Question

1. Describe how rRNA is transcribed, methylated, and cleaved into individual rRNAs

2. Describe the connection between some antibiotics and translation

Answer 1

1. rRNA is one of the components of the ribosome and plays an essential role in the synthesis of proteins in cells of both prokaryotic as well as eukaryotic origin. rRNA are synthesized in the densest part of the cell nucleus called the nucleolus. rRNA are synthesised from rDNA of which multiple copies exist in the cell since rRNA are required in large numbers. rRNA synthesis is initiated by the enzyme called RNA polymerase 1. This enzyme has a very high efficiency and allows rapid transcription of rRNAs. Membrane free clusters exist in the nucleolus where RNA polymerase complex rapidly synthesizes rRNA. The rRNA are synthesized polycistronically (in a long continuous strand). This strand has multiple spacer sequences where the strand is excised into multiple small rRNA with the help of enzymes called Small Nucleolar RNAs (snoRNA)
Methylation : Methylation is a post translational method of stopping the expression of a certain region in the nucleic acid sequence, also known as silencing. Methylation is of three kinds -

• Ribose Methylation - It occurs at the 2' hydroxyl position on the sugar backbone and takes place early in rRNA processing and plays a role in rRNA folding. It is found more frequently in eukaryotes and a few archaebacteria. In the latter, ribose methylation provides thermostability in the extreme environments. This methylation occurs in highly conserved regions adjacent to the sites where pseudouridylation occurs.
• Nitrogenous Base Methylation - It is the most evolutionally conserved methylation in its number and position and is found in all organisms (prokaryotic and eukaryotic). It occurs late in the process of ribosomal maturation.
• Pseudouridylation - They are common in eukaryotes and rare in bacteria. They are found in the most conserved regions of the DNA . These regions cover mostly the peptidyl transfer center of the ribosome at the smaller subunit. Methylation in these regions in known to improve translational efficiency.

2. Antibiotics are a group of natural and synthetic compounds that are used to inhibit or destroy bacterial cells. They are naturally produced by various fungi species, and infact the first antibiotic to be discovered was Penicillin from teh fungus Penicillium notatum by Alexander Fleming who was awarded the Nobel prize for the same in 1945. The antibiotics exclusively destroy bacterial (prokaryotic) cells and leave the eukaryotic cells without any harm. For this reason they are very effective at clearing out infections and diseases of prokaryotic origin. Antibiotics may be bacteriostatic (inhibits the growth of bacterial cells) or bacteriocidal (kills the bacterial cells).

Antibiotics have various modes of action, they may -

• Inhibit Cell wall Synthesis in Bacteria.
• Inhibit Protein Syntheis.
• Inhibit Nucleic acid Replication.
• Destroy Plasma membrane.
• Inhibit synthesis of essential metabolites.
  
  Translation is the process when the mRNA transcribed from DNA or RNA, is read and gives rise to its corresponding amino acid and ultimately to the protein the DNA/RNA sequence codes for.
  Many antibiotics like Chloramphenicol, Streptomycin, Erythromycin and Tetracyclins inhibit the synthesis of certain proteins which leads to the breakdown of cellular machinery of the cell and it eventually dies.
  Since prokaryotic cells have 70S ribosomes, their subunits (50S & 30S) are selectively targeted by these antibiotics and subsequently destroyed.
  Different antibiotics have different ways to inhibit protein synthesis -
• Chloramphenicol - It binds to the 50S subunit of the ribosome and inhibits the formation of the peptide bond between translated amino acids.
• Streptomycin - It binds and distorts the shape of the 30S ribosome subunit causing the mRNA to be read incorrectly leading to production of a non functional protein.
• Tetracycline - Inhibits the attachment of tRNA to mRNA-ribosome complex.