Ribosomal RNA (rRNA)

Ribonucleic acid (RNA) is a molecule that is present in the majority of living organisms and viruses. It is made up of nucleotides that are made up of ribose sugars attached to nitrogenous bases and phosphate groups. The nitrogenous bases include adenine, guanine, uracil, and cytosine. 

RNA mostly exists in the single-stranded form, but there are special RNA viruses that are double-stranded. The RNA molecule can have a variety of lengths and structures. Three main types of RNA are involved in protein synthesis. These are messenger RNA (mRNA), transfer RNA (tRNA), and ribosomal RNA (rRNA).

1.0What is Ribosomal RNA(rRNA)?

• The RNA found in ribosomes and responsible for catalyzing protein synthesis, is known as ribosomal RNA (rRNA). It is the most stable type of RNA. It was discovered by Kuntze. Ribosomal RNA is 80 % of the total cellular RNA. Over 60-80% of the weight of the ribosome is made up of ribosomal RNA. It is essential for all of the ribosomal activities, including binding to mRNA, attracting tRNA, and catalyzing the formation of peptide bonds between amino acids.
• In the eukaryotic cells the nucleus has unique structures known as nucleoli, where ribosomal RNA is transcribed. rRNA forms ribosomes, which are essential in protein synthesis. 
• A ribosome contains a large and small ribosomal subunit.The ribosomes contain an exit (E), peptidyl (P), and acceptor (A) site to bind aminoacyl-tRNAs and link amino acids together to create polypeptides. 

2.0Types of rRNA

The types of ribosomal RNA (rRNA) vary depending on whether the cell is a prokaryotic or eukaryotic.

In Prokaryotes:

• Prokaryotic cells have a 70s type of ribosomes and its subunits are 50s and 30s.
• Prokaryotic cells have three types of rRNA molecules: 5s, 16s, and 23s. 
• The 5s and 23s rRNAs are found in the larger subunit of  ribosome, whereas 16s rRNA is in the smaller subunit of ribosome.

In Eukaryotes:

• Eukaryotic cells have a 80s type of ribosomes and its subunits are 60s and 40s.
• Eukaryotic cells have four types of rRNA molecules: 5s, 5.8s, 18s, and 28s. 
• The 28s, 5.8s, and 5s rRNAs are found in the larger subunit of ribosome whereas 18s rRNA is in the smaller subunit of ribosome.

3.0Transcription of rRNA  and Ribosomal units

• Transcription of rRNA and ribosome formation are related processes that involve the following steps: 
• Transcription: RNA polymerases (RNA pol) I and III transcribe pre-rRNAs, which are then processed by a series of reactions that remove spacers. 
• Transcription of genes that code for ribosomal protein:RNA polymerase II transcribes the genes that encode ribosomal proteins outside of the nucleolus. 
• Ribosomal protein and rRNA assembly: The rRNAs associate with the ribosomal proteins to form large and small ribosomal subunits. 
• Ribosome assembly: The subunits assemble in the cytosol to form a functioning ribosome. Ribosomes are made up of about 60% ribosomal RNA (rRNA) and 40% ribosomal proteins. rRNA is the physical component of the ribosome that forces transfer RNA (tRNA) and messenger RNA (mRNA) to process and translate into proteins. 

Diagramatic representation of transcription of rRNA and ribosomal units

4.0Functions of rRNA

• Protein synthesis is the primary function of rRNA: The unusual three-dimensional structure of rRNA creates  A, P, and E sites within the ribosome, which has internal helices and loops. By attaching to messenger RNA and transfer RNA, these molecules assure that the codon sequence of the mRNA is appropriately translated into the amino acid sequence of proteins.
• Structural Component of Ribosomes: rRNA is a fundamental part of ribosomes, the cellular machinery responsible for translating messenger RNA (mRNA) into proteins. Ribosomes consist of two subunits, each composed of rRNA and proteins. The rRNA molecules provide the structural framework for the ribosome and help maintain its shape.
• Catalytic Activity (Ribozyme Function): rRNA has catalytic properties and acts as a ribozyme, meaning it can catalyze biochemical reactions. 28S rRNA in eukaryotic ribosomes  whereas 23S rRNA in prokaryotic ribosomes catalyzes the formation of peptide bonds between amino acids during protein synthesis. This is essential for the assembly of polypeptide chains.
• mRNA Binding and Translation: rRNA facilitates the binding of mRNA to the ribosome. It ensures that the mRNA is properly aligned with the ribosome for accurate translation. rRNA also helps in the decoding of mRNA sequences into the corresponding amino acid sequences.
• Regulation of Translation: rRNA contributes to the regulation of translation by influencing the interactions between the ribosome and various translation factors, ensuring that the translation process is efficient and accurately controlled.