Structure of the Eukaryotic Chromosome and the Karyotype

Abstract

Higher organisms are eukaryotes in contrast to bacteria and blue-green algae, which are prokaryotes. The eukaryotic chromosome is a complicated structure that contains DNA and several different types of proteins. Chromosomes of higher organisms are studied most frequently at mitotic metaphase. This is the stage at which the chromosomes reach their greatest condensation. Microscopic study of chromosomes generally utilizes cells that have been treated with a fixative to preserve the structure of chromosomes and other cell components. Then the chromosomes are stained with one or more of a variety of dyes. Because these treatments can alter the structure of cellular components, it may be necessary to study cells treated with several different fixatives and stains in order to ascertain that features seen under the microscope are real. The natural condensation of mitotic chromosomes is increased by a prefixation treatment with various drugs, for example, colchicine. During mitotic metaphase, the condensed chromosomes appear in identifiable shapes (the karyotype) characteristic of the species being studied.