Haploid: Definition, Life Cycle & Example

Instructor: Dominic Corsini
This lesson addresses the concept of haploid cells. It describes haploid cells and the role they play in various life cycles. Illustrations, real-world examples, and a brief quiz are included.

Why Haploid Cells?

Below are two pictures. What do you suppose they have in common?

Fern Gametophyte
Fern Gametophyte

Sperm Cells
Sperm Cells

At first glance, it probably appears that this plant and these sperm cells have very little in common. And for the most part, this is true. However, one characteristic they do share is that they're both haploid. Haploid is a condition where cells contain only one set of chromosomes. Chromosomes are tightly coiled structures made of DNA. The overwhelming majority of cells in your body contain two sets of chromosomes, as do most plant cells.

So, why do the images above depict cells with only one set? It has to do with an organism's life cycle. A life cycle is defined as the series of changes a living thing goes through from the beginning of its life until its death. Haploid cells are part of our life cycle, as well as the life cycle of plants.

Haploid Cells and Life Cycles

Let's start by investigating how haploid cells fit into the life cycle of animals, including humans. The cells of an animal's body normally contain two copies of each chromosome. This is a condition known as diploid.

In humans, the diploid number of chromosomes is 46. In other words, we have 23 pairs of chromosomes, or 46 total. Too many or too few chromosomes can result in physical and/or mental abnormalities or even death. Before a sperm and egg can join to form a new person, the chromosome number of each must be reduced by half. This halving creates the condition where a sperm (now with 23 chromosomes) and an egg (also with 23 chromosomes) can fuse to produce a new cell with 46 chromosomes.

So you see, having haploid cells is vital for reproduction. Without them, we (or any other sexually reproducing animal) would not be able to donate the correct number of chromosomes to our offspring. If sperm and egg were to each contain 46 chromosomes, they would fuse to create offspring with 92 chromosomes. This simply doesn't work, and that offspring would not survive.

As often happens with biology, however, just when it seems you've figured things out, they change. Enter plants into the haploid discussion. Plants undergo a life cycle termed alternation of generations. Alternation of generations is when a diploid plant (called a sporophyte) gives rise to a haploid plant (called a gametophyte), then the haploid plant produces gametes (also haploid), which combine to create another diploid plant (sporophyte). Sound confusing? Here's a graphic that might help you understand.

Alternation of Generations
Alternation of Generations

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