One of the chains of DNA found in cells. Chromosomes contain genes, each encoded as a subsection of the DNA chain. Chromosomes are usually present in all cells in an organism, even though only a minority of them will be active in any one cell.
Chromosomes are large biomacromolecules and the strategic containers of DNA. Since DNA can be up to three feet long when stretched out in its entirety, chromosomes are an encapsulated or compressed form of DNA and other proteins that allow it to exist at the cellular level. Chromosomes consist of a centromere, a variety of genes, nucleotide sequences and regulatory elements. They are the carriers of heritable traits. In sexual organisms, each parent contributes half of the total number of chromosomes. In the case of eukaryotes, the uncondensed DNA is located inside the nucleus where it encircles histones and this composite is known as chromatin. The only time a chromosome can be viewed under a microscope in its natural state is during cell division, (known as mitosis) when it becomes compact.
During mitosis, or cell division, chromosomes are condensed and a "spindle" of microtubules is formed. They attach to chromosomes at structures called the kinetochores (not centromeres – centromeres do not exist), which helps give them structure to divide. This is the only time they are visible through an optical microscope. The ends of chromosomes are called telomeres; these structures shorten with age.
The chromosome was first noticed in the mid 19th century, but it was only in 1910 that it was shown to be the carrier of genes. As such, a person can have abnormalities in his chromosomes without them resulting in diseases, though such abnormalities can cause disorders in the chromosomes of an offspring such as Down syndrome, Turner syndrome and Cri du chat syndrome.
Different creatures have different numbers of chromosomes; chromosomes are always diploid, or found in matching pairs. The number of chromosomes is in no way related to the complexity of the organism. For instance, fruit flies have eight chromosomes, humans have forty-six, butterflies have 380, and ferns have a remarkable 1200 chromosomes.
In summary, DNA carrying genetic information is packaged into large macromolecules (or biomacromolecules) called chromosomes. A chromosome is, in essence, a very long and continuous piece of DNA that contains many genes, regulatory elements, "junk DNA," and other nucleotide sequences.
Web Resources On Chromosome
What are Chromosomes?
Book Resources On Chromosome
Chromosomes: Organization and Function by Adrian T. Sumner
Human Chromosomes by Orlando J. Miller, Eeva Therman