• Step 1
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Chapter 16. Chapter 16: Mutation Analysis in Neurospora

Unpacking the Problem
true
true
You must read each slide, and complete any questions on the slide, in sequence.

You are using nitrosoguanidine to "revert" mutant nic-2 (nicotinamide-requiring) alleles in Neurospora. You treat cells, plate them on a medium without nicotinamide, and look for prototrophic colonies. You obtain the following results from two mutant alleles.

a. With nic-2 allele 1, you obtain no prototrophs at all.

b. With nic-2 allele 2, you obtain three prototrophic colonies, A, B, and C, and you cross each separately with a wild-type strain of Neurospora. From the cross prototroph A × wild type, you obtain 100 progeny, all of which are prototrophic. From the cross prototroph B × wild type, you obtain 100 progeny, of which 78 are prototrophic and 22 are nicotinamide requiring. From the cross prototroph C × wild type, you obtain 1000 progeny, of which 996 are prototrophic and 4 are nicotinamide requiring.

Explain these results at the molecular level.

Unpack the Problem: Break this problem into several parts and arrive at a solution using this guided, step-by-step approach.

  • Part A (steps 1-9): Secure a basic understanding of the experimental protocol.
  • Part B (steps 10 and 11): Explain the results of mutagenizing Neurospora nic-2 allele 1 cells.
  • Part C (steps 12-14): Assemble background information for understanding the nic-2 allele 2 results.
  • Part D (steps 15-17): Explain the results of mutagenizing Neurospora nic-2 allele 2 cells.
1

Based on the number of sets of genes per cell, Neurospora is classified as

A.
B.
C.
D.

1

Neurospora is capable of undergoing asexual reproduction when its spores germinate. Is this statement true or false?

True
False

When Neurospora MAT-A and MAT-a cells come into contact, transient diploid nuclei form, which undergo meiosis. Is this statement true or false?

True
False

You are using nitrosoguanidine to "revert" mutant nic-2 (nicotinamide-requiring) alleles in Neurospora. You treat cells, plate them on a medium without nicotinamide, and look for prototrophic colonies. You obtain the following results from two mutant alleles.

a. With nic-2 allele 1, you obtain no prototrophs at all.

b. With nic-2 allele 2, you obtain three prototrophic colonies, A, B, and C, and you cross each separately with a wild-type strain of Neurospora. From the cross prototroph A × wild type, you obtain 100 progeny, all of which are prototrophic. From the cross prototroph B × wild type, you obtain 100 progeny, of which 78 are prototrophic and 22 are nicotinamide requiring. From the cross prototroph C × wild type, you obtain 1000 progeny, of which 996 are prototrophic and 4 are nicotinamide requiring.

Explain these results at the molecular level.

Unpack the Problem: Break this problem into several parts and arrive at a solution using this guided, step-by-step approach.

  • Part A (steps 1-9): Secure a basic understanding of the experimental protocol.
  • Part B (steps 10 and 11): Explain the results of mutagenizing Neurospora nic-2 allele 1 cells.
  • Part C (steps 12-14): Assemble background information for understanding the nic-2 allele 2 results.
  • Part D (steps 15-17): Explain the results of mutagenizing Neurospora nic-2 allele 2 cells.
2

How does nitrosoguanidine work as a mutagen?

A.
B.
C.
D.

1

Does the structure of nitrosoguanidine resemble the structure of any DNA bases or base pairs?

Yes
No

Guanine and cytosine base pair using three hydrogen bonds. If the oxygen in guanine were unavailable for hydrogen bonding, would that make a guanine-thymine base pair more likely?

