Final answer:
The length of the RNA molecules detected on the northern blot would be 1500 base pairs, corresponding to the combined length of the five exons after intron removal during RNA processing.
Step-by-step explanation:
The question refers to the determination of the length of RNA molecules detectable by northern blot in a 6.0-kb DNA fragment with introns and exons. In eukaryotes, genes contain introns (noncoding segments) and exons (coding segments). During RNA processing, introns are removed and exons are spliced together to form a continuous mRNA transcript. The mRNA is then modified with the addition of a 5' cap and a poly-A tail.
Given that 5000 base pairs are transcribed and contain four introns totaling 3500 base pairs and five exons totaling 1500 base pairs, the resulting mRNA will consist solely of the exon sequences. Therefore, removal of the four introns (3500 base pairs) from the total transcribed region (5000 base pairs) would yield an mRNA of 1500 base pairs in length, corresponding to the aggregate length of the five exons. This is the size of the RNA molecule that would be detected on a northern blot.