Transcription – Quick-Grasp Notes 🧬

1. Big Picture ✨

Transcription copies genetic info from one DNA strand into RNA. Complementary base-pairing still rules, but adenine now pairs with uracil (A–U). Unlike replication (which copies the whole DNA), transcription targets only one strand of a specific segment :contentReference[oaicite:0]{index=0}.

2. Why Use Only One DNA Strand? 🤔

• If both strands served as templates, each would generate RNA with a different sequence, leading to two conflicting proteins from the same region.
• Those RNAs would complement each other and stick together, forming double-stranded RNA that can’t be translated.

Result: the cell wisely chooses just one strand as template :contentReference[oaicite:1]{index=1}.

3. Anatomy of a Transcription Unit 🏗️

A transcription unit has three key zones :contentReference[oaicite:2]{index=2}:

1. Promoter – binding site for RNA polymerase (upstream, 5′-end).
2. Structural gene – the part that actually gets copied.
3. Terminator – signals RNA polymerase to stop (downstream, 3′-end).

Coding and template strands:
Template runs \(3′ \rightarrow 5’\); RNA polymerase reads this to build RNA \(5′ \rightarrow 3’\). The opposite strand (same sequence as RNA except T ⇄ U) is called the coding strand :contentReference[oaicite:3]{index=3}.

4. Gene Talk: Exons & Introns ✂️

• Monocistronic genes (common in eukaryotes) code for one polypeptide.
• Polycistronic genes (usual in bacteria) code for many polypeptides.
• Eukaryotic genes are split: expressed parts are exons; non-coding gaps are introns :contentReference[oaicite:4]{index=4}.

5. Players in Transcription 🎭

5.1 In Bacteria 🦠

• One DNA-dependent RNA polymerase makes mRNA, tRNA, and rRNA.
• σ-factor helps the enzyme start; ρ-factor helps it stop :contentReference[oaicite:5]{index=5}.
• Transcription and translation happen together – ribosomes hop on the growing mRNA even before it’s finished.

5.2 In Eukaryotes 🐾

Primary hnRNA isn’t ready for action, so the cell:

• Caps the 5′ end with methyl-GTP 🧢
• Tails the 3′ end with ~200 adenines 🎀
• Splices out introns and stitches exons together 🎯

6. Step-by-Step Snapshot 🚀

1. Initiation – RNA polymerase + σ binds promoter, unwinds DNA.
2. Elongation – enzyme slides along template, adding ribonucleotides (energy from NTPs).
3. Termination – ρ factor or specific sequence cues release; RNA detaches and DNA helix zips up :contentReference[oaicite:6]{index=6}.

7. High-Yield NEET Nuggets 🏅

• Promoter orientation defines template vs coding strand – know the \(5′ \rightarrow 3’\) rule.
• σ and ρ factors regulate bacterial transcription start/stop.
• Three distinct RNA polymerases in eukaryotes – Pol I, II, III and their specific products.
• Post-transcriptional processing – capping, tailing, and splicing create functional mRNA.
• Coupled transcription-translation happens only in prokaryotes.

🙌 You’ve got this! Keep practicing and ace those NEET questions! 🙌