NEET Practice Questions (MCQs) with Answers & Solutions

DNA replication occurs during the S phase (Synthesis phase) of the cell cycle. In this phase, the cell synthesizes a complete copy of the DNA in its nucleus, ensuring that each daughter cell will have a complete set of chromosomes after cell division.

The heart of a cockroach is indeed made up of thirteen segments, which is a correct statement (S). The blood vascular system of the cockroach is of the open type, which is also a correct statement (R). However, R is not an explanation for S. Therefore, both S and R are correct, but R does not explain S.

In the context of a neuron, the cyton (cell body) does possess a large nucleus. The other options are incorrect as Nissl's granules are basophilic (basic), the cytoplasm is not restricted to only the cyton, and a neuron typically has one axon and multiple dendrites, not just one dendron.

Hydra has a nerve net, which is a simple form of a nervous system. It consists of interconnected nerve cells (neurons) that form a network throughout the body, allowing the organism to respond to stimuli in its environment.

The transmission of a nerve impulse is an electro-chemical process. It involves electrical signals (action potentials) traveling along the neuron and the release of chemical neurotransmitters at synapses to transmit the signal to the next neuron.

During the resting potential, the outside of the plasma membrane is positively charged and the inside is negatively charged. This is due to the difference in the concentration of ions, particularly sodium (Na+) outside and potassium (K+) inside the cell.

The correct order of events during the transmission of a nerve impulse is as follows: the nerve fiber first undergoes depolarization, leading to the generation of an action potential. Following this, repolarization occurs, and finally, the Na⁺ and K⁺ pumps are activated to restore the resting membrane potential. This sequence ensures the proper propagation of nerve impulses along the nerve fiber.

The opening and closing of ion channels are primarily controlled by electrical changes (voltage-gated ion channels) and chemical changes (ligand-gated ion channels) across the plasma membrane. Electrical changes refer to the difference in charge across the membrane, while chemical changes involve specific molecules binding to the ion channels to trigger their opening or closing.