The is the form of mRNA right after it has been transcribed from DNA and before it is translated into a protein. Segments of the mRNA, called introns are removed leaving exons, the portion of the mRNA that gets translated. To push the analogy a bit further, the pre-mRNA could be thought of as a 40,000 word essay cut down to 35,000 words. But the number of word/bases can very tremendously depending on the organism in question.
From the link:
"The amount and size of introns varies greatly. The mammalian DHFR has 6 exons that total about 2000 bases, yet the gene is 31,000 bases. Likewise, the alpha-collagen has 50 exons that range from 45-249 bases and the gene is about 40,000 bases. Clearly two genes of the same size can have different number of introns, and introns that vary in size."
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