Question

Which component of transcribed RNA in eukaryotes is present in the initial transcript but is removed before translation occurs:

Select one:

a. Ribosome binding site

b. codons coding for the protein to be produced

c. Introns

d. 5’ cap

e. 3’ Poly A tail

Answer 1

The gene in eukaryotes is made of several pieces of base sequences coding for amino acids called exonic DNA, separated by stretches of non-coding sequences, commonly called intronic DNA. Thus, the information on the eukaryotic genes for assembling aprotein is not continuous but split. The coding DNA sequences are called exons and the intervening non-coding DNA sequences are called introns. The nascent RNA synthesized by RNA polymerase II is called hnRNA (heterogenous nuclear RNA) or primary transcript. It contains both unwanted base sequences (introns) alternated with useful base sequences (exons). This primary transcript is converted into functional RNA after post-transcriptional processing which involves three steps namely, modification of 5' end by capping, tailing and splicing. Capping at 5' end occurs with the addition of an unusual nucleotide, i.e methyl guanosine triphosphate to the 5' end of hnRNA, catalyzed by enzyme guanyl transferase. Cap is essential for formation of mRNA-ribosome complex. Translation is not possible if cap is lacking because cap is identified by 18SrRNA of ribosome unit. Tailing is the addition of adenylate residues about 200-300 at 3' end in a template independent manner on newly formed hnRNA with the help of Poly A polymerase. It protects the mRNA from enzymatic degradation in the cytoplasm during translation and export it from the nucleus and translated into a protein by ribosomes in the cytoplasm. Splicing is the process of removal of introns and joining of exons in a defined order. Introns are removed by small nuclear RNA (SnRNA) and protein complex called small nuclear ribonucleoproteins or SnRNPs (Snurps). The fully processed hnRNA is now called mRNA and it is transported out of the nucleus for translation.

The ribosome consists of structural RNAs and about 80 different proteins. Ribosomes have two binding sites for binding amino acyl tRNA, P-site (peptidyl site) and A-site (aminoacyl). When the small subunit encounters an mRNA, the process of translation of the mRNA to protein begins. In translation, when a new aminoacyl tRNA complex reaches the A-site, the codon of mRNA interacts with the anticodon of tRNA and forms codon-anticodon bonding.

So, introns is the component of transcribed RNA in eukaryotes which is present in the initial transcript but is removed before translation occurs. Option (c) is the correct answer.