Transfer RNA, Ribosomal RNA, and MicroRNA

 

📌 Previous Notes:

https://mednotesbyrazaullah.blogspot.com/2026/08/genes-dna-structure-and-genetic-code.html

Transfer RNA (tRNA) and Anticodons

Transfer RNA (tRNA) plays the vital adapter role during translation, bridging the genetic instructions carried by messenger RNA (mRNA) and the physical assembly of proteins. Each tRNA molecule is a relatively small polynucleotide chain folded into a characteristic cloverleaf configuration that contains two crucial functional ends. At one terminus, an acceptor arm specifically binds to a single corresponding amino acid through an energy-dependent process catalyzed by aminoacyl-tRNA synthetase enzymes. At the opposing loop of the tRNA molecule lies the anticodon a specific triplet sequence of nucleotide bases designed to recognize and pair with a complementary codon on the mRNA strand.

During protein synthesis inside the ribosome, the alignment between codons and anticodons guarantees high fidelity in translating genetic code into functional proteins. As the ribosome advances along the mRNA template, consecutive tRNA molecules enter the ribosomal complex, matching their specific anticodon base triplets to the exposed mRNA codons through hydrogen bonding. Once aligned, the amino acid attached to the tRNA forms a peptide bond with the growing polypeptide chain, and the uncharged tRNA is released. This highly coordinated molecular interaction ensures that amino acids are added in the exact sequential order specified by the original gene blueprint.

• Transfer RNA transports amino acids during protein synthesis.
• Each tRNA combines specifically with one amino acid.
• tRNA carries amino acids directly to ribosomes.
• tRNA recognizes specific mRNA codons inside ribosomes.
• tRNA delivers amino acids to proper protein positions.
• tRNA contains approximately 80 nucleotides.
• tRNA forms a cloverleaf-shaped folded nucleotide chain.
• Adenylic acid resides at one tRNA end.
• Amino acids attach to adenylic acid ribose hydroxyls.
• tRNA specificity requires a three-base anticodon sequence.
• Anticodons are located near the middle of cloverleaf structures.
• Anticodons hydrogen-bond loosely with mRNA codons.
• Sequential codon-anticodon pairing aligns amino acids properly.

Ribosomal RNA (rRNA) and Ribosome Assembly:

• Ribosomal RNA constitutes 60 percent of ribosomes.
• Ribosomes contain approximately 75 types of proteins.
• Ribosomal proteins act as structural and enzymatic components.
• Ribosomes act as physical protein-manufacturing structures.
• Ribosomes require mRNA and tRNA to function.
• mRNA supplies proper amino acid sequence information.
• Five chromosome pairs contain rRNA DNA genes.
• Duplicate rRNA genes ensure high production volume.
• Synthesized rRNA collects inside cell nucleoli.
• Nucleoli appear large in high-protein synthesizing cells.
• Nucleoli process rRNA with ribosomal proteins.
• Primordial ribosomal subunits form within nucleoli.
• Subunits exit nucleoli through nuclear envelope pores.
• Subunits assemble into mature ribosomes in cytoplasm.
• Mature functional ribosomes exist only in cytoplasm.

MicroRNA (miRNA) and Small Interfering RNA (siRNA):

• MicroRNAs are single-stranded noncoding RNA fragments.
• MicroRNAs contain 21 to 23 nucleotides.
• MicroRNAs regulate cellular gene expression levels.
• DNA transcribes pri-miRNAs as primary transcripts.
• Microprocessor complexes process pri-miRNAs to pre-miRNAs.
• Pre-miRNAs form 70-nucleotide stem-loop structures.
• Cytoplasmic dicer enzymes generate functional miRNAs.
• Dicer helps assemble RNA-induced silencing complexes (RISC).
• MicroRNAs repress translation or promote mRNA degradation.
• Altered miRNA function links to cancer and heart disease.
• Small interfering RNAs are short double-stranded molecules.
• Synthetic siRNAs contain 20 to 25 nucleotides.
• Synthetic siRNAs bypass nuclear microprocessor processing.
• Cytoplasmic siRNAs activate RISC to block translation.
• Custom siRNAs silence specific targeted gene sequences.

Process of Translation on Ribosomes:

• Messenger RNA contacts and passes through ribosomes.
• Translation begins at chain-initiating RNA codons.
• Ribosomes synthesize protein chains reading mRNA codons.
• Ribosomes operate similarly to tape recorder heads.
• Stop codons signal end of protein synthesis.
• Completed protein molecules release into cytoplasm.

Polyribosomes and Ribosomal Non-Specificity:

• Single mRNA molecules contact multiple ribosomes simultaneously.
• Ribosome clusters on single mRNA form polyribosomes.
• Polyribosomes contain three to ten attached ribosomes.
• Proteins exist in different stages across polyribosomes.
• Ribosomes show no specificity for specific proteins.
• Ribosomes serve as universal protein manufacturing plants.

Attachment to Endoplasmic Reticulum:

• Forming protein sequences attach to endoplasmic receptors.
• Penetrating proteins enter the endoplasmic matrix.
• Entering proteins produce granular rough endoplasmic reticulum.
• Non-glandular cell proteins release mostly into cytosol.
• Cytosolic proteins form enzymes and internal structures.

Chemical Steps and Energy in Protein Synthesis:

• ATP combines with amino acids forming complexes.
• Activation consumes two high-energy phosphate bonds.
• Activated amino acids combine with specific tRNAs.
• tRNA anticodons bond temporarily with mRNA codons.
• Peptidyl transferase enzyme forms successive peptide bonds.
• Adding each amino acid consumes four phosphate bonds.
• Protein synthesis represents high cellular energy expenditure.

Formation of Peptide Linkages:

• Successive amino acids combine via peptide linkages.
• Hydroxyl radicals remove from carboxyl amino ends.
• Hydrogen ions remove from adjacent amine groups.
• Removed hydroxyls and hydrogens form water molecules.
• Reactive amino acid sites form covalent bonds.