Q.Describe the molecular mechanism of RNA modification.

Tailing is the post-transcriptional addition of a poly-A tail to the 3' end of a pre-mRNA, added independently of the DNA template.

Among the post-transcriptional processing steps of eukaryotic hnRNA (heterogeneous nuclear RNA, the primary transcript), 'tailing' refers specifically to the addition of about 200–300 adenylate (adenine nucleotide) residues at its 3' end, forming a poly-A tail. Unlike normal transcription, this addition occurs independently of the DNA template. Splicing (a) is the removal of introns and joining of exons, capping (c) is the addition of a …

Splicing edits out the non-coding intron sequences of a pre-mRNA and stitches the coding exons together to make mature mRNA.

In eukaryotes, genes are typically 'split genes', composed of alternating coding sequences (exons) and non-coding sequences (introns). The primary transcript (hnRNA) produced by transcription contains both exons and introns. Splicing is the process, occurring within the nucleus, by which the intervening intron sequences are precisely removed and the exon sequences are joined together, producing the final, mature mRNA m …

The removal of introns and the joining of exons in a defined order, during processing of the RNA transcript, is called splicing.

Working

In eukaryotes, the RNA polymerase first produces a primary transcript (pre-mRNA or hnRNA) that contains both coding regions (exons) and intervening non-coding regions (introns), copied directly from the gene. Before this transcript can serve as a template for translation, it must be processed: the intron sequences are precisely excised, and the remaining exon sequences are joined together end to end, in their original, defined order, to produce the shorter, mature mRNA. This excision-and-joining process is called splicing, and it is carried out inside the nucleus by a large ribonucleoprotein machine called the spliceosome, which recognises specific sequence signals at each intron-exon boundary. Splicing is essential because if the i …

The removal of introns and joining of exons in a defined order during mRNA processing is called splicing, carried out by the spliceosome.

Working

The primary transcript produced by RNA polymerase II (hnRNA/pre-mRNA) in eukaryotes contains alternating coding sequences (exons) and non-coding intervening sequences (introns). Before the transcript can be translated, it must be processed: the introns are precisely excised, and the exons are spliced (joined) together in the correct, defined order to produce the mature mRNA. This process, called splicing, is carried out by a large ribonucleoprotein complex called the spliceosome, which recognises specific sequences at intron-exon boundaries. It is essential because untranslated intron sequences …

Eukaryotic genes are typically "split," containing alternating coding and non-coding stretches; only the coding stretches — exons — are retained in the mature mRNA after splicing and are ultimately translated into protein.

Reasoning

Many eukaryotic genes are interrupted, containing both coding sequences (exons, which are Expressed and retained in the mature mRNA) and intervening, non-coding sequences (introns, which are Intervening and are removed/spliced out before translation). After the primary RNA transcript (hnRNA) undergoes RNA splicing, the introns are excised and the remaining exons are joined together t …

In a split (eukaryotic) gene, the coding/expressed sequences that appear in mature mRNA are called exons; the non-coding sequences that are removed are called introns.

Most eukaryotic genes are 'split genes' — their coding sequence is interrupted by non-coding stretches.

• Exons are the sequences that appear/are expressed in the mature, processed mRNA. They contain the actual coding information used for translation.
• Introns (intervening sequences) are the non-coding stretches that interrupt the coding sequence; they are transcribed into the primary (hnRNA) transcript but are removed during RNA splicing before the mRNA leaves the nucleus.
• Splicing is the process by which introns are cut out and exons are joined together to form the mature, translatable mRNA. …