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 functional rRNA and tRNA.
Next, messenger RNA undergoes processing (maturation):
- Splicing — removal of introns (non-coding regions) and joining of exons (coding regions).
- End modification — attachment of special sequences that protect RNA against exonucleases and help it 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 determine which regions will be active ("on") and which will be silent ("off").
Preservation and Replication 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 prior to cell division (except for the second meiotic division) via a semiconservative principle — each new molecule contains one old and one new strand.
- Chromosome Condensation. Compact packaging of genetic material at the onset of cell division. This prevents tangling and breakage of chromosomes as they segregate to opposite poles.
Destruction of Genetic Material
The nucleus can trigger programmed cell death — apoptosis. This process activates intranuclear enzymes (endonucleases) that fragment DNA.
Causes of this self-destruction:
- Accumulation of a critical number of errors and DNA damage that cannot be repaired.
- Receipt of specific extracellular chemical signals through receptors on the cell membrane.
Features of Germ Cell Nuclei
In maturing germ cells, stricter requirements are applied to genome preservation to prevent passing defects to offspring. Repair is more intensive, and damaged cells more frequently undergo apoptosis.
During meiosis, synapsis (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, "hypercondensation" is observed — maximal DNA packaging achieved by replacing conventional proteins with specific ones.
Evidence of Transcription in the Nucleus
RNA synthesis occurring specifically within the nucleus can be demonstrated experimentally using autoradiography.
A radioactive marker, 3H-uridine, is introduced into the organism. Uridine is used rather than UTP because phosphate groups prevent the molecule from passing through the cell membrane. Furthermore, unlike thymidine (a marker for DNA replication), uridine is incorporated exclusively during RNA synthesis. On histological sections covered with photoemulsion, radioactive decay leaves silver grains (black dots) that localize strictly over the cell nuclei.