Assertion (A): Genetic variation is essential for evolution. Reason (R): Without variation, natural selection cannot occur because there are no differences to select from. MCQ with Answer and Explanation

Explanation:
The juxtaglomerular apparatus (JGA) is a specialized structure where the distal convoluted tubule (DCT) comes into contact with the afferent arteriole. It consists of juxtaglomerular cells (modified smooth muscle cells in the afferent arteriole that secrete renin), macula densa cells (modified DCT cells that sense NaCl concentration), and extraglomerular mesangial cells. It plays a crucial role in regulating blood pressure and glomerular filtration rate via the renin-angiotensin-aldosterone system.

Explanation:
The cerebellum is a highly folded structure located at the back of the brain, below the cerebrum. It is primarily responsible for coordinating voluntary movements, maintaining balance, posture, and equilibrium. Damage to the cerebellum results in ataxia (lack of muscle coordination). The medulla controls vital involuntary functions like breathing and heart rate.

Explanation:
Magnesium (Mg²⁺) is the central atom in the porphyrin ring of chlorophyll, crucial for light absorption. Iron is in hemoglobin.