GAMETOGENESIS

Sexual duplicate genuinely starts off evolved with the formation of the gametes. By definition, the male gamete is little and motile and called a spermatozoon (sperm), and therefore the female gamete is large and immotile and called an egg or ovum. Each gamete contributes a haploid (1n) chromosome set therefore the zygote is diploid (2n), containing a maternal- and a paternal-derived copy of every chromosome. The gametes are formed from germ cells within the embryo. The germ cells are mentioned collectively because the germline, consisting of cells that will or can become the longer term gametes, and everyone other cell are mentioned because of the somatic tissues or soma. The importance of the germline is that its genetic information is often passed to the subsequent generation, while that of the soma cannot.

This means that a germline mutation is one occurring within the DNA of germ cells, which can be carried to the subsequent generation. Against this, a somatic mutation may occur during a cell at any stage of development and should be important within the lifetime of the individual animal, but it cannot affect subsequent generation. it's often the case that the long run germ cells become committed to their fate at an early stage of animal development. In some cases, there's a cytoplasmic determinant present within the egg that programs cells that inherit it to become germ cells. this is often related to a clear specialization of the cytoplasm called plasm. It occurs in Caenorhabditis elegans where the cells inheriting the polar granules become the P lineage and thereafter the germ cells. It occurs in Drosophila, where cells inheriting the pole plasm become pole cells and later germ cells. It also occurs in Xenopus where there's a vegetally localized plasm rich in mitochondria. In other species, there could also be no visible plasm within the egg but germ cells still appear at a comparatively early stage of development.

During embryonic development, germ cells undergo a period of multiplication and may also often undergo migration from the location of their formation to the gonad, which can be a long way away. The gonad arises from the mesoderm and is initially composed entirely of somatic tissues. After the germ cells arrive they become fully integrated into its structure, and in postembryonic life undergo gamete formation or gametogenesis. At some stage in mid-development, the key decision of sex determination is formed and therefore the gonad is decided to become either an ovary or a testis. The molecular mechanism of this is often, somewhat surprisingly, different for each of the principal experimental model species. But the upshot is that within the male the germ cells will need to become sperm and within the feminine, they go to need to become eggs. Unlike the other model organisms, C. elegans is usually a hermaphrodite and thus the germ cells will produce both sperm and eggs within an equivalent individual. However there are also male individuals of C. elegans, and thus the sex determination mechanism controls the male–hermaphrodite decision rather than a male-female decision.

Meiosis

The critical cellular event in gamete production is meiosis. This is often a modified sort of cell cycle during which the amount of chromosomes is reduced by half. As in mitosis, meiosis is additionally preceded by an S-phase during which each chromosome becomes replicated to make two identical sister chromatids, therefore the process starts with the nucleus possessing a complete DNA content of 4 times the haploid complement. In mitosis, the sister chromatids segregate into two identical diploid daughter cells. But meiosis involves two successive cell divisions. Within the first the homologous chromosomes, which are the equivalent chromosome derived from mother and father, pair with one another. At this stage, the chromosomes are mentioned as bivalents, and each consists of 4 chromatids, two maternal-derived and two paternal-derived. crossover can occur between these chromatids, bringing about recombination of the alleles present at different loci. Hence, alleles present at two different loci on the same chromosome of 1 parent may become separated into different gametes and be found in several offspring. The frequency with which alleles on the same chromosome are separated by recombination is roughly related to the physical separation of the loci, and this is often why the measurement of recombination frequencies is that the idea of genetic mapping. Recombination can also occur between sister chromatids but here the loci should all be identical because they have just been formed by DNA replication, so there aren't any genetic consequences.

Within the primary meiotic division, the four-stranded bivalent chromosomes separate into homologous pairs, which are segregated to the two daughter cells. there is no further DNA replication and within the second meiotic division, the two chromatids of each chromosome become separated into individual gametes.

It should be noted that the terms haploid (1n) and diploid (2n) are normally used to ask the number of homologous chromosome sets within the nucleus rather than the actual amount of DNA. After DNA replication a nucleus contains twice the utmost amount of DNA as before but retains the same ploidy designation.

Oogenesis

The process of the formation of eggs is named oogenesis. Following sex determination to female, the germ cells become oogonia, which continue mitotic division for a period. After the ultimate mitotic division, the reproductive cell becomes referred to as an oocyte. It's called a primary oocyte until completion of the primary meiotic division, and a secondary oocyte until completion of the second meiotic division. After this, it's referred to as an unfertilized egg or ovum. altogether the vertebrate organisms considered during this book, fertilization occurs before completion of the second division, so it's technically an oocyte instead of an egg that's being fertilized. However, the term “egg” is usually used rather loosely to ask about oocytes, fertilized ova, and even early embryos.

Eggs are larger than sperm and therefore the process of oogenesis involves the buildup of materials within the oocyte. Usually, the first oocyte may be a rather long-lived cell that undergoes a substantial increase in size. Its growth could also be assisted by the absorption of materials from the blood, like the yolk proteins of fish or amphibians that are made within the liver. it's going to even be assisted by direct transfer of materials from other cells. this is often seen in Drosophila where the last four mitoses of every oogonium produce an egg chamber containing one oocyte and 15 nurse cells. The nurse cells then produce materials that are exported to the oocyte. Animals that produce plenty of eggs usually maintain a pool of oogonia throughout life capable of generating more oocytes. Mammals differ from this pattern as they produce all their primary oocytes before birth. In humans, no more oocytes are produced after the seventh month of gestation, and therefore the primary oocytes then remain dormant until puberty.

Ovulation refers to the resumption of the meiotic divisions and therefore the release of the oocyte from the ovary. it's provoked by hormonal stimulation and involves a breakdown of the oocyte nucleus (the germinal vesicle) and therefore the migration of the cellular division spindle to the periphery of the cell. The meiotic divisions don't divide the oocyte into two halves, but instead, end in the budding off of small polar bodies. the first meiotic division divides the primary oocyte into the secondary oocyte and therefore the first cell, which may be a small projection containing a replicated haploid chromosome set (i.e. 1n information content and 2x DNA content). The second meiotic division divides the secondary oocyte into an egg and a second cell, which consists of another small projection enclosing a haploid chromosome set (in this case 1n information content and 1x DNA content). The polar bodies soon degenerate and play no further role in development.

Spermatogenesis

If the method of sex determination yields a male then the germ cells undergo spermatogenesis. Mitotic germ cells within the testis are referred to as spermatogonia. A number of these are stem cells which will both produce more of themselves and also produce progenitor cells, which divide a variety of times before differentiation into sperm. After the last mitotic division, the male reproductive cell is understood as a primary spermatocyte. Meiosis is equal, the primary division yielding two secondary spermatocytes and therefore the second division yielding four spermatids, which mature to become motile spermatozoa.

References :

Essential Developmental Biology By Jonathan M.W. Slack.