Gamete DefinitionGametes are haploid reproductive cells in sexually reproducing organisms that fuse together during fertilization. Fertilization creates a diploid cell that goes through repeated rounds of cell division to produce a new individual. Gametes are the physical carriers of genetic information from one generation to the next. They carry the recombinant chromosomes that are produced at the end of meiosis.
Oftentimes, sexually reproducing species have two morphologically different types of individuals producing different gametes. The large gamete produced by the female is usually called an egg or egg cell. The smaller one is the sperm. There are similar differences in the plant world, with the female gamete being referred to as the egg cell and the male gamete as pollen.
What are the types of Gametes?
Many species have two types of gametes with different shapes and functions. For example, in humans and other mammals, the egg cell is much larger than the sperm. Sperm have a distinctive tadpole-like appearance with special adaptations for their primary function of traversing the female reproductive tract and fertilizing eggs. Similarly, the egg cell exhibits several structural adaptations that support the process of precise fertilization and subsequent implantation. Species that show a clear difference in the appearance of the gametes are said to have anisogamy.
In addition, most species are also heterozygous – each gamete species has a different set of chromosomes. In mammals, the female gamete contains a single X chromosome in addition to 22 somatic chromosomes. On the other hand, the male gamete, the sperm, can have either an X or a Y chromosome as the 23rd chromosome. Depending on the chromosome present in the sperm, the resulting diploid zygote can be either a female (XX) or a male (XY). In birds, this form of heterosexuality is reversed. Females produce gametes, which can have W or Z chromosomes, and males produce only one type of gamete.
Examples of Gametes
The two most common gametes are the sperm and the egg. These two haploid cells can undergo internal or external fertilization and can differ from one another in size, shape, and function. Some species produce both sperm and eggs within the same organism. They are called bisexual. However, most sexually reproducing organisms are of different sexes, each producing only one type of gamete.
Sperm structure and function
Human spermatozoa are highly specialized cells that have gone through a long phase of differentiation.
As shown in the picture, spermatozoa have four morphological regions – head, neck, midsection and tail. These generic terms actually refer to various subcellular organisms that have been adapted to support the sperm in their function.
For example, the “head” contains genetic material. The DNA in a mature sperm is highly condensed, has practically no transcriptional activity, and all chromosomes are firmly condensed. They also have special proteins called protamines to hold DNA tighter than histones. The head is also surrounded by a cap-like structure that contains hydrolytic enzymes called an acrosome. Acrosomal enzymes act on the outer egg cells and enable the DNA in the sperm to reach the egg cell’s plasma membrane.
The sperm neck consists of two centrioles. The proximal nucleus penetrates the egg cell during fertilization and even replicates within the zygote. The distal centriole creates filamentous structures that form the whip tail of the sperm.
The tail is made up of flagella that allows this cell to travel along the female reproductive tract – from the cervix through the uterus to the fallopian tubes where fertilization can take place. This mobility is also necessary for species that are subject to external fertilization. Sperm flagella consist of a central axonal filament of the cytoskeleton, which is surrounded by 2 fibrous sheaths. The axon contains a pair of elongated microtubules that mediate movement through a motor protein called dynein.
The energy for the flagellum movement is provided by mitochondria arranged in a spiral in the tubular centerpiece. Some of the energy is also obtained from glycolysis, which takes place in the fiber covering of the flagellum. The carbohydrates required for glycolysis, aerobic respiration, and oxidative phosphorylation are transported from the mucous membranes of the female genital tract to the spermatozoa.
Sperm doesn’t have many organelles that are normally seen in most cells. For example, spermatozoa do not have an endoplasmic reticulum or ribosomes because most of the protein and lipid synthesis is completed during spermatogenesis. But even after prolonged differentiation, sperm must go through another process, so-called post-ejaculation, before they are fully functional. It usually involves membrane changes, activation (and inactivation) of certain enzymes, and protein modification.
One major difference between male and female gametes, especially in humans, is the way they are produced in the body. Spermatogenesis begins in the testes after puberty and can persist for the rest of the individual’s life if there is no disease or disorder. Sperm mother cells, also known as spermatogonia, can continuously divide through mitosis, creating cells that differentiate into mature sperm after meiosis. Each diploid spermatocyte can result in 2 haploid cells with one X chromosome and 2 haploid cells with one Y chromosome. All of these 4 nuclei remain connected to each other by cytoplasmic bridges so that spermatids with a Y chromosome can also benefit from the proteins produced by X chromosome gene expression.
The egg cell (ovum, plural: egg cell) is the female germ cell. This is usually an immobile cell. In birds, reptiles, amphibians and invertebrates, the egg is either fertilized from the outside or the egg is laid before a new organism is formed. In mammals, both fertilization and embryonic development take place in the female.
The egg cell is made from oogonia, or egg “mother cells,” through a process called oogenesis in the ovary. The egg cell is not only one of the largest cells in the body, it is also specialized in ensuring exact fertilization by exactly one sperm cell. The egg also contains nutrients that a growing zygote initially receives. In many organisms, these nutrients are viewed as fatty egg yolks and protein-rich albumin. In mammals, however, the egg is implanted in the uterus and, after the first few rounds of mitotic replication, draws its nutrients directly from the mother’s body.
Protective membranes of the egg cell
The human egg contains two important protective layers – the corona radiata, which contains follicle cells, and the zona pellucida. The corona radiation can consist of 2 or 3 layers of cells, while the zona pellucida is a clear, thick membrane made of glycoproteins. The corona radiata has to be overcome enzymatically by sperm before it reaches the zona pellucida. The binding of the sperm to this inner glycoprotein membrane induces the release of hydrolytic enzymes from the acrosome. This mediates the fusion of the sperm membrane with the plasma membrane of the egg cell and facilitates the fertilization of the two haploid nuclei. The release of digestive enzymes and the subsequent steps are known as the acrosomal response and it also provokes a response of the egg membranes. The egg cell forms a vitelline membrane that prevents other sperm from entering. Egg membranes are also believed to play a role in maintaining species specificity during fertilization by preventing the egg membranes from being reached by sperm from another species.
Sex determination in birds
In birds (as well as some fish) the female lays two different types of eggs because they are heterogametic sex. This means that the diploid body cell in adult female birds has two different types of sex chromosomes. These two chromosomes are called the Z and W chromosomes to distinguish them from the XY sex determination system. Men have two Z chromosomes and therefore produce sperm that all have only one Z chromosome. In short, it is the genetic makeup of the egg that determines the sex of the offspring, in direct contrast to the genetics of humans and many other animals.
Each haploid gamete should contain exactly half the number of chromosomes in the somatic diploid cell. However, errors during meiosis can result in gametes having fewer or more chromosomes. When such gametes take part in fertilization, the resulting zygote is aneuploid. Many aneuploid zygotes are not viable. That is, they do not complete the development of the embryo and lead to spontaneous abortion. However, sometimes aneuploidy can lead to disorders that only become apparent after birth. The most common of these is trisomy 21, also known as Down syndrome. It occurs when a haploid gamete contains 2 copies of chromosome 21 – either the entire DNA molecule or large parts of it.
If sex chromosome aneuploidy occurs, it can result in an individual having more than 2 sex chromosomes. Sometimes it can also result in a person having only one X chromosome in all of their cells. These individuals are usually sterile and their external sexual characteristics are often different from their internal genetic structure.
Gamete Definition, Types, Examples Sperm structure, and Function