Early microscopic observations on the number and size of chromosomes and their staining patterns led to the discovery of many important general characteristics of chromosome structure
- In nondividing cells, individual chromosomes are not visible, even with the aid of histologic stains for DNA (e.g., Feulgen or Giemsa stains) or electron microscopy
- During mitosis and meiosis, but, the chromosomes condense and become visible in the light microscope
- Almost all cytogenetic work (i.e., studies of chromosome morphology) has been done with condensed metaphase chromosomes obtained from dividing cells—either somatic cells in mitosis or dividing gametes during meiosis
- Certain dyes selectively stain some regions of metaphase chromosomes more intensely than other regions, producing characteristic banding patterns that are specific for individual chromosomes
- G bands are produced when metaphase chromosomes are subjected briefly to mild heat or proteolysis and then stained with Giemsa reagent, a permanent DNA dye
- A recently developed method for visualizing each of the human chromosomes in distinct, bright colors, called chromosome painting, greatly simplifies differentiating chromosomes of similar size and shape
Interphase Polytene Chromosomes Arise by DNA Amplification
- The larval salivary glands of Drosophila species and other dipteran insects contain enlarged interphase chromosomes that are visible in the light microscope
- When fixed and stained, these polytene chromosomes are characterized by a large number of reproducible
- Well-demarcated bands that have been assigned a standardized number
- The highly reproducible banding pattern seen in Drosophila salivary gland chromosomes provides an extremely powerful method for locating specific DNA sequences along the lengths of the chromosomes in this species
- A generalized amplification of DNA gives rise to the polytene chromosomes found in the salivary glands of Drosophila
- This process, termed polytenization, occurs when the DNA repeatedly replicates, but the daughter chromosomes do not separate
Heterochromatin Consists of Chromosome Regions That Do Not Uncoil
- As cells exit from mitosis and the condensed chromosomes uncoil, certain sections of the chromosomes remain dark-staining
- The dark-staining areas, termed heterochromatin, are regions of condensed chromatin
- The light-staining, less condensed portions of chromatin are called euchromatin
- In mammalian cells, heterochromatin appears as darkly staining regions of the nucleus, often associated with the nuclear envelope