Question

A complete plant gene containing four introns and five exons is carried on a 6.0-kb DNA fragment. DNA sequencing analysis finds that this fragment contains 1000 base pairs that flank the transcribed region of the gene and 5000 base pairs that are transcribed. Four introns contain 3500 base pairs, and five exons contain 1500 base pairs. Northern blot analysis is performed on RNA of this gene using a probe that binds to a portion of one of the exons. RNA isolated from the cytoplasm of cells is compared to RNA isolated from cell nuclei on the northern blot. Do you expect that all the RNAs will be a uniform length, or will RNA molecules of multiple lengths be detected on the northern blot?

Answer 1

Multiple RNA lengths will be detected on the northern blot; pre-mRNA with introns and exons in the nucleus, and shorter, mature mRNA with only exons in the cytoplasm.

Step-by-step explanation:

RNA molecules of multiple lengths will be detected on the northern blot. In eukaryotic cells, introns, which are non-coding sequences, are removed from the pre-mRNA during processing to form a mature mRNA with only exons, the coding sequences. This splicing process occurs within the nucleus where the introns are excised and the exons are joined together. As the DNA fragment in question has introns and exons, the RNA found in the nucleus will include both, resulting in a longer pre-mRNA. However, RNA isolated from the cytoplasm should represent the mature mRNA, with only exons present, and be shorter. Consequently, the RNA from the cytoplasm will appear as uniform-sized bands on a northern blot, assuming the splicing is consistent, while the RNA from the nucleus will include bands representing both pre-mRNA and possibly some mature mRNA.

The process of RNA splicing ensures that non-functional intron sequences are removed accurately before protein synthesis occurs. If any error is made during this process, such as a single nucleotide not being correctly excised, it could alter the exons' reading frame, leading to nonfunctional proteins. Therefore, the mature mRNA, after successful splicing, should be shorter, exclusively containing the exons, and this is what we expect to see in cytoplasmic RNA, as opposed to the nuclear RNA, which is likely to show a heterogeneous mix of spliced and unspliced RNA.