Based on the pathway diagram above, it appears that one mutant affects transport

Question

Based on the pathway diagram above, it appears that one mutant affects transport of precursors in both the pteridine and ommochrome pigment pathways. Whereas there is a mutant that affects transport of the precursor in just the pteridine pathway. Each mutant strain is the result of different single-gene mutations. Explain this observation. (Why might one mutation act generally and the other mutation is more specific?

Xanthopterin Isoxanthogterin violethue) brown Pteridine pathway GTP Biopterin blue) KEL-- 2-amino-4. hydroxypteridine Niue) Sepia PDA synthase Pyrimido Tetrahydro -biooterin Drosopterin (red-orange) diazepine | white Sepianterin Isosepiaoterin vermilion Scarlet Ommochrome pathway Tryptophan Tryptochan Kynurenine Oxygenase Tryptoghan OKynurenine hydroxylase Xanthommatin (brown)

Explanation / Answer

The ommochrome pathway is responsible for the formation of eye pigment colour in frui-fly. Tryptophan leads to formation of formylkynurenine (catalyzed by the enzyme tryptophan oxidase), formylkynurenine leads to formation of kynurenine (catalyzed by the enzyme kynurenine formidase), kynurenine leads to formation of 3-hydroxyikynurenine (catalyzed by the enzyme kynurenine 3-hydroxylase), xanthommatin or ommins are then formed 3-hydroxyikynurenine. If xanthommatin is formed brown pigment formation results and ommin is responsible for white pigment. It is concluded, after a series of experiments that the initiation of xanthommatin synthesis in Drosophila is controlled subsequent to the formation of 3-hydroxy-kynurenine, probably at the level of its bimolecular condensation (Rosemary L et al, 1974). In the diagram above it appears that mutation in the enzymes kynurenine hydroxylase and tryptophan oxygenase (oxidase) leads to the formation of other eye pigment colours i.e scarlet or vermilon respectively.

In the Pteridine pathway GTP is the initiator molecule From the figure it appears that 2-amino,4 hydroxypteridine is formed from GTP, which ultimately leads to the synthesis of pteridines via the formation of other molecules. Pteridine is of two types Drosopterin and Sepiaptein. Drosopterin is responsible for red-orange colour and Sepiaptein is responsible for yellow colour. If the mutation has occurred in the gene encoding an enzyme, structural canges are expected to occur. Now, the position of the enzyme in the pathway will determine whether the mutation will act generally or specifically.

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