(a) Sketch out a basic transcription unit and illustrate the RNA polymerase II associated assembly of a transcription initiation…

Eukaryotic Transcription Unit and RNA Polymerase II Assembly

A standard eukaryotic transcription unit comprises regulatory and coding elements arranged in a 5' to 3' polarity: upstream distal enhancers or silencers, a core and proximal promoter harboring the CAAT box (-75 to -80 bp) and TATA box (-25 to -30 bp), the transcription start site (+1), structural gene segments containing alternating coding exons and non-coding introns, and a downstream polyadenylation/termination signal.

`` [Enhancer]----[CAAT Box]--[TATA Box]----[+1 TSS]---[Exon 1]-[Intron]-[Exon 2]----[Poly-A Site] ``

Initiation by RNA Polymerase II involves the sequential assembly of the Pre-Initiation Complex (PIC). First, the TATA-binding protein (TBP), a subunit of Transcription Factor II D (TFIID), binds the minor groove of the TATA box, bending the DNA helix. Transcription factors TFIIA and TFIIB then join, stabilizing TFIID and recruiting RNA Polymerase II complexed with TFIIF to the promoter. Subsequently, TFIIE and TFIIH are recruited to complete the PIC. TFIIH acts as an ATP-dependent helicase to melt the DNA duplex at the start site, forming the open promoter complex. Its cyclin-dependent kinase subunit phosphorylates the Ser-5 residues within the heptapeptide repeats (Tyr-Ser-Pro-Thr-Ser-Pro-Ser) of the Carboxy-Terminal Domain (CTD) of RNA Polymerase II, releasing the enzyme from the promoter to commence productive elongation.

Construction of Recombinant c-DNA from m-RNA

Unlike genomic DNA libraries that retain non-coding introns, complementary DNA (cDNA) libraries represent exclusively processed, mature mRNA transcripts, making them indispensable for recombinant protein expression in prokaryotic systems.

`` mRNA: 5'-Cap====================AAAAA-3' Oligo(dT) Priming: <---TTTTT-5' Reverse Transcr.: 5'-Cap====================AAAAA-3' 3'------------------------TTTTT-5' (cDNA 1st strand) RNase H & Pol I: 3'------------------------TTTTT-5' 5'========================AAAAA-3' (cDNA 2nd strand) Linker Ligation: [Linker]-=================[Linker] -> Restriction -> Vector Cloning ``

The construction follows definite biochemical steps:

• Isolation and Priming: Polyadenylated eukaryotic mRNA is isolated and hybridized with a synthetic oligo-dT primer complementary to its 3' poly(A) tail.
• First-Strand Synthesis: Avian myeloblastosis virus (AMV) or Moloney murine leukemia virus (M-MuLV) reverse transcriptase synthesizes a complementary DNA strand using dNTPs, generating an mRNA-cDNA hybrid.
• Second-Strand Synthesis: RNase H introduces single-strand nicks into the RNA template. DNA Polymerase I utilizes these fragments as primers to synthesize the second DNA strand via nick translation. Residual hairpin loops or single-stranded overhangs are trimmed by S1 nuclease.
• Adapter Ligation and Cloning: Chemically synthesized linkers or adapters containing specific restriction endonuclease recognition sequences are ligated to the blunt-ended double-stranded cDNA using T4 DNA ligase. Following restriction endonuclease cleavage to expose cohesive ends, the recombinant cDNA is ligated into an expression vector (e.g., pUC19) and transformed into competent bacterial hosts.

Mechanisms of Apoptotic Induction and DNA Fingerprinting

Intrinsic Mechanism of Apoptotic Induction: The intrinsic (mitochondrial) apoptotic pathway is triggered by intracellular stressors like DNA damage and oxidative stress, contrasting with the extrinsic pathway activated by transmembrane death receptors (FasL/TNF-α). The pathway is governed by the Bcl-2 protein family. Pro-survival proteins (Bcl-2, Bcl-xL) are inhibited by BH3-only proteins (Puma, Bim, Noxa), leading to the homooligomerization of pro-apoptotic effectors Bax and Bak at the outer mitochondrial membrane. This causes Mitochondrial Outer Membrane Permeabilization (MOMP), releasing Cytochrome c and Smac/DIABLO into the cytosol.

Cytochrome c binds the adaptor protein Apaf-1 (Apoptotic Protease Activating Factor-1) in the presence of dATP, polymerizing into a heptameric wheel-like structure called the apoptosome. The apoptosome recruits and cleaves procaspase-9 into active caspase-9. Activated caspase-9 then cleaves and activates downstream executioner caspases (caspase-3, caspase-6, and caspase-7), initiating structural protein degradation and CAD-mediated internucleosomal DNA fragmentation. Fundamental insights into these caspase regulatory networks have been extensively investigated at institutions such as the Centre for Cellular and Molecular Biology (CCMB), Hyderabad.

Principle of DNA Fingerprinting: DNA fingerprinting relies on the vast inter-individual sequence polymorphism present in non-coding regions of the eukaryotic genome, specifically Variable Number Tandem Repeats (VNTRs/minisatellites), Short Tandem Repeats (STRs/microsatellites), and Single Nucleotide Polymorphisms (SNPs).

High-molecular-weight genomic DNA is extracted and cleaved with restriction endonucleases (or specific loci are amplified using multiplex PCR). The resulting fragments are fractionated by size using agarose gel electrophoresis and denatured. The single-stranded DNA fragments are transferred onto a nylon or nitrocellulose membrane via Southern blotting. The membrane is then incubated with radioactively or fluorescently labelled DNA probes complementary to hypervariable minisatellite sequences (such as BKm probes pioneered in India by Lalji Singh). Following unhybridized probe wash-off, autoradiography produces an individual-specific banding profile.

The match probability between samples is determined by statistical population genetics based on Hardy-Weinberg equilibrium. In the Indian applied context, this methodology provides definitive forensic identification in criminal investigations and disputed parentage cases conducted by the Central Forensic Science Laboratories (CFSL) at Hyderabad and Kolkata, and serves as an evidentiary tool in citizenship and lineage verification within the National Register of Citizens (NRC) framework in Assam.