Nucleolus Quiz

Nucleolus

• Silver stain is used to visualize the nucleolus.
• The diameter of the nucleolus is around 0.5-5.0 micrometers.
• The main component of the nucleolus is ribosomal RNA (rRNA).
• The function of the nucleolus is to synthesize and assemble ribosomes.
• The nucleolus is well developed in cells that require a high rate of protein synthesis.

Nucleolus Structure

• The nucleolus has three main components: the fibrillar center, the dense fibrillary component, and the granular component.
• The dense fibrillary component contains repeated genes for rRNAs.
• The function of the dense fibrillary component is to synthesize rRNA.
• The fibrillar center is the site of rRNA gene transcription.
• The granular component is the site of ribosome assembly.

rRNA Synthesis

• RNA polymerase I is responsible for the transcription of the 5.8S, 18S, and 28S rRNAs.
• There are 200-400 copies of the gene that encodes 5.8S, 18S, and 28S rRNAs present in the human genome.
• The genes for 5.8S, 18S, and 28S rRNAs are located in the nucleolus organizer regions (NORs) on chromosomes 13, 14, 15, 21, and 22 in the human genome.
• RNA polymerase III is responsible for the transcription of the 5S rRNA.
• There are 200-400 copies of the gene that encodes 5S rRNA present in the human genome.
• The 5S rRNA genes are located in the nucleolus organizer regions (NORs) on chromosome 1 in the human genome.

rRNA Processing

• The Large 45S pre-rRNA is transcribed by RNA polymerase I.
• During processing, the 45S pre-rRNA undergoes modifications, including methylation, pseudouridylation, and cleavage.
• The enzyme responsible for the chemical modification of pre-rRNAs is pseudouridine synthase.

Nucleus

• The nucleus is surrounded by a double membrane structure called the nuclear envelope.
• The nuclear envelope is perforated with nuclear pores, which act as a selective permeability barrier.
• The nuclear lamina is a protein structure that provides mechanical support to the nucleus.
• Nuclear pores are complexes that allow the passage of molecules between the nucleus and cytoplasm.
• There are 2,000-5,000 nuclear pore complexes typically found in a nucleus.
• The nuclear pore complex is composed of 30-50 nucleoporin proteins.
• The nuclear pore complex allows the passage of molecules up to 60 kDa in size.
• Molecules that are smaller than 60 kDa can freely enter the nucleus through the nuclear pores.
• Large molecules require a nuclear localization signal to pass through the nuclear pore complex.

Miscellaneous

• Anucleate cells are cells that lack a nucleus.
• The nucleus is not responsible for protein synthesis.
• The nucleoplasm is the region of the nucleus between the nuclear envelope and the nucleolus.
• The nuclear lamina acts as a selective barrier, regulating the passage of molecules between the nucleus and cytoplasm.