Question

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

Answer 1

Function of each component of the unit:

1. Promoter: It is a specific DNA sequence located at the start of the transcription unit. It provides the binding site for RNA polymerase. It helps in deciding where transcription starts and which strand will act as the template strand.
2. Transcription start site: It is the point from where RNA synthesis actually begins. RNA polymerase starts adding ribonucleotides from this site.
3. Structural gene: This is the DNA region that contains the information to be copied into RNA. It is transcribed to produce mRNA, tRNA or rRNA, depending on the gene.
4. Template strand: This DNA strand is read by RNA polymerase in the 3′ → 5′ direction. RNA is synthesised complementary to this strand in the 5′ → 3′ direction.
5. Coding strand: This strand does not take part directly in transcription. Its sequence is the same as the RNA transcript, except that thymine (T) is present instead of uracil (U).
6. Terminator: It is the DNA sequence present at the end of the transcription unit. It gives the signal to stop transcription. After reaching the terminator, RNA polymerase detaches and the newly formed RNA is released.