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18. The effect of various mutations in a gene x coding for a protein X, essential for the synthesis of leucine, is studied in a haploid yeast. The beginning and the end of the complete sequence of the coding strand of x is given below. Indicate the following statement is true. (A) Cells with a CT mutation at position 13 produce shorter mRNA of x (B) Cells with a AT mutation at position 37 produce shorter version of X (C) Cells with a AT mutation at position 16 are able to grow on a medium lacking leucine. (D) Cells with an additional G between positions 33 and 34 produce functional X.

19. The high osmolarity of the renal medulla is maintained by all the following except (A) active transport of salt from the upper region of the ascending limb (B) the spatial arrangement of juxtamedullary nephrons (C) diffusion of urea from the collecting duct (D) diffusion of salt from the descending limb of the loop of Henle. 

20. Mycorrhizae enhance plant nutrition mainly by (A) absorbing water and minerals through the fungal hyphae (B) providing sugar to root cells, which have no chloroplasts (C) converting atmospheric nitrogen to ammonia (D) enabling the roots to parasitize neighboring plants.

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