GS Paper III · Science & Technology · Molecular Biology · Genetics
🧬 Central Dogma of Molecular Biology
DNA → RNA → Protein · Transcription · Translation · Reverse Transcription · Genetic Code · Gene Expression · RNA Interference · PYQs & MCQs
📖
What is the Central Dogma?
Francis Crick 1958 · DNA → RNA → Protein · Gene expression · Information flow
Central Dogma — The Big Picture. Genetic information flows in one direction: DNA → RNA → Protein. DNA stores the blueprint; RNA carries the message; Proteins carry out the work. This one-way flow is the central dogma proposed by Francis Crick in 1958.
📖 Definition
The Central Dogma of Molecular Biology describes how genetic information flows within a biological system. Proposed by Francis Crick in 1958, it states: genetic information stored in DNA is transcribed into RNA, which is then translated into Protein. It explains how the instructions encoded in genes are used to make the proteins that carry out all cellular functions.
🧠 Simple Analogy — The Recipe Book
Imagine DNA as a recipe book locked in a library (nucleus). You cannot take the original book out. So you photocopy the relevant recipe page (Transcription → mRNA). You take the photocopy to the kitchen (cytoplasm → ribosome) where a chef (ribosome + tRNA) follows the recipe to cook the dish (Translation → Protein). The dish (protein) does the actual work — builds muscles, fights infections, speeds up reactions. The recipe book (DNA) stays safe and unchanged in the library.
Central Dogma — Complete Flow. Replication: DNA copies itself (circular arrow) — each daughter cell gets a complete genome. Transcription: DNA → RNA (right arrow) — RNA polymerase reads the DNA template. Reverse Transcription: RNA → DNA (left arrow) — carried out by reverse transcriptase enzyme in retroviruses (HIV). Translation: RNA → Protein (right arrow) — ribosomes read mRNA codons and assemble amino acids into a protein chain (shown as folded blue protein).
💾
DNA — The Storage
Master copy of all genetic information. Stays safe in nucleus (eukaryotes). Double-stranded helix. Stable, permanent. Contains all genes — but only specific genes are expressed at any time depending on cell type and signals.
📨
RNA — The Messenger
Temporary working copy. mRNA carries the code from DNA (nucleus) to ribosomes (cytoplasm). Single-stranded. Short-lived — degraded after use. Acts as the "photocopy" that can be taken out of the vault.
⚙
Protein — The Worker
The final functional product. Made of amino acids. Enzymes (catalyse reactions), structural proteins (collagen, keratin), hormones (insulin), antibodies (immunity), transporters (haemoglobin). Proteins do virtually all the work in the cell.
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Steps of Central Dogma — Transcription & Translation
RNA polymerase · Promoter · Codons · Ribosomes · tRNA · Peptide bond · Amino acids
STEP 1 — Replication (DNA → DNA)
1
DNA Replication — Before Everything Else
Before a cell divides, it must copy all its DNA so each daughter cell gets a complete genome. Semi-conservative replication (proven by Meselson & Stahl, 1958): the double helix unwinds → each strand serves as template → DNA polymerase synthesises a new complementary strand → two identical DNA molecules produced, each with one old and one new strand.
Key enzyme: DNA polymerase (reads 3'→5', synthesises 5'→3'). DNA helicase unwinds the helix. DNA ligase joins Okazaki fragments on the lagging strand.
Key enzyme: DNA polymerase (reads 3'→5', synthesises 5'→3'). DNA helicase unwinds the helix. DNA ligase joins Okazaki fragments on the lagging strand.
STEP 2 — Transcription (DNA → RNA)
2
Transcription — Making RNA from DNA
Where: Nucleus (eukaryotes); cytoplasm (prokaryotes)
Enzyme: RNA polymerase (DNA-dependent RNA polymerase)
Template: Template strand (antisense strand) — read 3'→5'
Product: mRNA (messenger RNA) — synthesised 5'→3'
Three stages:
Initiation: RNA polymerase binds to the Promoter region on DNA (specific sequence upstream of gene). In eukaryotes, transcription factors assist. The DNA double helix locally unwinds (~12–14 bp).
