What is the nucleolus primarily composed of?

The nucleolus is primarily composed of RNA and proteins, which play crucial roles in the synthesis of ribosomes. The nucleolus is a distinct structure within the cell nucleus where ribosomal RNA (rRNA) is transcribed and combined with proteins to form ribosomal subunits. These subunits are then transported out to the cytoplasm, where they participate in protein synthesis. The specific types of RNA present in the nucleolus include rRNA, while the proteins are mainly ribosomal proteins that contribute to the formation of the ribosomes.

Understanding the composition of the nucleolus is vital because it highlights its function in the cell. The synthesis of ribosomes is essential for protein production, making the nucleolus a critical component of cellular activity. The other options, which involve substances like phospholipids, carbohydrates, cellulose, and lignin, are not relevant to the nucleolus's structure or function.