44. Black stem rust in wheat is caused by which organism?
[AIPMT Mains, 2010]
(A) Alternaria solani
(B) Ustilago nuda
(C) Puccinia graminis
(D) Xanthomonas oryzae
Answer: (C) Puccinia graminis
Explanation: Puccinia graminis is a basidiomycete rust fungus infecting wheat stems and other plant parts. Ustilago nuda causes loose smut, Alternaria solani early potato blight, and Xanthomonas oryzae a bacterial rice disease.
43. Which combination of statements is correct:
(a) methanogens are archaea;
(b) Nostoc fixes nitrogen;
(c) chemosynthetic bacteria make cellulose from glucose as their defining process;
(d) mycoplasmas have no cell wall?
[AIPMT Mains, 2010]
(A) Only (b) and (c)
(B) Only (a), (b), (c)
(C) Only (b), (c), (d)
(D) Only (a), (b), (d)
Answer: (D) Only (a), (b), (d)
Explanation: Methanogens produce methane and belong to Archaea; many Nostoc strains fix nitrogen; and mycoplasmas lack cell walls. Chemosynthetic bacteria instead gain energy by oxidising inorganic substances, so statement (c) is incorrect.
42. Organisms surviving extremely high temperatures and very acidic conditions belong chiefly to which pair?
[AIPMT Prelims, 2010]
(A) Liverworts and yeasts
(B) Eubacteria and archaea
(C) Cyanobacteria and diatoms
(D) Protists and mosses
Answer: (B) Eubacteria and archaea
Explanation: Certain bacteria and archaea possess specialised proteins and membranes enabling survival in hot, acidic environments. Most eukaryotic organisms in the other choices cannot withstand such combined extremes.
41. Which infectious agents consist of abnormal proteins?
[AIPMT Prelims, 2010]
(A) Satellite viruses
(B) Gemini viruses
(C) Prions
(D) Viroids
Answer: (C) Prions
Explanation: Prions are misfolded infectious proteins associated with transmissible spongiform encephalopathies. They differ from viroids, which are naked RNA, and viruses, which contain nucleic acid protected by a protein coat.
40. In Whittaker’s system, most unicellular eukaryotes are included in:
[AIPMT Prelims, 2010]
(A) Monera
(B) Protista
(C) Fungi
(D) Archaea
Answer: (B) Protista
Explanation: The traditional kingdom Protista includes a great variety of mostly single-celled eukaryotes. Monera comprises prokaryotes, while Fungi is predominantly multicellular, except for species such as yeasts.
39. Which disease-causing organism pair is wrong?
[AIPMT Prelims, 2009]
(A) Black wheat rust-Puccinia graminis
(B) Loose wheat smut-Ustilago nuda
(C) Vegetable root knot-Meloidogyne
(D) Potato late blight-Alternaria solani
Answer: (D) Potato late blight-Alternaria solani
Explanation: Late blight of potato is caused by Phytophthora infestans, while Alternaria solani causes early blight. The other disease associations are correct. Properly identifying pathogens involves distinguishing fungal, fungus-like and nematode parasites.
38. T. O. Diener discovered which infectious entity?
[AIPMT Prelims, 2009]
(A) Naked infectious DNA
(B) An infectious protein
(C) A bacteriophage
(D) Naked infectious RNA
Answer: (D) Naked infectious RNA
Explanation: Diener identified viroids as remarkably small RNA pathogens that lack the protein coat of ordinary viruses. Potato spindle tuber disease is a well-known example of a viroid-induced plant disorder.
37. Which organism performs photosynthesis that liberates oxygen?
[AIPMT Prelims, 2009]
(A) Oscillatoria
(B) Rhodospirillum
(C) Chlorobium
(D) Chromatium
Answer: (A) Oscillatoria
Explanation: Oscillatoria is a cyanobacterium using oxygenic photosynthesis with water as an electron donor. The other listed photosynthetic bacteria are known for forms of anoxygenic photosynthesis that do not release oxygen.
36. Thermococcus, Methanococcus and Methanobacterium are examples of organisms with which characteristic?
[AIPMT Prelims, 2008]
(A) They are bacteria equipped with mitochondria
(B) They are archaea with histone-like proteins related to eukaryotic histones
(C) They have no histone-like proteins at all
(D) They are bacteria with typical eukaryotic cytoskeletons
Answer: (B) They are archaea with histone-like proteins related to eukaryotic histones
Explanation: These genera belong to Archaea, many of which possess DNA-binding histone proteins related to eukaryotic core histones. Their genetic machinery combines distinctive archaeal characteristics with certain molecular similarities to eukaryotes.
35. Which microorganism generally has cellulose-rich cell walls?
[AIPMT Prelims, 2008]
(A) Saccharomyces
(B) Pythium
(C) Xanthomonas
(D) Pseudomonas
Answer: (B) Pythium
Explanation: Pythium belongs to the oomycetes, fungus-like organisms with cellulose-rich walls. True fungi such as Saccharomyces have walls rich in glucans and chitin, while bacterial walls generally contain peptidoglycan.
