Question

Most eukaryotic mRNAs are shorter than the genes that encode them. The reason for this is _______?

1) mRNA molecules undergo post-transcriptional modifications that remove non-coding regions.
2) mRNA molecules are synthesized from only a portion of the gene sequence.
3) mRNA molecules are degraded more rapidly than the genes that encode them.
4) mRNA molecules are transcribed from introns within the gene sequence.

Answer 1

Eukaryotic pre-mRNA is processed to remove non-coding introns through splicing, shortening the final mRNA. This modified mRNA is stable and ready for translation, resulting in it being shorter than the initial gene due to the removal of introns.

Step-by-step explanation:

Eukaryotic pre-mRNA undergoes extensive processing before it can be translated into protein. In eukaryotic cells, genes consist of exons (protein-coding sequences) and introns (non-coding sequences). The process of converting pre-mRNA into mature mRNA involves splicing, which precisely removes the introns. This splicing is performed by a complex called the spliceosome. The introns are degraded, leaving only the exons, which are then reconnected to form the final mRNA that can be translated. This processed mRNA is shorter than the gene it's transcribed from because the introns have been removed. The additional steps in mRNA processing not only shorten the molecule but also increase its stability, resulting in a much longer half-life compared to prokaryotic mRNA.

Through alternative splicing, multiple mRNA molecules with different arrangements of exons can be produced from a single gene, allowing for the synthesis of various protein isoforms. This vastly increases the diversity of proteins a single gene can encode for. The final mature mRNA, with a 5' cap and a poly-A tail, is transported out of the nucleus and is ready for the process of translation in the cytoplasm.