The diagram below contains a pre-mRNA and its corresponding mRNA of a eukaryotic
ID: 95483 • Letter: T
Question
The diagram below contains a pre-mRNA and its corresponding mRNA of a eukaryotic gene. The gene is made up of 3 exons and 2 introns. a. Which segment (1-8) represents the 5' cap? b. Which segment represents the 5' UTR? c. Which segment represents the 3' UTR? d. Which segments represent introns? e. Which segments represent exons? f. In which segment would a single nucleotide insertion be most detrimental to the function of the encoded protein? g. In which segment does the start codon occur? h. Why do you not expect to find a TATA box in any segment?Explanation / Answer
The processing of mRNA starts with the addition of a 5'-cap and a 3'-poly(rA) tail at the 5' end and 3' end of the pre-mRNA respectively. The protein coding region contains coding sequences called exons. Splicing is the editing of the pre-mRNA which removes introns and the exons (non-coding sequences) are ligated to form the mature mRNA.
a) Segment 1 represents the 5' cap.
b) Segment 2 represents 5' UTR.
5' UTR or leader sequence is the region of an mRNA that is present directly upstream from the start codon.
c) Segment 8 represents 3' UTR.
3' UTR is the region of an mRNA that is present downstream from the stop codon. It follows the translation termination codon.
d) Segment 3, 5 and 7 represent exons.
e) Segment 4 and 6 represent introns.
f) A single nucleotide insertion in segments 3, 5 and 7 i.e in exons would be most detrimental to the function of the encoded protein as exons are the protein encoding regions. Any change in the composition of exons may result in the formation of a different protein and hence, can alter the functioning of encoded protein.
g) Start codon occurs at the 5' end of every exon present in the gene i.e segments 3,5 and 7. The start codon is the first codon of mRNA that is translated by a ribosome.
h) TATA box is present in the promoter of a eukaryotic gene and nearly one-half of all known promoters are TATA-less. The core promoter of this gene may be Inr (initiator element). This has been reported that promoters with a functional Inr are more likely to lack a TATA box or to possess a degenerate TATA sequence. Therefore, TATA box is not found in any segment.