Reproductive cell / Sex Cell

Sex cells, or gametes, are unique to organisms that reproduce sexually. In animals and plants (fungi are somewhat different in this regard) there are two types of sex cells: male and female. The male sex cells are sperm, while the female sex cells are eggs. Sex cells are formed from special body cells that are typically located in sex organs. In most animals, sperm are formed in the testes of males, and eggs are formed in the ovaries of females.

Sex cells contain only half of the hereditary material present in the body cells that form them. This is important because male and female sex cells ultimately join to become a fertilized egg, which gives rise to a new organism, or offspring. In order for the offspring to resemble its parents, its first cell must receive the entire genome from its two parents. For humans, we know there are 46 chromosomes in body cells existing as 23 pairs. A fertilized egg must therefore contain this same number and arrangement. In an elegant process called meiosis, each sex cell receives one member of each chromosome pair—23 total. When sperm fertilizes egg, these singles unite to reform pairs, with half the genome coming from each parent. With a few exceptions, this pattern holds true for all sexually reproducing organisms.

How are sex cells produced

Sex cells are produced from special body cells that contain the entire genome. The process by which the genome is halved is very precise — it’s not just a matter of randomly dividing the chromosomes into two sets. The process involves two cell divisions. Before the first occurs, all of the chromosomes are duplicated just as they are in body cell reproduction, but what happens next is different: the two duplicated strands remain attached to each other as the members of each chromosome pair move alongside each other. During the cell division that follows, only one member of each pair is transferred to each daughter cell—this is where the number of chromosomes is halved. The two strands of each chromosome are then separated during the second cell division, still maintaining half the number that existed in the parent cell. This results in four daughter cells — sperm or egg — that contain one member of each chromosome pair. This process is called meiosis.

Organisms that reproduce sexually do so via the production of sex cells also called gametes. These cells are very different for the male and female of a species. In humans, male sex cells or spermatozoa (sperm cells), are relatively motile. Female sex cells, called ova or eggs, are non-motile and much larger in comparison to the male gamete. When these cells fuse in a process called fertilization, the resulting cell (zygote) contains a mix of inherited genes from the father and mother. Human sex cells are produced in the reproductive system organs called gonads. Gonads produce sex hormones needed for the growth and development of primary and secondary reproductive organs and structures.

Human Sex Cell Anatomy

 The male gametes (sperm) are approaching a female gamete (an unfertilized egg) prior to conception. Credit: Science Picture Co/Subjects/Getty Images

Male and female sex cells are dramatically different from one another in size and shape. Male sperm resembles long, motile projectiles. They are small cells that consist of a head region, midpiece region, and tail region. The head region contains a cap-like covering called an acrosome. The acrosome contains enzymes that help the sperm cell penetrate the outer membrane of an ovum. The nucleus is located within the head region of the sperm cell. The DNA within the nucleus is densely packed, and the cell does not contain much cytoplasm. The midpiece region contains several mitochondriawhich provide the energy for the motile cell. The tail region consists of a long protrusion called a flagellum that aids in cellular locomotion.

Female ova are some of the largest cells in the body and are round in shape. They are produced in the female ovaries and consist of a nucleus, large cytoplasmic region, the zona pellucida, and the corona radiata. The zona pellucida is a membrane covering that surrounds the cell membrane of the ovum. It binds sperm cells and aids in the fertilization of the cell. The corona radiata are outer protective layers of follicular cells that surround the zona pellucida.

Sex Cell Production

 Four daughter cells are produced as a result of meiosis. Ed Reschke/Photolibrary/Getty Images

Human sex cells are produced by a two-part cell division process called meiosis. Through a sequence of steps, the replicated genetic material in a parent cell is distributed among four daughter cells. Meiosis produces gametes with one-half the number of chromosomes as the parent cell. Because these cells have one-half the number of chromosomes as the parent cell, they are haploid cells. Human sex cells contain one complete set of 23 chromosomes.

