The answer solutions says D is incorrect because the table shows exons are still in the transcript but option D says "introns remain in the final transcript"? I thought all introns had to be spliced out as a step in post transcriptional modification?

Hello,

Answer choice A is true, alternative splicing increases the number of possible proteins from a set number of genes and that is described in the passage. Answer choice B is false, and therefore the correct answer, Drosophila melanogaster is one of many species from the same lineage however the ancient form (nor any outgroup) is not shown on the tree. Answer choice C is true, the passage implies that isoforms and mRNAs are deposited to the egg and it is these transcripts that are most heavily spliced before the onset of zygotic transcription. Answer choice D is also true, as is shown in Table 1 where retained exons can be found in final transcripts.

Introns are crucial because the protein repertoire or variety is greatly enhanced by alternative splicing in which introns take partly important roles. Alternative splicing is a controlled molecular mechanism producing multiple variant proteins from a single gene in a eukaryotic cell. Each different species will remove different introns to end up with different proteins!

Hope this helps