Chapter 11: Translation

What is the process of Translation?

The synthesis of a polypeptide from the information in an mRNA molecule.

How are proteins constructed?

From 20 different amino acids.

What is the role of ribosomes in protein synthesis?

Ribosomes are large molecular machines that carry out the complex task of protein synthesis.

What are the two components of ribosomes?

Ribosomal RNA (rRNA) and ribosomal proteins (r-proteins).

What are the two subunits of a ribosome?

A small subunit and a large subunit.

What is the function of the small ribosomal subunit?

Responsible for deciphering the mRNA.

What is the function of the large ribosomal subunit?

Mediates the formation of peptide bonds between amino acids.

What is the overall mass composition of a ribosome?

Roughly two-thirds RNA and one-third protein.

What are the two essential processes of translation?

Deciphering triplet codons in mRNA and incorporating amino acids encoded by the triplets into a growing polypeptide chain.

What are transfer RNAs (tRNAs)?

Molecules that link the mRNA code to protein sequences.

What is the length range of tRNA molecules?

75 to 94 nucleotides in length.

How many different tRNAs are there?

Many different tRNAs, each specific to a particular amino acid.

What determines the specificity of tRNAs to amino acids?

Aminoacyl-tRNA synthetases.

What is the structure of tRNA?

A cloverleaf structure in two dimensions with four base-paired stems or arms and single-stranded loops.

What is the acceptor stem in tRNA?

Located at the 5' and 3' ends with a conserved 3' CCA tail, which is the attachment point for amino acids.

What is the anticodon loop in tRNA?

Contains the anticodon, a three-nucleotide sequence that base-pairs with mRNA codons.

What is wobble pairing in the context of tRNA and mRNA?

Occurs at the third position of the codon and allows certain non-Watson-Crick pairings, contributing to the overall decoding flexibility.

What are aminoacyl-tRNA synthetases?

Enzymes that attach the correct amino acid to its cognate tRNA.

What is the process of aminoacylation?

The accurate attachment of amino acids to tRNAs by aminoacyl-tRNA synthetases.

How many steps are involved in the loading process of amino acids onto tRNA?

Two steps: activation of the amino acid by attachment of AMP, and transfer of the amino acid to tRNA.

What is the mass range of ribosomes in bacteria and eukaryotes?

2.5 MDa in bacteria to more than 4 MDa in eukaryotes.

What is the main structure that the ribosome utilizes to facilitate translation?

It has a large subunit and a small subunit, with specific roles in translation.

What are the three binding sites of the ribosome for tRNA?

The aminoacyl (A) site, the peptidyl (P) site, and the exit (E) site.

What occurs during the initiation phase of translation?

The ribosome, mRNA, and initiator tRNA assemble at the start codon (AUG).

What is the function of initiation factors in translation?

Assist in the binding of the ribosome and initiator tRNA to the mRNA.

What is the role of elongation factors during elongation?

Facilitate the addition of amino acids to the growing polypeptide chain.

What is the significance of stop codons in translation?

Indicates the end of protein synthesis.

What are the three stop codons?

UAA, UAG, and UGA.

What role do release factors play in termination?

Recognize stop codons and promote the release of the completed polypeptide chain.

What is the purpose of ribosome recycling?

Disassembling the ribosome, mRNA, and remaining tRNA after protein release.

What is the no-go decay (NGD) pathway?

A cellular response pathway that promotes degradation of stalled mRNA and incomplete protein.

What is tmRNA and its function in bacteria?

A unique RNA that rescues ribosomes stalled on truncated mRNAs by tagging incomplete polypeptides for degradation.

What is nonsense-mediated decay (NMD)?

An mRNA surveillance pathway that targets mRNAs with premature termination codons for degradation.

What are programmed stop codon read-through and frameshifting?

Exceptions to standard decoding rules where ribosomes bypass stop codons or shift reading frames.

What are some antibiotics that target the ribosome?

Erythromycin, Chloramphenicol, and Streptomycin.

How does erythromycin function as an antibiotic?

Binds in the exit tunnel of the large subunit, blocking the growing peptide chain.

What is the mechanism of action of streptomycin?

Induces conformational changes in the ribosome, leading to incorrect amino acid incorporation.

What are common mechanisms of antibiotic resistance in ribosomal proteins?

Mutations that affect binding sites for antibiotics, often leading to decreased drug efficacy.

What is the role of the 5' cap and 3' poly(A) tail in eukaryotic translation initiation?

Facilitates ribosome binding and enhances translation efficiency.

What happens during the scanning mechanism in eukaryotic translation initiation?

The ribosome scans the mRNA from the 5' cap to locate the AUG start codon.

What is the primary function of the peptidyl transferase center?

Catalyzes peptide bond formation between amino acids during translation.

What is the significance of the hybrid states model in translation?

Suggests that tRNAs ratchet through the ribosome, maintaining contact with one subunit while moving relative to another.

How does eukaryotic translation termination differ from bacterial termination?

It employs different release factors and requires additional steps for ribosome recycling.

What role do GTPases play in the translation cycle?

They cycle between active and inactive states, facilitating various steps of translation.

How do cells respond to arrested ribosomes?

By targeting the mRNA for decay, proteolysis of the incomplete polypeptide, and recycling the stalled ribosome.

What is the role of Pelota and Hbs1 in ribosome rescue in eukaryotes?

They promote the recycling reaction that dissociates the ribosomal subunits and releases the mRNA.