Question 1

What are the two major steps cells use to synthesize proteins from genes?

Accepted Answer

Cells use transcription and translation to synthesize proteins. Transcription transfers genetic information from DNA to messenger RNA (mRNA) via RNA polymerase II. Translation then occurs in the ribosome, where the mRNA is decoded with help from transfer RNA (tRNA) molecules to synthesize a chain of amino acids that forms a polypeptide.

Question 2

How does mRNA processing prepare the molecule for translation?

Accepted Answer

Pre-mRNA undergoes three processing steps in the nucleus: 5' capping, 3' polyadenylation, and splicing. These modifications create mature mRNA, which undergoes a quality check before transport through nuclear pore complexes to the cytoplasm. In the cytoplasm, the ribosome performs another quality check, and incorrectly processed mRNAs are degraded.

Question 3

What role do tRNAs play during translation?

Accepted Answer

Transfer RNA (tRNA) molecules assist the ribosome in decoding mRNA during translation. They recognize triplet codons on the mRNA and deliver the corresponding amino acids. The ribosome catalyzes peptide bond formation between amino acids, building a polypeptide chain whose sequence depends on the mRNA's codon sequence.

Question 4

How do molecular chaperones contribute to protein function?

Accepted Answer

After the polypeptide chain is synthesized, molecular chaperones and protein folding mechanisms fold it into a functional tertiary structure. This three-dimensional shape is essential for the protein to perform its biological functions. Proper folding ensures the protein becomes active and functional within the cell.

Question 5

What happens when translation errors produce misfolded proteins?

Accepted Answer

Errors during translation can result in incorrect polypeptide chains and misfolded proteins that are toxic to cells. Cells tag these abnormal proteins with ubiquitin molecules, marking them for degradation in proteasomes. This quality control mechanism prevents accumulation of defective proteins that could harm cellular function.

Question 6

How does the genetic code determine amino acid sequence in proteins?

Accepted Answer

The nucleotide sequence in DNA defines the genetic code of mRNA through transcription. During translation, the ribosome reads triplet codons in the mRNA sequence, and each codon specifies a particular amino acid. This direct relationship between mRNA codons and amino acids ensures the polypeptide chain has the correct sequence for proper protein function.

Question 7

Why do cells perform quality checks at multiple stages of protein synthesis?

Accepted Answer

Cells enforce quality checks at various stages—in the nucleus after pre-mRNA processing, in the cytoplasm when mRNA enters, and during translation—to ensure synthesis of correct proteins. Any defective RNA or protein is degraded through pre-defined mechanisms. These multiple checkpoints prevent errors from propagating and protect cells from toxic or nonfunctional proteins.

Protéines : des gènes à la dégradation

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