Genetic imprinting

Levels of Organization and Molecular Mechanisms

Genetic imprinting can manifest at several levels of genetic material organization. Modern biology distinguishes genome-wide imprinting, chromosome-specific imprinting, and gene-specific imprinting. To date, about 20 such genes have been identified in mammals, although their total number is estimated to be small (about 0.1% of all organismal genes).

The universal molecular mechanism underlying this process is methylation of nitrogenous bases. A unique, specific pattern of methylated cytosine residue distribution forms in the promoter region of genes. It is this pattern that determines whether the gene will be active.

It is important to note that this pattern is not constant throughout all stages of an organism's development. During gametogenesis and germ tissue formation, this specific pattern is completely erased. Subsequently, during the formation of new sex cells, it is re-established depending on the individual's sex. For example, a gene received from the father may be heavily methylated, rendering it inactive. At the same time, the homologous gene received from the mother turns out to be lightly methylated and is therefore actively transcribed.

Meiotic drive: disruption of classical segregation

In addition to imprinting, there are other deviations from standard inheritance, notably meiotic drive. This phenomenon represents a deviation in the ratio of gametes produced by a heterozygous organism from standard Mendelian segregation (which should be 50:50). The deviation always occurs in favor of one of the alleles.

Let us examine this mechanism using the classic example of oogenesis in females:

1. Suppose that alongside the usual wild-type allele, a new mutant allele arises within a population of organisms.
2. During meiosis, out of the four resulting haploid cells, only one becomes a functional ovum. The remaining three cells turn into polar bodies and normally degenerate.
3. If, due to certain reasons, the mutant allele ends up in the surviving ovum more frequently than the wild-type allele, we observe meiotic drive.

The result of this process is a strong positive selection pressure acting on the mutant allele. In the long term, this inevitably leads to its fixation in the population, despite the fact that initially the alleles' chances seemed equal.

Progressive amplification and triplet expansion

Another important genetic phenomenon is progressive amplification. This is a specific type of gene mutation caused by multiple duplications of individual triplets (both in DNA introns and in coding regions).

A striking clinical example is Huntington's disease:

• The mutation affects the huntingtin protein gene.
• There is a pathological increase in the number of CAG triplets that encode the amino acid glutamine.
• Normally, the number of such repeats ranges from 11 to 34.
• In pathology, the number of repeats exceeds 36.

The pathogenesis of the disease lies in the fact that an excess of triplets results in the synthesis of a pathologically elongated polyglutamine tract. This leads to the formation of mutant huntingtin protein, which exerts a pronounced toxic effect on the body's cells.

Another example is myotonic dystrophy, in which the number of genetic repeats can reach colossal values—from 50 to 4,000. A characteristic feature of progressive amplification is its dynamics: the number of pathological repeats tends to increase from generation to generation, worsening the clinical picture.

Briefly: answers to topical questions

Class and exam questions

What else to study on this topic

• Detailed structure of gene promoter regions and localization of cytosine residues.
• Interaction of "silencer genes" with other regions of the mammalian genome.
• Cellular mechanisms of directed polar body death during oogenesis.
• Causes of the increasing number of triplet repeats in myotonic dystrophy across generations.

Other topics in «Elementary evolutionary factors»

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