Q.With regard to mature mRNA in eukaryotes:

In eukaryotes, the primary RNA transcript, known as heterogeneous nuclear RNA (hnRNA), contains both coding sequences called exons and non-coding sequences called introns.

During RNA processing, a crucial step called splicing occurs. In this process:

• The introns, which are intervening sequences, are removed from the hnRNA.
• The exons, which are the expressed sequences, are then ligated (joined) together in a specific order. …

Life's fundamental processes are orchestrated by proteins, and the blueprint for these proteins resides in our DNA. The journey from DNA to a functional protein is known as gene expression, a meticulously regulated process that ensures the right proteins are made at the right time and in the right amounts. This journey typically follows the central dogma: DNA is transcribed into RNA, and RNA is then translated into protein.

However, the genetic landscape in eukaryotes is more complex than in prokaryotes. Unlike the continuous coding sequences found in many prokaryotic genes, eukaryotic genes are often fragmented. They contain specific regions that code for proteins, interspersed with non-coding regions. These distinct regions are termed:

• Exons: These are the 'expressed sequences' or coding regions. They contain the genetic information that will ultimately be translated into amino acids, forming the protein.
• Introns: These are 'intervening sequences' or non-coding regions. They are present within the gene but do not carry information for protein synthesis.

When a eukaryotic gene is transcribed, the initial RNA molecule produced is called the primary transcript, or heterogeneous nuclear RNA (hnRNA). This hnRNA is a direct copy of the gene, meaning it contains both the exons and the introns.

Before this hnRNA can leave the nucleus and participate in protein synthesis, it must undergo several crucial processing steps, collectively known as RNA processing or post-transcriptional modification. One of the most vital of these steps is splicing. During splicing, the non-coding introns are precisely removed from the hnRNA, and the coding exons are then ligated (joined) together in a specific order. This intricate process ensures that only the meaningful coding information remains.

In addition to splicing, hnRNA also undergoes other modifications: …

Track what happens to the hnRNA molecule across its three processing edits — capping, tailing, and splicing — and ask which one deletes sequence versus which ones only add material at the ends. Capping and tailing add a cap and a poly-A tail; neither removes anything from the middle of the transcript. Splicing is the only edit that discards internal sequence, and it specifically discards introns. Since only i …