Step 1: Understand what a dipeptide is. A dipeptide is formed when two amino acids join together through a peptide bond ($-CO-NH-$) with the loss of one water molecule. The name of a dipeptide tells us which amino acids it contains. Step 2: Parse the name glycylalanine. The name follows the convention: the first amino acid in a dipeptide name gets the suffix changed to '-yl', and the second keeps its full name. So glycylalanine = GLYCINE (first) + ALANINE (second). Step 3: Recall the structures of glycine and alanine. Glycine: $H_2N-CH_2-COOH$ (simplest amino acid, $R = H$). Alanine: $H_2N-CH(CH_3)-COOH$ ($R = CH_3$). The peptide bond forms between the carboxyl group of glycine and the amino group of alanine. Step 4: Write the dipeptide structure. \[ H_2N-CH_2-CO-NH-CH(CH_3)-COOH \] The N-terminal end is glycine and the C-terminal end is alanine. Step 5: Identify option 1 from the image-based question. Option 1 shows the structures corresponding to glycine (with $R = H$) and alanine (with $R = CH_3$). These are the two amino acids that form glycylalanine, confirming option 1 is correct. Step 6: Confirm the answer. Glycylalanine is a dipeptide of glycine and alanine, which is represented by the structures in option 1. \[ \boxed{\text{Glycine and Alanine (Option 1)}} \]
