Chromosome Behavior in Meiosis and Combinative Variability
The biological significance of chromosomes is most clearly manifested in the process of meiosis. This type of cell division performs a critical function: maintaining the constancy of the karyotype. Thanks to meiosis, the doubling of the chromosome number in each new generation is prevented, which guarantees the preservation of the species-specific chromosome set.
In addition, chromosomes serve as the cytological basis for combinative variability. During meiosis, the processes of conjugation and crossing over take place. Homologous chromosomes exchange their segments, which carry different gene alleles. An important feature of this process is that the genes themselves and the general structure of the genotype do not undergo changes. However, due to recombination, completely new complexes of traits are formed.
Together with fertilization, when sex cells fuse and a zygote is formed, chromosome recombination ensures colossal genetic diversity. It is this mechanism that makes the appearance of two genotypically identical organisms impossible (except for monozygotic twins), creates a variety of offspring in the same parents, and ultimately increases the survival rate of the entire species in constantly changing environmental conditions.
Genome "Rejuvenation" Hypothesis
In addition to their classical genetic role, chromosomes in meiosis participate in a vital cytophysiological process. There is a genome "rejuvenation" hypothesis proposed by researchers M.M. Vilenchik and N.N. Mushkambarov (the "ANEREM" concept). According to this theory, meiosis represents a mechanism for a peculiar "overhaul" of chromosomes, which is completely inaccessible to ordinary somatic cells.
Complete restoration of the genome and its release from accumulated damage is possible exclusively during chromosome conjugation (in meiosis or during endomixis in protozoa). In prophase of meiosis I, parallel to conjugation and crossing over, repair processes are activated. Enzymes of the complex "recombinase" system perform not only the function of gene shuffling, but also carry out thorough DNA repair, including the cleavage and subsequent ligation of chains.
The prolonged stay of homologous chromosomes in a conjugated state exerts a pronounced protective effect on DNA molecules. It is assumed that upon the resumption of meiosis, immediately prior to ovulation, repairing processes receive additional activation. In germ cells, the intensity of DNA repair is sharply increased. If chromosome damage turns out to be irreparable, the probability of apoptosis in such cells increases significantly. The elimination of defective gametes occurs, which reliably protects future offspring from the transmission of genetic breakdowns.
Cytogenetic Studies of Chromosomes
To study the structure and detect chromosomal abnormalities in modern clinical practice, molecular cytogenetic studies are actively used. One of the leading approaches is the FISH method (fluorescence in situ hybridization).
This study has strict indications. In particular, it is prescribed to all patients with newly diagnosed multiple myeloma (MM), as well as during the development of the first and second relapses of the disease. Plasma cells are used as the biological material for analysis. The level of recommendation certainty for this method is assessed as C (LOC 4).
The main goal of applying the FISH method is to identify prognostically important chromosomal abnormalities. The spectrum of detected disorders is quite broad. The method allows for the precise diagnosis of:
- Various translocations, including t(4;14), t(14;16), t(6;14), and t(11;14).
- Deletions, for example, del 17p13.
- Specific changes in chromosome 1.
- General assessment of cell ploidy.
Chromosomal Abnormalities: X Chromosome Trisomy Syndrome
Anomalies in the number of chromosomes lead to the development of specific syndromes. One of them is X chromosome trisomy syndrome (XXX), also known as triple-X syndrome. This pathology occurs with a frequency of 1 in 1,000 girls and is characterized as a borderline state between absolute normality and pathology.
For the cytological diagnosis of the syndrome, epithelial cells of the oral mucosa are examined. The key marker is the detection of two Barr bodies, whereas normally only one should be present. Extremely rare karyotype variants consisting of 48 chromosomes (four X chromosomes at once) have also been described in medical literature. In such unique cases, three Barr bodies are cytologically detected.
The clinical picture and phenotype of women with the XXX karyotype are quite variable. Outwardly, patients often have no noticeable deviations from the norm. Two main variants of the syndrome's course are distinguished:
- Asymptomatic: women maintain a regular menstrual cycle and normal fertility, allowing them to successfully bear children.
- Symptomatic: irregular menstrual cycle, spontaneous abortions, infertility, and early menopause are observed. The main cause of these clinical manifestations is a pronounced endocrine imbalance leading to impaired normal ovarian function.
Meiosis can be imagined as an "overhaul" of chromosomes: recombinase enzymes not only shuffle genes for the diversity of offspring, but also ligate DNA breaks, purging the genome of accumulated errors.
Class and exam questions
How does the number of Barr bodies change in triple-X syndrome?
Normally, one Barr body is detected in oral mucosa cells. With the XXX karyotype, their number increases to two, and in rare cases with 48 chromosomes (four X chromosomes) to three.
What is the cytophysiological significance of meiosis?
According to the genome "rejuvenation" hypothesis, prolonged conjugation of homologous chromosomes promotes efficient DNA repair. Cells with irreparable defects undergo apoptosis, which prevents the transmission of mutations to offspring.
What chromosomal abnormalities are detected using the FISH method?
The fluorescence in situ hybridization method makes it possible to detect prognostically important translocations (e.g., t(4;14), t(11;14)), deletions (del 17p13), changes in the first chromosome, and to assess the overall cell ploidy.
What else to study on this topic
- Morphology and Chemical Composition of Chromosomes
- Levels of chromatin compaction
- Karyotype and Classifications of Human Chromosomes
- Chromosomal theory of heredity
- Molecular Genetic and Biochemical Methods
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