Frameshift mutation from EK 1001 Bio

[engineeredout]

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Question 165: "Which of the following DNA lesions will NOT result in a frameshift mutation?"

Answers:

A. 1 Missing base pair
B. 2 Altered base pairs
C. 3 deleted base pairs
D. 2 Inserted base pairs

Answer: "B is correct. MCAT writers love asking questions about frameshift mutations. A frameshift mutation occurs when there is an addition or deletion of 1 or 2 base pairs (a 3 base pair addition or deletion will cause an in-frame mutation, not a frameshift mutation, because each codon is coded by 3 nucleotides), which causes the ribosome to read all downstream codons in the wrong frame. Frameshift mutations usually result in truncated, non-functional proteins.

Altered base pairs (known as point mutations) do not cause the ribosome to read out of frame. Point mutations may cause silent, missense, or nonsense mutations. Answer choices A and C can be eliminated because both are examples of base pair deletion."




Now from that explanation, I see why B would be a correct answer because it is not actually adding or deleting anything. But don't they also state in the explanation that a 3 base pair deletion causes an in-frame mutation, NOT a frameshift mutation? It seems to be that C would also be a correct answer. Am I missing something here?

 

[rocuronium]

I agree with you. Deleting 3 base pairs should not cause a frameshift mutation.

 

[JA Prufrock]

I believe a 3 base pair deletion could still cause a frameshift mutation, depending on which 3 base pairs are deleted. If the deletion preserves the frame (that is, deletes one codon), then there is no frameshift mutation. If the 3 base pairs are deleted, but occur across two future codons then there is still a frameshift mutation. I hope that makes sense.

 

[engineeredout]

I believe a 3 base pair deletion could still cause a frameshift mutation, depending on which 3 base pairs are deleted. If the deletion preserves the frame (that is, deletes one codon), then there is no frameshift mutation. If the 3 base pairs are deleted, but occur across two future codons then there is still a frameshift mutation. I hope that makes sense.

Thats a good point and I see what you're saying. If three random base pairs are deleted at three random points then it would cause a frameshift? Still though, everywhere I've looked for frameshift mutations, whether EK or wiki, they make no mention of the fact that the three base pairs had to be removed next to each other for it to be nonframeshift.

 

[JA Prufrock]

Thats a good point and I see what you're saying. If three random base pairs are deleted at three random points then it would cause a frameshift? Still though, everywhere I've looked for frameshift mutations, whether EK or wiki, they make no mention of the fact that the three base pairs had to be removed next to each other for it to be nonframeshift.

Yeah, now that I think about it more, I'm not sure whether it matters if the base pairs need to be deleted next to each other or not. The frame seems to be preserved regardless. Hopefully, someone else can shed some light on this.

 

[cwfergus]

A framshift mutation is when one or more bases are deleted or inserted.

JUST from taht you can clear away Altered.
use process of elimination, when they talk about frameshift, they want insertions or deletions not alterations (such as a G being changed to an A)

 

[Kaustikos]

Yeah, now that I think about it more, I'm not sure whether it matters if the base pairs need to be deleted next to each other or not. The frame seems to be preserved regardless. Hopefully, someone else can shed some light on this.

From what it seems, they were asking you which would NEVER causea frameshift. Deletions/Insertions can cause frameshifts, even if they are 3 basepairs. Alterations conserve the number of basepairs by switching out for other basepairs. Thus, B is correct.

 

[NigerianBull87]

Off the top of my head, I remember a frameshift mutation being deleted or inserted base pairs in intervals other than three. The fact that B clearly states that the action is an altered base pair not in intervals of three gives the "most correct" answer. If 3 base pairs were deleted, then it would cause a frameshift mutation depending upon when the mutations occurred. In the other choices, time could be a huge factor because no matter when any of the other answer choices occurred a frame mutation could still result. That is just the logic I have gathered from it.

 

[rockybalboa08]

In their answer, I think they were asking for something that would cause the ribosome to read out of frame (but asked for a frameshift mutation in the question). C would be incorrect because the deletion of 3 base pairs will disrupt the reading frame between the first and the last mutation.

 

[engineeredout]

Bah. Still a crappy question IMO