The nucleus is a prominent, membrane-bound organelle in eukaryotic cells, distinguishing them from prokaryotes. It serves as the central command center, orchestrating cellular activities. The nucleus is the primary repository for the cell’s genetic information and controls cell growth, metabolism, and reproduction by regulating gene expression.
The Nuclear Envelope
The nuclear envelope is a double-layered membrane enclosing the nucleus, separating its contents from the cytoplasm. A narrow perinuclear space (10-50 nm wide) exists between these membranes. The outer membrane is continuous with the endoplasmic reticulum, facilitating molecular exchange.
This double membrane acts as a protective barrier, maintaining a distinct internal nuclear environment. Numerous nuclear pores, complex protein structures, are embedded within the envelope. These pores regulate the selective passage of molecules, such as proteins and RNA, ensuring necessary molecules are available for nuclear functions while preventing unwanted substances from entering.
Genetic Material: DNA and Chromatin
The nucleus houses deoxyribonucleic acid (DNA), the cell’s genetic material and fundamental blueprint. DNA is not found as free strands but is organized and packaged by associating with proteins, primarily histones, to form chromatin. This packaging allows long DNA molecules to fit compactly within the nuclear space.
Chromatin exists in two forms, reflecting condensation and gene activity. Euchromatin is a less condensed, loosely packed form, appearing as lightly stained regions. Its open structure makes DNA readily accessible for gene expression and active transcription. Genes within euchromatin are frequently expressed, producing RNA and proteins.
In contrast, heterochromatin is a highly condensed, tightly packed form, appearing as darkly stained clumps. Its compact nature restricts access for transcription machinery, making genes largely inactive or silenced. Heterochromatin maintains genome stability and regulates gene expression by repressing certain DNA sequences. Both forms are crucial for regulating DNA replication, repair, and gene expression.
The Nucleolus
The nucleolus is a prominent, distinct structure within the nucleus that lacks a membrane. It is composed of ribosomal RNA (rRNA) genes, proteins, and RNA molecules. It is the primary site of ribosome synthesis and assembly.
Ribosome biogenesis begins with the transcription of precursor ribosomal RNA (pre-rRNA) by RNA polymerase I. This pre-rRNA undergoes processing and modification. Ribosomal proteins, synthesized in the cytoplasm, are imported into the nucleolus. They combine with processed rRNA to form small and large ribosomal subunits.
Once assembled, these subunits are transported to the cytoplasm. There, they combine to form functional ribosomes, the cellular machinery for protein synthesis. Continuous ribosome production is essential to meet the cell’s demand for protein synthesis, supporting growth and metabolism.
The Nucleoplasm and Nuclear Bodies
The nucleoplasm is the viscous, jelly-like substance filling the nucleus, analogous to cytoplasm. It suspends chromatin, the nucleolus, and other nuclear components. It facilitates the transport of molecules, enzymes, nucleotides, and other substances involved in nuclear processes.
Beyond major structures, the nucleoplasm contains dynamic, non-membrane-bound nuclear bodies. These are involved in specific nuclear functions. For instance, nuclear speckles (interchromatin granule clusters) are enriched with pre-messenger RNA (pre-mRNA) splicing factors. They serve as sites for the storage, assembly, and modification of these factors, crucial for processing RNA.
Nuclear speckles influence gene activity and regulate gene expression. Other nuclear bodies, like Cajal bodies, are involved in the biogenesis and assembly of small nuclear ribonucleoproteins (snRNPs), important for RNA processing. These nuclear bodies contribute to various cellular processes, are dynamic, and change in size and number based on cell activity.