eggs are very large, immobile cells. Many authors report that they are one of the largest cells in the body of a mammalian animal such as humans..
These female sex cells are produced in animals by organs known as ovaries..
In flowering plants, the ovules are also produced in ovaries, and they are part of a structure called gynoecium..
These cells normally have an abundant cytoplasm (much more abundant than that of male gametes), a large nucleus and, in animals, two well-defined regions are distinguished: the zona pellucida and the corona radiata..
The zona pellucida is a thin membrane that covers the cell membrane of the ovum and participates in its union with the sperm, meanwhile the corona radiata represents a set of layers of cells that overlap the zona pellucida.
This portion of the structure is also of utmost importance for the ovum during fertilization..
Plants, as we said, also have female sex cells called ovules. When the ovules of a flowering plant are mature, they consist of a large cell that is surrounded by nutritional tissue and that, at the same time, is surrounded by integuments that will later become the seed coat..
The gynoecium, which is the female structure of the flowers, consists of a stigma (where the pollen grain initially arrives), a style (a tube that communicates the stigma with the ovary) and an ovary, where one or more ovules are housed.
Eggs are also known as embryonic sacs, as the cell inside them (haploid) divides by mitosis to give rise, generally to 7 cells: 2 cells called synergists, 1 “egg” cell, 3 cells called antipodes and an additional cell with two cores.
Each one of the cells inside the embryo sac fulfills a certain function during the fertilization process and later during the development of the embryo and the seed..
Sex cells are produced by a type of cell division known as meiosis, which differs from "normal" cell division (mitosis) in that the genetic material of the "parent" cell divides twice.
The process of formation and maturation of these cells is called gametogenesis.
Before division, the genetic material contained in the nucleus (DNA, which is in the form of chromosomes) is duplicated, so that each copy and the original “template” are joined in one place..
The duplicated pairs of chromosomes are then aligned in the center of the cell, where the individual chromosomes are separated, in such a way that the "templates" and their replicas are separated from their counterparts towards both poles of the cell, which divides at the same time. half.
Subsequently, a second cell division occurs, through which the copies of each chromosome are separated, in a very similar way to what we just described. Thus, each progenitor cell can give rise to 4 sex cells, each of which has only half of the "original" genetic material..
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