The human disease caused by the deficiency of vitamin B₃ (niacin) is: MCQ with Answer and Explanation

Explanation:
Chiasmata become clearly visible during the diplotene stage of Prophase I. While the actual exchange of genetic material (crossing over) occurs during the pachytene stage, the homologous chromosomes begin to repel each other and move apart in diplotene, remaining connected only at the sites of crossing over, which appear as X-shaped chiasmata. Exam tip: Pachytene = crossing over; Diplotene = chiasmata visible.

Explanation:
B-lymphocytes (B cells) differentiate into plasma cells that secrete antibodies, providing humoral immunity. T-lymphocytes (T cells) are responsible for cell-mediated immunity, directly attacking infected or cancerous cells. Vaccines are prophylactic; they stimulate the immune system to develop memory and prevent future infections, but they do not cure active, ongoing viral infections. Thus, statements 1 and 2 are correct.

Explanation:
The liver contains enzymes (cytochrome P450 system) that metabolize and detoxify alcohol, drugs, and other xenobiotics, converting them to less toxic, excretable forms. The kidneys excrete waste, stomach digests, pancreas secretes enzymes.