Each chromosome is made up of DNA tightly coiled many times around proteins called histones that support its structure. 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.
Down syndrome is the most common genetic condition in the United States. It was first described in 1866 and is named after John Langdon Down, the doctor who first identified the syndrome. The cause of Down syndrome, also known as trisomy 21, was discovered in 1959.
In eukaryotes, a centromere is a region of DNA that is responsible for the movement of the replicated chromosomes into the two daughter cells during mitosis and meiosis. There is one centromere on each chromosome, and centromeres are responsible for two major functions.
A submetacentric chromosome with an interstitial band midway between the telomeric band and the centromeric band; there were no interstitial bands located on the long arms, although occasionally a tertiary constriction can be found on the long arms of the chromosomes.
Beaumount and Zouros (1991) postulated that the Chlamys nobilis karyotype (n = 16), which has 3 metacentric and 13 telocentric chromosomes (Komaru and Wada, 1985), may be derived from the n = 19 karyotype, by Robertsonian fusion of three pairs of telocentric chromosomes.
Wolf-Hirschhorn syndrome is caused by a deletion of genetic material near the end of the short (p) arm of chromosome 4. This chromosomal change is sometimes written as 4p-. The size of the deletion varies among affected individuals; studies suggest that larger deletions tend to result in more severe intellectual disability and physical abnormalities than smaller deletions.