Why Do Most Humans Have 23 Pairs of Chromosomes?

To understand what chromosomes are, you first have to understand what DNA is. Formally known as deoxyribonucleic acid, DNA is a complex molecule found in all plants and animals. It's found in almost every cell of an organism's body, and it contains all the information necessary to keep that organism up and running (and developing and reproducing). DNA also is the primary way organisms pass on hereditary information. In the process of reproduction, a portion of DNA is passed along from parent to offspring. In short, DNA is what tells the story of your totally unique biology.

As you can probably imagine, DNA has to be pretty long to hold all that important info. And it is — if you stretched the DNA in just one cell all the way to its full length, it would be about 6.5 feet (2 meters) long, and if you put the DNA in all your cells together, you'd have a chain about twice the diameter of the solar system.

Luckily, cells are pretty smart and have an ingenious way of packaging all that info into space-efficient parcels. Enter: the chromosome.

With its name rooted in the Greek words for color (chroma) and body (soma), the chromosome is a cell structure (or body) that scientists can spot under a microscope by staining it with colored dyes (get it?). Each chromosome is made up of protein and — you guessed it — DNA.

Every chromosome contains exactly one molecule of DNA, to be exact, and that long string of genetic info is tightly wrapped around the protein (called a histone), which acts like a spool, efficiently bundling the lengthy, info-rich molecule into the perfect size and shape to fit inside the nucleus of a cell. Every human cell has 23 pairs of chromosomes for a total of 46 (aside from sperm and egg cells, which each contain only 23 chromosomes).

The magic number of 46 (23 pairs) per cell isn't universal among living things. First, though, humans also happen to be a "diploid" species, which means that most of our chromosomes come in matched sets called homologous pairs (the two members of each pair are called homologues). A lot of animals and plants are diploid, but not all of them have a total number of 46. Mosquitoes, for example have a diploid chromosome number of six, frogs have 26 and shrimp have a whopping 508 chromosomes!

But why do humans have 23 pairs? It happened during evolution. "Humans have 23 pairs of chromosomes, while all other great apes (chimpanzees, bonobos, gorillas and orangutans) have 24 pairs of chromosomes," Belen Hurle, Ph.D., says via email. Hurle is a research fellow at the National Human Genome Research Institute at the National Institutes of Health. "This is because in the human evolutionary lineage, two ancestral ape chromosomes fused at their telomeres [tips], producing human chromosome 2. Thus, humans have one fewer pair of chromosomes. This is one of the main differences between the human genome and the genome of our closest relatives."

Now let's go back to that sperm and egg issue — these cells only have one homologous chromosome from each pair and are considered "haploid." Here's why: When a sperm and egg fuse, they combine their genetic material to form one complete, diploid set of chromosomes. And if you think about it, that makes perfect sense. It means each parent contributes one homologue to a homologous pair of chromosomes in their child's cells.

Consider blood type as a clear example: People with AB blood type inherited two different gene variations on their two homologous chromosomes — one for A and one for B — that, when combined, produced AB.

Now you know the textbook example of a healthy human has 23 pairs of chromosomes in almost every cell of their body, but life isn't always a textbook. What happens if something causes more or less chromosomes to develop? A gain or loss of chromosomes from the standard 46 (called aneuploidy) occurs either during the formation of reproductive cells (sperm and egg), in early fetal development or in any other cell of the body after birth.

One of the more common forms of aneuploidy is "trisomy," which is the presence of an extra chromosome in the cells. One well-known result of trisomy is Down syndrome, which is a condition caused by three copies of chromosome 21 in each cell. This extra chromosome leads to a total of 47 chromosomes per cell, rather than 46.

The loss of one chromosome in a cell is called "monosomy," and describes a condition in which people have just one copy of a specific chromosome per cell as opposed to two. Turner syndrome, in which women have only one copy of the X chromosome per cell versus the regular two, is considered a form of monosomy.

There are other variations of aneuploidy as well, and in extreme cases, they may compromise a person's life. Additionally, cancer cells also have alterations in their number of chromosomes. Unlike the variations that happen in reproductive cells, these changes occur in other cells of the body, so they're not inherited.

Originally Published: Jul 3, 2019