The gland that secretes a hormone that regulates calcium levels by lowering blood calcium is: MCQ with Answer and Explanation

Explanation:
The human heart is a four-chambered muscular organ consisting of two upper atria (receiving chambers) and two lower ventricles (discharging chambers), ensuring complete separation of oxygenated and deoxygenated blood. The lungs are a pair of spongy, air-filled organs located on either side of the chest, serving as the primary site for gas exchange (respiration). The brain, the control center of the nervous system, is encased and protected by the bony cranium (skull). All statements are correct.

Explanation:
Vaccination introduces antigens (weakened or killed pathogens) to stimulate the body's own immune system to produce antibodies and memory cells without causing disease. This is artificial active immunity. Natural active immunity is acquired by recovering from infection. Artificial passive immunity is direct administration of antibodies (e.g., antiserum). Natural passive immunity is transfer of maternal antibodies to fetus.

Explanation:
Helicase unwinds the DNA helix at the replication fork by breaking hydrogen bonds between base pairs. DNA polymerase adds nucleotides, ligase seals Okazaki fragments, primase synthesizes RNA primer. This coordinated action ensures accurate semi-conservative replication. Without helicase, the strands would not separate. Energy for unwinding comes from ATP hydrolysis.