Gametes

Gametes are the reproductive cells of an organism. These cells allow species to produce offspring through a process known as fertilization. A type of cell division called meiosis produces gametes in the reproductive organs of certain species. Meiosis is a two-part process that involves many phases. A male gamete and a female gamete unite to form a zygote during sexual reproduction. This sexual reproduction involves sets of chromosomes from each gamete.

Overview

Gametes, which are also called sex cells, are found in species of the Eukaryota domain. Members of this domain include many species of plants, animals, fungi, and protists. Protists are a large group of species that are not classified as plants, animals, or fungi. Specific examples of members of the Eukaryota domain include humans, monkeys, trees, plankton, and mushrooms. All of the species in this domain have cells called eukaryotic cells, which have nuclei that contain chromosomes. A chromosome is a long, stringy collection of genes. Genes are segments of DNA, or hereditary information. The Eukaryota domain is the only domain that has organisms composed of eukaryotic cells.

A gamete is a haploid cell, which means that it contains one set of chromosomes. There is one set of twenty-three chromosomes in a haploid human cell. On the other hand, a diploid cell has two sets of chromosomes. Therefore, a diploid human cell contains two sets of twenty-three chromosomes, or forty-six chromosomes in all.

Meiosis

A type of cell division called meiosis produces gametes. Meiosis occurs only in the reproductive organs of eukaryotic organisms. During the first part of meiosis, which is called meiosis I, four gametes are created. These gametes are called daughter cells. First, a diploid parent cell undergoes DNA replication. Next, there are two cycles of nuclear division, which produces the four gametes. In this sense, meiosis actually involves two cell divisions. In a phase called prophase I, chromosomes pair up and form synapses. These paired chromosomes are referred to as bivalents. Each bivalent has two chromosomes and four chromatids. Chromatids are the two identical chromosomal strands that result when a chromosome splits. Chiasmata also form during this phase. Chiasmata are points of overlap in paired chromatids where fusion and exchange of genetic material occurs. In prometaphase I, the nuclear membrane disappears, and the chromosomes that are attached to spindle fibers move. During metaphase I, bivalents align randomly. In anaphase I, chiasmata split, and chromosomes move to opposite poles.

There are two possible outcomes in telophase I—the nuclear envelopes will reform or the cell will begin meiosis II. During cytokinesis, two daughter cells are formed. The second part of meiosis, called meiosis II, then begins in these two new cells. Meiosis II is similar to meiosis I. In prophase II of meiosis II, the nuclear membrane and nuclei may separate. Additionally, the chromosomes move to the center of the cell. During metaphase II, the chromosomes align at the cell’s center. In anaphase II, the sister chromatids split, and they move to opposite poles. In telophase II, nuclei are formed at the poles. Lastly, during cytokinesis, two additional daughter cells form.

Once meiosis is complete, there are four gametes. Each gamete contains a single set of chromosomes, which is half of the total number of chromosomes in the original diploid parent cell. Half of the chromosomes come from the mother and half come from the father. Although the parent cell that produces the gametes has two varieties of each gene, the gametes themselves have only one copy of each gene. These genes are chosen randomly during meiosis, so one of the two varieties is passed on to the offspring.

Male and Female Gametes

Male gametes are referred to as sperm, while female gametes are referred to as ova or eggs. A single sperm cell is called a spermatozoon, and a single egg cell is called an ovum. Sperm are produced in the testes, which are the two reproductive organs found in males. Ova are produced in the ovaries, the two reproduction organs found in females. Sperm have a flagellum, which is a tail-shaped projection that allows the sperm to move. Ova, on the other hand, do not move and are much larger than sperm.

Fertilization

A spermatozoon and ovum join together during sexual reproduction in a process called fertilization. In general, only gametes from the same species can unite and produce offspring. The united spermatozoon and ovum form a cell called a zygote, which is a diploid cell. The zygote, therefore, contains two sets of chromosomes. One set of chromosomes is from the spermatozoon, and the other set is from the ovum. In other words, half of the zygote’s chromosomes come from the mother, and the other half come from the father. The zygote eventually develops into an organism. The zygote’s chromosomes ultimately determine how the organism will develop.

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