Final answer:
The most useful type of library to start from when cloning a gene in order to deduce the amino acid sequence of the protein from the DNA is a cDNA library.
Step-by-step explanation:
A cDNA library is made using mRNA molecules instead of genomic DNA. This is beneficial because different tissues express different genes, so by using mRNA as a template, the library will only contain DNA from the expressed genes in the cell. This allows for a focus on the translated sequences and excludes introns, control sequences, and DNA not destined to be translated into proteins, making it easier to study the sequence and structure of the gene.
By using a cDNA library, researchers can clone the cDNA insert, sequence it, and infer the amino acid sequence encoded by its cDNA. This allows for the identification and study of the gene of interest.