1.8:
The Nucleus
In eukaryotic cells, the nucleus is a membrane-bound organelle that houses the cell's genetic information – the DNA.
However, the organization of the DNA within the nucleus depends on the cell cycle phase.
For instance, when the cell is not dividing, the DNA protein complexes, known as chromatin fibers, are suspended loosely in the nucleoplasm, allowing easy access to DNA replication and transcription enzymes.
In contrast, when the cell is about to divide, chromatin fibers are compressed into chromosomes.
The nucleolus is a distinct region within the nucleus specialized for the assembly of the ribosomes – RNA-protein complexes responsible for protein translation.
The nucleus is surrounded by a double membrane barrier, the nuclear envelope, which is lined with nuclear pore complexes or NPCs.
These NPCs control the exchange of molecules between the nucleus and the cytoplasm.
While most cells in the body have one nucleus, some, like the red blood cells, are anucleated, i.e. they lack a nucleus. Whereas, others, such as skeletal muscle cells, are multinucleated due to structural and functional needs.
1.8:
The Nucleus
The nucleus is a membrane-bound organelle that acts as a control center in a eukaryotic cell. It contains chromosomal DNA, which controls gene expression and precisely regulates the production of proteins within the cell. In contrast, the DNA inside the mitochondria and chloroplast only carries out functions that are specific to those organelles.
Arrangement of DNA within Nucleus
The regulation of gene expression inside the nucleus is dependent on many factors, including the DNA structure. The DNA in the nucleus is wrapped around proteins called histones, creating a DNA-protein complex called chromatin. When cells are not dividing, i.e., during the interphase, the chromatin is organized diffusely. This allows for easy access to the DNA during transcription. When the eukaryotic cell is about to divide, the chromatin condenses tightly into distinct, linear chromosomes, which can be easily segregated into daughter cells.
Nucleoli
The nucleolus is a membrane-less organelle that forms around the chromosomal loci of the ribosomal RNA within the nucleus. Therefore, nucleoli are the site for rRNA synthesis and assembly of ribosomal subunits. These subunits are later exported out of the nucleus for assembly into fully functional ribosomes.
Additional Sub-nuclear Structures
The nucleus also contains many other nuclear bodies, such as Cajal bodies, speckles, and paraspeckles. These nuclear bodies help to spatially compartmentalize the nuclear environment and create distinct sites for specific biological reactions. Besides, their membrane-less structure allows for a better exchange of contents with the nucleoplasm.