What is Zygote
Zygote is the diploid cell resulting from the union of the nuclei of two mutually compatible cells . It is the product of sexual reproduction .
Segmentation of a Zygote
Immediately after fertilization, the zygote (ovum) begins to divide by mitosis . These initial divisions of the zygote are called cleavage or cleavage .
In general, it is slower in the region where the greatest amount of yolks is concentrated , which corresponds to the vegetative pole. The region where the nucleus is located corresponds to the animal pole.
Types of Zygote
In animals, the zygote is called an egg and is the result of the union of two gametes: the oocyte and the sperm .
It is a totipotent cell , that is, it is capable of storing the genetic characteristics of the parents, being able to generate all the cell lines of the adult organism.
Through several mitotic divisions it gives rise to a new individual (embryo).
The quantity and distribution of the yolk varies according to the organism considered and even depends on the type of development of the embryo.
In humans, zygote formation occurs after fertilization , in which the released oocyte and a sperm combine to form a single diploid (2n) cell, the zygote.
As soon as fertilization begins, that is, when the sperm enter the oocyte , the second meiosis divides into two new cells: a haploid cell with only 23 chromosomes , almost all of the cytoplasm, and the sperm pronucleus (ovum). ) and another much smaller haploid cell, the polar body, which soon undergoes apoptosis.
In the ovum, the DNA is replicated in the two separate pronuclei of the oocyte and the sperm , so for a brief moment the zygote is tetraploid (4n).
About 30 hours after fertilization, the pronuclei fuse, and the first cleavage produces two diploid (2n) cells, called blastomeres.
Between fertilization and nesting , the embryo is considered a preimplantation concept.
Immediately after the end of fertilization, it moves along the uterus while continuing to cleave. After four divisions, the conceptus consists of 16 blastomeres and is called a morula.
Through the processes of compaction, cell division, and blastulation , on the fifth day of development, the conceptus takes the form of a blastocyst as it approaches the nesting point.
When the blastocyst separates from the zona pellucida, it can implant in the uterine endometrium and initiate embryonic development.
In green plants, the zygote may be polyploid if fertilization occurs between meiotically unreduced gametes.
It forms in a chamber called the archegonium . In seedless plants, the archegonium is usually shaped like a glass globe with a long, hollow tube through which the male gamete enters.
The zygote is then severed and grows within the archegonium .
The zygote will give rise to the sporophyte , the spore-producing stage; The germination of these spores gives rise to the gametophyte, capable of producing haploid gametes.
In spermatophytes , the zygote first develops into an embryo within the seed, while in pteridophytes, the zygote develops directly into the adult plant.
In fungi, the process of karyogamy occurs , in which the fusion of the nuclei of two sexually compatible gametes gives rise to a zygospore , which produces a new hypha whose nuclei may, depending on the life cycle of the species, undergo mitosis or meiosis to generate spores capable of developing into new individuals.
When subjected to environmental stress, such as nitrogen deprivation, in the case of Chlamydomonas , the cells are induced to form gametes.
In the Plasmodium vivax protista , one of its infective forms is called a merozoite , whose cells can generate gametes after meiosis.
The Anopheles mosquito aspirates the sexual forms (macrogametes and microgametes) when it bites an infected human.
In the mosquito intestine, the microgamete exflagellates and fuses with the macrogamete, generating a zygote.
It differentiates into the ookinete , a motile form that crosses the stomach wall and lodges in the basement membrane, differentiating into oocysts and developing into sporozoites, bursting the oocysts and migrating to the salivary glands of the insect, from which they invade a new human host. .
There may be many oocysts in the stomach of Anopheles , but the damage to the stomach wall when the oocysts hatch does not seem to have a negative effect on the longevity of the mosquitoes.
Homozygotes are called ” pure ” because they are characterized by pairs of identical allele genes.
That is, analogous alleles will produce only one type of gamete represented by the same letters (AA, aa, BB, bb, VV, vv), while uppercase letters are called dominant, while lowercase letters are recessive. .
In other words, homozygotes are composed of the same alleles, but they can be recessive or dominant, as is the case with the evidence resulting from Mendel’s Laws.
The so-called heterozygotes or “ hybrids ” correspond to individuals that have different pairs of alleles that determine this characteristic.
To the extent that in heterozygotes the pairs of alleles are different, they are represented by the union of uppercase and lowercase letters, for example, Aa, Bb, Vv.
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