Index
Also called a cytogenetic map, it is a generic term that refers to the visual appearance of a chromosome once it is stained and examined under a microscope.
Critical is the visually distinct regions, called light and dark bands, that give each chromosome a unique appearance.
This characteristic allows a person’s chromosomes to be studied in a clinical test known as a karyotype, which will enable scientists to look for chromosomal abnormalities.
The karyotype bands’ size, shape, and pattern are schematically depicted in a karyotype.
The graphic representation must be ordered in a karyogram and pairs of homologous chromosomes.
As the chromosomes are the same in all cells, it can be done in any, although blood cells are usually used.
Cytogenetics
Cytogenetics is the field of genetics that studies chromosomes, their structure, composition, and role in the evolution and development of disease.
The history of cytogenetics begins in 1903, when the American Walter Sutton and Boveri the German Teodor established the chromosomal theory of inheritance, proposing that genetic material is found in chromosomes.
Sutton and Boveri initiated a scientific revolution that allowed the characterization of numerous previously unexplained malformation syndromes and the emergence of new specialties in Human Genetics with applications in prevention, diagnosis, and medical therapy.
Also in cytogenetics are research on biodiversity, studies of genetic improvement of plants and animals, and even studies of fertility, both human and animal.
Chromosome classification
In 1956, researchers Tjio and Levan showed that 46 chromosomes make up the human karyotype, 23 pairs.
Chromosomes can be differentiated by size and shape.
In 1960 a group of researchers ordered the human chromosomes from largest to smallest by size, and within the same size, by the position of the centromere.
This set of characteristics such as shape, size, the position of the centromere, and the bands that show when staining, among others, allows the chromosomes of the various species to be distinguished; it is called a karyotype.
The karyotype and the number of chromosomes are particular to each species.
Human beings have 46 chromosomes, or 23 pairs, in the nucleus of each diploid cell, made up of 22 pairs of autosomal chromosomes and one pair of sex chromosomes (XY and XX).
Somatic chromosomes were later classified into seven groups distinguished by the letters A to G, organized into seven groups:
- Group A: is made up of pairs 1, 2, and 3, which are very large chromosomes, where 1 and 3 are metacentric chromosomes, and 2 is a submetacentric chromosome.
- Group B: consists of pairs 4 and 5, which are large and submetacentric chromosomes.
- Group C: is made up of pairs 6, 7, 8, 9, 10, 11, and 12, all medium and submetacentric chromosomes.
- Group D: is formed by pairs 13, 14, and 15, acrocentric medium chromosomes with satellite.
- Group E: is made up of pairs 16, 17, and 18; they are small chromosomes; 16 is a metacentric and submetacentric chromosome, 17 and 18.
- Group F: is formed by pairs 19 and 20; they are small and metacentric chromosomes.
- Group G: is formed by pairs 21 and 22; they are small and acrocentric chromosomes.
Pair 23 comprises the sex chromosomes, an X chromosome of a medium metacentric size similar to group C chromosomes, and a Y chromosome, small and acrocentric, identical to group G chromosomes.
Score and cytogenetic card
Each human chromosome has a short arm (“p” for petit in French, meaning small) and a long arm (“q” queue in French for tailor tail), separated by a centromere.
The ends of the chromosome are called telomeres.
Each arm of the chromosome is divided into regions or cytogenetic bands visible under the microscope using certain stains.
The bands are called p1, p2, p3, q1, q2, q3, etc., from the centromere to the telomere.
At higher resolution, subbands appear within bands.
The subbands are numbered in the same order.
For example, the location of the CFTR gene on the cytogenetic map is 7q31.2, that is:
- Chromosome 7, q arm, band 3, subband one, and sub-sub band 2.
The ends of the chromosome are called peel and well. The notation 7qtel indicates the end of the long arm of chromosome 7.
Procedure for the study of chromosomes
The procedure for studying human chromosomes is to culture lymphocytes and, at the time of division, treat them with colchicine.
Colchicine disrupts mitosis at metaphase.
From the shape and number of chromosomes of a species, especially mitotic chromosomes, in the metaphase phase, the karyotype of the species can be described.
This is the most auspicious moment for the observation of chromosomes. They are then dipped in a hypotonic solution to swell and disperse. They are eventually stained, which is known as chromosome painting.
Chromosome painting is a technique used to mark or “paint” the chromosome sample to be analyzed.
Fluorescent-labeled samples are called DNA “probes,” in analogy to space-traveling probes.
As the chromosomes are arranged in pairs, the probe will form a hybrid with its corresponding team, which will be easily identified by color.
This procedure is known as fluorescent in situ hybridization.
With this technique, it is possible to map the genome of various organisms for evolutionary studies, biological classification studies, and studies of genetic alterations in diseases.
The so-called conventional stains such as Giemsa, Acetic Orcein, Schiff’s reagent, hematoxylin, or eosin, among others, can be used in the methodologies that identify the “bands” of chromosomes.
More recently, with the advent of various techniques in Molecular Biology, cytogenetics has accumulated refinements, especially those related to the location of genes or repetitive DNA sequences.
These studies are reflected in the karyogram or cytogenetic map in drawings or photographs.
Chromosome study uses
From the study of the karyogram, genetic investigations of that individual can be carried out, which allows the detection of possible syndromes.
As the chromosomes are the same in all cells, it can be done in any, although blood cells are usually used.
To carry out these types of studies in the fetus, the cells are obtained by amniocentesis, taking the cells from the umbilical cord blood or the amniotic fluid.
The karyogram is used to detect chromosomal abnormalities in the following cases:
- Genetic disorders.
- Congenital malformations.
- Mental retardation.
- A delay in puberty.
- Gonadal disorders.
- A study of couples with a history of recurrent reproductive loss.
- An analysis of teams with a history of malformed or stillborn children.
As a consequence, the alterations presented in the chromosomes can cause fertility or health problems.
The alterations in the chromosomes can be due to the absence of a chromosome, a duplication, or the loss of some “piece.”
In those cases when a chromosome is missing, we are in the case of aneuploidy.
The most common aneuploidies are trisomy on chromosome 21 or Down syndrome, trisomy on chromosome 18 or Edwards syndrome, and trisomy on chromosome 13 or Patau syndrome.
