Goals and objectives of genetic cloning
In modern medical practice, DNA cloning technologies open up wide opportunities for human genome analysis. The main vectors of development include:
- Genetic mapping — compiling a detailed genetic map of the organism.
- Establishing chromosomal localization of defects leading to pathologies.
- Prenatal diagnostics of severe hereditary diseases before birth.
Thanks to these approaches, scientists have precisely determined the chromosomal localization of many severe diseases, such as sickle cell anemia, phenylketonuria, and Duchenne muscular dystrophy.
Restriction fragment length polymorphism (RFLP) method
The foundation of the RFLP method is changes in the primary DNA structure resulting from various mutations (point substitutions, deletions, or insertions of additional nucleotides).
The mechanism of action is that any rearrangements of the primary sequence inevitably shift, destroy, or create new restriction sites. When biological material is treated with restriction enzymes, mutant DNA is cut into fragments whose size significantly differs from the length of a healthy person's DNA fragments.
In practice, the RFLP phenomenon is actively used to screen patients for hidden carriage of pathological genes. For example, the method reliably identifies heterozygous parents in families with a high risk of transmitting sickle cell anemia to offspring.
Analysis using allele-specific probes
Another powerful tool of molecular diagnostics is the use of synthetic short oligodeoxynucleotides acting as probes. To visualize the results, these molecules are labeled with the P³² isotope.
The testing procedure requires the preparation of two types of probes:
- A probe containing the normal (unchanged) nucleotide sequence.
- A probe complementary to the DNA sequence with the target mutation.
During the analysis, the patient's genetic material is tested using these specific molecules. Hybridization of the sample with a specific probe makes it possible to draw an unambiguous conclusion about the presence or absence of a specific genetic defect.
Genetic polymorphism and clinical manifestations
Individual human responses to environmental factors (diet, toxins, medications, infectious agents) are due to multiple genetic polymorphisms. First of all, this concerns various isoforms of detoxification enzymes.
Clinical examples of polymorphism:
- Lactose intolerance: a genetically determined inability to digest and absorb fresh milk.
- Alcohol sensitivity: pronounced ethnic differences (particularly in Mongoloids) associated with mutations in the alcohol dehydrogenase gene.
- Solanine sensitivity: a toxic glycoside from green potato tubers inhibits the enzyme pseudocholinesterase; the response to the toxin depends on the genetic variant of this enzyme.
Hereditary predisposition Multiple polymorphisms of receptors, detoxification enzymes, and cell cycle regulatory proteins underlie the familial predisposition to a number of multifactorial pathologies. These include diabetes mellitus, atherosclerosis, oncological processes, mental disorders, as well as lung, heart, and kidney diseases. It is important to note that genetic predisposition is realized and clinically manifested only when exposed to certain environmental conditions.
Briefly: answers to topical questions
Which specific enzymes belong to the first and second phases of xenobiotic detoxification in the liver?
Sources name the following enzymes and enzyme systems for the detoxification phases.
Phase I enzymes:
- Cytochrome P450 monooxygenases — a microsomal oxidation system involved in substrate hydroxylation.
- The complex includes cytochrome P450 and flavins cytochrome P450 reductase.
Phase II enzymes (conjugation):
- Transferases — attach conjugates to hydrophilic groups of detoxified substances.
- Glutathione S-transferase — an enzyme for conjugation with glutathione.
- Glucuronyltransferase — an enzyme for the conjugation of bilirubin to form bilirubin glucuronides.
What method is used to separate and visualize DNA fragments after treatment with restriction enzymes?
DNA fragments after enzymatic cleavage are separated using gel electrophoresis and visualized by probe detection.
Stages of analysis:
- Gel electrophoresis — used for the spatial separation of the resulting DNA fragments depending on their length.
- Hybridization (detection) — carried out using radioactively labeled oligonucleotide probes, which allow the identification of specific regions on the electrophoregram.
What function does the protein dystrophin perform, the gene of which mutates in Duchenne muscular dystrophy?
Dystrophin performs structural and anchoring functions.
Main functions and localization:
- Structure binding — as an anchor protein, dystrophin binds F-actin to the plasma membrane.
- Maintaining the shape and contractile activity of muscle fibers — dystrophin is located under the sarcolemma, maintains the shape of muscle fibers, and ensures their contractile activity.
When deprived of dystrophin, muscle fibers are damaged and die, and connective tissue develops in their place.
For quick memorization of pathology localization: Duchenne — 10 (D-D, sounding similar), Anemia (sickle cell) — 11 (two ones like two sickle cells), Phenylketonuria — 12 (P-12).
Class and exam questions
On the modification of which structure is the RFLP method based?
The RFLP method is based exclusively on changes in the primary DNA structure caused by insertions, deletions, or nucleotide substitutions.
Why do some people react strongly to green potatoes?
Green potatoes contain the glycoside solanine, which inhibits the enzyme pseudocholinesterase. Sensitivity depends on the genetic polymorphism of this enzyme.
Is the development of a multifactorial disease inevitable in the presence of a predisposition?
No. Hereditary predisposition (polymorphism of enzymes and receptors) turns into a disease only when exposed to specific environmental factors.
What else to study on this topic
- Mechanisms of action of restriction enzymes in the formation of restriction fragments
- Molecular basis of synthesis and hybridization of allele-specific probes
- The role of cell cycle regulatory protein polymorphism in oncogenesis
- Pharmacogenetics: the effect of detoxification enzyme isoforms on drug metabolism
- Ethnic genetic features of enzyme systems (using alcohol dehydrogenase as an example)
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