Gamete cells, also known as sex cells, are the cells responsible for sexual reproduction. A male gamete is called sperm (spermatozoa) and is a haploid cell formed through Spermatogenesis. A female gamete is called an ova or egg cells (Oocytes), which are haploid cells carrying one copy of each chromosome.
Gametes are necessary for DNA to be passed from one organism down to the next generation. How exactly are gametes created, and how do they function? What differences are there between gametes and other types of cells in a body?
Gametes are divided into two different types, male and female. Though there are similarities between male and female gametes, both types of gametes also differ in key ways.
The Function of Gametes
Let’s take a look at the similarities between male and female gametes before moving on to the differences. Both types of gametes are sex cells, and both are involved in sexual reproduction. Both gametes are necessary to create a zygote, a clump of cells made out of the DNA from both gametes that will go on to become a fetus. Both male and female gametes contain a single set of chromosomes. The function of gametes is to combine with the other type of gamete and thus combine their single set of chromosomes to create a new complete double set of chromosomes.
Sperm cells have a layer called the “acrosome” which is made up of enzymes that let the sperm penetrate the membrane of an egg cell. Photo: Public domain
Male gametes are a type of haploid cell. Haploid cells are cells which only contain a single set of chromosomes. People’s cells contain 46 chromosomes normally, except for haploid cells which only contain 23. Male gametes, male sex cells, are called sperm cells. Sperm is created in the testes, and it is a tiny cell that can propel itself with a long flagellum. This gives it the ability to move towards a female sex cell. The “head” of a sperm cell is covered in a layer called “acrosome” which consists of a number of enzymes which enable the sperm and to penetrate the membrane of an egg cell. Since both male and female gametes are haploid cells, once the sperm cell does penetrate the egg cell the two cells fuse creating a diploid cell with 46 chromosomes.
Much like male gametes, gametes in females are haploid cells. In females, they are referred to as egg cells. In many species, including humans, male gametes are much smaller than female gametes. Female gametes also typically stationary, unlike the male gametes which can move with flagella. Egg cells are produced in the ovaries, and they are usually produced in small numbers, with a woman having all the egg cells she will have in her life by the time of puberty.
Different types of life have different ways of combining sperm and egg cells. In mammals, sexual intercourse usually consists of the male ejaculating sperm into the female and the sperm then seeks out the female’s egg. Plants are stationary, and thus they use pollen to reproduce. Male gametes are produced within pollen sacs on the plant and are released into the environment. This pollen is frequently carried by the wind or by insects to the female gametes of another plant.
Most of the cells within the human body, as well as the bodies of other animals, reproduce through a system known as mitosis. Mitosis is what allows non-sex cells to reproduce, and create copies of themselves. In mitosis, one cell referred to as a mother cell divides itself to create two new cells, the daughter cells. Mitosis is typically divided into four different phases known as prophase, metaphase, anaphase, and telophase. The job of mitosis is to ensure that the daughter cells receive a full set of 46 chromosomes as if something goes wrong and a cell has fewer chromosomes it frequently dies or causes cancer. While mitosis is responsible for creating the vast majority of cells within a person’s body, sex cells are created through a process known as meiosis.
Meiosis is similar to mitosis in some ways and different in others. Similar to mitosis, meiosis goes through different stages or phases which implement many of the same strategies that mitosis uses to replicate cells. Meiosis separates sister chromatids, which are the two different halves of the same chromosome, and it must also divide the pairs of nonidentical chromosomes that the cell received from its two parent cells. The chromosome pairs that a gamete received from its parent cells are referred to as homologous chromosomes.
While mitosis is divided into four categories, meiosis undergoes these four phases twice. The first instance of cell division separates homologous chromosomes and it is referred to as Meiosis I. The second round of cell division separates the sister chromatids, and it is referred to as Meiosis II. During Meiosis I, the diploid cell is converted into two haploid cells, and each haploid cells will have non-identical sister chromatids. This process is called chromosomal reduction, and it is important because if the chromosomes were not divided before reproduction, the offspring would have four identical sets of chromosomes instead of the desired one complete set with variation. Meiosis II is more like mitosis, and it produces four haploid daughter cells out of the two already existing mother haploid cells.
When both female and male gametes merge the process is referred to as fertilization. Fertilization occurs within the fallopian tubes, a region of the reproductive tract in females. During sexual intercourse, millions of sperm are released into the fallopian tubes and they will travel up to the egg. Typically only one sperm out of these millions will penetrate the egg, though rarely more than one sperm will manage to penetrate it. This happens because the egg is covered in a layer referred to as the “zona pellucida”. The zona pellucida transforms into a barrier once a sperm successfully penetrates the membrane and then begins to merge with the egg. If multiple sperm were to penetrate an egg, the condition is referred to as polyspermy and the result is that the zygote has extra chromosomes, in which case it is no longer viable.
Once fertilization happens the two haploid cells become one zygote, with a full 46 chromosomes. This zygote will continue to divide via the process of mitosis until it matures into a functioning baby. The sex of the individual is determined by which sex chromosomes the zygote inherits. A cell that inherits only one X chromosome will have a Y chromosome as well, and it will be a male. A zygote that inherited two X chromosomes will be a female.
After fertilization, the fertilized egg will remain in the fallopian tube for about 3 to 4 days where it rapidly divides into many different cells through mitosis. It will keep undergoing mitotic division as it moves through the fallopian tube and into the uterus, where it will attach to the uterine wall and continue to divide as it develops into a fetus.