Which Of The Following Would Result In A Frameshift Mutation

A frameshift mutation, a particularly impactful type of genetic mutation, arises from the insertion or deletion of nucleotide bases in a DNA sequence. This alteration disrupts the established reading frame of the genetic code, leading to a cascade of changes in the resulting protein. Understanding the mechanisms that cause frameshift mutations and their consequences is crucial in comprehending the intricacies of molecular biology and genetics.

Understanding Frameshift Mutations

The genetic code functions by reading sequences of three nucleotides, called codons, each specifying a particular amino acid in the protein. A frameshift mutation shifts this reading frame, altering the entire downstream amino acid sequence from the point of mutation. This often leads to a premature stop codon, resulting in a truncated and non-functional protein.

Frameshift mutations are primarily caused by:

• Insertions: The addition of one or more nucleotide bases into the DNA sequence.
• Deletions: The removal of one or more nucleotide bases from the DNA sequence.

It is important to note that insertions or deletions that involve multiples of three nucleotides do not cause frameshift mutations. Instead, they add or remove entire codons, potentially altering the protein but maintaining the original reading frame. These mutations are called in-frame insertions or in-frame deletions.

Examples of Frameshift-Causing Scenarios

Let's explore specific examples to illustrate which scenarios would result in a frameshift mutation:

1. Insertion of a Single Nucleotide

Inserting a single nucleotide into the DNA sequence will invariably cause a frameshift mutation.

Original Sequence:

5'- AUG-GUC-AUC-GUA-UGA -3'
   (Met-Val-Ile-Val-Stop)

Mutated Sequence (with insertion of 'C' after AUG):

5'- AUG-CGU-CAU-CGU-AUG-A -3'
   (Met-Arg-His-Arg-Met)

As demonstrated, the insertion of a single nucleotide ('C') after the start codon (AUG) shifts the reading frame. All subsequent codons are misread, resulting in a completely different amino acid sequence. This is a classic example of a frameshift mutation with severe consequences for the protein's structure and function. In this case, the original stop codon is bypassed and the translation will continue until a stop codon is encountered in the new frame.

2. Deletion of Two Nucleotides

The deletion of two nucleotides also leads to a frameshift mutation.

Original Sequence:

5'- AUG-GUC-AUC-GUA-UGA -3'
   (Met-Val-Ile-Val-Stop)

Mutated Sequence (with deletion of 'UC' after AUG):

5'- AUG-GAI-CGU-AUG-A -3'
   (Met-Asp-Arg-Met)

The deletion of 'UC' after the start codon shifts the reading frame. All codons after the deletion are misread, resulting in a different amino acid sequence. Again, this exemplifies a frameshift mutation disrupting the proper protein synthesis. Similar to the insertion example, the original stop codon is bypassed.

3. Insertion of Three Nucleotides

The insertion of three nucleotides does not cause a frameshift mutation.

Original Sequence:

5'- AUG-GUC-AUC-GUA-UGA -3'
   (Met-Val-Ile-Val-Stop)

Mutated Sequence (with insertion of 'AAA' after AUG):

5'- AUG-AAA-GUC-AUC-GUA-UGA -3'
   (Met-Lys-Val-Ile-Val-Stop)

In this case, an entire codon (AAA, coding for Lysine) is inserted. The reading frame for the rest of the sequence remains unchanged. This is an in-frame insertion. The resulting protein will be slightly longer, with an additional amino acid, but its overall structure and function might still be largely preserved, depending on the location and properties of the inserted amino acid.

4. Deletion of Six Nucleotides

The deletion of six nucleotides does not cause a frameshift mutation.

Original Sequence:

5'- AUG-GUC-AUC-GUA-UGA -3'
   (Met-Val-Ile-Val-Stop)

Mutated Sequence (with deletion of 'GUC-AUC' after AUG):

5'- AUG-GUA-UGA -3'
   (Met-Val-Stop)

Here, two complete codons (GUC and AUC) are deleted. The reading frame is maintained; however, the protein will be shorter by two amino acids. This is an in-frame deletion. The effect on the protein's function depends on the significance of the deleted amino acids for its structure and activity.

5. A Combination of Insertion and Deletion that Results in a Net Non-Multiple of Three

If a sequence undergoes both insertion and deletion, whether or not a frameshift occurs depends on the net change in the number of nucleotides.

• If the total number of inserted and deleted nucleotides is a multiple of three (e.g., insertion of 4 and deletion of 1, net change of +3), the reading frame will be maintained, and it will not be a frameshift mutation. It would, however, lead to changes in the amino acid sequence where the insertion and deletion occurred.
• If the total number of inserted and deleted nucleotides is not a multiple of three (e.g., insertion of 1 and deletion of 2, net change of -1), then a frameshift mutation will occur.

Example (Frameshift):

Original Sequence:

5'- AUG-GUC-AUC-GUA-UGA -3'
   (Met-Val-Ile-Val-Stop)

Mutated Sequence (insertion of 'C' after AUG, deletion of 'AU' after GUC):

5'- AUG-CGU-GCG-UA-UGA -3'
   (Met-Arg-Ala-Val-Stop)

Here, the insertion of 'C' and the deletion of 'AU' result in a net change of -1, causing a frameshift mutation from the 'GUC' codon onwards.

Example (No Frameshift):

Original Sequence:

5'- AUG-GUC-AUC-GUA-UGA -3'
   (Met-Val-Ile-Val-Stop)

Mutated Sequence (insertion of 'AAA' after AUG, deletion of 'GUC' after AAA):

5'- AUG-AAA-AUC-GUA-UGA -3'
   (Met-Lys-Ile-Val-Stop)

Here, the insertion of 'AAA' and the deletion of 'GUC' result in a net change of 0, so there is no frameshift mutation. Instead, the Valine (GUC) is replaced by Lysine (AAA) in the protein.

