Context and molecular mechanism
The molecular events of genomic imprinting unfold against the background of the passive movement of the secondary oocyte (and subsequently the formed zygote) along the uterine tube toward the uterine cavity.
The phenomenon is based on selective DNA methylation. This process involves the chemical attachment of methyl groups to strictly defined genes of the Impr group. Methylation occurs in advance — during the development of germ cells (in the course of gametogenesis and meiosis).
The main consequence of methylation is the alteration of gene expression:
- Methylated genes transition into an inactive state (are completely switched off).
- Unmethylated genes of the same group, conversely, begin to function actively.
A crucial feature of imprinting is its striking stability. Following fertilization, the predetermined methylation status is reliably preserved in all somatic cells of the developing organism. With each cell division and DNA replication, the new nucleotide strand is methylated in strict accordance with the template of the old strand.
Functional specialization of parental genomes
Genomic imprinting ensures a clear division of biological functions between chromosomes received from the father and the mother. The presence of both genomes is critical for normal development.
Genes active in the paternal genome: This is a group including approximately 11 genes. In maternal chromosomes, they are in a methylated (inactive) state, whereas in paternal chromosomes, they are active. Their main task is to control the development of extraembryonic (provisional) organs necessary to support the embryo's viability. Experimental evidence: If a zygote containing exclusively two female pronucleus is created (complete absence of the paternal genome), development will begin. However, it is inevitably interrupted due to critical underdevelopment of extraembryonic structures.
Genes active in the maternal genome: This is a group consisting of approximately 5 genes. In the male chromosomes, they are switched off, while in the female chromosomes, they are expressed. This group of genes determines the development of the embryo's body itself. Experimental proof: When artificially creating a zygote with two male pronuclei (without the maternal genome), the reverse picture is observed: extraembryonic organs develop relatively normally, but the body of the embryo itself turns out to be profoundly defective.
Reprogramming in germ cells
To transmit correct imprinting to the next generation, a mechanism of remethylation (reprogramming) exists.
In the organism's somatic cells, the initial distribution of paternal and maternal marks remains unchanged throughout life. However, in primordial germ cells (PGCs), a global reset of settings occurs, consisting of two stages:
- Demethylation. Methyl marks are removed from all genes of the Impr group. Information regarding both paternal and maternal marks is erased.
- De novo methylation. Completely new marks are established that now strictly correspond to the genetic sex of the given individual. In spermatocytes, new marks are laid down according to the male pattern, and in oocytes, exclusively according to the female pattern.
Briefly: answers to topical questions
Which specific extraembryonic (provisional) organs develop thanks to the paternal genome?
Thanks to the paternal genome (a group of approximately 11 active genes), a complex of extraembryonic (provisional) organs necessary to support embryogenesis develops. In early human embryogenesis, four key structures are formed:
- Yolk sac (saccus vitellinus) — a temporary (regressing) organ.
- Allantois (allantois) — a temporary organ functioning in the first weeks of development.
- Chorion — a permanent villous membrane.
- Amnion — a permanent organ participating in the formation of fetal membranes.
What are the main genes subject to genomic imprinting in humans?
Genes subject to genomic imprinting are designated in sources by the general symbol Impr. It is also noted that about 80 imprinted genes have been identified in the human genome. Among the specifically named genes in sources related to genomic imprinting disease — Beckwith–Wiedemann syndrome — IGF2 and H19, localized in the 11p15 region, are cited.
Which genetic diseases arise from disruptions in genomic imprinting?
Disorders of imprinted genes — microdeletions, uniparental disomies, or epigenetic labeling anomalies, including DNA methylation defects — result in genomic imprinting disorders. Sources cite the following examples:
- Prader–Willi syndrome — upon deletion of the 15th chromosome region (q11–q12) inherited from the father.
- Angelman syndrome — upon deletion of the same region of the 15th chromosome inherited from the mother.
- Beckwith–Wiedemann syndrome — associated with the 11p15 region; manifestations include macroglossia and an increased predisposition to tumors.
"Dad builds the house, mom raises the child": paternal genes (~11) are responsible for extraembryonic organs (the "house" for the embryo), while maternal genes (~5) are responsible for the embryo's body itself. In germ cells, the old house deeds are canceled (demethylation) and new ones are issued strictly according to the owner's sex.
Class and exam questions
In which cells does the methylation status remain unchanged after fertilization?
The methylation status is stably preserved in all somatic cells of the new organism throughout its life.
What will happen to the embryo in the absence of the maternal genome (two male pronuclei)?
Extraembryonic provisional organs will develop relatively normally, but the body of the embryo itself will form defective and unviable.
At what stage of ontogenesis does the primary establishment of imprinting marks occur?
Selective methylation of genes in the Impr group occurs at the stage of germ cell development (gametogenesis, meiosis).
Why is demethylation necessary in primordial germ cells?
It is necessary for "resetting" old parental memory in order to subsequently establish new marks (de novo) corresponding to the sex of the individual themselves.
What else to study on this topic
- Topography and passive movement of secondary oocyte along the uterine tube during molecular rearrangements.
- Biochemical foundations of selective DNA methylation and the mechanism of gene expression suppression.
- Features of DNA replication in somatic cells: copying methyl marks to the new strand.
- Molecular triggers of demethylation and de novo methylation in primordial germ cells.
- Morphology of extraembryonic organs forming under the control of the paternal genome.
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