What is Frameshift Mutation?

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Grade Level:

Class 12

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Definition

What is it?

A frameshift mutation is a type of gene mutation where the addition (insertion) or removal (deletion) of nucleotides changes the 'reading frame' of the genetic code. This shift causes all the codons downstream of the mutation to be read incorrectly, leading to a completely different protein or a non-functional one.

Simple Example

Quick Example

Imagine a message written in groups of three letters, like a secret code for your friends: 'THE BIG RED CAT ATE THE RAT'. If you accidentally remove one letter, say the 'B' from 'BIG', the message shifts: 'THE IGR EDC ATA TET HER AT'. Now, the entire message after the 'B' is messed up and makes no sense, just like a frameshift mutation messes up the genetic code.

Worked Example

Step-by-Step

Let's use a simple DNA sequence and see how a frameshift mutation changes it.

Step 1: Original DNA sequence (read in groups of three, called codons): AGC TAG CTA GCT AGA
---Step 2: The corresponding mRNA sequence (replace T with U): UCG AUC GAU CGA UCU
---Step 3: The amino acid sequence (protein) formed: Ser-Ile-Asp-Arg-Ser (Each three letters codes for one amino acid)
---Step 4: Now, let's add one extra nucleotide, 'G', after the first 'AGC'. The new DNA sequence becomes: AGC GTA GCT AGC TAG A (We inserted 'G' after the first 'C')
---Step 5: The new mRNA sequence: UCG CAU CGA UCG AUC U
---Step 6: The new amino acid sequence: Ser-His-Arg-Ser-Ile (Notice how all amino acids after the insertion have changed completely!)
---Answer: The insertion of a single nucleotide 'G' caused a frameshift, completely altering the protein produced from that point onwards.

Why It Matters

Understanding frameshift mutations is crucial in medicine for diagnosing and treating genetic diseases like cystic fibrosis or certain cancers. In biotechnology, scientists use this knowledge to engineer organisms or develop gene therapies. Future doctors and biotech engineers will use this to find cures and improve human health.

Common Mistakes

MISTAKE: Thinking that a frameshift mutation only affects the single nucleotide inserted or deleted. | CORRECTION: A frameshift mutation changes the entire 'reading frame' from the point of mutation onwards, affecting all subsequent codons and amino acids.

MISTAKE: Confusing frameshift mutations with point mutations (like substitutions). | CORRECTION: A frameshift mutation involves the addition or deletion of nucleotides, causing a shift in the reading frame. A point mutation is a change in only one nucleotide, which might or might not change the amino acid, but doesn't shift the frame.

MISTAKE: Believing that inserting or deleting three nucleotides always causes a frameshift. | CORRECTION: If exactly three nucleotides (or a multiple of three) are inserted or deleted, the reading frame might be restored or remain intact, though specific amino acids might be added or removed. This is not a frameshift mutation because the reading frame itself doesn't shift.

Practice Questions

Try It Yourself

QUESTION: What type of mutation occurs if a single nucleotide is removed from a DNA sequence? | ANSWER: Frameshift mutation (specifically, a deletion frameshift)

QUESTION: A gene has the sequence 'AUG CCA UGG GUA'. If 'A' is inserted after the first 'G' in 'AUG', what will be the new mRNA sequence from that point onwards? | ANSWER: AUG AGC CAU GGG UAA

QUESTION: If a DNA sequence 'TAC GAT CCA' codes for amino acids, and two nucleotides 'AG' are inserted after 'GAT', describe the impact on the resulting protein. | ANSWER: The insertion of 'AG' (two nucleotides) will cause a frameshift mutation. This will change the reading frame from 'CCA' onwards, leading to completely different amino acids being produced after the 'GAT' sequence, likely resulting in a non-functional protein.

MCQ

Quick Quiz

Which of the following is most likely to cause a frameshift mutation?

Substitution of one nucleotide for another

Deletion of three consecutive nucleotides

Insertion of a single nucleotide

Duplication of an entire gene

The Correct Answer Is:

A frameshift mutation occurs when the addition or deletion of nucleotides changes the reading frame. Inserting a single nucleotide (Option C) will shift the frame. Deleting three nucleotides (Option B) would remove an entire codon, but the frame would likely remain intact. Substitution (Option A) only changes one nucleotide without shifting the frame. Duplication (Option D) adds an entire section, not shifting the existing frame.

Real World Connection

In the Real World

In India, understanding frameshift mutations is vital for genetic counselling centres, especially for families with a history of genetic disorders like thalassemia or sickle cell anemia. Medical researchers at institutions like AIIMS or CCMB use this knowledge to develop diagnostic tests and potential gene therapies, aiming to improve the lives of many patients.

Key Vocabulary

Key Terms

NUCLEOTIDE: The basic building block of DNA and RNA | CODON: A sequence of three nucleotides that together form a unit of genetic code in a DNA or RNA molecule | READING FRAME: The way a sequence of nucleotides is divided into codons for translation | INSERTION: The addition of one or more nucleotide base pairs into a DNA sequence | DELETION: The removal of one or more nucleotide base pairs from a DNA sequence

What's Next

What to Learn Next

Now that you understand frameshift mutations, explore 'Point Mutations' next. This will help you compare different types of genetic changes and see how even small alterations can have big impacts on our bodies and health.