question_answer

(i) Which property of DNA double helix led Watson and Crick to hypothesise semiconservative mode of DNA replication? Explain.

(ii) Draw the diagram of its double helical model.

Or

Draw the labelled schematic structure of a transcription unit. Explain the function of each component of this unit in the process of transcription.

Answer:

(i) Watson and Crick observed that the nitrogenous bases form complementary pairs between the two polynucleotide chains of DNA. Based on the X-ray diffraction data, they proposed that DNA consists of a double helix with two chains having sugar phosphate on the outside and nitrogen bases on the inner side. Further, they proposed that two chains are antiparallel with 5'-3' orientation of the other. The two chains are twisted helically.

This property of double helix model of DNA led them to hypothesise semiconservative mode of DNA replication where the two strands separate and act as a template for the synthesis of new complementary strand.

(ii) The diagram of double helical model of DNA is given below

Or

Schematic structure of a transcription unit It is as follows

Functions The functions of different components of transcription unit are as follows

(i) Promoter (DNA sequence) It provides binding site for the RNA polymerase.

(ii) Structural genes These codes for enzymes/proteins and transcribe the mRNA for the same.

(iii) Terminator (Sequence of bases) It defines the end of transcription process.

(iv) DNA strand with 3' - 5' polarity It acts as the template for transcription of mRNA.

(v) DNA strand with 5' - 3' polarity This is coding strand and does not code for RNA.