Chromosomes can be visible using a microscope just prior to cell division, when the DNA within the nucleus uncoils as it replicates. By visualizing a cell during metaphase, a stage of cell division or mitosis, researchers can take pictures of the duplicated chromosome and match the pairs of chromosomes using the characteristic patterns of bands that appear on the chromosomes when they are stained with a dye called giemsa. The resulting arrangement is called a karyotype. The ends of the chromosome are referred to as telomeres, which are required to maintain stablility and recently have been associated with aging. An enzyme called telomerase maintains the length of the telomere. Older cells tend to have shorter telomeres. The telomere has a repeated sequence (TTAGGG) and intact telomeres are important for proper DNA replication processes.
Karyotypes are useful in diagnosing some genetic conditions, because the karyotype can reveal an aberration in chromosome number or large alterations in structure. For example, Down syndrome is can be caused by an extra chromosome 21 called trisomy 21. A karyotype of a child with Down syndrome would reveal this extra chromosome.
A chromosome usually appears to be a long, slender rod of DNA. Pairs of chromosomes are called homologues. Each separate chromosome within the duplicate is called a sister chromatid. The sister chromatids are attached to each other by a structure called the centromere. Chromosomes appear to be in the shape of an X after the material is duplicated. The bottom, longer portion of the X is called the long arm of the chromosome (q-arm), and the top, shorter portion is called the short arm of the chromosome (p-arm).
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