4.6: 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.

The nucleus, a prominent feature of eukaryotic cells, is a membrane-bound organelle that houses genetic information, the DNA, which is organized differentially depending on the lifecycle phase of the cell.

For instance, DNA protein complexes, known as chromatin, are suspended in nucleoplasm, a gel-like substance, and are particularly concentrated in a region called the nucleolus. When the cell's not dividing, chromatin fibers are loosely arranged and resemble tangled strings, allowing for easier access to the DNA during transcription. In contrast, when the cell's about to divide, chromatin fibers are compressed into chromosomes.

The nucleolus is also where the sub-units of ribosomes, structures necessary for translation, are made. Surrounding the nucleus is a double membrane nuclear envelope that contains protein lined pores to control the flow of ions, molecules, ribosomal parts and messenger RNA between the inner nucleoplasm and the cytoplasm.

While most cells in the body have one nucleus, some, like red blood cells, are anucleated. They lack a nucleus. Whereas others, such as skeletal muscle cells, are multinucleated with more than one nucleus due to structural and functional needs.