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 Mesozoic era (about 250-65 million years ago) is called the 'Age of Reptiles' or 'Age of Dinosaurs' because reptiles, especially dinosaurs, dominated terrestrial ecosystems. It is divided into Triassic, Jurassic, and Cretaceous periods. The Cenozoic is the 'Age of Mammals', the Paleozoic saw the rise of fishes and amphibians, and the Proterozoic is earlier.

Explanation:
R.H. Whittaker's five-kingdom classification (1969) categorizes organisms into Monera, Protista, Fungi, Plantae, and Animalia. Monera includes all prokaryotes—bacteria, cyanobacteria, and archaebacteria—characterized by lack of true nucleus and membrane-bound organelles. Protista includes unicellular eukaryotes. Fungi are multicellular saprophytes. Plantae are autotrophic eukaryotes. Monera is the only kingdom of prokaryotes in this system.

Explanation:
Dominant alleles express their trait when present, masking recessive alleles. A gene is a DNA segment; alleles are its variants. Homozygous means identical alleles, recessive expressed only in homozygous state.