Karyotype and chromosome morphology
The karyotype is defined as the set of characteristics of a chromosome complement typical of a particular biological species. The main characteristics of a karyotype include the total number, size, shape of chromosomes, and their pairing (in diploid organisms).
Human somatic cells contain 46 chromosomes (diploid set), whereas sex cells (gametes) contain a haploid set of 23 chromosomes. To graphically represent the haploid set, an idiogram is used—a diagram demonstrating the characteristic banding pattern of each chromosome.
The basic structure of a chromosome includes:
- Short arm (designated by the Latin letter p — from short arm).
- Long arm (designated by the letter q — from long arm).
Chromosomes are numbered from 1 to 22 (these are autosomes), and sex chromosomes (X and Y) are distinguished separately. Segments and regions on the arms are designated by numbers (loci), which makes it possible to precisely indicate the location of genes.
Nucleolar organizer region (NOR)
Nucleolar organizer regions play a special role in cell function. They are secondary constrictions located on the short arms of acrocentric chromosomes.
In humans, such regions are localized on chromosomes numbered 13, 14, 15, 21, and 22. It is in these zones that the genes for ribosomal RNA (rRNA) are located. During interphase, a nucleolus forms around these regions, which is necessary for ribosome synthesis.
Spatial organization within the nucleus
In the interphase nucleus, chromosomes are not in a chaotic, decondensed state (they are not diffusely intermixed with each other like "spaghetti"). Conversely, chromatin forms loop domains, and each chromosome occupies a compact, strictly isolated volume—a chromosome territory. Homologous chromosomes can thus be located in completely different parts of the nucleus.
Chromosomes are firmly attached to the inner nuclear membrane (the nuclear lamina) at several points. This fixation and spatial orientation involve:
- Telomeres (terminal regions of the arms). They anchor the chromosome at one pole of the nucleus.
- Centromeres (primary constriction regions). They attach at the opposite pole.
The main functions of telomeres are to create a specific architecture and order within the interphase nucleus, as well as to prevent chromosome ends from fusing together.
Classification and nomenclature
To organize knowledge about human chromosomes, international standards have been developed:
- Denver classification (adopted in 1960 in the USA) — the first attempt to unify nomenclature.
- ISCN (introduced in 1978) — An International System for Human Cytogenetic Nomenclature (An International System for Human Cytogenetic Nomenclature), which is universally used in modern practice.
With routine (solid) staining, all human chromosomes are divided into 7 groups. This division is based on the following criteria:
- Chromosome size.
- Centromere position.
- Presence of a secondary constriction.
- Presence of satellites.
Today, differential staining methods are used to accurately identify the structure of each individual chromosome.
Briefly: answers to topical questions
Which chromosome groups are distinguished according to the Denver classification?
According to the Denver classification, human autosomes are divided into groups A–G (7 groups in total). Sex chromosomes are not assigned to a separate group. With routine staining, chromosomes are grouped according to the following criteria:
- chromosome size;
- centromere position;
- presence of a secondary constriction;
- presence of satellites.
Which human chromosomes are acrocentric?
Acrocentric chromosomes include human chromosome pairs 13, 14, 15, 21, and 22. The structure of these chromosomes features a number of specific characteristics:
- Position of the constriction — the centromere is strongly shifted toward the end;
- Arms — a very short arm and a very long arm are clearly distinguished;
- Nucleolar organizer regions (NOR) — located in the secondary constrictions of the short arms and contain numerous copies of ribosomal RNA genes.
Acrocentric chromosomes are prone to rearrangements: they can fuse, losing their short arms and forming a single metacentric or submetacentric chromosome.
What function does the centromere (primary constriction) perform during cell division?
During cell division, the centromere (primary constriction) connects two chromatids and serves as the attachment site for the fibers of the mitotic apparatus. The main functions during cell division are:
- Connection — holds sister chromatids together;
- Anchorage of the spindle — the kinetochore attaches to the centromere, acting as a structure for spindle fiber attachment;
- Spatial orientation — ensures chromosome movement, alignment at the equatorial plane in metaphase, and subsequent segregation to opposite poles of the cell when the fibers shorten in anaphase.
Class and exam questions
How does a karyotype differ from an idiogram?
Karyotype is the actual number and morphology of chromosomes in a cell of a specific organism. Idiogram is a schematic, graphical representation of a haploid set of chromosomes indicating their transverse banding pattern.
Where are chromosomes located in the interphase nucleus?
They are not chaotically intermixed. Each chromosome occupies its own isolated chromosome territory and is attached to the nuclear lamina via centromeres and telomeres.
On which human chromosomes is the nucleolar organizer region located?
Nucleolar organizer regions are localized on the short arms of acrocentric chromosomes: 13, 14, 15, 21, and 22.
What else to study on this topic
- Methods of differential chromosome staining and their types
- Detailed criteria for dividing chromosomes into 7 groups
- Chromatin structure and the formation of loop domains
- Chromosome visualization using FISH (spectral karyotyping)
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