Which of the following are synthesised in the nucleolus?

The correct option is rRNA.

To understand why this is the correct answer, let's look at the function of the nucleolus and how it relates to the synthesis of different nucleic acids:

1. What is the Nucleolus?
The nucleolus is a distinct, dense sub-structure located inside the nucleus of eukaryotic cells. Unlike many other organelles, it is not membrane-bound. Its primary role is to serve as the site for ribosome biogenesis.

2. Synthesis of Ribosomal RNA (rRNA):
Ribosomes are the cellular machinery responsible for protein synthesis, and they are composed of ribosomal proteins and ribosomal RNA (rRNA). The transcription (synthesis) of the precursor rRNA genes, as well as their processing and assembly with proteins to form ribosomal subunits, occurs specifically within the nucleolus. Thus, rRNA is directly synthesized in the nucleolus.

3. Why other options are incorrect:

• tRNA (Transfer RNA): Most tRNA molecules are transcribed in the nucleoplasm (the general fluid/space of the nucleus outside the nucleolus) by the enzyme RNA polymerase III, not within the nucleolus itself.
• DNA (Deoxyribonucleic Acid): DNA replication (synthesis of new DNA) occurs throughout the nucleoplasm during the S-phase of the cell cycle, not specifically in the nucleolus.
• All of the above: Since DNA and tRNA are not synthesized in the nucleolus, this option is incorrect.

Therefore, the nucleolus is specifically dedicated to the transcription and assembly of rRNA.