NEET 2013 question paper with solutions

1. Recall the gene order in the lac operon.

Given that the structural genes of the lac operon are arranged in the order $z$, $y$, $a$, coding respectively for beta-galactosidase, permease, and transacetylase, transcription and translation proceed sequentially along this order.

2. Understand the effect of a nonsense mutation.

Given that a nonsense mutation introduces a premature stop codon, translation halts at that point, meaning any genes coming after the mutated gene in the sequence also fail to be expressed properly.

3. Apply this to a mutation in the $y$ gene.

Since the mutation occurs in the $y$ gene, translation stops there, preventing the production of permease and the downstream transacetylase ($a$ gene).

4. Determine what is still produced.

Given that the $z$ gene comes before the $y$ gene in the sequence, its translation is unaffected, so beta-galactosidase is still produced normally.

Conclusion: Hence, the answer is D, $\beta$-galactosidase.

1. First, note the key detail in the question: the eyes of the octopus and cat have different structural patterns but perform a similar function (vision).

2. Structures that have different origin/structure but similar function are, by definition, called analogous organs.

3. When unrelated organisms (like octopuses, which are molluscs, and cats, which are mammals) independently evolve similar functional structures because they adapt to similar needs (in this case, the need for vision), this process is called convergent evolution.

4. This rules out homology, since homologous organs would need to share a common structural origin, which is not the case here.

5. Hence, the answer is B. Analogous organs that have evolved due to convergent evolution.

1. First, break down the description in the question: organisms with different evolutionary histories (i.e., not closely related) develop similar traits because they face a shared environmental challenge.

2. This is precisely the definition of convergent evolution, where unrelated lineages independently evolve similar structures (called analogous organs) to solve similar ecological problems.

3. A classic example is wings: flying insects, birds, and bats all independently evolved the ability to fly. All four serve the same function (flight) and appear structurally similar in a general sense, but their underlying anatomy is fundamentally different.

4. This is different from adaptive radiation, where a single ancestral species diversifies into many forms — here, the emphasis is on different starting ancestors converging on a similar solution.

5. Hence, the answer is A. Convergent evolution.