Yes
No

You are using nitrosoguanidine to "revert" mutant nic-2 (nicotinamide-requiring) alleles in Neurospora. You treat cells, plate them on a medium without nicotinamide, and look for prototrophic colonies. You obtain the following results from two mutant alleles.

a. With nic-2 allele 1, you obtain no prototrophs at all.

b. With nic-2 allele 2, you obtain three prototrophic colonies, A, B, and C, and you cross each separately with a wild-type strain of Neurospora. From the cross prototroph A × wild type, you obtain 100 progeny, all of which are prototrophic. From the cross prototroph B × wild type, you obtain 100 progeny, of which 78 are prototrophic and 22 are nicotinamide requiring. From the cross prototroph C × wild type, you obtain 1000 progeny, of which 996 are prototrophic and 4 are nicotinamide requiring.

Explain these results at the molecular level.

Unpack the Problem: Break this problem into several parts and arrive at a solution using this guided, step-by-step approach.

  • Part A (steps 1-9): Secure a basic understanding of the experimental protocol.
  • Part B (steps 10 and 11): Explain the results of mutagenizing Neurospora nic-2 allele 1 cells.
  • Part C (steps 12-14): Assemble background information for understanding the nic-2 allele 2 results.
  • Part D (steps 15-17): Explain the results of mutagenizing Neurospora nic-2 allele 2 cells.
3

What is a prototroph?

A.
B.
C.
D.

1

What is the composition of minimal medium?

Organic molecules, glucose, and water
Inorganic salts, a carbon source (e.g., glucose), and water
Inorganic salts, organic growth factors, and water

Which of the following organisms could be classified as prototrophic?

Bacteria, fungi, coral, and fruit flies
Fungi, lampreys, and arachnids
Yeast, bacteria, and filamentous fungi

You are using nitrosoguanidine to "revert" mutant nic-2 (nicotinamide-requiring) alleles in Neurospora. You treat cells, plate them on a medium without nicotinamide, and look for prototrophic colonies. You obtain the following results from two mutant alleles.

a. With nic-2 allele 1, you obtain no prototrophs at all.

b. With nic-2 allele 2, you obtain three prototrophic colonies, A, B, and C, and you cross each separately with a wild-type strain of Neurospora. From the cross prototroph A × wild type, you obtain 100 progeny, all of which are prototrophic. From the cross prototroph B × wild type, you obtain 100 progeny, of which 78 are prototrophic and 22 are nicotinamide requiring. From the cross prototroph C × wild type, you obtain 1000 progeny, of which 996 are prototrophic and 4 are nicotinamide requiring.

Explain these results at the molecular level.

Unpack the Problem: Break this problem into several parts and arrive at a solution using this guided, step-by-step approach.

  • Part A (steps 1-9): Secure a basic understanding of the experimental protocol.
  • Part B (steps 10 and 11): Explain the results of mutagenizing Neurospora nic-2 allele 1 cells.
  • Part C (steps 12-14): Assemble background information for understanding the nic-2 allele 2 results.
  • Part D (steps 15-17): Explain the results of mutagenizing Neurospora nic-2 allele 2 cells.
4

What is an auxotroph?

A.
B.
C.
D.

1

Nutritional mutants of microorganisms can be isolated by replica plating between minimal medium and minimal medium supplemented with one or more essential biochemical building blocks. Is this statement true or false?

True
False

Prototrophs contain biochemical pathway mutations that are absent in corresponding auxotrophs. Is this statement true or false?

True
False

You are using nitrosoguanidine to "revert" mutant nic-2 (nicotinamide-requiring) alleles in Neurospora. You treat cells, plate them on a medium without nicotinamide, and look for prototrophic colonies. You obtain the following results from two mutant alleles.

a. With nic-2 allele 1, you obtain no prototrophs at all.

b. With nic-2 allele 2, you obtain three prototrophic colonies, A, B, and C, and you cross each separately with a wild-type strain of Neurospora. From the cross prototroph A × wild type, you obtain 100 progeny, all of which are prototrophic. From the cross prototroph B × wild type, you obtain 100 progeny, of which 78 are prototrophic and 22 are nicotinamide requiring. From the cross prototroph C × wild type, you obtain 1000 progeny, of which 996 are prototrophic and 4 are nicotinamide requiring.

Explain these results at the molecular level.

Unpack the Problem: Break this problem into several parts and arrive at a solution using this guided, step-by-step approach.

