Lysine and arginine are called basic amino acids because their side chains are:

1. Amino acids are grouped by the chemistry of their R group, or side chain. 2. The side chains of lysine and arginine both carry a positive charge. 3. Positively charged side chains behave as bases, so these are the basic amino acids. 4. Negatively charged side chains would instead make an amino acid acidic. _Source: OpenStax Biology for AP(R) Courses (CC BY 4.0), Ch 3 'Biological Macromolecules', sections 3.1-3.5_