DNA and histone proteins are packaged into structures called chromosomes.
The 22 autosomes are numbered by size. The other two chromosomes, X and Y, are the sex chromosomes. This picture of the human chromosomes lined up in pairs is called a karyotype.
A chromosome is the organelle responsible for the physical transmission of hereditary information (see heredity) from one generation to the next. In bacteria and archaea, the chromosome consists of a single naked ring of DNA. In eukaryotes, each chromosome consists of a single linear DNA molecule composed of many genes.
Chromosomes are not visible in the cell's nucleus – not even under a microscope – when the cell is not dividing.
However, the DNA that makes up chromosomes becomes more tightly packed during cell division and is then visible
under a microscope. Most of what researchers know about chromosomes was
learned by observing chromosomes during cell division.
Each chromosome has a constriction point called the centromere, which divides the chromosome into two sections, or "arms." The short arm of the chromosome is labeled the "p arm." The long arm of the chromosome is labeled the "q arm." The location of the centromere on each chromosome gives the chromosome its characteristic shape, and can be used to help describe the location of specific genes.
In humans, each cell normally contains 23 pairs of chromosomes, for a total of 46. Twenty-two of these pairs, called autosomes, look the same in both males and females. The 23rd pair, the sex chromosomes, differ between males and females. Females have two copies of the X chromosome, while males have one X and one Y chromosome.
Chromosomal aberration is any abnormality in the chromosomes of a cell, such as a variation in the normal number or a change in the normal arrangement of the genetic material, which may produce an unexpected characteristic. Down's syndrome, for example, is associated with an extra chromosome. Some malformations have been associated with the absence of sections of certain chromosomes.