The hypothetical “stimulin” gene contains two exons that encode a protein of 100 amino acids. They are separated by an intron of 100 bp beginning after the codon for amino acid 10. Stimulin messenger RNA (mRNA) has 5′ and 3′ untranslated regions of 70 and 30 nucleotides, respectively. A complementary DNA (cDNA) made from mature stimulin RNA would have which of the following sizes?
a. 500 bp
b. 400 bp
c. 300 bp
d. 100 bp
e. 70 bp
Answers
400 bp-exons are the coding portions of genes and consist of trinucleotide codons that guide the placement of specific amino acids into protein. introns are the noncoding portions of genes that may function in evolution to provide shuffling of exons to produce new proteins. the primary rna transcript contains both exons and introns, but the latter are removed by rna splicing. the 5 (upstream) and 3 (downstream) untranslated rna regions remain in the mature rna and are thought to regulate rna transport or translation. a poly(a) tail is added to the primary transcript after transcription, which facilitates transport and processing from the nucleus. the discovery of introns complicated mendels idea of the gene as the smallest hereditary unit; a modern definition might be the colinear sequence of exons, introns, and adjacent regulatory sequences that accomplish protein expression. using these principles, one can determine the size of the stimulin gene. it contains a coding region of 300 bp (100 amino acids3 bp per amino acid), plus 100 bp in the intron, plus 70 + 30 = 100 bp in the untranslated regions (total = 500 bp). the mature rna contains the same number of bp except for the 100 bp in the intron (500 100 = 400 bp). transcription begins at the start of the 5 untranslated region (70 bp) and the splice site occurs 30 bp (10 x3) into the coding region at the beginning of the intron.
Answer:
b. 400 bp
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