You have discovered a gene (Figure 7-28A) that is alternatively spliced to produce several forms of mRNA in various cell types, three of which are shown in Figure 7-28B.The lines connecting the exons that are included in the mRNA indicate the splicing.From your experiments, you know that protein translation begins in exon 1.For all forms of the mRNA, the encoded protein sequence is the same in the regions of the mRNA that correspond to exons 1 and 10.Exons 2 and 3 are alternative exons used in different mRNA, as are exons 7 and 8.Which of the following statements about exons 2 and 3 is the most accurate? (A)

(B)

Figure 7-28

Question

You have discovered a gene (Figure 7-28A) that is alternatively spliced to produce several forms of mRNA in various cell types, three of which are shown in Figure 7-28B.The lines connecting the exons that are included in the mRNA indicate the splicing.From your experiments, you know that protein translation begins in exon 1.For all forms of the mRNA, the encoded protein sequence is the same in the regions of the mRNA that correspond to exons 1 and 10.Exons 2 and 3 are alternative exons used in different mRNA, as are exons 7 and 8.Which of the following statements about exons 2 and 3 is the most accurate? (A)

(B)

Figure 7-28

(B)

Figure 7-28

Quizplus · Accepted Answer

Since the protein sequence is the same in the regions corresponding to exons 1 and 10 for all forms of the mRNA, it means exons 2 and 3 must have the same reading frame as each other (otherwise the protein sequence encoded would be different). Codons are read in groups of three nucleotides, so the number of nucleotides in exons 2 and 3 must be divisible by 3. Therefore, the correct statement is that exons 2 and 3 must contain a number of nucleotides that when divided by 3, leaves the same remainder (that is, 0, 1, or 2), making choice C the most accurate.

You Have Discovered a Gene (Figure 7-28A) That Is Alternatively