  • Part A (steps 1-9): Secure a basic understanding of the experimental protocol.
  • Part B (steps 10 and 11): Explain the results of mutagenizing Neurospora nic-2 allele 1 cells.
  • Part C (steps 12-14): Assemble background information for understanding the nic-2 allele 2 results.
  • Part D (steps 15-17): Explain the results of mutagenizing Neurospora nic-2 allele 2 cells.
5

The Neurospora nic-2 mutants are

A.
B.
C.
D.

1

Why is nicotinamide required in the medium in order for Neurospora nic-2 mutants to grow?

The nic-2 mutants have lost the ability to use nicotinamide.
The nic-2 mutants have lost the ability to synthesize nicotinamide.
The nic-2 mutants have lost the ability to convert nicotinamide into NADH.

Recall from Steps 3 and 4 what distinguishes prototrophs from auxotrophs.

You are using nitrosoguanidine to "revert" mutant nic-2 (nicotinamide-requiring) alleles in Neurospora. You treat cells, plate them on a medium without nicotinamide, and look for prototrophic colonies. You obtain the following results from two mutant alleles.

a. With nic-2 allele 1, you obtain no prototrophs at all.

b. With nic-2 allele 2, you obtain three prototrophic colonies, A, B, and C, and you cross each separately with a wild-type strain of Neurospora. From the cross prototroph A × wild type, you obtain 100 progeny, all of which are prototrophic. From the cross prototroph B × wild type, you obtain 100 progeny, of which 78 are prototrophic and 22 are nicotinamide requiring. From the cross prototroph C × wild type, you obtain 1000 progeny, of which 996 are prototrophic and 4 are nicotinamide requiring.

Explain these results at the molecular level.

Unpack the Problem: Break this problem into several parts and arrive at a solution using this guided, step-by-step approach.

  • Part A (steps 1-9): Secure a basic understanding of the experimental protocol.
  • Part B (steps 10 and 11): Explain the results of mutagenizing Neurospora nic-2 allele 1 cells.
  • Part C (steps 12-14): Assemble background information for understanding the nic-2 allele 2 results.
  • Part D (steps 15-17): Explain the results of mutagenizing Neurospora nic-2 allele 2 cells.
6

What is the significance of using a medium lacking nicotinamide?

A.
B.
C.
D.

1

What type of Neurospora will grow on this medium?

All types of fungi will grow on this medium.
Only Neurospora will grow on this medium.
Only Neurospora that can synthesize nicotinamide will grow on this medium.

Plating cells onto medium lacking nicotinamide will select between cells that can synthesize nicotinamide and those that cannot. Is this statement true or false?

True
False

You are using nitrosoguanidine to "revert" mutant nic-2 (nicotinamide-requiring) alleles in Neurospora. You treat cells, plate them on a medium without nicotinamide, and look for prototrophic colonies. You obtain the following results from two mutant alleles.

a. With nic-2 allele 1, you obtain no prototrophs at all.

b. With nic-2 allele 2, you obtain three prototrophic colonies, A, B, and C, and you cross each separately with a wild-type strain of Neurospora. From the cross prototroph A × wild type, you obtain 100 progeny, all of which are prototrophic. From the cross prototroph B × wild type, you obtain 100 progeny, of which 78 are prototrophic and 22 are nicotinamide requiring. From the cross prototroph C × wild type, you obtain 1000 progeny, of which 996 are prototrophic and 4 are nicotinamide requiring.

Explain these results at the molecular level.

Unpack the Problem: Break this problem into several parts and arrive at a solution using this guided, step-by-step approach.

  • Part A (steps 1-9): Secure a basic understanding of the experimental protocol.
  • Part B (steps 10 and 11): Explain the results of mutagenizing Neurospora nic-2 allele 1 cells.
  • Part C (steps 12-14): Assemble background information for understanding the nic-2 allele 2 results.
  • Part D (steps 15-17): Explain the results of mutagenizing Neurospora nic-2 allele 2 cells.
7

How were the cells containing the two different nic-2 alleles obtained?

