विस्तृत व्याख्या:

Mechanism of DNA Replication on Lagging Strand (डीएनए प्रतिकृति एवं ओकाज़ाकी खंड):

• Assertion (A) is TRUE: During semiconservative DNA replication at the replication fork, the lagging strand is synthesized discontinuously as short segments known as Okazaki fragments (1,000–2,000 nucleotides in prokaryotes, 100–200 nucleotides in eukaryotes).
• Reason (R) is TRUE: All known DNA polymerases possess strict catalytic polarity: they can only add free deoxyribonucleotides to an existing 3'-OH group, thus extending growing DNA chains exclusively in the 5' $\rightarrow$ 3' direction.
• Official Answer Key Note: In the official NTA answer key, this question is keyed as Option 2 (Both (A) and (R) are true but (R) is NOT the correct explanation of (A)), emphasizing that the antiparallel orientation of parental DNA strands combined with unidirectional fork progression is additionally required to necessitate lagging strand discontinuity.

Correct Answer: Option 2.

विस्तृत व्याख्या:

Chargaff's Rules of Base Pairing (चारगाफ के नियम):

• According to Erwin Chargaff's rules for double-stranded DNA:

• The molar ratio of Adenine (A) equals Thymine (T): $[A] = [T]$
• The molar ratio of Guanine (G) equals Cytosine (C): $[G] = [C]$
• The sum of all four bases equals 100%:
  $$[A] + [T] + [G] + [C] = 100\%$$
  • Calculation:

• Given: $[A] = 30\%$
• Therefore: $[T] = [A] = 30\%$
• Total for $A + T = 30\% + 30\% = 60\%$
• Total remaining for $G + C = 100\% - 60\% = 40\%$
• Since $[G] = [C]$:

विस्तृत व्याख्या:

Endomycorrhizal Fungi (ग्लोमेरोमाइकोटा एवं वैम कवक):

• Arbuscular mycorrhizae (AM) / Vesicular-Arbuscular Mycorrhizae (VAM) represent the most widespread type of endomycorrhiza, forming mutualistic symbiotic associations with more than 80% of terrestrial vascular plants.
• The hyphae penetrate root cortical cells to form branched arbuscules (for nutrient exchange, notably phosphorus) and vesicles (for storage).
• All arbuscular mycorrhizal fungi belong exclusively to the phylum Glomeromycota (Glomeromycetes), including genera such as Glomus, Gigaspora, Acaulospora, and Rhizophagus.
• Ectomycorrhizae are predominantly formed by Basidiomycetes and Ascomycetes with forest trees.

Correct Answer: Option 4 (Glomeromycetes).

विस्तृत व्याख्या:

Azospirillum Physiology and Nitrogenase Oxygen Sensitivity (एज़ोस्पिरिलम कार्यिकी):

• Assertion (A) is TRUE: When Azospirillum lipoferum or A. brasilense is inoculated into nitrogen-free malate medium and subjected to vigorous rotary shaking, it completely fails to grow.
• Reason (R) is TRUE and correctly explains (A): In nitrogen-free medium, Azospirillum depends entirely on the activity of its nitrogenase enzyme to fix atmospheric $\text{N}_2$ for cellular growth. Nitrogenase is extremely oxygen-sensitive and irreversibly inactivated by oxygen. Because Azospirillum lacks specialized protective structures (such as heterocysts in cyanobacteria or high respiratory protection in Azotobacter), it is an obligate microaerophile for diazotrophic growth. Shaking saturates the medium with dissolved $\text{O}_2$, which destroys nitrogenase activity and completely halts growth.