Germ cells are unipotent stem cells that divide to produce gametes in sexually reproducing organisms. A germ cell undergoes meiotic cell division to produce genetically unique, haploid sex cells, which then fuse during fertilization to form a diploid zygote. In female organisms, germ cells give rise to egg cells and, in males, they produce sperm cells.
What is a Germ Cell?
Germ cells are the cells that give rise to gametes in all sexually reproducing organisms. In vertebrates, they are the precursors of male sperm cells and female egg cells. Collectively, all the germ cells in an organism are known as the germline.
The germline is separated from the somatic (AKA body) cell line in the early stages of embryonic development. The two cell lines have very different functions; somatic cells make up all of the body’s structures, whereas germ cells produce gametes for the transmission of genetic information to offspring. The key defining feature of germ cells is that they divide by meiosis, unlike all other cells of the body which divide mitotically.
Germ cells are unipotent stem cells, as they can only become one other type of cell (an egg or a sperm). Once fused together, however, they form a zygote with totipotent potential. (i.e., it can give rise to all other types of body cell).
Function of Germ Cells
Germ cells are the type of stem cell that gives rise to gametes. They are, therefore, the origin cells of all sexually reproducing organisms, and allow individual members of a species to pass on genetic information to their offspring. The inheritance of DNA is the driving force behind natural selection and evolution, and the fact that germ cells divide by meiosis ensures maximal genetic variation among gametes.
Development of Germ Cells
In humans, germ cells originate from stem cells called primordial germ cells (PGCs) in a region of the embryo called the epiblast. The PGCs migrate into the developing gonads, where they divide by meiosis and become either sperm or egg cells.
Meiosis produces non-identical, haploid cells that each contain a single set of chromosomes. During fertilization, haploid egg and sperm cells fuse to produce a zygote containing DNA from both parents. This genetic information is preserved in the germline of the new organism and passed on over further generations of reproduction.
Germ Cells and Meiotic Division
Germ cells are the only cells in the body to divide by meiosis, as opposed to mitosis. Whereas mitosis produces two, diploid, genetically identical daughter cells, meiosis gives rise to four, haploid, genetically unique daughter cells.
Germ cells are diploid cells that divide to become gametes, which must be haploid to ensure that the offspring ends up with the correct number of chromosomes. Therefore, the chromosome number of germ cells is halved by meiosis. The fact that meiosis produces genetically unique cells is also an important aspect of gamete production, as this ensures greater genetic diversity within the species.
Stages of Meiosis in Germ Cells
Meiosis takes place in two stages; meiosis I, and meiosis II. DNA replication takes place in the interphase stage of the cell cycle, before meiosis I begins. The germ cell divides twice during one complete round of meiosis, producing four haploid gametes as a result.
By the time a germ cell enters prophase I, it has already made copies of all its DNA. At this stage, each chromosome in the germ cell consists of two genetically identical sister chromatids, which are joined together by a centromere.
During prophase I, the meiotic spindle is formed and the chromosomes condense and form homologous pairs. The homologous chromosome pairs exchange bits of genetic material with one another in a process called crossing over. This ensures that the sister chromatids are no longer genetically identical.
The homologous pairs line up along the middle of the germ cell, and spindle fibers attach themselves to one chromosome from each pair.
The meiotic spindle fibers pull on the chromosomes, dragging one from each pair to opposite poles of the cell. At this point, the sister chromatids are still attached to one another.
Telophase I and Cytokinesis
The two sets of chromosomes gather at either end of the cell. The cytoplasm of the cell splits in two, creating two non-identical daughter cells(this is cytokinesis). The daughter cells produced by meiosis I are haploid, as they contain only half the number of chromosomes as the original germ cell.
The two haploid daughter cells produced by meiosis I undergo further cell division in meiosis II. During prophase II, the chromosomes condense once more, and the meiotic spindle reforms.
The chromosomes line up along the middle of the cell. Spindle fibers attach themselves to each of the sister chromatids.
The spindle fibers pull the sister chromatids apart, and one from each chromosome moves to either end of the cell.
Telophase II and Cytokinesis
The two cells divide to produce four, genetically unique cells, each with a single set of chromosomes. In male organisms, meiosis produces four sperm cells and is called spermatogenesis. In females, meiosis produces a single egg cell and four polar bodies (small, haploid cells that cannot be fertilized), and is called oogenesis.
Location of Germ Cells
Germ cells are found in the gonads (or sex organs) or an organism. In male vertebrates, they are found in the testes and, in female vertebrates, they reside in the ovaries.