Genetics Index Glossary
Chromosome Structure and Terminology
Cytogenetic analyses are almost always based on examination of chromosomes fixed during mitotic metaphase. During that phase of the cell cycle, DNA has been replicated and the chromatin is highly condensed. The two daughter DNAs are encased in chromosomal proteins forming sister chromatids, which are held together at their centromere. The centromere is the structure where the mitotic spindle attaches prior to segregation.
Metaphase chromosomes differ from one another in size and shape, and the absolute length of any one chromosome varies depending on the stage of mitosis in which it was fixed. However, the relative position of the centromere is constant, which means that that the ratio of the lengths of the two arms is constant for each chromosome. This ratio is an important parameter for chromosome identification, and also, the ratio of lengths of the two arms allows classification of chromosomes into several basic morphologic types:
Each species has a normal diploid number of chromosomes. Cytogenetically normal humans, for example, have 46 chromosomes (44 autosomes and two sex chromosomes). Cattle, on the other hand, have 60 chromosomes. Look the table Chromosome Numbers in Different Species for data on common animals.
Centromere position and arm ratios can assist in identifying specific pairs of chromosomes, but inevitably several or many pairs of chromosomes appear identical by these criteria. The ability to identify specific chromosomes with certainty was revolutionized by discovery that certain dyes would produce reproducible patterns of bands when used to stain chromosomes. Chromosome banding has since become a standard and indispensible tool for cytogenetic analysis., and several banding techniques have been developed::
- Q banding: chromosomes are stained with a fluorescent dye such as quinacrine
- G banding: produced by staining with Giemsa after digesting the chromosomes with trypsin
- C banding: chromosomes are treated with acid and base, then stained with Giesma stain
Each of these techniques produces a pattern of dark and light (or fluorescent versus non-fluorescent) bands along the length of the chromosomes. Importantly, each chromosome displays a unique banding pattern, analagous to a "bar code", which allows it to be reliably differentiated from other chromosomes of the same size and centromeric position. In the following figure, human chromosome pairs 1, 2 and 3 are seen with and without G banding.
Next Topic for Cytogenetics: Preparing a Karyotype
Last updated on December 08, 1996
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