Q.At what level does the control of gene expression occur?

To understand why splicing points to the RNA world, we need to step back and think about what splicing actually is. In eukaryotic cells, genes are split into coding sequences (exons) and non-coding sequences (introns). After transcription, the initial RNA transcript — called pre-mRNA — contains both exons and introns. Splicing is the process that removes the introns and joins the exons together to form a mature mRNA that can be translated into protein.

Now here is the remarkable part. For many years, biologists assumed that all biological reactions were catalysed by proteins (enzymes). But in the 1980s, Thomas Cech and Sidney Altman independently discovered that certain RNA molecules can act as catalysts. These are called ribozymes — RNA enzymes. The splicing of some introns, particularly self-splicing introns (like Group I introns), is carried out entirely by the RNA itself, without any protein help. The intron folds into a specific three-dimensional shape that brings the ends together and catalyses the cutting and rejoining reactions.

This dual ability — information storage and catalysis — is the core of the RNA world hypothesis. The idea is that before DNA and proteins evolved, early life may have used RNA as the primary molecule for both heredity and metabolism. DNA is chemically more stable and better for long-term storage, while proteins are more versatile catalysts. But RNA could have been the transitional molecule that made the leap from simple chemistry to the first living systems. …

Concept and Intuition

In eukaryotes, genes are split into coding sequences (exons) and non-coding intervening sequences (introns). When a gene is transcribed, the entire stretch — both exons and introns — is copied into a primary transcript (pre-mRNA). This pre-mRNA is not yet ready for translation because the introns would disrupt the protein-coding sequence. So, the cell performs splicing: a process that precisely cuts out the introns and joins the exons together. The reason splicing exists is exactly because introns are present in the primary transcript and are non-functional (they do not code for protein). Without splicing, the mRNA would contain useless or even harmful sequences.

Step-by-Step Reasoning

1. Understand the Assertion (A):

   "Primary transcripts in eukaryotes are subjected to splicing to remove the introns."

   This is a factual statement. In eukaryotic cells, the immediate product of transcription (pre-mRNA) contains both exons and introns. Splicing is the process that removes introns and joins exons, producing mature mRNA. So, Assertion (A) is true.

2. Understand the Reason (R):

   "Primary transcripts contain both exons and introns and the introns are non-functional in eukaryotes." …

Transcription is the fundamental process where genetic information from a DNA template is copied into an RNA molecule. In prokaryotes, this process is carried out by a single type of RNA polymerase, but it requires specific signals and accessory factors to ensure accuracy and regulation. Understanding the roles of these components is key to grasping how genes are expressed.

Here's a step-by-step breakdown of the components involved:

1. Identifying Region 'X': The Promoter

   • Transcription does not begin randomly on the DNA. It starts at a specific sequence called the promoter. The promoter acts as a recognition site for RNA polymerase, indicating where transcription should begin and which strand should be used as the template.
   • In prokaryotes, common promoter sequences include the -35 region (e.g., TTGACA) and the -10 region (Pribnow box, e.g., TATAAT), located upstream from the transcription start site.
   • Therefore, the region 'X' where transcription initiates is the promoter.

2. Identifying Factor 'Y': The Sigma (σ) Factor

   • While RNA polymerase is the enzyme that synthesizes RNA, it cannot, on its own, efficiently recognize and bind to the promoter sequence. This is where accessory factors come into play.
   • In prokaryotes, the sigma (σ) factor is a dissociable subunit of the RNA polymerase holoenzyme. Its primary role is to recognize and bind specifically to the promoter sequences (like the -35 and -10 regions). This binding guides the core RNA polymerase enzyme to the correct initiation site.
   • Once transcription has initiated and a short RNA strand is synthesized, the sigma factor typically dissociates from the core enzyme, allowing the core enzyme to proceed with elongation.
   • The Rho (ρ) factor, mentioned in option (b), is involved in termination of transcription in some cases, not initiation.
   • Therefore, the factor 'Y' involved in initiating transcription by recognizing the promoter is the sigma (σ) factor.

   RNA Polymerase Holoenzyme = Core Enzyme (α₂ββ'ω) + Sigma (σ) Factor

3. Identifying Enzyme 'Z': RNA Polymerase …