Six steps in a newly discovered biochemical pathway in Neurospora are shown belo

ID: 60617 • Letter: S

Question

Six steps in a newly discovered biochemical pathway in Neurospora are

shown below. Six strains, each carrying a single mutation (a, b, c, d, e, or f) were

independently isolated. Enzyme analysis showed them to be blocked in the

steps indicated in the diagram (Capital letters are the intermediates). Assume

that all arrows represent enzymes composed of a single polypeptide.

a d ec f b

--> L --> J --> H --> M --> K

A. Predict the complementation pattern.

B. Predict the growth responses on each compound and intermediates

accumulated for all six strains.

C. Assuming that complementing mutations assort independently in

crosses, what percentage of wild-type progeny are expected from the following

matings?

(1) a x e (single mutants)

(2) a f (double mutant) x d (single mutant)

(3) a b (double mutant) x a e (double mutant)

(4) a f (double mutant) x d e (double mutant)

A. Complementation pattern.

aa, dd,ec ec, ff, and bb.

B. Growth response on each compound and intermediates accumulated for all six strains.

Mutant in a - No production of L

Mutant in d- accumulation of L

Mutant in ec - accumulation of J

Mutant in f - accumulation of H

Mutant in b - accumulation of M.

C. For each single mutant the probability is 1/2,

1. a X e (Single mutants) - 1/2 X 1/2 =1/4

2. af (double mutant)X d(Single mutant) (1/2x1/2)x1/2= 1/8

3, ab (double mutant) X a e (double mutant)= 0 Since it is still homozygous for b and e hence no progeny

4. a f (double mutant) X d e (double mutant)= 1/4 X 1/4 = 1/16.

a d ec f b a + - - - - d - + - - - ec - - + - - f - - - + - b - - - - +

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