Chapter 3 - The Identification of human chromosomes

Abstract

This chapter discusses the identification of human chromosomes through different techniques. The chromosomes are arranged in descending order of length and, where the lengths are similar, they are further divided according to the position of the centromere. Secondary constrictions are narrow, stretched parts of the chromosomes which are more palely staining. They are called secondary to distinguish them from the primary constriction of the centromere. They are apparent in specific regions in some of the chromosomes but only under certain conditions of preparation. They may be accentuated by special techniques such as growing the cells in a calcium-free medium. As the positions of the constrictions are specific, some chromosomes may be further identified by their presence. When a chromosome replicates during interphase, it synthesizes new molecules of DNA. If growing cells are supplied with titrated thymidine, they incorporate this compound in their newly synthesized DNA. Tritium is a radioactive isotope of hydrogen emitting β particles, and the presence of the isotope may be located by placing special photographic film over the specimen. The film is developed and fixed. This autoradiograph then shows a pattern of radioactive labeling visible as silver grains over the chromosomes.