What are chromosomes?
Each species has a unique set of chromosomes. These chromosomes, together with mitochondrial DNA, make up an organism's genome.
Encyclopædia Britannica, Inc.
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Encyclopædia Britannica, Inc.Each species has a unique set of chromosomes which are the carriers of genetic information.
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Encyclopædia Britannica, Inc.An introduction to Austrian botanist, teacher, and Augustinian prelate Gregor Mendel's studies of heredity.
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Encyclopædia Britannica, Inc.The Human Genome Project, which operated from 1990 to 2003, provided researchers with basic information about the genetic content of the human organism, opening new avenues of discovery in fields such as cancer research.
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Encyclopædia Britannica, Inc.Time-lapse photography of a live plant cell nucleus undergoing mitosis.
News •
Weakened chromosome anchors may help explain age-related declines in egg quality
• Oct. 26, 2026, 1:40 PM ET (Medical Xpress)
Transcript
NARRATOR: Chromosomes, located inside the nucleus of cells, carry the genetic information of an organism. Every species of animal or plant has a characteristic number of chromosomes. This number has nothing to do with the size of the organism or how complicated it may appear. Human beings have 23 pairs of chromosomes, for a total number of 46. Fruit flies have only 4 pairs of chromosomes. Cats have 19 pairs. Some ferns have more than 300 pairs.
A newly divided cell has a complete set of chromosomes, which is not usually visible under the microscope at this stage. When special stains are added, however, the nuclei seem to contain dense tangles of threads. These threads, which become the chromosomes, are made up mostly of DNA (deoxyribonucleic acid). DNA is a long molecule that carries all of the genetic information for inherited traits.
A newly divided cell has a complete set of chromosomes, which is not usually visible under the microscope at this stage. When special stains are added, however, the nuclei seem to contain dense tangles of threads. These threads, which become the chromosomes, are made up mostly of DNA (deoxyribonucleic acid). DNA is a long molecule that carries all of the genetic information for inherited traits.
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