Correct option is C
Explanation-
While ribosomes generally synthesize peptides using only L-amino acids, D-amino acid–containing peptides (D-peptides) are often produced via an alternative biosynthetic pathway called Non-Ribosomal Peptide Synthesis (NRPS). This is a ribosome-independent process, primarily found in bacteria and fungi, where enzymes called non-ribosomal peptide synthetases directly incorporate D-amino acids during peptide synthesis. These NRPS pathways can incorporate D-amino acids directly using epimerase domains, which convert L- to D-amino acids during synthesis. Many antibiotics and peptide toxins (like gramicidin, tyrocidine) are synthesized this way.
Incorrect options-
Option a - "These peptides are produced by ribosomes by incorporating D-amino acids at specific positions."
Ribosomes can only incorporate L-amino acids during protein synthesis. The genetic code used by ribosomes is based entirely on L-amino acids. There is no known mechanism for direct incorporation of D-amino acids by ribosomes. Therefore, this statement is biologically inaccurate.
Option b - "Ribosome makes peptides with L-amino acids only. However, some of the amino acids in the peptides are replaced by D-amino acids by a pathway that involves excision of the L-amino acids."
This statement is partially correct in saying ribosomes make peptides from L-amino acids. But the idea that L-amino acids are excised and replaced with D-amino acids post-translationally is not the main mechanism for D-amino acid incorporation.
While rare enzymatic modifications like this can occur, it's not the primary pathway. Hence, this option is plausible but not the best or most widely supported answer.
Option d - "Peptides with D-amino acids exist only in archaea where they are made by the presence of racemases."
This option is factually incorrect . Peptides with D-amino acids are found in many organisms, especially bacteria, not just archaea.
For example, bacterial cell walls (like peptidoglycan) contain D-alanine and D-glutamate.
Racemases do exist and convert L- to D-amino acids, but these enzymes are not limited to archaea. Racemases are present in bacteria and even in eukaryotes. So, this option is both overly restrictive and misleading.
So, the correct answer is option c - The peptides with the D-amino acids are produced in a ribosome-independent manner.
