A 21-year-old woman presents with a 3-month history of malaise, joint pain, weight loss, and sporadic fever. Her temperature is 38°C (101°F). The serum antinuclear antibody (ANA) test is positive (1:640). Corticosteroids are prescribed to suppress the patient's immune system. Death of post-thymic lymphocytes induced by corticosteroid therapy is characterized by fragmentation of DNA, which can be detected by gel electrophoresis in the form of a "ladder." This finding represents sequential cleavage of which of the following cellular components?