While the interphase chromatin appears to be a tangled mass within the nucleus, the mitotic chromosome appears as an organized structure with many prominent features. These features include structures known as the centromere and the telomere.
The centromere is the region of the chromosome to which the spindle apparatus attaches during mitosis and meiosis. The spindle apparatus is the network of fibers along which the chromosomes move during cell division. It also contains the site at which sister chromatids are attached prior to their separation during the stage of cell division known as anaphase. The centromere is responsible for the movement of the chromosome. During mitosis and meiosis, the centromere is pulled by the spindle fibers toward the opposite ends of the dividing cell (poles), as the attached chromosome is dragged behind. The centromere is essential for segregation.
The telomere is a structure that occurs at the end of linear eukaryotic chromosomes and that confers stability. The first telomere to be sequenced was from the organism Tetrahymena thermophila, a type of single-cell eukaryotic organism, in 1978 by Elizabeth Blackburn and Joseph Gall. This telomere contains an AACCCC nucleotide sequence that is repeated thirty to seventy times. The sequences of telomeres from other species show the same pattern: a tandem array of a short nucleotide sequence, one DNA strand Grich and the other DNA strand C-rich. Telomeres are synthesized by an enzyme called telomerase, which adds telomeric sequences back onto chromosome ends, one base at a time.
Banding techniques allow every mitotic chromosome, as well as regions within individual chromosomes, to be distinguished. The first method used, known as Q-banding, uses flourescent derivatives of quinacrine, which for unknown reasons bind preferentially to some regions of chromosomes. When viewed under ultraviolet light, chromosomes appear with bright bands, corresponding to euchromatin, and dark bands, corresponding to heterochromatin. This banding method is useful in identifying some polymorphisms, which are gene variations (alleles) within the population of genomes. It is also useful for identifying the Y chromosome.
Later, another banding technique was developed, called G-banding. This technique employs a modified Giemsa stain, which is a dye that specifically binds to DNA. As in Q-banding, a series of identifiable light and dark bands is generated on the chromosome. Euchromatin stains lightly, while most heterochromatin stains darkly.
These banding methods have made it possible to diagrammatically represent each human chromosome, using designated nomenclature for specific chromosomal regions. These techniques have shown that the mitotic chromosome can be divided into a short arm, designated p, and a long arm, q. Each arm is then divided into one to three regions by landmarks, such as the ends of the arm, the centromere, and certain prominent bands. Regions are spaces between adjacent landmarks and are numbered consecutively within each region. These features allow genes to be designated to specific regions on the mitotic chromosome. For example, a gene at band 4q14 is localized to chromosome 4, the long arm, region 1, band 4.
- Eukaryotic Chromosome - Polytene Chromosomes
- Eukaryotic Chromosome - Heterochromatin Versus Euchromatin
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