Question

Study the diagram given below and answer the questions that follow.

1. Identify the parts marked ‘E’ and ‘F’.
2. Explain the post-transcriptional modifications that occur in part ‘F’.

Answer 1

(i) 

• E = Promoter: Area where RNA polymerase attaches to start transcription.
• F = hnRNA (Heterogeneous nuclear RNA)/Primary transcript: The freshly synthesised RNA strand prior to processing.

(ii) 

To produce functional mRNA in eukaryotes, hnRNA passes through three main post-transcriptional changes:

1. Splicing: The process of connecting coding sequences (exons) and removing non-coding sequences (introns). Spliceosomes are the catalyst.
2. 5' Capping: Adding methyl guanosine triphosphate (7-methyl guanosine), an uncommon nucleotide, to the 5' end. helps identify ribosomes and protects mRNA from degradation.
3. 3' Poly-A Tailing: The addition of a stretch of adenylate (poly-A) residues at the 3' end is known as 3' Poly-A Tailing. Increases the stability of mRNA and helps in its export from the nucleus.