Cell cycle is a series of cyclic changes through which a cell passes during its growth and division. In the cell cycle, the resting stage or interphase can be divided into three periods.

G₁ phase: RNA and Protein synthesised

S phase: DNA is formed from purines and pyrimidines. The DNA content of nucleus would be doubled.

G₂ phase: Synthesis of RNA and protein continues, but DNA synthesis stops.

Out of all the four options, proteins are most essential in the origin of life. 

Barr body is also known as sex chromatin. It is partially inactivated. Its development occurs in one of the two X chromosomes in interphase nuclei. Any of the two X-chromosomes can become heterochromatic.

In Fabaceae stamens have diadelphous arrangement i.e. (9) + 1. In total these are 10 in number.

Porous wood possesses abundant vessels. Due to the presence of vessels, hardwoods are called porous woods.

Respiratory centre controls rhe rate of respiration. It is located in medulla oblongata and pons. It has the following components:

(i) Inspiratory area

(ii) Pneumotaxic area

(iii) Expiratory area

(iv) Chemosensitive area

Chemoreceptors located on carotid and aortic bodies are sensitive to carbon dioxide levels in arterial blood. They send information to respiratory centre. Oxygen does not have a significant direct effect on the respiratory centre of the brain.

Plasmids are autonomous elements, whose genomes exist in the cell as the extra chromosomal unit. These are found in bacteria. They are used as cloning vectors due to their increased yield potential.

Plastids are semi autonomous cell organelles. They take part in storage and synthesis of organic compounds.

Mitochondria are called the power houses of the cell as they are the centres of Kreb's cycle of respiration liberating the maximum amount of energy.

Ribosomes are sub- microscopic cell organelles manufacturing protein.

A muscle fibre would contract only when it receives stimulation of certain intensity called threshold stimulus. Response of a muscle fibre to a stimulus is not proportional to its intensity. It is absent when the intensity is subliminal. Muscle fibre contracts to the maximum whether the stimulus has threshold value or supraliminal value.

For human RBC, 0.9% salt solution is isotonic. Therefore, a medium of 1.5 % salt solution would be hypertonic i.e. more concentrated. Hence, RBC shrinks due to exosmosis.

Jamming of wooden frames during rains is caused by swelling of wood due to imbibition. (The absorption of water by the solid particles of a substance without forming a solution is called imbibition).