Chromosome FAQs

What is a chromosome?

Chromosomes are tightly coiled microscopic rod-like structures of DNA and protein that are found in the nuclei of eukaryotic cells. Each chromosome contains a single molecule of DNA. Each strand of the DNA double helix is a linear arrangement of repeating similar units called nucleotides, which are each composed of one sugar, one phosphate, and a nitrogenous base. A DNA nucleotide contains one of four different nitrogenous bases: adenine (A), thymine (T), cytosine (C), and guanine (G). The order of bases along a strand of DNA is what determines the genome sequence. See a DNA Structure diagram.

How big are human chromosomes?

Human chromosomes range in length from 51 million to 245 million base pairs. With few exceptions (e.g., red blood cells), each of the trillions of cells in the human body contains a complete set of chromosomes--the genome. If all the bases in the human genome were spread out 1 millimeter apart, they would extend from Memphis to Los Angeles.

How many chromosomes are in the human genome?

The nucleus of most human cells contains two sets of chromosomes, one set given by each parent. Each set has 23 single chromosomes--22 autosomes and an X or Y sex chromosome. A normal female will have a pair of X chromosomes; a male will have an X and Y pair.

What are the sizes of the individual human chromosomes?

Why are the chromosomes shown in this Web site striped?

Chromosomes can be seen under a light microscope and, when stained with certain dyes, reveal several features. Such features include length, position of a constricted region (centromere), and the particular patterns of light and dark bands that result from treatment with stains. Differences in size and banding pattern allow the 24 chromosomes to be distinguished from each other, an analysis called a karyotype.

A few types of major chromosomal abnormalities, including missing or extra copies of a chromosome or gross breaks and rejoinings (translocations), can be detected by microscopic examination; Downs syndrome, in which an individual's cells contain a third copy of chromosome 21, is diagnosed by karyotype analysis.

Most changes in DNA, however, are too subtle to be detected by this technique and require molecular analysis. These subtle DNA abnormalities (mutations) are responsible for many inherited diseases such as cystic fibrosis and sickle cell anemia or may predispose an individual to cancer, major psychiatric illnesses, and other complex diseases.

What are genes? What role do they play in disease research?

Genes are chromosome pieces whose particular bases (e.g., ATTCGGA) determine how, when, and where the body makes each of the many thousands of different proteins required for life. Humans have an estimated 30,000 genes, with an average length of about 3,000 bases. Genes make up less than 2 percent of human DNA; the remaining DNA has important but still unknown functions that may include regulating genes and maintaining the chromosome structure.

Researchers hunt for disease-associated genes by looking for base changes found only in the DNA of affected individuals. Numerous disorders and traits mapped to particular chromosomes are displayed in this Web site. Some disorders, such as cystic fibrosis (chromosome 7) and sickle cell anemia (chromosome 11) are caused by base sequence changes in a single gene. Many common diseases such as diabetes, hypertension, deafness, and cancers have more complex causes that may be a combination of sequence variations in several genes on different chromosomes, in addition to environmental factors.

How will knowing the human genome sequence affect medicine?

Knowing the DNA sequence is important because it affects such attributes as appearance, response to particular medicines, and resistance to infections and toxins. It may even influence behavior. Some sequence variations also can cause or contribute to such disorders as those found on the chromosomes presented in this Web site.

These new data and powerful DNA analysis tools will usher in a new era of medicine that could allow doctors to detect disease at earlier stages, make more accurate diagnoses, and customize drugs and other medical treatments to fit an individual's own DNA sequence. The eventual understanding of a gene's normal functions and how the gene may change to cause or contribute to disease will lead to more focused and effective treatments with fewer side effects.

How many chromosomes are found in chickens? tomatoes? armadillos?

Go to Chromosome Number of Different Species Web site and use their search tool to find out the number of chromosomes in several different organisms.