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 t