A.
B.
C.
D.
E.

1

What is the purpose of inducing mutagenesis in this experiment?

It slows down growth rates, thus reducing the number of colonies per plate, so that it is easier to identify mutants.
It creates suppressors that convert the mutant phenotype to wild type.
It randomly creates mutant cells.

What is the purpose of selection in this experiment?

It is a screening method for identifying the presence of mutant cells that can no longer synthesize nicotinamide.
It is a screen for suppressor mutants that can now synthesize nicotinamide.
It is a screening method that distinguishes between fast- and slow-growing colonies.

You are using nitrosoguanidine to "revert" mutant nic-2 (nicotinamide-requiring) alleles in Neurospora. You treat cells, plate them on a medium without nicotinamide, and look for prototrophic colonies. You obtain the following results from two mutant alleles.

a. With nic-2 allele 1, you obtain no prototrophs at all.

b. With nic-2 allele 2, you obtain three prototrophic colonies, A, B, and C, and you cross each separately with a wild-type strain of Neurospora. From the cross prototroph A × wild type, you obtain 100 progeny, all of which are prototrophic. From the cross prototroph B × wild type, you obtain 100 progeny, of which 78 are prototrophic and 22 are nicotinamide requiring. From the cross prototroph C × wild type, you obtain 1000 progeny, of which 996 are prototrophic and 4 are nicotinamide requiring.

Explain these results at the molecular level.

Unpack the Problem: Break this problem into several parts and arrive at a solution using this guided, step-by-step approach.

  • Part A (steps 1-9): Secure a basic understanding of the experimental protocol.
  • Part B (steps 10 and 11): Explain the results of mutagenizing Neurospora nic-2 allele 1 cells.
  • Part C (steps 12-14): Assemble background information for understanding the nic-2 allele 2 results.
  • Part D (steps 15-17): Explain the results of mutagenizing Neurospora nic-2 allele 2 cells.
8

What does reversion mean?

A.
B.
C.
D.
E.
F.

1

Consider the general-purpose definition of the verb to revert.

The mutation under investigation is

the inability to grow on nicotinamide.
the inability to grow in the absence of nicotinamide.
the ability to grow on nicotinamide.

The starting population of Neurospora were all nic+. Is this statement true or false?

True
False

You are using nitrosoguanidine to "revert" mutant nic-2 (nicotinamide-requiring) alleles in Neurospora. You treat cells, plate them on a medium without nicotinamide, and look for prototrophic colonies. You obtain the following results from two mutant alleles.

a. With nic-2 allele 1, you obtain no prototrophs at all.

b. With nic-2 allele 2, you obtain three prototrophic colonies, A, B, and C, and you cross each separately with a wild-type strain of Neurospora. From the cross prototroph A × wild type, you obtain 100 progeny, all of which are prototrophic. From the cross prototroph B × wild type, you obtain 100 progeny, of which 78 are prototrophic and 22 are nicotinamide requiring. From the cross prototroph C × wild type, you obtain 1000 progeny, of which 996 are prototrophic and 4 are nicotinamide requiring.

Explain these results at the molecular level.

Unpack the Problem: Break this problem into several parts and arrive at a solution using this guided, step-by-step approach.

  • Part A (steps 1-9): Secure a basic understanding of the experimental protocol.
  • Part B (steps 10 and 11): Explain the results of mutagenizing Neurospora nic-2 allele 1 cells.
  • Part C (steps 12-14): Assemble background information for understanding the nic-2 allele 2 results.
  • Part D (steps 15-17): Explain the results of mutagenizing Neurospora nic-2 allele 2 cells.
9

Which of the following mechanism(s) could cause a reversion in phenotype?

A.
B.
C.
D.
E.
F.
G.

1

If a mutation within a coding sequence of an A-T base pair to G-C is later converted back to A-T, would the coding sequence be restored?

