Molecular basis of Inheritance, CBSE ClassXll

(1)Define cistron .

        Cistron is a segment of DNA coding for a polypeptide .

       (It is a segment of DNA containing the genetic information for the sequence of amino acids of a  polypeptide chain i.e. the blueprint of a protein is actually stored in DNA as a cistron).

(2)Define intron and exon .

          In Eukaryotes, the structural gene is monocistronic and coding sequences are interrupted by intervaining sequences.

     Coding sequences are expressed as exons and intervaining sequences are expressed as introns in pre -mRNA/ hn RNA during transcription process .Introns are excised by spliceosome at  processing time and only exons are appeared  and joined to make a mature/processed RNA.

 The monocistronic structural genes are also known as split gene.

(3) Define transcription .

       The process of copying/transfer of genetic information from template strand of DNA into RNA  maintaining "the principle of complimentarity" is known as transcription .

(4) Why both strands of DNA are not copied during transcription ?

   There are two reasons :

(i) If two strands of a structural genes act as template then the two transcribed RNA will be of different sequences , as a result of that transcribed proteins will differ from each other . Hence , a gene will be coding two different genes ,which contradicts the definition of a gene.

(ii) The transcribed RNA molecules will be complementary to each other , hence, they will form ds RNA and translation pracess will prevented .