Assertion (A): The SA node is called the pacemaker of the heart. Reason (R): It generates electrical impulses that initiate the heartbeat. MCQ with Answer and Explanation

Explanation:
The double helix structure of DNA was elucidated by James Watson (an American biologist) and Francis Crick (a British physicist) in 1953 at the University of Cambridge. Their model was based on X-ray diffraction data produced by Rosalind Franklin and Maurice Wilkins. They proposed that DNA consists of two antiparallel strands held together by hydrogen bonds between complementary base pairs (A-T, G-C).

Explanation:
Trypsinogen, secreted by the pancreas, is activated to trypsin by enterokinase (intestinal enzyme). Trypsin then activates other zymogens. Pepsin is active in the stomach, rennin in infants, amylase digests starch. This cascade prevents premature protein digestion in the pancreas.

Explanation:
The pineal gland secretes melatonin in a daily rhythm, peaking at night, thus regulating the sleep-wake cycle and seasonal biological rhythms. The pituitary controls other glands, thyroid metabolism, adrenals stress response. Light signals from the retina influence pineal activity.