The cells that form gametes are germ cells, as opposed to somatic cells. The ploidy (number of chromosomes) of germ cells is the same as somatic cells (only during the formation of gametes does meiosis occur, reducing the number of chromosomes to half). Gametogenesis Review. The formation of gametes The gametes are genetically different from the parent cells. During fertilisation, a sperm fuses with an ovum to form zygote. As the sperm & ovum have a haploid number of chromosomes (n = 23) each, the zygote formed will have a diploid number of chromosomes (2n = 46) During mitosis, the zygotes then divides mitotically to produce millions of cells, each cell having 46 chromosomes.
- Gametes
- What Happens During Gamete Formation
- Formation Of Gametes In Humans Body
- Role Of Gamete Formation
Gametes are reproductive cells (sex cells) that unite during sexual reproduction to form a new cell called a zygote. Male gametes are sperm and female gametes are ova (eggs). In seed-bearing plants, pollen is the male sperm producing gametophyte. Female gametes (ovules) are contained within the plant ovary. In animals, gametes are produced in male and female gonads. Sperm are motile and have a long, tail-like projection called a flagellum. However, ova are non-motile and relatively large in comparison to the male gamete.
Gamete Formation
Gametes are formed by a type of cell division called meiosis. This two-step division process produces four daughter cells that are haploid. Haploid cells contain only one set of chromosomes. When the haploid male and female gametes unite in a process called fertilization, they form what is called a zygote. The zygote is diploid and contains two sets of chromosomes.
Gamete Types
Some male and female gametes are of similar size and shape, while others are different in size and shape. In some species of algae and fungi, male and female sex cells are almost identical and both are usually motile. The union of these types of gametes is known as isogamy. In some organisms, gametes are of dissimilar size and shape. This is known as anisogamy or heterogamy (hetero-, -gamy). Higher plants, animals, as well as some species of algae and fungi, exhibit a special type of anisogamy called oogamy. In oogamy, the female gamete is non-motile and much larger than the male gamete.
Gametes and Fertilization
Fertilization occurs when male and female gametes fuse. In animal organisms, the union of sperm and egg occurs in the fallopian tubes of the female reproductive tract. Millions of sperm are released during sexual intercourse that travel from the vagina to the fallopian tubes. Sperm are specially equipped for fertilizing an egg. The head region contains a cap-like covering called an acrosome that contains enzymes which help the sperm cell penetrate the zona pellucida (outer covering of an egg cell membrane). Upon reaching the egg cell membrane, the sperm head fuses with the egg cell. Penetration of the zona pellucida triggers the release of substances that modify the zona pellucida and prevent any other sperm from fertilizing the egg. This process is crucial as fertilization by multiple sperm cells, or polyspermy produces a zygote with extra chromosomes. This condition is lethal to the zygote.
Upon fertilization, the two haploid gametes become one diploid cell or zygote. In humans, this means that the zygote will have 23 pairs of homologous chromosomes for a total of 46 chromosomes. The zygote will continue to divide by mitosis and eventually mature into a fully functioning individual. Whether or not this individual will be male or female is determined by the inheritance of sex chromosomes. A sperm cell may have one of two types of sex chromosomes, an X or Y chromosome. An egg cell only has one type of sex chromosome, an X chromosome. Should a sperm cell with a Y sex chromosome fertilize an egg, the resulting individual will be male (XY). Should a sperm cell with an X sex chromosome fertilize an egg, the resulting individual will be female (XX).
What is Oogenesis ?
Oogenesis is the process of the formation of a mature ovum from the oogonia in females. It takes place in the ovaries. During oogenesis, a diploid oogonium or egg mother cell increases in size and gets transformed into a diploid primary oocyte. This diploid primary oocyte undergoes first meiotic division i.e., meiosis I or reductional division to form two unequal haploid cells. The smaller cell is known as the first polar body, while the larger cell is known as the secondary oocyte. This secondary oocyte undergoes second meiotic division i.e., meiosis II or equational division and gives rise to a second polar body and an ovum. Hence, in the process of oogenesis, a diploid oogonium produces a single haploid ovum while two or three polar bodies are produced.
The three stages of Oogenesis :-
- Stage # 1. Multiplication Phase:
The primordial germinal cells divide repeatedly to form the oogonia. The oogonia multiply by the mitotic divisions and form the primary oocytes which pass through the growth phase.
- Stage # 2. Growth Phase:
The growth phase of the oogenesis is comparatively longer than the growth phase of the spermatogenesis. In the growth phase, the size of the primary oocyte increases enormously.
In the primary oocyte, large amount of fats and proteins becomes accumulated in the form of yolk and due to its heavy weight (or gravity), it is usually concentrated towards the lower portion of the egg forming the vegetative pole.The cytoplasm of the oocyte becomes rich in RNA, DNA, ATP and enzymes. Moreover, the mitochondria, Golgi apparatus, ribosomes, etc., become concentrated in the cytoplasm of the oocyte.During the growth phase, tremendous changes also occur in the nucleus of the primary oocyte. The nucleus becomes large due to the increased amount of the nucleoplasm and is called germinal vesicle.
Gametes
The chromosomes change their shape and become giant lamp-brush chromosomes which are directly related with increased transcription of messenger RNA (mRNA) molecules and active protein synthesis in the cytoplasm. When the growth of the cytoplasm and nucleus of the primary oocyte is completed, it becomes ready for maturation phase.
- Stage # 3. Maturation Phase:
The maturation phase is accompanied by the maturation or meiotic division. The maturation division of the primary oocyte differs greatly from the maturation division of the spermatocyte. Here after the meiotic division of the nucleus, the cytoplasm of the oocyte divides unequally to form a single large-sized haploid egg and three small haploid polar bodies or polocytes at the end.
What Happens During Gamete Formation
This type of unequal division has the great significance for the egg. If the equal divisions of the primary oocyte might have been resulted, the stored food amount would have been distributed equally to the four daughter cells and which might prove insufficient for the developing embryo.
Formation Of Gametes In Humans Body
Therefore, these unequal divisions allow one cell out of the four daughter cells to contain most of the cytoplasm and reserve food material which is sufficient for the developing embryo.
Role Of Gamete Formation
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