Germ Cells: Definition, Types, & Function

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Germ Cell Definition

The unipotent stem cells that can divide and produce gametes are called germ cells. The gametes of haploid sex cells are produced by a germ cell, which undergoes meiotic cell division and produces genetically unique gametes. These gametes fuse during fertilization and form diploid zygotes. The germ cells produce eggs in females and sperms in male organisms.

What is Germ Cell?

Gametes are formed by the germ cells in all sexually reproducing organisms. Invertebrates are the precursors of male sperm cells and female egg cells. The germline is collectively called to all the germ cells in an organism.

In the early stages of embryonic development, the germline separated from the somatic cell line. Both the cell lines have very different functions. All the body structures are made up of somatic cells, whereas the gametes are made of germ cells and help in the transmission of genetic information to offspring.

The main difference between germ cells and sperm cell is, the germ cells divide by meiosis whereas somatic cells divide by mitotic division. The term unipotent means they can only become one other type of cells such as germ cells such as an egg or a sperm.

The gametes fuse and form a diploid zygote during fertilization, which is a totipotent cell. The totipotent cell can give rise to all other types of body cells.

Function of Germ Cell

Germ cells can give rise to gametes. Thus, a person can pass the genetic information to their offspring by germ cells and they are the original cells of all sexually reproducing organisms. Natural selection and evolution occur due to the inheritance of DNA and maximal genetic variation among gametes are ensured by the fact that germ cells divide by meiosis.

Development of Germ Cell

In humans, the stem cells known as primordial germ cells (PGCs) give rise to the germ cells. They originate in the epiblast region of the embryo. After migration of PGCs into the developing gonads, they divide by meiosis and give rise to either sperm or egg cells.

The gametes produced by meiosis are non-identical, haploid cells, which contain a single set of chromosomes. The zygote is formed by the fusion of haploid egg and sperm during fertilization, which contains DNA from both parents. The germline preserves the genetic information in the offspring which further passes on over generations of reproduction.

Germ Cell and Meiotic Division

Meiotic division only takes place in the germ cells of the body. Meiosis produces four, haploid, non-identical gametes, while mitosis produces two, diploid, genetically identical daughter cells.

Originally, germ cells are diploid cells and become haploid after meiotic division to form gametes. The gametes should be haploid to ensure that the offspring get the correct number of chromosomes are fertilization. Thus, the germ cell undergoes meiotic division to halve the number of chromosomes.

The gametes produced by meiosis are genetically unique, which ensures the genetic diversity within the species.

Stages of Meiosis in Germ Cell

There are two stages in meiosis: meiosis I, and meiosis II. During the interphase stage of the cell cycle, the DNA replicates to make its copy. During a complete round of meiosis, the germ cell divides twice and produces four haploid gametes.

i. Meiosis I

Prophase I: The DNA has already replicated, when the cell enters prophase I. At this stage, each chromosome consists of two sister chromatids that are identical and connected by a centromere.

The formation of the meiotic spindle takes place during prophase I and after the condensation of chromosomes, homologous pairs are formed. The process of crossing over occurs between the homologous chromosome pairs, in which a few genetic material exchanges. This ensures that the sister chromatids are not genetically identical.

Metaphase I: During metaphase, the homologous pairs line up in the middle of the cell and the chromosomes attach to spindle fibers.

Anaphase I: The chromosomes are pulled by the meiotic spindle fibers and drag one chromosome from each pair to opposite poles of the cells. The sister chromatids are still attached.

Telophase I and Cytokinesis: The chromosomes reach the end of the cell and the cytoplasm of the cell splits by the process of cytokinesis. It creates two non-identical daughter cells. In meiosis I, the haploid cells are produced with half of the number of chromosomes as the original germ cell.

i. Meiosis II

Prophase II: In meiosis II, the daughter cells produced by meiosis I undergo further division. During prophase II, the condensation of chromosomes takes place, and meiotic spindle forms.

Metaphase II: The chromosome is lined up along the middle of the cell and then each sister chromatid attaches to the spindle fibers.

Anaphase II: The sister chromatids are pulled by the spindle fibers towards the opposite poles of the cell and one from each chromosome move to opposite ends of the cell.

Telophase II and Cytokinesis: During telophase, the chromosomes reach the ends of the cell, and then the cytoplasm of the divided by cytokinesis. The two cells, which undergo meiosis II, produce four non-identical haploid cells.

In male organisms, four sperm cells are produced by meiosis or the process called spermatogenesis, while in females, the germ cell produces a single egg cell and four polar bodies, which cannot be fertilized. The process is known as oogenesis in females.

Location of Germ Cell

In an organism, the germ cells are found in the gonads. They are found in the testes of male vertebrates and female vertebrates, they are located in the ovaries.

Germ Cell Citations

Germ Cells Are Forever. MINIREVIEW| VOLUME 132, ISSUE 4, P559-562, FEBRUARY 22, 2008.

Germ line is the sex cells


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