After fertilization, a zygote with an improper chromosome complement occurs. Translocation is the movement of a piece or a whole chromosome onto another chromosome. A spectral karyogram of a brain cancer glioblastoma illustrating multiple translocation problems and chromosome instability. Abnormalities in mitosis also occur and can result in diseases from translocations. Some organisms and cells have entire sets of chromosomes additional to the standard 2N diploid. Cells that have extra sets in the formula of 3N are called triploid.
If they are 4N, they are called tetraploid. Any time there are abnormal numbers of chromosomes, cells are referred to as aneuploid. Q-banding involves use of the fluorescent dye quinacrine, which alkylates DNA and is subject to quenching over time. Caspersson et al. Since then, researchers have developed a variety of other chromosome banding techniques that have largely supplanted Q-banding in clinical cytogenetics.
Today, most karyotypes are stained with Giemsa dye, which offers better resolution of individual bands, produces a more stable preparation, and can be analyzed with ordinary bright-field microscopy. The molecular causes for staining differences along the length of a chromosome are complex and include the base composition of the DNA and local differences in chromatin structure.
In G-banding , the variant of Giemsa staining most commonly used in North America, metaphase chromosomes are first treated briefly with trypsin, an enzyme that degrades proteins, before the chromosomes are stained with Giemsa.
Trypsin partially digests some of the chromosomal proteins, thereby relaxing the chromatin structure and allowing the Giemsa dye access to the DNA. In general, heterochromatic regions, which tend to be AT-rich DNA and relatively gene-poor, stain more darkly in G-banding. In contrast, less condensed chromatin—which tends to be GC-rich and more transcriptionally active—incorporates less Giemsa stain, and these regions appear as light bands in G-banding.
Most importantly, G-banding produces reproducible patterns for each chromosome, and these patterns are shared between the individuals of a species. An example of Giemsa-stained human chromosomes, as they would appear under a microscope, is shown in Figure 1a. Typically, Giemsa staining produces between and bands distributed among the 23 pairs of human chromosomes.
Measured in DNA terms, a G-band represents several million to 10 million base pairs of DNA, a stretch long enough to contain hundreds of genes. G-banding is not the only technique used to stain chromosomes, however. R-banding, which is used in parts of Europe, also involves Giemsa stain, but the procedure generates the reverse pattern from G-banding. In R-banding Figure 1c , the chromosomes are heated before Giemsa stain is applied.
The heat treatment is thought to preferentially melt the DNA helix in the AT-rich regions that usually bind Giemsa stain most strongly, leaving only the comparatively GC-rich regions to take up the stain.
R-banding is often used to provide critical details about gene-rich regions that are located near the telomeres. Yet another method is C-banding Figure 1d , which can be used to specifically stain constitutive heterochromatin , or genetically inactive DNA, but it is rarely used for diagnostic purposes these days. C-banding is a specialized Giemsa technique that primarily stains chromosomes at the centromeres, which have large amounts of AT-rich satellite DNA.
The first method to be used to identify all 46 human chromosomes was Q-banding Figure 1b , which is achieved by staining the chromosomes with quinacrine and examining them under UV light. This method is most useful for examining chromosomal translocations, especially ones involving the Y chromosome. Taken together, these banding techniques offer clinical cytogeneticists an arsenal of staining methods for diagnosing chromosomal abnormalities in patients.
In order to maximize the diagnostic information obtained from a chromosome preparation, images of the individual chromosomes are arranged into a standardized format known as a karyotype, or more precisely, a karyogram Figure 1a-c. According to international conventions, human autosomes, or non-sex chromosomes, are numbered from 1 to 22, in descending order by size, with the exceptions of chromosomes 21 and 22, the former actually being the smallest autosome.
The sex chromosomes are generally placed at the end of a karyogram. Within a karyogram, chromosomes are aligned along a horizontal axis shared by their centromeres. Individual chromosomes are always depicted with their short p arms—p for "petite," the French word for "small"—at the top, and their long q arms—q for "queue"—at the bottom. Centromere placement can also be used to identify the gross morphology, or shape, of chromosomes.
For example, metacentric chromosomes, such as chromosomes 1, 3, and 16, have p and q arms of nearly equal lengths.
Submetacentric chromosomes, such as chromosomes 2, 6, and 10, have centromeres slightly displaced from the center. Acrocentric chromosomes, such as chromosomes 14, 15, and 21, have centromeres located near their ends. Arranging chromosomes into a karyogram can simplify the identification of any abnormalities.
Note that the banding patterns between the two chromosome copies, or homologues, of any autosome are nearly identical. Some subtle differences between the homologues of a given chromosome can be attributed to natural structural variability among individuals. Occasionally, technical artifacts associated with the processing of chromosomes will also generate apparent differences between the two homologues, but these artifacts can be identified by analyzing 15—20 metaphase spreads from one individual.
It is highly unlikely that the same technical artifact would occur repeatedly in a given specimen. Today, G-banded karyograms are routinely used to diagnose a wide range of chromosomal abnormalities in individuals. Although the resolution of chromosomal changes detectable by karyotyping is typically a few megabases, this can be sufficient to diagnose certain categories of abnormalities.
For example, aneuploidy , which is often caused by the absence or addition of a chromosome, is simple to detect by karyotype analysis. Cytogeneticists can also frequently detect much more subtle deletions or insertions as deviations from normal banding patterns.
Likewise, translocations are often readily apparent on karyotypes. When regional changes in chromosomes are observed on karyotypes, researchers often are interested in identifying candidate genes within the critical interval whose misexpression may cause symptoms in patients. This search process has been greatly facilitated by the completion of the Human Genome Project , which has correlated cytogenetic bands with DNA sequence information. Lawrence C. Brody, Ph. Featured Content.
Introduction to Genomics. Reprints and Permissions. Zheng, Js. Karyotype of mitotic metaphase and meiotic diakinesis in non-heading Chinese cabbage. Plant Syst Evol , — Download citation. Received : 05 April Accepted : 14 July Published : 20 August Issue Date : February Anyone you share the following link with will be able to read this content:. Sorry, a shareable link is not currently available for this article.
Provided by the Springer Nature SharedIt content-sharing initiative. Skip to main content. Search SpringerLink Search. Abstract Non-heading Chinese cabbage [ Brassica rapa L. Nature Genet — Xiong ZY, Pires JC Karyotype and identification of all homologous chromosomes of allopolyploid Brassica napus and its diploid progenitors. Acknowledgments This work was supported financially by grants from: 1. View author publications.
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