What are introns and exons?
Introns and exons are nucleotide sequences that carry genetic information within a gene.
Introns are non-coding sequences and are removed by RNA splicing. Therefore introns are not expressed in the final mRNA product.
If introns are not removed then final mRNA product contain extra sequence that leads to production of wrong protein during translation.
Introns are not considered to be useless as they have evolutionary significance that produces different kinds of protein products with the help of alternative splicing.
Exons are coding sequences that are joined with each other by covalent bonding and produces mature mRNA.
Transcription of mRNAs starts and ends with an exon and introns are present between the exons. This is shown as E-I-E-I-E-I-E (where, E is exon and I is intron).
When mRNA is transcribed, it is called as pre-mRNA and contains extra information called as introns. Introns are removed and will not present in mature mRNA. Only exons will remain in the mature mRNA.
The exon sequences may translated into final protein. It also contains 5′ and 3′ untranslated sequences (UTRs). These are present before and after the open reading frame. But this 5′ and 3′ UTR are not translated but maintains the stability of the mRNA. Therefore, exons contain translated and untranslated sequences.