Genetics For Dummies by Rene Fester Kratz & Lisa J. Spock

Author:Rene Fester Kratz & Lisa J. Spock [Kratz, René Fester & Spock, Lisa J.]
Language: eng
Format: epub
ISBN: 9781394210206
Publisher: Wiley
Published: 2023-11-24T00:00:00+00:00

Big enough to see

One way a geneticist counts chromosomes is with the aid of microscopes and special dyes to see the chromosomes during metaphase — a time in the cell cycle when the chromosomes take on a fat, easy-to-see, sausage shape. In order to examine chromosomes, a sample of cells is obtained. Almost any sort of dividing cell works as a sample, including root cells from plants, blood cells, or skin cells. These cells are then cultured — given the proper nutrients and conditions for growth — to stimulate cell division. Some cells are removed from the culture and treated to stop mitosis during metaphase, and dyes are added to make the chromosomes easy to see. Finally, the cells are inspected under a microscope. The chromosomes are sorted, counted, and examined for obvious abnormalities.

This process of chromosome examination, with the identification, pairing, and ordering of the chromosomes, is called karyotyping. A karyotype (shown in Chapter 6) reveals exactly how many chromosomes are present in a cell, along with some details about the chromosomes’ structure. Scientists can only see these details by staining the chromosomes with special dyes. In traditional images of a karyotype, the chromosomes appear to be striped because of the stain used. When examining this type of karyotype, a geneticist looks at each individual chromosome. Every chromosome has a typical size and shape and a very specific pattern of stripes (called the banding pattern). In addition, scientists identify chromosomes by the location of the centromere and the length of the chromosome arms (the parts on either side of the centromere).

In some disorders, part of one of the chromosome arms is misplaced or missing. Therefore, geneticists often refer to the chromosome number along with the letter p or q to communicate which part of the chromosome is affected (p refers to the short arm of the chromosome; q refers to the long arm). The bands that are present after staining also have numbers. Therefore, you may see something like 22q11.2 when referring to a chromosome problem. This means that the alteration involves the long arm of chromosome 22 and that it is region 11.2 that is affected (see Figure 12-6). Basically, the q11.2 is like a map coordinate, telling you exactly where on chromosome 22 there is a change.

Karyotyping is ideal for detecting numerical chromosome problems (aneuploidy or polyploidy) — it allows scientists to determine whether there are too many or too few chromosomes. In addition, because each chromosome has its own unique size and shape, as well as its own unique banding pattern when stained, they can determine specifically which chromosome is extra or missing. Because of those unique features, scientists can also use karyotyping to detect and identify chromosomal changes that involve huge sections of DNA, such as large deletions, duplications, inversions, and translocations. Unfortunately, karyotyping is unable to detect small changes in structure, such as deletions that are only a few thousand base pairs in length. With such a small deletion, the chromosome involved would look no different under the microscope than its normal homolog.

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