Basics of parent-specific expression
In classical genetics, it is generally accepted that both gene alleles (received from the mother and the father) are equal and expressed equally. However, in genomic imprinting, this rule is violated. In this case, the cell "remembers" the origin of the genetic material.
As a result, one allele remains active and information is read from it, while the other is permanently repressed (turned off). Thus, the manifestation of a trait depends entirely on whether this allele was received via the egg cell or the spermatozoon.
Mechanism of repression
The main molecular tool by which the cell implements imprinting is DNA methylation.
The process occurs in the regulatory zone of the gene, which is called the promoter region. The attachment of methyl groups to DNA in this zone acts as a physical barrier: it causes a persistent blockade of expression. The methylated allele becomes inaccessible to enzymes, and the gene "goes silent".
Evolutionary significance
Imprinting is a critically important evolutionary change characteristic of mammalian cells. It is believed that this mechanism arose because natural selection acts differently on maternal and paternal genomes, forming the unique balance of gene activity necessary for normal organism development.
Imprinting disorders
Since only one parental allele functions during imprinting, the organism becomes extremely vulnerable to mutations in this region.
The etiology of imprinting disorders boils down to damage to the specific gene that should normally be active (expressed on only one allele). If its expression is repressed or a DNA segment is completely lost (deletion), the cell has no backup working gene because the second allele is turned off by methylation.
Localization: The most well-known pathologies associated with this mechanism arise due to defects in genes located on chromosome 15.
Briefly: answers to topical questions
What specific diseases (syndromes) belong to classical imprinting disorders associated with chromosome 15?
Classical imprinting disorders caused by the localization of affected genes on chromosome 15 include Prader-Willi and Angelman syndromes.
- Prader-Willi syndrome — occurs due to alterations in the paternal chromosome. Clinically, it is accompanied by muscular hypotonia, intellectual disability, strabismus, and obesity.
- Angelman syndrome — occurs due to alterations of the same genes in the maternal chromosome. It manifests as more pronounced intellectual disability, jerky movements, and sleep disorders.
Which enzymes carry out the attachment of methyl groups to DNA during imprinting?
The attachment of methyl groups to specific DNA regions is carried out by enzymes called DNA methyltransferases.
DNA methylation is the mechanism of imprinting: methylation in the promoter region leads to the blockade of gene expression. Methylation typically occurs at cytosine followed by guanine; such pairs are designated as CpG. MeCPs proteins bind to methylated CpGs, recruiting repressor complexes. The result is the suppression of gene expression without altering its sequence.
Association: "Imprinting is a genetic seal (methyl mark) of one of the parents that forces the second allele to fall silent forever."
Class and exam questions
What determines gene expression during imprinting?
Exclusively from its parental origin. Only one allele is expressed (maternal or paternal), while the second is repressed.
How does the cell "turn off" the unneeded allele?
Through DNA methylation in the promoter region. This blocks the ability to read information from the gene.
What is the cause of imprinting disorders?
In the deletion or suppression of expression of the single parental allele that should normally be active.
On which chromosome are the genes located whose damage leads to classical imprinting disorders?
These genes are located on chromosome 15.
What else to study on this topic
- Molecular mechanisms of promoter region methylation
- Types of deletions in chromosome 15 genes
- The impact of natural selection on mammalian genomes
- Mechanisms of permanent allele repression
Master the entire topic in the "Biology" course
This is a summary. The course covers everything tested in classes and exams.
- Complete study guide on "Genomic Imprinting" and 3,793 other pages of the subject
- Assignments and quizzes after every topic — just like in a colloquium
- AI tutor answers questions on the topic anytime
- 129 video lessons on the subject
Other topics in "Molecular Genetics"
Free video lessons: BiologyShort lessons with study notes — no registration required.
Prepared by WhiteCoat Pro faculty in accordance with the medical school curriculum. For educational purposes only.