Realization of genetic information
This function includes processes that supply the cell with proteins and RNA. The main stage is gene expression, which begins with transcription (creation of primary transcripts on a DNA template) and the synthesis of service rRNA and tRNA.
Next, processing (maturation) of messenger RNA takes place:
- Splicing — removal of introns (non-coding regions) and joining of exons (coding regions).
- End modification — attachment of special sequences that protect RNA from exonucleases and help bind to the ribosome.
In addition, ribosomal subunits are assembled in the nucleus (from rRNA and cytoplasmic proteins). Gene activity is regulated by chemical modifications of DNA and histones, which determines which regions will "work" and which will remain "silent".
Preservation and reproduction of the genome
Maintaining DNA integrity is based on three processes:
- Repair. Constant detection and correction of errors. Its efficiency declines with age.
- Replication (duplication). Occurs before division (except for the second meiotic division) according to the semi-conservative principle — each new molecule contains one old and one new strand.
- Chromosome condensation. Compact packaging of genetic material at the start of cell division. This prevents tangling and breakage of chromosomes as they pull apart to the poles.
Destruction of genetic material
The nucleus is capable of triggering programmed cell death — apoptosis. This process activates intranuclear enzymes (endonucleases) that fragment DNA.
Causes of such self-destruction:
- Accumulation of a critical number of errors and damage in DNA that cannot be repaired.
- Reception of specific external chemical signals via receptors on the cell membrane.
Features of germ cell nuclei
In maturing germ cells, stricter requirements are imposed on genome preservation to avoid passing defects on to offspring. Repair is more intensive, and damaged cells are more likely to undergo apoptosis.
During meiosis, conjugation (pairing) and crossing over (exchange of segments) of homologous chromosomes occur, which not only increases genetic diversity but also, according to the "rejuvenation" hypothesis, improves defect repair. During spermatogenesis, "supercondensation" is observed — maximum packing of DNA by replacing standard proteins with specific ones.
Proof of transcription in the nucleus
RNA synthesis specifically in the nucleus can be proven experimentally by the autoradiography method.
A radioactive marker, H³-uridine, is introduced into the organism. Uridine is used instead of UTP because phosphate groups prevent the molecule from passing through the cell membrane. Furthermore, unlike thymidine (a marker of DNA replication), uridine is incorporated exclusively during RNA synthesis. On histological sections covered with photoemulsion, radioactive decay leaves silver grains (black dots) localized strictly over the cell nuclei.
Briefly: answers to topical questions
By what specific chemical modifications of DNA and histones is gene activity regulated?
Gene activity is regulated by DNA methylation, as well as methylation, acetylation, and phosphorylation of histones.
- DNA methylation — the addition of methyl groups transitions genes into an inactive state.
- Histone methylation and acetylation — occurs at amino groups in lysine and arginine side chains.
- Histone phosphorylation — takes place at the serine residue.
These histone modifications reduce their overall positive charge, which weakens the binding of proteins to DNA in nucleosomes. As a result, chromatin transitions into a potentially active state (euchromatin), granting access to transcription enzymes.
Through which specific proteins does DNA supercondensation occur during spermatogenesis?
DNA supercondensation during spermatogenesis occurs due to the replacement of typical nuclear histones with specialized basic proteins.
- Protamines — specific highly basic proteins that replace histones.
- Specific histones — proteins significantly enriched in arginine and cysteine.
Such protein replacement ensures ultra-dense packing of chromatin, in which the nucleosomal organization is modified or completely disappears, and the nuclear volume decreases by approximately 30 times. This protects the genetic material in the sperm head.
Class and exam questions
UTP cannot enter the cell due to the phosphate groups, so the nucleoside uridine is used instead. Thymidine is used to track DNA replication, while uridine is specifically incorporated only into RNA during transcription.
UTP cannot enter the cell due to the phosphate groups, so the nucleoside uridine is used instead. Thymidine is used to track DNA replication, while uridine is specifically incorporated only into RNA during transcription.
What is splicing?
This is a stage of mRNA maturation (processing) in which non-coding regions (introns) are removed from the molecule, and the remaining coding fragments (exons) are spliced together.
What is the essence of the semi-conservative principle of replication?
During DNA duplication, each of the two resulting daughter molecules consists of one old parental strand and one completely new strand synthesized according to the principle of complementarity.
What else to study on this topic
- Mechanisms of histone chemical modification
- Structure and functions of chromatin in the interphase nucleus
- Feulgen staining of DNA
- Types of exonucleases and mRNA protection
- Specific proteins in supercondensation during spermatogenesis
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