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Sex Cells

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Sex Cells
Sex Cells

Sex Cells - Sperm Cell and Egg Cell

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Sex Cells

Organisms that reproduce sexually do so via the production of sex cells 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 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

Male and female sex cells are dramatically different from one another in size and shape. Male sperm resemble 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 a 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 mitochondria which 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

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

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 therfore 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.

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