The genetic material of the sperm is responsible for the traits of the progeny like the color of eyes, hair, and skin. Sperm contains X and Y chromosome that determines the sex of the progeny. The egg cell or the ovum is the ovoid or spherical non-motile gamete produced in the female reproductive structure known as the ovaries. The size of an egg is larger than the sperm.
A human egg usually has a diameter of about 0. In fish and frogs, it is aroundmm. Egg or ova is a haploid gamete and in humans, it contains 23 chromosomes. Egg contains cytoplasm which is known as ooplasm. The cytoplasm of the egg has two parts i. The human egg is described as alecithal as it contains a very small quantity of nutritive yolk. This is a contrasting feature to the eggs of birds wherein the cytoplasm is rich in nutritive yolk which is made up of lipoproteins, pigment granules, and water.
The cytoplasm contains the nucleus of the egg, known as the germinal vesicle, and a vacuole known as the germinal spot. The nucleus of the ovum is large, swollen with nucleoplasm , and is eccentric in position. Thus, giving polarity to the human ovum i. The animal pole is the side where the nucleus and polar body of the ovum is present, while the opposite side to it is known as the vegetal pole.
The cytoplasm is enclosed in a peripheral layer known as the cortex, which is made up of microvilli and cortical granules. The ovum is encased in a thick, transparent envelope known as the zona striata or zona pellucida.
Below the zona pellucida, is a thin layer known as the vitelline membrane. There is a narrow space between the zona pellucida and the vitelline membrane which is known as perivitelline space.
While the outermost layer, which is above the zona pellucida is known as the corona radiata , which radiates from the egg surface. The primary function of the ovum is to carry the genetic material, i. It also provides the required environment that enables its fertilization by the sperm. Post-fertilization, the nutrients of the ovum are required for the growth of the zygote.
Oogenesis is the process of differentiation of the ovum that occurs in the female reproductive organ known as the ovaries. The process of oogenesis differs from species to species. In humans, ova are developed from the germ cells that are present from the time of the birth of a female child. Hence, there are two broad stages of the oogenesis that occur:. During each menstrual cycle, the maturation of an oocyte occurs resulting in the formation of an ovum through division.
This division results in cells of unequal size i. Genetically, all mammals are diploids as a result of the fusion of two haploid gametes. The haploid gamete cells ensure that in every generation the genetic content or the number of the chromosomes remains constant. If the gamete cells are not haploid, then every subsequent generation will have double the chromosome number or the genetic material of the previous generation.
It is important to note that a non-diploid state is the characteristic of cancer cells. The excess or a lack of a pair of chromosomes could result in destabilization during the cell replication process. This, in turn, can eventually result in a disease state, such as cancer.
Ploidy alteration in humans and most mammals is usually fatal. Apart from this, haploids are used for crop improvement especially in rice and tobacco as haploids can be produced in a very short span of time. Haploids are thus helpful in reducing the breeding cycle and creating novel genetic compositions for crop improvement. Haploids are also a very useful cytological tool especially for studying mutations and genetic disorders.
In mammals, primary sex determination is the determination of the gonads and is determined by chromosomes and not the environment. It is the presence of a Y chromosome that determines sex in placental mammals. Normally, female cells contain two X chromosomes, i. Thereby, each of the eggs from females contains single X chromosomes while the male sperms are of two types, one that contains the X chromosome and the other which contains the Y chromosome. Thus, the fusion of the egg containing the X chromosome with the sperm containing the X chromosome results in the formation of female progeny with XX chromosomal composition.
Conversely, the male gamete containing the Y chromosome that fuses with the ovum containing the X chromosome will result in the formation of a zygote with sex chromosomes, XY, which will develop into a male offspring. The Y chromosome carries an SRY gene encoding the testis-determining factor resulting in the formation of testes in the male offspring.
In birds, Z and W chromosomes are the sex determinants and the females are the ones that are heterogametic i. The size of the Z chromosome is larger than W chromosomes. Two genes, i. This process of sex determination is not clear in chickens. However, like mammals, in chickens, the differentiation of gonads into male or female reproductive organs occurs after some time post-birth.
Chicken needs estrogen for sex determination. Interestingly, if eggs are injected with estrogen during the developmental phase, then a male chicken can be converted to a female chicken.
Some reptiles, fishes, and amphibians also exhibit ZW chromosomal sex determination. Different insects have different patterns of sex determination. In butterflies and moths order Lepidoptera , females are heterogametic and males are homogametic.
One division occurs to make two cells. Four stages of this division. Two divisions occur to make four cells. Eight stages in these divisions. Similar or homologous chromosomes do not pair.
Homologous chromosomes pair during prophase l. Pairing is called synapsis. Sexual reproduction uses the process of meiosis , which creates gametes. The process of meiosis happens in the male and female reproductive organs. As a cell divides to form gametes:. Gametes have half the total number of chromosomes that the organism needs to develop and are referred to as haploid.
For example, humans need 46 chromosomes to develop, therefore a human gamete has 23 chromosomes. Fertilisation is the fusion of the nucleus of a male gamete with the nucleus of a female gamete. These mechanisms include DNA methylation, histone tail modifications and association with Polycomb and Trithorax proteins. Several cells within the organism must however maintain or regain developmental competence while they are highly specialized.
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