MoBio Chromosomes and Karyotype Chapter 1

In a non-dividing cell, chromosomes are not visible by light microscopy, because chromatin spreads throughout the nucleus. During the metaphase of cell division, the chromatin condenses and becomes visible as chromosomes. At this time, each chromosome has been duplicated. A chromosome becomes two sister chromatids attached at the centromere.

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Figure 1-C-2. Schematic drawing of chromosomes. (a) During the metaphase of cell division, a chromosome becomes two sister chromatids attached at the centromere. (b) Notations about the chromosome bands. This figure uses human chromosome 17 as an example.

Chromosome banding

To see chromosomes by microscope, they are normally treated with chemical dyes, such as Giemsa. The chromosome will appear as a series of alternate dark and light bands. If Giemsa is used, the dark band is called G-band or G-positive band, and the light band is named G-negative band. Similar banding patterns can be observed by using another dye, Quinacrine. However, if chromosomes were treated in a hot alkaline solution before staining with Giemsa, a reverse pattern will be observed, namely, the original dark band will become light band, and vice versa. For this reason, the G-negative band is also known as the R-band.

Chromosome bands are named as follows. Each chromosome consists of two arms separated by the centromere. The long arm and short arm are labeled q (for queue) and p (for petit), respectively. At the lowest resolution, only a few major bands can be distinguished, which are labeled q1, q2, q3; p1, p2, p3, etc., counting from the centromere. Higher resolution reveals sub-bands, labeled q11, q12, q13, etc. Sub-sub-bands identified by even higher resolution are labeled q11.1, q11.2, q11.3, etc. Traditionally, the short arm (p) is displayed on top of the long arm (q). See Figure 1-C-5 for the banding pattern of the entire human chromosomes.

Karyotype

Karyotype is the representation of entire metaphase chromosomes in a cell, arranged in order of size.

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Figure 1-C-3. The karyotype of human somatic cells. A human somatic cell contains two sets of homologous chromosomes, which may be divided into two types: autosomes and sex chromosomes. Autosomes are further divided into seven groups: A to G. During the metaphase of cell division, each chromosome has been duplicated. Therefore, this karyotype consists of 92 chromosomes.