Gonad Definition

A gonad is a specialized organ which contains germ cells. Germ cells are responsible for producing the haploid cells required for sexual reproduction. Typically in sexually reproducing animals, each organism has a set number of chromosomes. Each chromosome has two parts, one from the mother and one from the father. Every cell in an organism contains all the chromosomes, with both parts.

Consider humans for example. Humans have 23 pairs of chromosomes in their cells. You received 23 chromosomes from your mother and 23 from your father. Because these chromosomes represent the same portions of DNA, the pairs that are alike group into 1 chromosome, containing sister chromatids. Therefore, humans have 46 chromosomes in each cell.

Here’s where the gonad comes in. Imagine if two humans reproduced without dividing their chromosomes first. 46 + 46 = 92 chromosomes! While some organisms like plants are benefitted by too many copies of the same chromosome, it is detrimental in humans and many other animals. Therefore, before reproducing, an animal must use the germ cells in a gonad to reduce the DNA to 23 chromosomes that do not have pairs. Cells like these are known as haploid cells or gametes, as opposed to the typical diploid cells.

Function of a Gonad

To make haploid cells in humans, a gonad facilitates a special type of cell division known as meiosis. During meiosis, a diploid cell’s DNA is doubled, creating 92 chromosomes, which are then split into 4 cells. Each cell is unique, and contains 23 unpaired chromosomes. These cells will become the eggs and sperm involved in sexual reproduction.

A gonad in another organism functions in the exact same way, although the number of chromosomes in each organism varies. In organisms with complex hormonal systems, the gonads are often involved in the regulation of hormones. This can be seen in humans. Male testes have been found to be involved in the production and regulation of testosterone, while female ovaries produce a variety of hormones involved in ovulation and pregnancy, the main hormone of which is estrogen.

The gonad is often connected to the sexual organ, and aids in the release of gametes. This is seen in species that rely on internal fertilization, or the act of copulating. Other species rely on casting their gametes into the environment. In these species, the gonads themselves release the gametes. This can be seen in flowers. The gonad that produces the pollen is the same organ used to release the pollen into the air and onto bees. The pollen is then transferred to the stigma, or female gonad, of another flower. This is where fertilization occurs.

Examples of Gonads

A gonad can exist in many forms throughout the animal kingdom, but a general pattern of males having testes and females having ovaries can be seen. In animals that are sexually differentiated, each gender contains only one type of gonad (male or female). In some hermaphroditic animals, such as earthworms, each organism contains both male and female gonads. In other organisms

Example #1: A Typical Gonad

Through a human lens, the “typical” arrangement of gonads is that of two distinct genders. This is known as sexual dimorphism. Many sexually dimorphic creatures are also sexually differentiated. This means that each gender receives 1 type of gonad, and produces 1 type of gamete. In many animals this can be seen as males and females, males producing sperm and females producing eggs.

This pattern of life is used in the smallest of creatures to the largest, and is by far the most common form of reproduction. In this form, two main types of gonad exist, ovaries and testis. Males use testis to produce sperm. These haploid gametes typically have tails which can power them toward the eggs, increasing the chances of fertilization. Female eggs are always produced in the ovaries, but can take a variety of forms. In humans, the eggs remain in the ovary until ovulation, when the egg descends towards the uterus and can become fertilized. In birds, the egg is fertilized, protected in a hard shell, and kept warm in a nest while the embryo develops. In many sea creatures, eggs and sperm are simply cast into the environment on a synchronous cue. However, in all these forms, the simple gonad is the organ producing all of the gametes.

Example #2: Earthworms

While it may seem like the above plan of sexual dimorphism is the only way to go, there are many organisms that exist with both male and female gonads. The most common of these is the typical earthworm. A single earthworm contains a male gonad and a female gonad. The male gonad may give gametes to another worm, and the female gonad can take gametes from the same worm.

One theory is that this gives earthworms an added benefit when mating because it allows the passage of genetics to go both ways. This is important to animals that may rarely meet underground, though they might only be inches from each other. This way, not 1, but 2 mating events can occur every time a pair of earthworms meet. Oftentimes it is necessary in organisms that are immobile, have poor senses, or travel individually to develop other means for reproducing successfully. This is often seen in the gonad of the animal becoming specialized in some way. In earthworms it can be seen as the possession of both types of gonad.

Example #3: Wrasses

The wrasse is a common coral-reef dwelling fish, of which there are many species. Certain species have become what are known as sequential hermaphrodites. While all individuals possess both male and female gonads, only one individual looks unlike the others. In this individual, the now dominant female, her male gonad is suppressed, and her female gonad is active. She produces the eggs for the whole group, and the smaller fish fight to fertilize them.

Wrasse

Wrasse

The smallest fish, now male, will eventually become the largest and oldest of the males. At that time, if the female dies, it will be his turn to assume the role of head female. As such, he will undergo a sexual metamorphosis in which his male gonad is suppressed and his female gonad is activated. The new female often becomes a different color and grows rapidly, probably in response to the quick change of hormones. This sequential hermaphroditism can be seen many fishes including clown fish and wrasses, as well as in some reptiles.

Related Biology Terms

  • Diploid – A cell with the full number and pairs of chromosomes for a given species.
  • Haploid – A cell with the full number of chromosomes, but none of the pairs, for a given species.
  • Gamete – A haploid cell produced by a gonad in many organisms.
  • Sexual-dimorphism – When two organisms of the same species look or function differently based on gender.

Quiz

1. What is the function of a gonad?
A. To clean the blood of toxins.
B. To produce and regulate hormones.
C. To create gametes, or haploid cells.
D. Some of B and C.

Answer to Question #1

2. Congrats! You found a new species. Now you have to describe it for Science! When watching the organism reproduce, you notice that the individuals mating look the same. There is usually one mating that happens, then both individuals give birth to an offspring. Without killing to creatures to look inside at their gonads, how would you describe them?
A. Sexually-Differentiated.
B. Hermaphroditic.
C. Sequentially Hermaphroditic.

Answer to Question #2

3. Inside of a gonad, cells that have undergone meiosis often reside close to cells that have not underdone meiosis yet. If you take a cross-section of a gonad, and look at it under the microscope, you can see two cells close to each other, the first has twice as much DNA as the second cell. How do we describe the amount of DNA, scientifically?
A. First Cell: Haploid; Second Cell: Diploid
B. First Cell: Diploid; Second Cell: Haploid
C. First Cell: Duplicated; Second Cell: Singular

Answer to Question #3
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