Haploid cells are cells that contain half of the number of chromosomes that are usually found per cell of an organism. In the case of diploid organisms, haploid cells would contain one set of chromosomes, rather than the standard two sets of homologous chromosomes.
Understanding Ploidy
Ploidy is the term used to refer to the number of sets of homologous chromosomes normally found per cell in an organism. There are many different types of ploidy found in nature, from diploidy to polyploidy.
- Homologous definition: When referring to chromosomes, homologous means that the chromosomes were paired at meiosis, and that the two chromosomes are similar in terms of structural features, such as size, and that they share the same pattern of genes (i.e. they have the same order and location of alleles; alleles are different forms of a gene)
“For each chromosome contributed by the sperm there is a corresponding chromosome contributed by the egg; there are two chromosomes of each kind, which together constitute a pair.” — Thomas Hunt Morgan
Chromosomes
As a brief aside, let’s first cover a bit of background pertaining to chromosomes. Chromosomes are bundles of genetic material, called deoxyribonucleic acid (DNA), and are stored in the nucleus of each cell. Packaging the long strands of DNA into chromosomes allows all of the DNA to fit into the cell’s tiny nucleus. The DNA strands are wound up very tightly around histones, which are proteins that help to maintain the chromosome structure.
The common visual representation of a chromosome is the X-shape seen in the figure below. This shape is actually a chromosome that has copied itself and the two copies have joined together at a point called the centromere. The typical X-shaped representation of a chromosome is due to the fact that chromosomes cannot be seen even under a microscope until they have copied themselves and condensed in preparation for cell division.
What is the Difference Between Haploid and Monoploid?
It is easy to become confused about the difference between haploid and monoploid cells, as haploid cells are commonly associated with diploid cells and so haploid cells are said to contain one set of chromosomes. While it is true that the corresponding haploid cells of diploid cells have one set of chromosomes, this does not mean that all haploid cells have only one set of chromosomes. In other words, not all haploid cells are monoploid cells.
Monoploid cells are cells that contain one unique set of chromosomes, rather than one half of the number of chromosome sets. The chromosomes in monoploid cells do not have one or more homologous sets of chromosomes in another cell.
What are Diploid Cells?
Considering the fact that human cells are diploid cells, you may be somewhat familiar with this term. Very simply, diploid cells contain two sets of chromosomes that are homologous to one another. Both of your parents each contributed a set of chromosomes when making you, leading to each of your cells containing two sets of chromosomes. Humans have forty-six chromosomes, with twenty-three chromosomes from the mother and twenty-three chromosomes from the father. The only cells found in the human body that are not diploid cells are the gametes, which are haploid cells.
Examples of Haploid Cells
Gametes
As mentioned above, gametes are the only cells found in the human body which are haploid cells. All other human cells are diploid cells. Gametes are also known as the sex cells, or the sperm and eggs. Gametes fuse together to create a zygote, from which a fetus is developed. When the gametes fuse together, the twenty-three chromosomes in each haploid gamete cell combine to a total of forty-six chromosomes in the diploid zygote cells.
Gametes are formed in a process called meiosis.
Meiosis
Meiosis is the process by which cells divide to produce gametes and occurs as part of the cell cycle.
Meiosis occurs in two stages, meiosis I and meiosis II. During meiosis I, the chromosomes duplicate themselves, and homologous chromosomes pair together. While the homologous chromosomes are paired together, they exchange pieces of themselves in a process called recombination which results in greater genetic diversity; this is the key process of meiosis I. The paired chromosomes are then aligned along the center of the cell before being pulled to either end. The centrioles are an important organelle during meiosis, as they organize the movement of chromosomes during cell division. After the pairs of chromosomes have been separated to either end of the cell, the cell divides into two cells. These two cells are haploid cells.  However, the process is not over; meiosis II occurs next.
During meiosis II, each of the haploid cells formed during meiosis I is divided like in mitosis. The end result of meiosis is four haploid cells that are each genetically different.
“I think I was dealt a good hand. I have happy genes.” — Maeve Binchy
Insect Drones
In insects such as ants, bees, and wasps, the sex of each organism is determined by a system called haplodiploidy. In the haplodiploidy system, female organisms are diploid and males are haploid. The female organisms are formed via typical sexual reproduction, where the male and female gametes fuse to form a diploid daughter. The male organisms are formed from unfertilized eggs so they are haploid and have genes only from a mother.
Plants, Algae, and Fungi
Most plants, algae, and fungi species exhibit some form of a haploidic stage in their lifecycles. A lot of fungi and algae have lifecycles during which the organisms are dominantly haploid, while plants and some algae have lifecycles during which each generation of organisms switches between a dominantly haploid and dominantly diploid stage.
Fungi and algae that are dominantly haploid only form diploid cells when two mating-type haploid cells fuse to form a diploid zygote which then re-forms haploid spore cells by meiosis. Organisms that live predominantly as haploid cells create a diploid zygote in order to increase genetic diversity.
“Diversity may be the hardest thing for a society to live with, and perhaps the most dangerous thing for a society to be without.” — William Slone Coffin
Plants and algae that switch between haploid and diploid (or in the case of some plants, polyploid) states exist as gametophytes which are haploid and sporophytes which are diploid. The gametophytes produce gametes that fuse to form the diploid zygote that develops into the sporophyte. The sporophytes produce haploid spores via meiosis and the spores grow into gametophytes, and the cycle continues.
Haploid cells are important as their existence allows organisms to recombine and diversify their genetic material.