הגרעין

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.