During which phase of protein synthesis is a base substitution most likely to have an effect?

A base substitution primarily affects the process of translation, as this is the phase where the sequence of nucleotides in mRNA is translated into a sequence of amino acids in a protein. If a base substitution occurs during transcription, it can lead to a change in the mRNA sequence, which may subsequently alter the amino acid sequence during translation. Therefore, the impact of this mutation becomes significant during translation when the ribosome reads the mRNA and assembles the corresponding amino acids. Changes in the coding sequence, due to base substitutions, can result in different amino acids being incorporated into the growing polypeptide chain, potentially altering the functionality of the protein.

In contrast, while transcription is where the initial base substitution occurs, its effects are indirectly realized during translation. Replication is primarily concerned with copying DNA, while processing involves modifying the pre-mRNA into mature mRNA through splicing and addition of caps and tails, both of which are less directly influenced by base substitutions in the context of protein synthesis impact. Thus, the best choice regarding the direct effects of a base substitution aligns with translation, as that is where the final protein product could be altered.