What Is a Chromosome? DNA Packaging Explained Simply

A chromosome begins with DNA, the molecule that carries genetic instructions. DNA is extremely long compared with the tiny space inside a cell nucleus, so it cannot simply float around as one loose thread. It winds around proteins called histones and folds into a compact structure called chromatin. This packaging keeps DNA organised while still allowing sections to be opened when the cell needs to use particular genes.

When people ask what a chromosome is, they often picture the familiar X shape from a textbook. That shape is most obvious after DNA has been copied and condensed for cell division. At other times, chromosomes are less tightly packed and can look more like loose chromatin. The chromosome is still there even when it is not shaped like a neat X.

Genes are sections of DNA found along chromosomes. A chromosome can carry many genes, along with regions that help control when genes are active. The whole structure is therefore more than a container. It is an organised way for a cell to manage a very large set of instructions.

Different species usually have characteristic chromosome numbers. Human body cells typically have 46 chromosomes arranged in 23 pairs. One chromosome in each pair usually came from one biological parent and the other from the other parent. The pairs contain many of the same genes in the same general positions, although the versions of those genes can differ.

Chromosome number is especially important during mitosis and meiosis. Before a body cell divides by mitosis, its chromosomes are copied so each new cell can receive a complete set. During meiosis, the chromosome number is reduced by half to make sex cells. Fertilisation then combines two half sets and restores the usual number for the new organism.

This is another useful answer to what a chromosome is. It is a unit that helps genetic information move reliably from one cell generation to the next. Mistakes can happen when chromosomes fail to separate correctly, which is one reason cells have checkpoints and other control systems during division.

A few simple distinctions can keep chromosome vocabulary clear:

• •DNA is the genetic molecule that stores information.
• •A gene is a section of DNA with a particular biological role.
• •A chromosome is one long DNA molecule packaged with proteins.
• •A copied chromosome can contain two identical sister chromatids joined together.
• •A genome is the complete genetic information of an organism.

A useful way to study a chromosome is to draw the levels as a simple ladder. Start with one DNA double helix, show it wrapping around histone proteins, then zoom out to a condensed chromosome. Mark a short gene segment on the DNA so you can see that a gene is not a separate object floating beside the chromosome. Finally, sketch a copied chromosome with two sister chromatids. This picture connects vocabulary to structure and also explains why diagrams can look different at different stages of the cell cycle. If two textbook images seem inconsistent, first ask whether they show DNA before or after replication and whether the chromatin is loose or condensed.

For around three decades, textbooks stated that human cells contained 48 chromosomes. Early microscope preparations were crowded and smeared, with chromosomes overlapping, so an accurate count was very difficult. In 1956 Joe Hin Tjio and Albert Levan, working in Sweden, improved the method: they treated cells so that chromosomes stayed condensed and spread apart in a single layer. They counted 46, and other laboratories quickly confirmed it. Today a karyotype, a sorted picture of a cell's chromosomes, is a routine test for missing, extra or rearranged chromosomes.

The old figure of 48 was not random, as it happens. Chimpanzees, gorillas and orangutans do have 48 chromosomes. Human chromosome 2 turns out to be the product of two ancestral chromosomes that fused end to end at some point in our lineage after it split from the other great apes. You can see the evidence in its DNA: the remains of telomere sequences, which normally cap the ends of chromosomes, sit in the middle of chromosome 2, along with traces of a second, inactive centromere.

It is tempting to assume that more chromosomes means a more complex organism, but the numbers show no such pattern. Dogs have 78 chromosomes, and potatoes have 48, two more than humans. The fruit fly used in genetics laboratories has just 8, and in one species of Australian ant the females carry only a single pair. At the other extreme, some ferns have well over a thousand. What matters is how much useful genetic information exists and how it is controlled, not how many pieces the DNA happens to be divided into.

Number does matter for reproduction, though. During meiosis, matching chromosomes must find and pair with each other before they separate, and that works smoothly only when the two sets correspond. A horse has 64 chromosomes and a donkey 62, so their offspring, the mule, inherits 63. The mule grows and lives normally, but when its body tries to make sex cells, the mismatched horse and donkey chromosomes pair poorly, so it almost never produces viable eggs or sperm. Differences in chromosome number and structure can therefore help keep closely related species apart.

Female mammals have two X chromosomes and males one X and one Y, which raises a problem: females would seem to get a double dose of every gene on the X. The answer, proposed by the British geneticist Mary Lyon in 1961, is that early in a female embryo's development each cell silences one of its two X chromosomes more or less at random. The silenced X is packed into a tight clump called a Barr body, after Murray Barr, who spotted it in cat nerve cells. Every descendant of that cell keeps the same X switched off.

Cats show the result on their fur. In domestic cats, a gene affecting orange versus black colour sits on the X chromosome. A female carrying the orange version on one X and the black version on the other becomes a patchwork, because each patch of skin descends from cells that silenced different X chromosomes. That is the tortoiseshell or calico pattern. A typical male has only one X, so he is usually orange or black, not both. The rare male calicos almost always carry an extra X, with the chromosomes XXY, and they are usually sterile.

A chromosome is a packaged DNA molecule that helps a cell store, use, copy, and distribute genetic information. Remember the scale: genes are sections of DNA, DNA is packed into chromosomes, and all the chromosomes together form a large part of the genome. Once those levels are separated, many genetics questions become much easier.