Genetics And Evolution Molecular Basis Of Inheritance 9

Protein Synthesis:#

Enzymes and Charging of tRNA:

• Enzymes called aminoacyl-tRNA synthetases play a crucial role in charging tRNA molecules with their corresponding amino acids.
• This process ensures that each tRNA molecule carries the correct amino acid to the ribosome during translation.

Codon and Initiation Codon:

• A codon is a sequence of three nucleotides in mRNA that codes for a specific amino acid during translation.
• The initiation codon, also known as the start codon, is AUG, and it marks the beginning of protein synthesis.

Stop Codon:

• Stop codons (UAA, UAG, and UGA) signal the termination of protein synthesis during translation.
• When a ribosome encounters a stop codon, it releases the newly synthesized protein.

Role of Ribosomes:

• Ribosomes are cellular structures where protein synthesis occurs.
• They consist of two subunits: a small subunit and a large subunit.
• Ribosomes facilitate the pairing of tRNA anticodons with mRNA codons, ensuring the correct amino acid sequence in the growing protein.

Initiation of Protein Synthesis:#

Bacterial Ribosome:

• Bacterial ribosomes consist of a small 30S subunit and a large 50S subunit.
• They are involved in protein synthesis in prokaryotic cells.

Eukaryotic Ribosomes:

• Eukaryotic ribosomes are larger and more complex, consisting of a small 40S subunit and a large 60S subunit.
• They are involved in protein synthesis in eukaryotic cells.

Ribosomes:

• Ribosomes play a central role in translation, where they read the mRNA and coordinate the binding of tRNA molecules carrying amino acids.

Initiator Factors:

• In prokaryotes, initiation of protein synthesis involves initiation factors that help assemble the small ribosomal subunit on the mRNA.

Steps of Initiation in Prokaryotes:#

1. mRNA Binding: The mRNA binds to the small subunit of the ribosome, positioning the start codon (AUG) at the P site.
2. Initiator tRNA: The initiator tRNA carrying methionine (AUG) binds to the mRNA through base pairing between its anticodon and the start codon.

Steps of Initiation in Eukaryotes:#

1. mRNA Binding: The mRNA binds to the small ribosomal subunit, with the help of initiation factors.
2. Small Ribosomal Subunit Assembly: The small ribosomal subunit, along with the initiator tRNA carrying methionine, assembles on the mRNA.

Elongation of Protein Chain:#

• Elongation occurs in three phases: codon recognition, peptide bond formation, and translocation.

Delivery of Charged tRNA to the A Site:#

• During elongation, a charged tRNA carrying the correct amino acid enters the A site of the ribosome, guided by the mRNA codon.

Termination of Protein Synthesis:#

• Termination occurs when a stop codon is encountered.
• Release factors recognize the stop codon, causing the ribosome to release the completed protein.

Deconstruction of Ribosomal Complex:#

• After termination, the ribosomal complex disassembles, releasing the mRNA and tRNA molecules.