The primary transcript (hnRNA) of a eukaryotic gene is made of, in order: exon 1 of 210…
Contributed by StemPy LabTry it on paper first. A hint, the key idea and the worked solution are here when you need them.
The primary transcript (hnRNA) of a eukaryotic gene is made of, in order: exon 1 of nucleotides, intron 1 of , exon 2 of , intron 2 of and exon 3 of . The first nucleotides of exon 1 are the untranslated region, and the last nucleotides of exon 3 are the untranslated region. All the rest of the exons, from the start codon to the stop codon, is read in codons.
- How many nucleotides does the hnRNA have?
- How many nucleotides are left after splicing, before the tail is added?
- What percentage of the hnRNA is removed by splicing?
- How many amino acids does the polypeptide have?
- In tailing, adenylate residues are added. How many nucleotides does the mature mRNA then have, not counting the cap?
The worked solution
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Work it out on paper first. It is fine to get stuck or make a mistake: that is how we learn. When you are ready, go through the solution one step at a time, and check each step against your own.
(a) hnRNA. The primary transcript has both exons and introns: nucleotides.
(b) After splicing. Only the exons remain: nucleotides.
(c) Removed. The introns, nucleotides, are of the hnRNA.
(d) Amino acids. The coding part is nucleotides, or codons. The last is the stop codon, so the polypeptide has amino acids.
(e) Mature mRNA. Tailing adds the adenylate residues at the -end without a template: nucleotides. Capping adds methyl guanosine triphosphate at the -end; it is not counted here.
Answer: (a) nucleotides. (b) nucleotides. (c) . (d) amino acids. (e) nucleotides.
Check. The coding part, nucleotides, divides by exactly, as a reading frame must. Adding back the untranslated regions and the introns gives , the length of the hnRNA.
Watch out. Untranslated regions are part of the exons: they stay in the mature mRNA, but they code for no amino acids. Introns never appear in the mature mRNA.
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