Gametes

Beginning of Life
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Gametes are reproductive cells or sex cells that unite during sexual reproduction to form a new cell called a zygote. Male gametes are called sperm and female gametes are ova (eggs). Sperm are motile and have a long, tail-like projection called a flagellum. Ova are non-motile and relatively large in comparison to the male gamete.

In seed-bearing plants, pollen is a male sperm-producing gametophyte and female sex cells are contained within plant ovules. In animals, gametes are produced in male and female gonads, the cite of hormone production. Read to learn more about how gametes divide and reproduce.

Gamete Formation

Gametes are formed through a process of cell division called meiosis. This two-step division process produces four haploid daughter cells. Haploid cells contain only one set of chromosomes. When the haploid male and female gametes unite in a process called fertilization, they form what is called a zygote. The zygote is diploid and contains two sets of chromosomes.

Gametes and Fertilization

Fertilization occurs when male and female gametes fuse. In animal organisms, the union of sperm and egg occurs in the fallopian tubes of the female reproductive tract. Millions of sperm are released during sexual intercourse and these travel from the vagina to the fallopian tubes.

Fertilization

Sperm are specially equipped with burrowing catalysts and mechanisms for fertilizing an egg. The head region contains a cap-like covering called an acrosome that contains enzymes that help the sperm cell penetrate the zona pellucida, the outer covering of an egg cell membrane.

When a sperm reaches the egg cell membrane, its head fuses with the egg. This triggers the release of substances that modify the zona pellucida to prevent any other sperm from fertilizing the egg. This process is crucial as fertilization by multiple sperm cells, or polyspermy, produces a zygote with extra chromosomes. Polyspermy is lethal to a zygote.

Development

Upon fertilization, two haploid gametes become one diploid zygote. A human zygote has 23 pairs of homologous chromosomes and 46 chromosomes total—half from the mother and half from the father. The zygote continues to divide by mitosis until a fully functional individual is formed. The biological sex of this human is decided by the sex chromosomes it inherits.

A sperm cell may either have an X or Y sex chromosome, but an egg cell can only have an X chromosome. A sperm cell with a Y sex chromosome results in a male (XY) and a sperm cell with an X sex chromosome results in a female (XX).

Types of Sexual Reproduction

The type of sexual reproduction of an organism is largely dependent on the size and shape of its gametes. Some male and female gametes are of similar size and shape, while others are vastly different. In some species of algae and fungi, for example, male and female sex cells are almost identical and both are usually motile. The union of similar gametes is known as isogamy.

The process of gametes of dissimilar size and shape joining is called anisogamy or heterogamy. Higher plants, animals, and some species of algae and fungi exhibit a special type of anisogamy called oogamy. In oogamy, the female gamete is non-motile and much larger than the fast-moving male gamete. This is the type of reproduction that occurs in humans.