Question

7. Identify each of the mentioned nuclear subcompartments and and function(s) that each performs.

8. Describe the functions of the nucleolus.

9. Briefly describe how the rRNA molecules are made and processed and assembled into ribosomal subunits

10. Describe the process of making tRNAs.

Answer 1

answer;

Main function of nucleolus is the ribosome formation,thus it is called ribosome factory of the cell.The protein of ribosome are synthesised in cytoplasm but diffuses into nucleus and reach at nucleolus.

Here r-RNA and ribosomal proteins are assembled to form ribosomes which move to cytoplasm through nuclear pores

Note that large and more numerous nucleoli are present in cells actively carrying out protein synthesis.

9-RNA Synthesis and Processing

The nucleolar part of the nucles consits genes coding for the r RNA and the genes coding for rRNA are highly repetitious and mammalian cells contain 100 to 2000 copies per cell.

Each transcription unit contains sequences coding for 18S, 5.8S and 28S rRNA. The genes are organised in transcription units separated by non-transcribed spacers.
The transcription units are transcribed by RNA polymerase I into giant RNA molecules, primary transcripts, that in addition to the sequences corresponding to 18S, 5.8S and 28S rRNA contains external and internal transcribed spacer sequences.

Ribosome Assembly

The Ribosome assembly takes place in the nucleolus with the help of Small nuclear rna (snoRNA). The assembly of the 40S subunit starts by the production of larger primary transcript followed by the intiation of snoRNA-dependent cleavage of the primary transcript. The other snoRNAs directs the modification of specific nucleotides in the rRNA. The folding of the rRNA proceeds with the help of primary rRNA binding ribosomal proteins associated to the rRNA.

The ribosomal proetins which donot bind directly to rRNA forms the rRNA-protein complex thereby forming the 40S ribosomal subunit.

The 5S rRNA a play vital role in the sythesis of 60S rRNA as this 5s rRNa coordinate for the production of 60S sub unit.

10-Synthesis of tRNA. In eukaryotic cells, tRNA aremade by a special protein that reads the DNA code and makes an RNA copy, or pre-tRNA. This process is called transcription and for making tRNA, it's done by RNA polymerase III. Pre-tRNA are processed once they leave the nucleus.