Yes
No

If a base pair within a coding sequence is deleted, then a frameshift would occur in the transcript. If a different base pair is later inserted downstream of the deletion, could the original amino acid sequence be restored?

Yes
No

A mutation in gene X leads to loss of a protein X binding to protein Y, which blocks a biosynthetic pathway. Later a mutation in gene Y restores binding between proteins X and Y. Could this remove the block in the biosynthetic pathway?

Yes
No

You are using nitrosoguanidine to "revert" mutant nic-2 (nicotinamide-requiring) alleles in Neurospora. You treat cells, plate them on a medium without nicotinamide, and look for prototrophic colonies. You obtain the following results from two mutant alleles.

a. With nic-2 allele 1, you obtain no prototrophs at all.

b. With nic-2 allele 2, you obtain three prototrophic colonies, A, B, and C, and you cross each separately with a wild-type strain of Neurospora. From the cross prototroph A × wild type, you obtain 100 progeny, all of which are prototrophic. From the cross prototroph B × wild type, you obtain 100 progeny, of which 78 are prototrophic and 22 are nicotinamide requiring. From the cross prototroph C × wild type, you obtain 1000 progeny, of which 996 are prototrophic and 4 are nicotinamide requiring.

Explain these results at the molecular level.

Unpack the Problem: Break this problem into several parts and arrive at a solution using this guided, step-by-step approach.

  • Part A (steps 1-9): Secure a basic understanding of the experimental protocol.
  • Part B (steps 10 and 11): Explain the results of mutagenizing Neurospora nic-2 allele 1 cells.
  • Part C (steps 12-14): Assemble background information for understanding the nic-2 allele 2 results.
  • Part D (steps 15-17): Explain the results of mutagenizing Neurospora nic-2 allele 2 cells.
10

Why were no prototrophs detected when cells containing nic-2 allele 1 were mutagenized and plated onto selective medium?

A.
B.
C.

1

Nitrosoguanidine always mutates specific nucleotides within DNA. Is this statement true or false?

True
False

The failure to include nicotinamide in the medium prevented the identification of prototrophs. Is this statement true or false?

True
False

You are using nitrosoguanidine to "revert" mutant nic-2 (nicotinamide-requiring) alleles in Neurospora. You treat cells, plate them on a medium without nicotinamide, and look for prototrophic colonies. You obtain the following results from two mutant alleles.

a. With nic-2 allele 1, you obtain no prototrophs at all.

b. With nic-2 allele 2, you obtain three prototrophic colonies, A, B, and C, and you cross each separately with a wild-type strain of Neurospora. From the cross prototroph A × wild type, you obtain 100 progeny, all of which are prototrophic. From the cross prototroph B × wild type, you obtain 100 progeny, of which 78 are prototrophic and 22 are nicotinamide requiring. From the cross prototroph C × wild type, you obtain 1000 progeny, of which 996 are prototrophic and 4 are nicotinamide requiring.

Explain these results at the molecular level.

Unpack the Problem: Break this problem into several parts and arrive at a solution using this guided, step-by-step approach.

  • Part A (steps 1-9): Secure a basic understanding of the experimental protocol.
  • Part B (steps 10 and 11): Explain the results of mutagenizing Neurospora nic-2 allele 1 cells.
  • Part C (steps 12-14): Assemble background information for understanding the nic-2 allele 2 results.
  • Part D (steps 15-17): Explain the results of mutagenizing Neurospora nic-2 allele 2 cells.
11

Why were no prototrophs detected when cells containing nic-2 allele 1 were mutagenized and plated onto selective medium?

A.
B.
C.
D.
E.
F.
G.

1

Nitrosguanidine mutates DNA by

replacing a base in DNA.
altering a base in DNA, so that a specific mispairing occurs.
damaging a base, so that it can no longer pair with any base.

Nitrosoguanidine mutates nucleotides within DNA more or less randomly. Is this statement true or false?

