In the process of gene expression, transfer RNA (tRNA) is specifically necessary during the translation step. While I cannot see the diagram you are referring to, I can provide some context to help you identify the correct step.
Gene expression typically involves two main stages:
- Transcription (Step A): The synthesis of messenger RNA (mRNA) from a DNA template.
- Translation (Step B): The process where the mRNA is decoded by the ribosome, and tRNA brings amino acids to the ribosome to build a polypeptide chain (protein).
Therefore, if you are looking for the step where tRNA is necessary, it would typically be during the translation process. If your diagram labels the steps as A, B, C, D, look for the option that corresponds to translation.