Biology (A)

Chromosomes

Chromosomes are coiled structures made of DNA and proteins. Chromosomes are the form in which the genetic material of a cell goes through cell division. During other phases of the cell cycle, DNA is not coiled into chromosomes. Instead, it exists as a grainy material called chromatin.

Chromatids and the Centromere

DNA condenses and coils into the familiar X-shaped form of a chromosome, shown in Figure below, only after it has replicated. (You can watch DNA coiling into a chromosome at the link below). Because DNA has already replicated, each chromosome actually consists of two identical copies. The two copies are called sister chromatids. They are attached to one another at a region called the centromere.

DNA Coiling into a ChromosomeSource: http://www.hhmi.org/biointeractive/ The programs may be broadcast, duplicated, and distributed within educational systems for not-for-profit educational purposes. Any other usage requires the written permission of the Howard Hughes Medical Institute. Duplication of the programs for resale or any other commercial purpose is strictly prohibited.

Chromosomes and Genes

The DNA of a chromosome is encoded with genetic instructions for making proteins. These instructions are organized into units called genes. Most genes contain the instructions for a single protein. There may be hundreds or even thousands of genes on a single chromosome.

Human Chromosomes

Human cells normally have two sets of chromosomes, one set inherited from each parent. There are 23 chromosomes in each set, for a total of 46 chromosomes per cell. Each chromosome in one set is matched by a chromosome of the same type in the other set, so there are actually 23 pairs of chromosomes per cell. Each pair consists of chromosomes of the same size and shape that also contain the same genes. The chromosomes in a pair are known as homologous chromosomes.

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CK-12 Foundation, Biology. http://creativecommons.org/licenses/by-nc-sa/3.0/