Question

Draw and explain the processing process of eukaryotic cell hnRNA.

Answer 1

HnRNA: The Eukaryotic mRNAs are monocistronic. The primary transcript is much larger than the mature mRNA and is called Heterogeneous nuclear RNA (hnRNA) which contains several sequences that do not exist in the mature mRNA.The hnRNA undergoes processing and produces the mature mRNA therefore, therefore it is also called as “mRNA precursor” or “pre-mRNA”.

HnRNA processing involves the following changes:

1. Packaging of hnRNA with protein: The hnRNAs are packed with protein to form a ribonucleoprotein complex (RNA and protein).

2. Polyadenylation or Poly (A) tail formation: Poly A tail is added at the 3'-end of the hnRNA synthesized from DNA via Transcription with the help of poly (A) polymerase. Poly (A) polymerase adds around 200 Adenine to the 3'-end. The poly-A tail helps in the transport of the hnRNA from the nucleus to the cytoplasm for translation or other functions. It also helps to prevent the intracellular digestion of the hnRNA while transporting from the nucleus to cytoplasm by RNases.

3. RNA splicing: The whole hnRNA does not have the coding sequence. It involves exons and introns. The exons are the coding region whereas introns are non-coding regions. So the introns are removed and the exons are combined in a specific manner. During splicing the length of the primary transcript is too much reduced because of removal of introns. Some ribonucleotides play a key role in slicing that is known as snurps.

4. 5′ Capping: The 5'-end is capped by methylation. The cap has methylated sugars at 2' position which is connected by a triphosphate bridge to 7-methylguanosine (m⁷G). The cap prevents the degradation of hnRNA at 5'-end.