What is frameshift mutation?
A frameshift mutation occurs when nucleotides are inserted into or deleted from the DNA and cause a "shift" in the reading of mRNA codons.
Frameshift mutations are insertions or deletions of nucleotides in DNA that change the reading frame (the grouping of codons).
Recall that a codon is a group of 3 nucleotides that corresponds with a specific amino acid.
We can demonstrate a frameshift mutation using this example:
Say we have a letter sequence of
THE FAT CAT SAT
If one letter (similar to one nucleotide), like an extra E, is inserted into this sequence, we would get
Just by inserting one extra E, the entire reading of the sentence changed. Note how it caused a shift in the pattern, and affected every single letter after it. Now the words are just gibberish.
Similarly, if this was the code for a protein, the ribosomes would have a difficult time translating this sequence correctly.
What if the F was deleted from the sequence?
THE ATC ATS AT
Like the insertion, a deletion can also cause a shift in the pattern and affect all subsequent letters.
It is important to note that frameshifts only occur if the insertion or deletion of nucleotides is not a multiple of 3. Since codons are read in groups of 3, an insertion of 3 or 6 would not cause a shift in the positions of all subsequent nucleotides.
If a frameshift mutation occurs, then the mRNA codons will code for different amino acids. Since amino acids make up proteins, the resulting protein will not function properly (or not function at all).
A mutation that occurs early in the sequence could have more adverse effects, since it affects more nucleotides down the line.
It is also possible that the mutated code contains an early stop codon (UAA, UAG, or UGA) and stops translation early, resulting in an abnormally short polypeptide.