True
False

You are using nitrosoguanidine to "revert" mutant nic-2 (nicotinamide-requiring) alleles in Neurospora. You treat cells, plate them on a medium without nicotinamide, and look for prototrophic colonies. You obtain the following results from two mutant alleles.

a. With nic-2 allele 1, you obtain no prototrophs at all.

b. With nic-2 allele 2, you obtain three prototrophic colonies, A, B, and C, and you cross each separately with a wild-type strain of Neurospora. From the cross prototroph A × wild type, you obtain 100 progeny, all of which are prototrophic. From the cross prototroph B × wild type, you obtain 100 progeny, of which 78 are prototrophic and 22 are nicotinamide requiring. From the cross prototroph C × wild type, you obtain 1000 progeny, of which 996 are prototrophic and 4 are nicotinamide requiring.

Explain these results at the molecular level.

Unpack the Problem: Break this problem into several parts and arrive at a solution using this guided, step-by-step approach.

  • Part A (steps 1-9): Secure a basic understanding of the experimental protocol.
  • Part B (steps 10 and 11): Explain the results of mutagenizing Neurospora nic-2 allele 1 cells.
  • Part C (steps 12-14): Assemble background information for understanding the nic-2 allele 2 results.
  • Part D (steps 15-17): Explain the results of mutagenizing Neurospora nic-2 allele 2 cells.
12

When a prototrophic revertant is crossed with wild-type Neurospora, how many progeny should typically come from wild-type ascospores?

A.
B.
C.
D.

1

Meiocytes are

always haploid.
always diploid.
can be either haploid or diploid in any organism.
are haploid in some organisms and diploid in others.

During meiosis, one set of 2n meiocytes are converted to

a set of two n gametes or spores.
a set of two 2n gametes or spores.
a set of four n gametes or spores.
a set of four 2n gametes or spores.

You are using nitrosoguanidine to "revert" mutant nic-2 (nicotinamide-requiring) alleles in Neurospora. You treat cells, plate them on a medium without nicotinamide, and look for prototrophic colonies. You obtain the following results from two mutant alleles.

a. With nic-2 allele 1, you obtain no prototrophs at all.

b. With nic-2 allele 2, you obtain three prototrophic colonies, A, B, and C, and you cross each separately with a wild-type strain of Neurospora. From the cross prototroph A × wild type, you obtain 100 progeny, all of which are prototrophic. From the cross prototroph B × wild type, you obtain 100 progeny, of which 78 are prototrophic and 22 are nicotinamide requiring. From the cross prototroph C × wild type, you obtain 1000 progeny, of which 996 are prototrophic and 4 are nicotinamide requiring.

Explain these results at the molecular level.

Unpack the Problem: Break this problem into several parts and arrive at a solution using this guided, step-by-step approach.

  • Part A (steps 1-9): Secure a basic understanding of the experimental protocol.
  • Part B (steps 10 and 11): Explain the results of mutagenizing Neurospora nic-2 allele 1 cells.
  • Part C (steps 12-14): Assemble background information for understanding the nic-2 allele 2 results.
  • Part D (steps 15-17): Explain the results of mutagenizing Neurospora nic-2 allele 2 cells.
13

A suppressor mutation that suppresses the effect(s) of the first mutation can restore the original wild-type phenotype. Is this statement true or false?

A.
B.

1

In this experiment what is the wild-type phenotype being followed?

The ability to synthesize nicotinamide
The inability to synthesize nicotinamide
The ability to resist nitrosoguanidine

Look up the definition of a suppressor in your textbook glossary.

You are using nitrosoguanidine to "revert" mutant nic-2 (nicotinamide-requiring) alleles in Neurospora. You treat cells, plate them on a medium without nicotinamide, and look for prototrophic colonies. You obtain the following results from two mutant alleles.

a. With nic-2 allele 1, you obtain no prototrophs at all.

b. With nic-2 allele 2, you obtain three prototrophic colonies, A, B, and C, and you cross each separately with a wild-type strain of Neurospora. From the cross prototroph A × wild type, you obtain 100 progeny, all of which are prototrophic. From the cross prototroph B × wild type, you obtain 100 progeny, of which 78 are prototrophic and 22 are nicotinamide requiring. From the cross prototroph C × wild type, you obtain 1000 progeny, of which 996 are prototrophic and 4 are nicotinamide requiring.

