Chromosomes

You might be familiar with the term chromosomes, but what are they—and what do chromosomes do? Chromosomes are packets of genetic material—that is, your DNA—found in your cells. They store all of the instructions for making and running a living thing, including you.

Chromosomes are also critical for cells to be able to make new copies of themselves and for life’s instructions to be passed down from one generation to the next. To reproduce, a cell will divide, resulting in two new cells. Before the division, each chromosome can be copied so that a complete chromosome  can be moved into each of the two newly forming cells.

This process ensures that your cells have the complete instructions for life. In addition, chromosomes can be copied and moved into reproductive cells (sperm and eggs) so that genetic material passes from generation to generation.

Chromosomes consist of neatly organized DNA strands wrapped tightly around small, spool-like proteins called histones. The wrapping of DNA around a histone is what allows DNA molecules to be so compact that they fit inside your cells. Incredibly, fitting all of the DNA in one human cell's nucleus is the equivalent of packing 24 miles (about 40 km) of very thin thread into a tennis ball.

Types of Chromosomes

Our cells contain different types of chromosomes. Humans typically have 23 distinct chromosomes in a set. Because most of our cells have two sets of chromosomes, most cells have a total of 46—that is, 23 pairs.

Of these 23 pairs, one pair stands out. These are the sex chromosomes, X and Y. Biological males have an X and a Y chromosome, while biological females have two X chromosomes. The X chromosome is one of the larger chromosomes and contains hundreds of distinct stretches of DNA called genes. The much smaller Y chromosome has many fewer genes. But it does have one important gene (SRY) that causes most Y- containing embryos to develop as biologically male. The X chromosome does much more than turn an embryo into a biological male. It has genes involved in many other processes as well.

The other 22 pairs, which altogether contain tens of thousands of genes, don't have central roles in sex development. These chromosomes are known as autosomal chromosomes, which means “body chromosomes." They were originally numbered 1-22 based on size with 1 being the largest and 22 being the smallest. Later it was found that chromosome 21 was smaller than 22.

There's one more type of chromosome in human cells—mitochondrial DNA (mtDNA). Each of the mitochondria—the cell structures that turn sugars into energy in cells—contains its own circular chromosome. Everyone inherits mitochondrial chromosomes only from their mother.

What Are Chromosomes Made of?

Chromosomes are made of strands of DNA wrapped around spools of protein called histones and organized into distinct packets. The packets are accessible for copying in the cell and are transportable. When a cell splits to make two new cells, a copy of each chromosome can be moved into each of the newly forming cells.

Genes and Chromosomes

What do genes and chromosomes have to do with each other? In chromosomes, stretches of DNA units called genes are arranged in a predictable order along each DNA strand. These genes contain most of the instructions found in the DNA.

Where in the Cell Are Chromosomes Located?

Where in a human cell the chromosomes are located depends on when you look. In a human cell that's busy doing its job—such as being a muscle cell, for example—the chromosomes are locked up in a cellular vault called the nucleus. The nucleus keeps the chromosomes—and the genes they contain— safe from outside interference.

When a cell is ready to split into two cells, it first copies its chromosomes. Then the nucleus will temporarily break down, leaving the chromosomes briefly unprotected but available to be moved around. This opening allows for a full complement of chromosomes to be shifted into each of the two new cells. Once the chromosomes are in their new spaces, the nuclear boundaries quickly re-form around them.

How Many Chromosomes Does a Human Have?

The answer to the question of how many chromosomes a human typically has depends on where you look. In most cells in most people, the nucleus contains 46 chromosomes—in the form of 23 matched pairs. An exception is biological males, where one of the pairs is mismatched (the X and the Y).  In addition to these 46 chromosomes, each of the mitochondria in the cell has a circular chromosome.

Some people have fewer (or more) than 46 nuclear chromosomes because a different number was present at conception—or they gained or lost one very early in embryonic development. For example, people with Turner syndrome have 45 chromosomes, having a single X chromosome rather than a pair of Xs or an X and Y.

People with Down syndrome have 47 chromosomes because they have three copies of chromosome 21 instead of a pair. But not everyone with extra chromosomes has such obvious symptoms. For example, people with an extra X or Y chromosome often have no obvious symptoms at all.

Although most body cells have two matched sets of chromosomes, there are two cell types that are exceptions: reproductive cells and mature red blood cells.  Reproductive cells—that is, sperm or eggs—have only a single chromosome set. In this way, when a sperm and egg meet and fuse, the result is a new cell with the two paired sets. Mature red blood cells have no nucleus, so they have no autosomal or sex chromosomes—only the chromosome present in their mitochondria.

Finally, cells have different numbers of mitochondria, depending on energy needs, which means that the number of mitochondrial chromosomes differs from cell to cell.