Cell signaling in eukaryotes is often dependent on receptor proteins located in the plasma membrane. During the production of the mature mRNA molecules coding for these receptors, pre-mRNA molecules are processed to remove introns and to connect exons together. The exons contain the sequences that code for proteins. In certain instances, different mature mRNA molecules can be formed from the same pre-mRNA by alternative splicing, which results in different protein sequences in the resulting polypeptides. Figure 1 represents the expression of a gene with 5 exons that can be alternatively spliced to produce receptor protein A and receptor protein B. 

(a) Explain how ligand A and ligand B can cause identical cellular responses in a cell. 

(b) Predict the most likely effect of a two-nucleotide deletion in the middle of the intron located between exons 4 and 5 on the structure of protein A. Justify your prediction.

Exon DNA Intron MRNA OR 2 4 5 3. Ligand A Ligand B Protein Protein A Plasma Protein B Plasma Membrane Membrane 2nd Messenger