Consequences of Frameshift Mutations

Frameshift mutations can have profound consequences on the protein encoded by the affected gene. Since the reading frame is altered, the resulting amino acid sequence downstream of the mutation site is completely different from the intended sequence. This often leads to:

• Premature Stop Codons: The altered reading frame may introduce a stop codon earlier than intended, resulting in a truncated protein.
• Non-Functional Proteins: Even if a premature stop codon is not encountered, the altered amino acid sequence is unlikely to produce a functional protein. The protein's structure, folding, and active sites will be disrupted.
• Altered Protein Interactions: The mutated protein may be unable to interact properly with other proteins or cellular components, disrupting cellular processes.
• Disease: Frameshift mutations can cause a variety of genetic diseases, depending on the gene affected.

Examples of Diseases Caused by Frameshift Mutations

Several genetic diseases are caused by frameshift mutations. Here are a few prominent examples:

• Cystic Fibrosis (CF): While the most common mutation in CF is a deletion of a single phenylalanine residue (an in-frame deletion), frameshift mutations in the CFTR gene (Cystic Fibrosis Transmembrane Conductance Regulator) can also cause the disease. These frameshift mutations lead to a non-functional CFTR protein, resulting in the characteristic symptoms of CF, such as thick mucus buildup in the lungs and digestive system.
• Tay-Sachs Disease: Some cases of Tay-Sachs disease are caused by frameshift mutations in the HEXA gene, which encodes the alpha subunit of hexosaminidase A. The enzyme hexosaminidase A is crucial for breaking down certain lipids in the brain. Frameshift mutations in HEXA lead to a deficiency of this enzyme, causing a buildup of lipids and ultimately leading to neurodegeneration.
• Beta-Thalassemia: Beta-thalassemia is a blood disorder characterized by reduced or absent production of beta-globin, a component of hemoglobin. Frameshift mutations in the HBB gene (encoding beta-globin) can cause beta-thalassemia by disrupting the synthesis of functional beta-globin protein.
• Some Cancers: Frameshift mutations can occur in tumor suppressor genes or oncogenes, contributing to cancer development. For example, frameshift mutations in genes involved in DNA repair or cell cycle control can lead to uncontrolled cell growth and division.

Mechanisms Leading to Frameshift Mutations

Several mechanisms can lead to frameshift mutations:

• Slippage During DNA Replication: During DNA replication, the DNA polymerase enzyme can "slip" on the template strand, causing it to insert or delete nucleotides. This slippage is more likely to occur in regions with repetitive sequences, such as microsatellites.
• Unequal Crossing Over: During meiosis, homologous chromosomes can exchange genetic material through crossing over. If the alignment of chromosomes is not precise, unequal crossing over can occur, leading to insertions or deletions in the resulting chromosomes.
• Intercalating Agents: Certain chemical compounds, known as intercalating agents, can insert themselves between the nucleotide bases in DNA. This intercalation can distort the DNA structure and increase the likelihood of insertions or deletions during replication. Examples of intercalating agents include ethidium bromide and acridine dyes.
• Transposons: Transposons, also known as "jumping genes," are mobile genetic elements that can insert themselves into different locations in the genome. If a transposon inserts itself into a gene, it can disrupt the reading frame and cause a frameshift mutation.
• Spontaneous Mutations: Frameshift mutations can also occur spontaneously, without any known external cause. These spontaneous mutations are thought to arise from errors in DNA replication or repair.

Identifying Frameshift Mutations

Several methods are used to identify frameshift mutations:

• DNA Sequencing: The most direct way to identify a frameshift mutation is by sequencing the DNA of the gene in question. DNA sequencing will reveal any insertions or deletions that have occurred.
• Polymerase Chain Reaction (PCR): PCR can be used to amplify a specific region of DNA. If a frameshift mutation is present, the PCR product may be a different size than expected.
• Gel Electrophoresis: Gel electrophoresis can be used to separate DNA fragments based on their size. If a frameshift mutation has altered the size of a DNA fragment, it can be detected by gel electrophoresis.
• Protein Analysis: If a frameshift mutation affects the protein product of a gene, protein analysis techniques, such as Western blotting or mass spectrometry, can be used to detect the altered protein.

Repair Mechanisms for Frameshift Mutations

Cells have several DNA repair mechanisms that can correct frameshift mutations:

• Mismatch Repair (MMR): MMR is a major DNA repair pathway that corrects errors that occur during DNA replication, including small insertions and deletions.
• Nucleotide Excision Repair (NER): NER is a versatile DNA repair pathway that can remove a wide range of DNA damage, including bulky adducts and insertions/deletions that distort the DNA helix.
• Homologous Recombination (HR): HR is a DNA repair pathway that uses a homologous DNA template to repair double-strand breaks. HR can also be used to repair insertions or deletions by copying the correct sequence from the homologous template.

However, these repair mechanisms are not always perfect, and frameshift mutations can sometimes escape repair, leading to permanent changes in the genome.

Conclusion

Frameshift mutations, resulting from the insertion or deletion of nucleotides that are not multiples of three, are potent disruptors of the genetic code. They fundamentally alter the amino acid sequence of proteins, often leading to non-functional or truncated products. These mutations are implicated in various genetic diseases and contribute to the broader landscape of genetic variation. Understanding the mechanisms behind frameshift mutations, their consequences, and the methods for their identification is vital in the fields of genetics, molecular biology, and medicine.