Question

Illustration below is a DNA segment, which constitutes a gene:

i. Name the shaded and unshaded regions of gene.

ii. Explain how these genes are expressed.

iii. How is this gene different from prokaryotic gene in its expression? [HOTS]

Answer 1

(i) The shaded portions are introns and unshaded portions are exons. 

(ii) The primary RNA contains both introns and exons. By the mechanism of splicing, introns are removed and exons are joined to form functional mRNA after capping and tailing. 

Post-transcriptional modifications 

▪ The primary transcripts are non-functional, containing both the coding region, exon, and non-coding region, intron, in RNA and are called heterogenous RNA or hnRNA. 

▪ The hnRNA undergoes two additional processes called capping and tailing. 

▪ In capping, an unusual nucleotide, methyl guanosine triphosphate, is added to the 5′- end of hnRNA. 

▪ In tailing, adenylate residues (about 200–300) are added at 3′-end in a template independent manner. 

▪ Now the hnRNA undergoes a process where the introns are removed and exons are joined to form mRNA by the process called splicing.

(iii) In prokaryotes, the structural gene is continuous and is not differentiated into exons and introns unlike eukaryotes. In prokaryotes, transcription is followed by translation without RNA splicing mechanism.