The vitamin that is synthesized by gut bacteria in the human intestine is: MCQ with Answer and Explanation

Explanation:
Xylem tracheids and vessels develop lignified secondary walls and lose protoplasm, becoming dead tubes efficient for water conduction. R correctly explains A.

Explanation:
The nephron is the fundamental structural and functional unit of the kidney. Each kidney contains about one million nephrons. A nephron consists of two main parts: the renal corpuscle (Malpighian body, containing the glomerulus and Bowman's capsule) and the renal tubule (PCT, Loop of Henle, DCT). It is responsible for urine formation.

Explanation:
Retroviruses (e.g., HIV) possess two copies of single-stranded RNA and the enzyme reverse transcriptase, which transcribes the RNA into DNA upon infection. This DNA integrates into the host genome. Other viruses may have dsDNA, ssDNA, or dsRNA, but the defining feature of retroviruses is the RNA genome and reverse transcription.