There are two stages of meiosis: meiosis I and meiosis II. Prior to meiosis, the chromosomes replicate and exist as sister chromatids. At the end of meiosis I, two daughter cells are produced. The sister chromatids of each chromosome within the daughter cells are still connected at their centromere. At the end of meiosis II, sister chromatids separate and four daughter cells are produced. Each cell contains one-half the number of chromosomes as the original parent cell.

Meiosis is similar to the cell division process of non-sex cells known as mitosis. Mitosis produces two cells that are genetically identical to and contain the same number of chromosomes as the parent cell. These cells are diploid cells because they contain two sets of chromosomes. Human diploid cells contain two sets of 23 chromosomes for a total of 46 chromosomes. When sex cells unite during fertilization, the haploid cells become a diploid cell.

The production of sperm cells is known as spermatogenesis. This process occurs continuously and takes place within the male testes. Hundreds of millions of sperm must be released in order for fertilization to take place. The vast majority of sperm released never reach the ovum. In oogenesis or ovum development, the daughter cells are divided unequally in meiosis. This asymmetrical cytokinesis results in one large egg cell (oocyte) and smaller cells called polar bodies. The polar bodies degrade and are not fertilized. After meiosis I is complete, the egg cell is called a secondary oocyte. The secondary oocyte will only complete the second meiotic stage if fertilization begins. Once meiosis II is complete, the cell is called an ovum and can fuse with the sperm cell. When fertilization is complete, the united sperm and ovum become a zygote.

Sex Chromosomes

 This is a scanning electron micrograph (SEM) of human sex chromosomes X and Y (Pair 23). The X chromosome is much larger than the Y chromosome.

 Power and Syred/Science Photo Library/Getty Images

Male sperm cells in humans and other mammals are heterogametic and contain one of two types of sex chromosomes. They contain either an X chromosome or a Y chromosome. Female egg cells, however, contain only the X sex chromosome and are therefore homogametic. The sperm cell determines the sex of an individual. If a sperm cell containing an X chromosome fertilizes an egg, the resulting zygote will be XX or female. If the sperm cell contains a Y chromosome, then the resulting zygote will be XY or male.

Key Takeaways

• Sexual reproduction occurs through the union of sex cells or gametes.
• Gametes differ widely in males versus females for a given organism.
• For humans, male gametes are called spermatozoa while female gametes are called ova. Spermatozoa are also known as sperm and ova are also known as eggs.
• Sperm cells are produced through the process of spermatogenesis. Egg cells are produced by oogenesis.
• Sperm are highly motile and have three regions: head, mid-piece and tail. Ova are large round cells.
• Meiosis is the process by which both sperm and eggs are produced.
• In humans, sperm cells contain either one X chromosome or one Y chromosome. Egg cells always contain one X chromosome. Thus, sperm cells are heterogametic, while egg cells are homogametic.

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• reproductive cell - a spermatozoon or an ovum; a cell responsible for transmitting DNA tothe next generation germ cell, sex cell cell - (biology) the basic structural and functional unit of all organisms; they may existas independent units of life (as in monads) or may form colonies or tissues as in higherplants and animals gamete - a mature sexual reproductive cell having a single set of unpaired chromosomes genital system, reproductive system - organs and tissues involved in the production and maturation ofgametes and in their union and subsequent development as offspring

Gametes are an organism's reproductive cells. They are also referred to as sex cells. Female gametes are called ova or egg cells, and male gametes are called sperm. Gametes are haploid cells, and each cell carries only one copy of each chromosome. These reproductive cells are produced through a type of cell division called meiosis. During meiosis, a diploid parent cell, which has two copies of each chromosome, undergoes one round of DNA replication followed by two separate cycles of nuclear division to produce four haploid cells. These cells develop into sperm or ova. The ova mature in the ovaries of females, and the sperm develop in the testes of males. Each sperm cell, or spermatozoon, is small and motile. The spermatozoon has a flagellum, which is a tail-shaped structure that allows the cell to propel and move. In contrast, each egg cell, or ovum, is relatively large and non-motile. During fertilization, a spermatozoon and ovum unite to form a new diploid organism  

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