Understanding the Human Karyotype: Methods of Its Examination
What is a karyotype? What is a human karyotype? What is your karyotype? Just joking. But we all have a karyotype and all of this karyotype analysis will make sense by the end of this post.
Every one of us has a set of 23 chromosome pairs. These chromosomes house all of the genes that make us who are. In other words, chromosomes are carriers of our most precious possession: our unique identity.
So, while karyotyping refers to the laboratory procedure whereby a doctor can examine your set of chromosome pairs to look for possible disorders, a karyotype denotes the actual collection of all of your chromosomes that are examined. More specifically, it refers to both the number as well as the appearance of all of the chromosomes in a human cell.
The test itself looks for the number of chromosomes as well as the shape of each of the 23 pairs under a light microscope. In particular, the tests look for irregularities in several characteristics. These include chromosome length, the position of the centromeres, the banding pattern, for any difference between the sex chromosomes as well as any additional physical characteristics that may point to possible disease.
As we mentioned earlier, the standard number of chromosomes in an individual is a set of 23 pairs. This number can be referred to as the somatic number because the frequency is based on the number in somatic cells, is different in the germline (the sex cells), in which the chromosome number is n. This means that in humans, that number is 2n = 46.
The investigation of karyotypes has several important applications in cell biology and genetics where they have several purposes including the study of chromosomal aberrations, cellular function, taxonomic relationships, as well as to find out information that pertains to past evolutionary events.
Understanding the frequency of chromosomes is important as it may point to diseases that are termed as aneuploidy, whereby individuals either have fewer (monosomy) or more (trisomy) chromosomes. For example, one of the most commonly known aneuploidies is Down syndrome that manifests itself as trisomy 21, whereby humans have an extra copy of chromosome 21. This surplus genetic material leads to Down syndrome symptoms such as delayed development as well as several other physical appearance anomalies. An example of monosomy is Cri du chat syndrome that is marked by the deletion of chromosome 5 and thereby a lack of chromosomal material and, therefore, missing genetic material. In either case, both too much genetic material, as well as missing genetic information, can lead to disorders.
Another way to look for disease via karyotyping is by means of banding. Banding refers to chromosome appearance after they have been treated with certain stains. This appearance, as the name states, is in bands. More specifically, it refers to alternating light and dark banding pattern along the lengths of the chromosomes. Specific banding patterns can help identify chromosomes as well as diagnose chromosomal aberrations. These include chromosome breakage, loss, duplication or inverted segments. There are a total of six banding patterns, namely G-bands, R-bands, C-bands, Q-bands, T-bands, and NOR-bands. This is because chromosomal anomalies are typically difficult to see with the naked eye under the light microscope, and stains help the visualization process.
This method refers to staining with a dye called Giemsa stain that is performed after chromosomes have been treated with the enzyme trypsin. This will typically yield anywhere between 300–400 bands in a normal human genome, whereby the light bands refer to regions that are rich in guanine and cytosine, whereby the dark bands denote adenine and thymine rich regions.
This method is essentially the reverse of G-banding, whereby the staining yields dark bands of regions rich in guanine and cytosine and light bands that refer to adenine and thymine rich areas.
This method also involves the Giemsa stain, but it is primarily focused on staining centromeres. The name is taken from centromeric or constitutive heterochromatin. Chromosomes are denatured and then stained, producing banding patterns that are most suitable for characterizing plant chromosomes.
The “Q” in this procedure stands for quinacrine, which is a fluorescent dye that yields a fluorescent pattern. Similar to the staining pattern of G-banding, the bands are recognized by a fluorescing yellow color that differs in intensity. While the dye binds to both the guanine and cytosine as well as the thymine and adenine regions, it is the latter that fluoresces and creates a visible difference between the two regions.
The T in this procedure refers to the visualization of telomeres, which are localized at the end of each chromosome. The function of the telomeres is to protect the chromosomes from deterioration as well as potential fusion to neighboring or nearby chromosomes.
Silver is used for staining a region called the nucleolus organizer regions (NOR). These regions have a vital role in the formation of the nucleolus, a structure that is located deep in the cell within the nucleus. The NORs are located on the short arms of chromosomes in humans.
Preparation of karyotypes is typically done during the metaphase or prometaphase step of the cell cycle (during which the cell divides and genetic material is copied and then segregated). This is because, at that point, the chromosomes are at their most condensed conformations. Once the staining has been completed, the chromosomes are organized in what is called a karyotype or a karyogram, whereby they are aligned starting from chromosome 1 to 22, or in descending order by size. An exception of this are chromosomes 21 and 22, the two smallest autosomes. Sex chromosomes are placed at the very end of the karyogram.
There you have it. Almost everything you need to know about your karyotype. This should make things a bit more understandable the next time someone asks you about it.