Elongation: RNA polymerase moves along template strand (3'→5'), adding complementary ribonucleotides to growing RNA chain (5'→3'). A pairs with U in RNA (not T). G pairs with C.
Termination: RNA polymerase reaches the Terminator sequence. The mRNA transcript is released.
Eukaryote post-transcriptional processing (not in prokaryotes):
• 5' Capping: 7-methylguanosine cap added (protects mRNA, helps ribosome binding)
• 3' Poly-A tail: ~200 adenine nucleotides added (increases mRNA stability)
• Splicing: Introns (non-coding sequences) removed by spliceosome; Exons (coding sequences) joined → mature mRNA exported to cytoplasm
Enzyme: RNA polymerase (DNA-dependent RNA polymerase)
Template: Template strand (antisense strand) — read 3'→5'
Product: mRNA (messenger RNA) — synthesised 5'→3'
Three stages:
Initiation: RNA polymerase binds to the Promoter region on DNA (specific sequence upstream of gene). In eukaryotes, transcription factors assist. The DNA double helix locally unwinds (~12–14 bp).
Elongation: RNA polymerase moves along template strand (3'→5'), adding complementary ribonucleotides to growing RNA chain (5'→3'). A pairs with U in RNA (not T). G pairs with C.
Termination: RNA polymerase reaches the Terminator sequence. The mRNA transcript is released.
Eukaryote post-transcriptional processing (not in prokaryotes):
• 5' Capping: 7-methylguanosine cap added (protects mRNA, helps ribosome binding)
• 3' Poly-A tail: ~200 adenine nucleotides added (increases mRNA stability)
• Splicing: Introns (non-coding sequences) removed by spliceosome; Exons (coding sequences) joined → mature mRNA exported to cytoplasm
STEP 3 — Translation (RNA → Protein)
3
Translation — Building Protein from RNA
Where: Cytoplasm (both prokaryotes and eukaryotes) — at ribosomes
Machinery: Ribosome (rRNA + ribosomal proteins), mRNA (template), tRNA (amino acid carrier), Amino acids (building blocks)
Energy: ATP and GTP (active process)
Three stages:
Initiation: Small ribosomal subunit (30S prokaryote / 40S eukaryote) binds to mRNA at the start codon AUG (codes for Methionine). Large subunit (50S/60S) joins. Initiator tRNA (carrying Met) base-pairs with AUG in the P-site.
Elongation: Ribosome moves along mRNA codon by codon (5'→3'). Each codon read by aminoacyl-tRNA (anticodon–codon matching). The ribosome has three sites: A-site (incoming tRNA), P-site (tRNA carrying growing peptide), E-site (exit site for empty tRNA). Peptide bond formed between amino acids (by rRNA — peptidyl transferase activity). Ribosome translocates one codon forward. Chain grows.
Termination: Ribosome reaches a stop codon (UAA, UAG, or UGA). No tRNA matches stop codons. Release factors bind → polypeptide chain released → ribosome disassembles. Polypeptide folds into functional 3D protein (sometimes aided by chaperone proteins).
Machinery: Ribosome (rRNA + ribosomal proteins), mRNA (template), tRNA (amino acid carrier), Amino acids (building blocks)
Energy: ATP and GTP (active process)
Three stages:
Initiation: Small ribosomal subunit (30S prokaryote / 40S eukaryote) binds to mRNA at the start codon AUG (codes for Methionine). Large subunit (50S/60S) joins. Initiator tRNA (carrying Met) base-pairs with AUG in the P-site.
Elongation: Ribosome moves along mRNA codon by codon (5'→3'). Each codon read by aminoacyl-tRNA (anticodon–codon matching). The ribosome has three sites: A-site (incoming tRNA), P-site (tRNA carrying growing peptide), E-site (exit site for empty tRNA). Peptide bond formed between amino acids (by rRNA — peptidyl transferase activity). Ribosome translocates one codon forward. Chain grows.
