Gametes: Definition, Formation, and Types

Beginning of Life
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Gametes are reproductive cells (sex cells) that unite during sexual reproduction to form a new cell called a zygote. Male gametes are sperm and female gametes are ova (eggs). In seed-bearing plants, pollen is the male sperm producing gametophyte. Female gametes (ovules) are contained within the plant ovary. In animals, gametes are produced in male and female gonads. Sperm are motile and have a long, tail-like projection called a flagellum. However, ova are non-motile and relatively large in comparison to the male gamete.

Gamete Formation

Gametes are formed by a type of cell division called meiosis. This two-step division process produces four daughter cells that are haploid. 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.

Gamete Types

Some male and female gametes are of similar size and shape, while others are different in size and shape. In some species of algae and fungi, male and female sex cells are almost identical and both are usually motile. The union of these types of gametes is known as isogamy. In some organisms, gametes are of dissimilar size and shape. This is known as anisogamy or heterogamy (hetero-, -gamy). Higher plants, animals, as well as 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 male gamete.

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 that travel from the vagina to the fallopian tubes. Sperm are specially equipped for fertilizing an egg. The head region contains a cap-like covering called an acrosome that contains enzymes which help the sperm cell penetrate the zona pellucida (outer covering of an egg cell membrane). Upon reaching the egg cell membrane, the sperm head fuses with the egg cell. Penetration of the zona pellucida triggers the release of substances that modify the zona pellucida and 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. This condition is lethal to the zygote.

Upon fertilization, the two haploid gametes become one diploid cell or zygote. In humans, this means that the zygote will have 23 pairs of homologous chromosomes for a total of 46 chromosomes. The zygote will continue to divide by mitosis and eventually mature into a fully functioning individual. Whether or not this individual will be male or female is determined by the inheritance of sex chromosomes. A sperm cell may have one of two types of sex chromosomes, an X or Y chromosome. An egg cell only has one type of sex chromosome, an X chromosome. Should a sperm cell with a Y sex chromosome fertilize an egg, the resulting individual will be male (XY). Should a sperm cell with an X sex chromosome fertilize an egg, the resulting individual will be female (XX).