Variation in Chromosome Size Help
Variation in Chromosome Size
In general, the chromosomes of most organisms are too small to be seen microscopically, except when highly condensed during metaphase of mitosis. Until the advent of molecular biology and DNA sequencing technology, the microscope was the only way to study chromosome structures. Now we can study chromosome size, structure and composition using molecular sequence analysis tools as well as new kinds of microscopy, such as confocal microscopy. Confocal microscopy utilizes laser light point illumination to visualize fluorescently stained specimens that are thicker than the focal plane. A detector receives the excited fluorescent light emission and a computer creates a three-dimensional image of the specimen, such as condensed chromosomes, microscopic organisms or whole cellular structures. Because fluorescent dyes can be attached to specific molecules, particular chromosomal regions, such as telomeres, or cellular proteins, such as actin, can be easily identified. The visualization of chromosomes using this technique is called FISH, or Flourescent In-Situ Hybridization.
Drosophila has long been a favorable organism for genetic studies because it produces large numbers of progeny within the confines of a small bottle in a short period of time. There are only four pairs of chromosomes in the diploid complement of D. melanogaster, but their size in reproductive cells and most body cells is quite small. Unusually large chromosomes, 100 times as large as those in other parts of the body, are found in the larval salivary gland cells. Each giant polytene chromosome (Fig. 7.2) is composed of hundreds of chromatids paired along their identical DNA sequences throughout their length. Furthermore, each pair of homologous polytene chromosomes is also constantly synapsed in these somatic cells. Distinctive crossbandings (appearing when chromosomes are stained) represent regions (called chromomeres) of the chromatid bundle containing highly coiled or condensed DNA that is interspersed between regions of less condensation. The crossbanding pattern of each chromosome is characteristic of each species, but the pattern may change in a precise sequence at various stages of development. Chromosomal aberrations (translocations, inversions, duplications, deletions, etc.) can often be easily recognized in these polytene chromosomes under the light microscope.
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