Gametogenesis Study Guide

Introduction

In 1678, Anton van Leeuwenhoek, a Dutch microscopist who co-discovered sperm, believed semen consists of parasitic material. He thought that sperm had nothing to do with reproduction of new life. But, with time and experiments, he understood that each sperm consists of an embryo.

Gametogenesis is derived from two Greek words, “gamete” and “genein.” “Gamete” means “wife”, “gamein” means “to marry” whereas “genein” means “to produce.” We are going to learn about gametogenesis and the various processes involved.

What is Gametogenesis?

Gametogenesis is a biological process in which diploid and haploid precursor cells undergo cell division to form new organisms. They form mature haploid gametes by the meiotic division of diploid gametes. The male gametes, or as we refer to them as sperms, are produced by testes. The male reproductive organ consists of two round globular shaped sac-like structures located below the penis. The female gametes called ova or eggs are produced by ovaries. They are two oblong orange-like structures present on each side of the uterus, in the lower abdomen.

The primitive germ cells present in female and male reproductive organs initiate gametogenesis. The germ cell consists of 46 chromosomes, which hold the DNA or the genetic information, and this genetic information influences the body structure and intellect of the new organism.

Process of Gametogenesis

The genetic code determines the sex of the organism. It also helps determine the height, eye color, hair type, etc. All this information is stored in chromatin fibers. Chromatin is a complex of DNA and protein.

The genetic information is condensed in the nucleus of each cell by tightly coiled chromatin around proteinaceous scaffolds, which are known as histones. There are 23 pairs of chromosomes present in somatic cells — chromosomes 1 to 22 occur in duplicates, while the remaining two chromosomes are sex chromosomes X and Y.

Gametogenesis in the Human Body

Gametogenesis consists of different phases in generating the new organism from the male and female gametes. Depending on the biological life cycle of the organism, gametogenesis occurs by meiotic division of diploid gametocytes into various gametes, or by mitosis.

Meiosis is the first cell division between four haploid cells and one diploid cell. It is different from other cell divisions like mitosis. Meiosis occurs in two distinctive phases: meiosis I and meiosis II.

Meiosis I:

Interphase

It is the growth phase of the cell, where the DNA is present in heterochromatin or euchromatin. During the interphase, each chromosome is replicated to produce two identical copies, called sister chromatids, that are held together at the centromere by cohesin proteins. Interphase also represents the time of cellular growth where the cell duplicates in preparation for cell division. Hence, the parent cell is converted from diploid state to tetraploid state, i.e., having 92 chromosomes.

Prophase I

After completing the interphase, the cells enter the prophase I state. This phase consists of the alignment of homologous pairs of chromosomes and the exchange of DNA.

Prophase I can be further divided into the following sub-stages:

• Leptotene stage — The beginning of the exchange of genetic materials occurs from the parent cells.

• Zygotene stage — The homologous chromosomes get attached near the telomere region. This phenomenon is known as synapsis.

• Pachytene stage — The chromosomes appear as one structure joined at the synaptonemal complex. In this phase, recombination of the chromosomes occurs.

• Diplotene stage — The synaptonemal complex begins to disappear. Here the chromosomes become shorter. This stage occurs during the 5th gestational week in the female body. The gametes are arrested in diplotene until the beginning of puberty, where one ovum will complete meiosis before the onset of the menstrual cycle.

• Diakinesis stage — This stage us analogous to the prometaphase stage of meiosis. It is characterized by the termination of the condensing of the chromosomes. The bivalents attached to the spindle fibers begin to move along the equatorial plate of metaphase.

Metaphase I

After the completion of prophase I (the completion of diakinesis), is the beginning of metaphase I. In this stage, the nuclear envelope is completely disintegrated, the spindle fibers from opposing centrosomes connect to bivalents and align them along the center of the cell.

Anaphase I

Disjunction occurs, and the bivalents split. Homologous chromosomes move to opposite poles of the cell.

Telophase I

The nuclear membrane might reform, cytokinesis (cell division) occurs to form two haploid daughter cells. The two daughter cells will have genetically different chromosomes. This stage occurs differently in males and females. The cytoplasmic division is asymmetric in females. It produces larger oocytes whereas, in males, the spermatocytes hang on to the cytoplasmic bridge.

Meiosis II:

In meiosis, four gametes are formed from a single cell. Hence, after the completion of telophase I, meiosis II (the second round of cell division) occurs. Meiosis II resembles mitosis more than meiosis I as it is a simple division with no crossing over. No DNA replication occurs, and the number of chromosomes remains unchanged during meiosis II.

Types of Gametogenesis

###Spermatogenesis

It is basically sperm formation. In males, testes are the producers of immature germ cells. When puberty hits these germ cells also known as spermatogonia are transformed into sperms with the help of “Spermatogenesis” process.

Primary spermatocytes experience meiosis and create haploid cells that are known as secondary spermatocytes. Afterwards, the secondary spermatocytes experience 2nd meiotic division to create spermatids.

Next, the spermatids go through Spermatogenesis to transform into sperms. There are various hormones involved in reviving Spermatogenesis. These hormones include LH, GnRH, Androgens, and FSH.

###Oogenesis

It is basically ovum formation. In females, the process of oogonia transforming into mature ovum is known as Oogenesis. In the ovary, there are a huge number of oogonia formed while the foetus is developing. These oogonia goes through meiotic division which rests at prophase-I and further lead to produce primary oocytes.

The primary oocytes are engraved within the primary follicles and these follicles are surrounded by granulosa cell layers, which ultimately form secondary follicles. Now, the secondary follicles become the tertiary follicle.

When puberty hits, the primary oocytes complete meiosis in the tertiary follicles and form haploid or secondary oocytes. Then the tertiary follicle goes through functional and structural changes and produces Graafian follicles.

Finally the secondary oocytes go through 2nd meiotic division and form ovum. The ovum is released by the Graafian follicle when the menstrual cycle is going on. Hence, this release of ovum is called ovulation and it is controlled by female reproductive hormones.

###Significance of Gametogenesis

Gametogenesis is essential because it produces eggs and sperms via meiosis or mitosis from a diploid or haploid cell respectively. Moreover, these gametes are essential because they allow the formation of genetically-varied organisms or offsprings.

Conclusion

The process of gametogenesis shows us the different stages that occur when a parent cell divides and forms a new cell in the body. It gives a complete idea of the phases invloved in the formation of mature haploid gametes.

FAQs:

1. What is the process of gametogenesis?

The process of gametogenesis involves the production of gametes from haploid precursor cells. It is different in human and plant bodies, and it involves a lot of stages.

2. What is human gametogenesis?

The development of male and female germ cells in the human body, which is required to form an individual organism, is known as human gametogenesis.

3. What are the two types of gametogenesis?

Spermatogenesis and oogenesis are the two types of gametogenesis.

4. What is gametogenesis and oogenesis?

Gametogenesis is the formation of new organisms from the male and female germ cells. In contrast, oogenesis is the growth process of primary eggs or ova into mature ova in the female reproductive organs.

5. What are homogametes?

Homogametes are those gametes that resemble both anatomically and morphologically. They are very similar to each other.

6. What are meiocytes? Explain their role.

Meiocytes are specialized cells in diploid organisms that differentiates into a gamete through the process of meiosis. Through meiosis, the diploid meiocyte divides into four genetically different haploid gametes and thus help maintain the chromosome number.

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