Question

Which of the following statements is not an effect of codon-anticodon recognition in the ribosome? a. The 305 subunit changes from an open to closed conformation. b. The aminoacyl-tRNA structure is distorted. c. EF-Tu hydrolyzes GTP. d. The large subunit dissociates from the small subunit. e. EF-Tu dissociates from the ribosome.

Short answer

d. The large subunit dissociates from the small subunit.

Step-by-step solution

Understand Codon-Anticodon Recognition

In the ribosome, codon-anticodon recognition is essential for ensuring the correct tRNA matches with the mRNA codon. This interaction typically occurs in the smaller 30S subunit of the ribosome, causing conformational changes that ensure the proper reading of the mRNA.

Analyze Statement a

"The 30S subunit changes from an open to closed conformation." Codon-anticodon recognition often involves conformational changes in the ribosome for accurate translation, so this is indeed an effect.

Analyze Statement b

"The aminoacyl-tRNA structure is distorted." Accurate base pairing can indeed cause minor structural adjustments in the tRNA to fit correctly, so this is another potential effect.

Analyze Statement c

"EF-Tu hydrolyzes GTP." The elongation factor Tu (EF-Tu) is involved in bringing charged tRNAs to the ribosome and uses GTP hydrolysis to facilitate accurate recognition, so this is an effect.

Analyze Statement d

"The large subunit dissociates from the small subunit." There is no dissociation of the large and small subunits during normal codon-anticodon recognition, making this unlikely to be an effect.

Analyze Statement e

"EF-Tu dissociates from the ribosome." Post recognition and GTP hydrolysis, EF-Tu does dissociate from the ribosome, allowing the tRNA to engage fully with the ribosome, indicating this is an effect.

Conclusion: Identify the Incorrect Statement

Based on the analysis, the statement that does not fit the effects of codon-anticodon recognition is "The large subunit dissociates from the small subunit." This is not a part of the normal mechanism of codon-anticodon recognition in translation.

Key concepts

Ribosome Structure

The ribosome is a molecular machine that plays a critical role in translating genetic information into proteins. It consists of two subunits: a small subunit (30S in prokaryotes; 40S in eukaryotes) and a large subunit (50S in prokaryotes; 60S in eukaryotes). These subunits come together to form the functional ribosome where protein synthesis occurs.

Each subunit contains ribosomal RNA (rRNA) and proteins. The small subunit is responsible for decoding the mRNA, while the large subunit is involved in forming peptide bonds between amino acids.

• The small subunit has a crucial role in reading the mRNA and ensuring the correct transfer RNA (tRNA) is paired with the mRNA codon's sequence.
• The large subunit provides the enzymatic activity needed to bind amino acids together, forming a growing peptide chain.

During translation, the ribosome assembles around the mRNA and proceeds to translate the entire mRNA strand into a polypeptide, with the help of tRNA and other factors. This arrangement is key to efficient and correct protein synthesis.

Translation Mechanism

The translation mechanism is the process by which the ribosome reads the mRNA and builds a protein with the help of tRNA molecules. This mechanism is divided into three main phases: initiation, elongation, and termination.

**Initiation**:
In this first phase, the small ribosomal subunit binds to the mRNA. The initiation factors help the tRNA carrying the amino acid methionine (which corresponds to the start codon AUG) recognize and bind to the start of the mRNA sequence.

**Elongation**:
During elongation, tRNA molecules bring specific amino acids that match the codons on the mRNA. The ribosome forms peptide bonds between these amino acids, producing a growing polypeptide chain.

• The elongation factor EF-Tu plays a crucial role by ensuring tRNAs are correctly matched to the mRNA codon and helps catalyze the peptide bond formation using energy from GTP hydrolysis.
• After GTP is hydrolyzed, EF-Tu dissociates from the ribosome, allowing the process to continue smoothly.

**Termination**:
This final phase occurs when a stop codon is reached on the mRNA. Release factors then help disassemble the ribosomal subunits and release the newly synthesized protein.
The precise orchestration of these steps ensures proteins are synthesized accurately and efficiently.

tRNA Interaction

Transfer RNA (tRNA) is essential in translating the genetic code from mRNA into an amino acid sequence in a protein. Each tRNA molecule has two important regions: the anticodon loop and the acceptor stem.

- **Anticodon Loop**: This region contains a sequence of three nucleotides that can base-pair with the complementary codon sequence on the mRNA. This ensures that the correct amino acid is added to the growing polypeptide chain. - **Acceptor Stem**: This part of the tRNA binds to a specific amino acid, activated by its corresponding aminoacyl-tRNA synthetase.

The interaction between the codon on the mRNA and the anticodon on the tRNA is crucial for accurate translation. It ensures that the ribosome incorporates the correct amino acids based on the sequence of the mRNA.

• During translation, tRNA molecules move through three sites on the ribosome: A (aminoacyl) site, P (peptidyl) site, and E (exit) site.
• The structure of tRNA allows it to pivot and adjust slightly (a process known as "wobble") to accommodate minor variations in the pairing, helping maintain translation fidelity.

Through these precise interactions, tRNA molecules play a critical role in converting genetic information into functional proteins.