Several mutants are isolated, all of which require compound...

Several mutants are isolated, all of which require compound G for growth. The compounds (A to E) in the biosynthetic pathway to G are known, but their order in the pathway is not known. Each compound is tested for its ability to support the growth of each mutant (1 to 5). In the following table, a plus sign indicates growth and a minus sign indicates no growth. $$\begin{array}{rcccccc} {5}{c} {\text {compound tested}} \\ \ { 2 - 6 }{1}{c} {} & &\mathrm{A} & \mathrm{B} & \mathrm{C} & \mathrm{D} & \mathrm{E} & \mathrm{G} \\ \hline \text { Mutant } & 1 & - & - & - & + & - & + \\ & 2 & - & + & - & + & - & + \\ & 3 & - & - & - & - & - & + \\ & 4 & - & + & + & + & - & + \\ & 5 & + & + & + & + & - & + \\ \hline \end{array}$$ a. What is the order of compounds A to Ein the pathway? b. At which point in the pathway is each mutant blocked? c. Would a heterokaryon composed of double mutants 1,3 and 2,4 grow on a minimal medium? Would 1,3 and $3,4 ?$ Would 1,2 and 2,4 and $1,4 ?$

Several mutants are isolated, all of which require compound G for growth. The compounds (A to E) in the biosynthetic pathway to G are known, but their order in the pathway is not known. Each compound is tested for its ability to support the growth of each mutant (1 to 5). In the following table, a plus sign indicates growth and a minus sign indicates no growth.
$$\begin{array}{rcccccc}
{5}{c} {\text {compound tested}} \\
\ { 2 - 6 }{1}{c} {} & &\mathrm{A} & \mathrm{B} & \mathrm{C} & \mathrm{D} & \mathrm{E} & \mathrm{G} \\
\hline \text { Mutant } & 1 & - & - & - & + & - & + \\
& 2 & - & + & - & + & - & + \\
& 3 & - & - & - & - & - & + \\
& 4 & - & + & + & + & - & + \\
& 5 & + & + & + & + & - & + \\
\hline
\end{array}$$
a. What is the order of compounds A to Ein the pathway?
b. At which point in the pathway is each mutant blocked?
c. Would a heterokaryon composed of double mutants
1,3 and 2,4 grow on a minimal medium? Would 1,3 and
$3,4 ?$ Would 1,2 and 2,4 and $1,4 ?$

Key Concepts

Heterokaryosis in Fungal Genetics

Heterokaryosis refers to a condition where cells contain genetically distinct nuclei within a common cytoplasm. In genetic studies, particularly with fungi, creating heterokaryons by combining different mutant strains can lead to complementation between defective nuclei, allowing the recovery of a complete biosynthetic pathway. This technique is a powerful tool for studying interactions between different gene products and their roles in metabolic pathways.

Mutant Blockage Analysis

Mutant blockage analysis is used to determine the specific step in a biosynthetic pathway where a mutation has inhibited the flow of intermediates. By testing the ability of various intermediates to bypass the block, it is possible to map the defective step relative to other steps in the pathway. This approach is critical in understanding enzyme function and regulation within the pathway.

Genetic Complementation

Genetic complementation involves combining two different mutants to determine if they can restore a lost function. In the context of biosynthetic pathways, if two mutants deficient in different steps are combined, their resultant phenotype can reveal whether the blocks occur at different steps. This analysis helps in clarifying the relationships between pathway components and the functional redundancy or specificity of enzymes.

Biosynthetic Pathways

Biosynthetic pathways are sequences of enzymatic reactions leading to the formation of a final product. Understanding their order is essential in determining how an organism synthesizes complex molecules from simpler precursors. The organization of these pathways helps in predicting the flow of intermediates and identifying points where the pathway may be interrupted.

Feeding Experiments

Feeding experiments involve supplementing different intermediates to an organism or mutant strain deficient in a biosynthetic pathway. By observing whether the addition of a particular intermediate restores growth or function, researchers can deduce the order of reactions and locate where a mutant's block exists in the pathway.

Transcript

00:01 If we put our table here and then our genes across the top, or mute, i always want to put f in there, but there's no f.

00:18 So a is only functional at 5.

00:23 And b is functional at 2, 4, and 5.

00:26 2, 4, 5.

00:29 C is functional only at 4 and 5.

00:34 And d is functional at 1, 2, 4, and 5.

00:39 And e is not functional at all, and then g is functional at every spot.

00:46 Okay, so let's add these up, just so we can see.

00:49 Here we have one, functional, three, two, four, zero, and five.

00:58 That tells us the order of the pathway, because the more mutants of compound supports, the later in the pathway it occurs.

01:06 So that tells us that g, which supports the growth of five, is the last one in the pathway, whereas e supports no growth.

01:16 So that must be the first step in the pathway.

01:19 And we can just order them like that.

01:22 And so the first step is going to be e with zero.

01:26 The second step is going to be a with one, and then c with two, and then b with three, and then d with four, and then g with five.

01:46 And that's our pathway.

01:48 And then the b part of the question asks us to put the mutants that's there.

01:54 So we can do that.

01:56 I'll just do vertical lines like this, and then we can determine what mutants is there.

02:02 And so e is going to be mutant five because it has zero growth.

02:09 A is four.

02:12 So this pathway is mutant four.

02:14 This here, you've got nothing that's going on.

02:17 And then c is mutant two, mutant two right here, that guy, and then b to d is mutant one, so this one for d, and then d to g is mutant three.

02:41 And so those are the mutants over here.

02:48 Okay, then c part asks about heterocarions.

02:52 Remember, heterocaryons are in organisms such as fungi that fuse the nuclei.

03:01 So you effectively have two nuclei that support the cells function.

03:09 And so here if we have a heterocaryon with one and three and two and four...

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