Explain these results at the molecular level.

Unpack the Problem: Break this problem into several parts and arrive at a solution using this guided, step-by-step approach.

  • Part A (steps 1-9): Secure a basic understanding of the experimental protocol.
  • Part B (steps 10 and 11): Explain the results of mutagenizing Neurospora nic-2 allele 1 cells.
  • Part C (steps 12-14): Assemble background information for understanding the nic-2 allele 2 results.
  • Part D (steps 15-17): Explain the results of mutagenizing Neurospora nic-2 allele 2 cells.
14

When the progeny in a cross reveal a recombination frequency of less than 50%, then the genes being analyzed are linked. Is this statement true or false?

A.
B.

1

What is the relationship between recombination frequency and the distance between genes that are located on the same chromosome?

The greater the distance between two genes, the greater the recombination frequency.
The smaller the distance between two genes, the greater the recombination frequency.
Recombination frequency and gene distance are independent of each other.

What does it mean when two genes are linked?

They are adjacent to each other on a chromosome.
During meiosis, they behave as if they are one gene.
During meiosis, they may not assort independently.

You are using nitrosoguanidine to "revert" mutant nic-2 (nicotinamide-requiring) alleles in Neurospora. You treat cells, plate them on a medium without nicotinamide, and look for prototrophic colonies. You obtain the following results from two mutant alleles.

a. With nic-2 allele 1, you obtain no prototrophs at all.

b. With nic-2 allele 2, you obtain three prototrophic colonies, A, B, and C, and you cross each separately with a wild-type strain of Neurospora. From the cross prototroph A × wild type, you obtain 100 progeny, all of which are prototrophic. From the cross prototroph B × wild type, you obtain 100 progeny, of which 78 are prototrophic and 22 are nicotinamide requiring. From the cross prototroph C × wild type, you obtain 1000 progeny, of which 996 are prototrophic and 4 are nicotinamide requiring.

Explain these results at the molecular level.

Unpack the Problem: Break this problem into several parts and arrive at a solution using this guided, step-by-step approach.

  • Part A (steps 1-9): Secure a basic understanding of the experimental protocol.
  • Part B (steps 10 and 11): Explain the results of mutagenizing Neurospora nic-2 allele 1 cells.
  • Part C (steps 12-14): Assemble background information for understanding the nic-2 allele 2 results.
  • Part D (steps 15-17): Explain the results of mutagenizing Neurospora nic-2 allele 2 cells.
15

When organisms containing nic-2 allele 2 were mutagenized and plated onto selective medium, three prototrophic colonies were isolated. What is/are likely molecular explanation(s) for why 100% of the progeny were prototrophic from the cross of prototroph A with wild-type Neurospora?

A.
B.
C.
D.

1

Fifty percent of the progeny from this cross came from wild-type ascospores. Is this statement true or false?

True
False

As you evaluate each answer choice, think like Mendel and consider that the outcome of a genetic cross has probabilistic underpinnings. Therefore, in order for 100% of the progeny to be prototrophic (that is, having reverted to wild type), what event is most likely to have happened?

The loss of function is restored by a mutation of a gene on a different chromosome.
The loss of function is restored by a mutation of a gene on the same chromosome.
The loss of function is restored by a mutation of the original gene but in a different part of the sequence.
The loss of function is restored by a precise reversal of the original mutation in the original gene.

You are using nitrosoguanidine to "revert" mutant nic-2 (nicotinamide-requiring) alleles in Neurospora. You treat cells, plate them on a medium without nicotinamide, and look for prototrophic colonies. You obtain the following results from two mutant alleles.

a. With nic-2 allele 1, you obtain no prototrophs at all.

b. With nic-2 allele 2, you obtain three prototrophic colonies, A, B, and C, and you cross each separately with a wild-type strain of Neurospora. From the cross prototroph A × wild type, you obtain 100 progeny, all of which are prototrophic. From the cross prototroph B × wild type, you obtain 100 progeny, of which 78 are prototrophic and 22 are nicotinamide requiring. From the cross prototroph C × wild type, you obtain 1000 progeny, of which 996 are prototrophic and 4 are nicotinamide requiring.

