What is the likely consequence of a mutation that alters the branch point within an intron?

Question

A.  no effect, since introns are not expressed
B.  failure to form a lariat
C.  failure of snRNPs to recognize the 5' end of intron
D.  no exon shuffling
E.  failure of snRNAs to combine with protein and form snRNPs

Clarify Question
· What is the key concept addressed by the question?
· What type of thinking is required?
· What key words does the question contain and what do they mean?

Gather Content
· What do you already know about splicing?

Consider Possibilities
· Consider the different answer options. Which can you rule out?

Choose Answer
· Given what you now know, what information and/or problem solving approach is most likely to produce the correct answer?

Reflect on Process
 · Did your problem-solving process lead you to the correct answer? If not, where did the process break down or lead you astray? How can you revise your approach to produce a more desirable result?

Answer 1

B.  failure to form a lariat

Clarify Question
· What is the key concept addressed by the question?
        o This question addresses splicing.
· What type of thinking is required?
        o This question is asking you to analyze the information given, using logic, to dissect the problem and determine the answer.
· What key words does the question contain and what do they mean?
        o An intron is a portion of the sequence of a pre-mRNA that is removed by splicing.
        o The branch point is a particular adenosine nucleotide that lies within the intron and is recognized by snRNPs during the splicing process.

Gather Content
· What do you already know about splicing?
        o Splicing is the process that removes introns from pre-mRNA.
        o Splicing is carried out by the spliceosome, a complex formed of snRNPs.
        o The snRNPs bind to specific sites on the 5’ end of the intron and to the branch point near the 3’ end. The snRNPs come together, forming the intron into a lariat, which is cut as the exons are joined together.

Consider Possibilities
· Consider the different answer options. Which can you rule out?
        o Is there no effect, since introns are not expressed? Well, the effect is likely to be that the intron will become expressed – this will have a big effect on the resulting protein.
        o Will it affect exon shuffling? Not likely, since exon shuffling is an evolutionary process that is not dependent on the branch point.
        o Will snRNAs fail to combine with their proteins? No, because that happens independently of any particular intron.

Choose Answer · Given what you now know, what information and/or problem solving approach is most likely to produce the correct answer?
        o The snRNPs bind at the 5’ end and at the branch point near the 3’ end. Then by coming together, the two sets of snRNPs create the lariat which is cleaved as the exons are ligated.
        o If the snRNPs couldn’t recognize the branch point, they could still recognize and bind the 5’ end.
        o However, the lariat would fail to form.

Reflect on Process · Did your problem-solving process lead you to the correct answer? If not, where did the process break down or lead you astray? How can you revise your approach to produce a more desirable result?
        o This question asked what would result if snRNPs couldn’t recognize the branch point of an intron.
        o The question required you to analyze the information given, using logic, to dissect the problem and determine the answer.
        o Did you recognize that the branch point is near the 3’ end?
        o Did you recognize that binding the branch point is critical for lariat formation?