Explain these results at the molecular level.

Unpack the Problem: Break this problem into several parts and arrive at a solution using this guided, step-by-step approach.

  • Part A (steps 1-9): Secure a basic understanding of the experimental protocol.
  • Part B (steps 10 and 11): Explain the results of mutagenizing Neurospora nic-2 allele 1 cells.
  • Part C (steps 12-14): Assemble background information for understanding the nic-2 allele 2 results.
  • Part D (steps 15-17): Explain the results of mutagenizing Neurospora nic-2 allele 2 cells.
16

When organisms containing nic-2 allele 2 were mutagenized and plated onto selective medium, three prototrophic colonies were isolated. What is/are likely molecular explanation(s) for why 78% of the progeny were prototrophic (and 22% were auxotrophic) from the cross of prototroph B with wild-type Neurospora?

A.
B.
C.
D.

1

The number of progeny, 78 prototrophs to 22 auxotrophs, approximates what well-known Mendelian ratio?

9:1
3:1
4:1
15:1

Prepare a Punnett square for two genes having two alleles of each gene in a haploid organism.

You are using nitrosoguanidine to "revert" mutant nic-2 (nicotinamide-requiring) alleles in Neurospora. You treat cells, plate them on a medium without nicotinamide, and look for prototrophic colonies. You obtain the following results from two mutant alleles.

a. With nic-2 allele 1, you obtain no prototrophs at all.

b. With nic-2 allele 2, you obtain three prototrophic colonies, A, B, and C, and you cross each separately with a wild-type strain of Neurospora. From the cross prototroph A × wild type, you obtain 100 progeny, all of which are prototrophic. From the cross prototroph B × wild type, you obtain 100 progeny, of which 78 are prototrophic and 22 are nicotinamide requiring. From the cross prototroph C × wild type, you obtain 1000 progeny, of which 996 are prototrophic and 4 are nicotinamide requiring.

Explain these results at the molecular level.

Unpack the Problem: Break this problem into several parts and arrive at a solution using this guided, step-by-step approach.

  • Part A (steps 1-9): Secure a basic understanding of the experimental protocol.
  • Part B (steps 10 and 11): Explain the results of mutagenizing Neurospora nic-2 allele 1 cells.
  • Part C (steps 12-14): Assemble background information for understanding the nic-2 allele 2 results.
  • Part D (steps 15-17): Explain the results of mutagenizing Neurospora nic-2 allele 2 cells.
17

When organisms containing nic-2 allele 2 were mutagenized and plated onto selective medium, three prototrophic colonies were isolated. What is/are likely molecular explanation(s) for why 99.6% of the progeny were prototrophic from the cross of prototroph C with wild-type Neurospora?

A.
B.
C.
D.

1

In this cross of prototroph C and wild type, is it still true that 50% of the 1000 progeny come from wild-type ascospores?

Yes
No

Does the ratio of prototrophs to auxotrophs among the progeny correspond to a Mendelian ratio?

Yes, it is roughly 3:1.
Yes, it is roughly 15:1.
No, the number of prototrophs is much greater than expected.

In this experiment, when the ratio among progeny phenotypes deviates significantly from what is expected, then what might the cause be?

Recombination between genes that are relatively far apart
Linkage between two genes that are very close to each other on a chromosome
Complementation between related genes
Recessive epistasis

Conclusion

Inducing mutations is a powerful tool that geneticists use for generating phenotypic variants. These variants can then be studied to identify the mutated genes as well as the gene products that the wild-type genes encode. These studies can be extended, in some cases, to unravel biochemical pathways or identify protein interactions. The creation of revertants can be used to identify additional genes (e.g., suppressors) and their gene products.