Termination: Ribosome reaches a stop codon (UAA, UAG, or UGA). No tRNA matches stop codons. Release factors bind → polypeptide chain released → ribosome disassembles. Polypeptide folds into functional 3D protein (sometimes aided by chaperone proteins).
🧠 Memory — Transcription vs Translation
Trans-SCRIPT-ion = writing a script (mRNA) from DNA. Happens in nucleus (eukaryotes). Enzyme = RNA polymerase.
Trans-LATE-ion = converting the language of RNA into protein. Happens later in the cytoplasm. Machinery = ribosome.
Flow of base pairing:
DNA template strand → RNA: A→U, T→A, G→C, C→G
mRNA codon → tRNA anticodon: A→U, U→A, G→C, C→G (complementary and antiparallel)
Trans-LATE-ion = converting the language of RNA into protein. Happens later in the cytoplasm. Machinery = ribosome.
Flow of base pairing:
DNA template strand → RNA: A→U, T→A, G→C, C→G
mRNA codon → tRNA anticodon: A→U, U→A, G→C, C→G (complementary and antiparallel)
🦠
Prokaryote vs Eukaryote — Where it Happens High Yield
Nucleus · Coupled transcription-translation · Post-transcriptional processing · Ribosomes
| Feature | 🦠 Prokaryotes (Bacteria) | 🧬 Eukaryotes (Plants, Animals, Fungi) |
|---|---|---|
| Nucleus | No nucleus (nucleoid region) | True membrane-bound nucleus |
| Where is transcription? | Cytoplasm (no nucleus) | Nucleus |
| Where is translation? | Cytoplasm | Cytoplasm (at ribosomes; rough ER for secretory proteins) |
| Are they simultaneous? | ✅ YES — coupled transcription-translation. Translation begins before transcription finishes (ribosomes attach to mRNA even as it is being made). | ❌ NO — transcription in nucleus first → mRNA processed → exported to cytoplasm → then translation begins. Sequential, not simultaneous. |
| mRNA processing | No processing — mRNA is used directly (polycistronic mRNA — one mRNA can code for multiple proteins) | Extensive processing: 5' cap, 3' poly-A tail, intron splicing → mature mRNA (monocistronic — one mRNA = one protein) |
| Introns? | Rarely present | Present — removed by spliceosomes during processing |
| Ribosomes | 70S (30S + 50S) | 80S (40S + 60S). Mitochondria/chloroplasts have 70S! |
| RNA polymerases | One RNA polymerase (for all RNA types) | Three: RNA Pol I (rRNA), RNA Pol II (mRNA), RNA Pol III (tRNA, 5S rRNA) |
| Operons | Yes — genes clustered and co-regulated (e.g. Lac operon, Trp operon) | No operons — each gene has its own promoter |
⚠ Most Common UPSC Mistake — Statement 1 in Practice Question
❌ WRONG: "In eukaryotic organisms, transcription and translation both occur in the cytoplasm."
✅ CORRECT: In eukaryotes — Transcription in NUCLEUS, Translation in CYTOPLASM. In prokaryotes (bacteria) — both in cytoplasm (no nucleus). This distinction is one of the most frequently tested UPSC facts in cell biology/molecular biology.
✅ CORRECT: In eukaryotes — Transcription in NUCLEUS, Translation in CYTOPLASM. In prokaryotes (bacteria) — both in cytoplasm (no nucleus). This distinction is one of the most frequently tested UPSC facts in cell biology/molecular biology.
🔄
Reverse Transcription — RNA → DNA UPSC Favourite
Reverse transcriptase · HIV · Retroviruses · cDNA · RT-PCR · Gene therapy
📖 Definition
Reverse Transcription is the process of synthesising DNA from an RNA template, carried out by the enzyme Reverse Transcriptase. This is the reverse of normal transcription (DNA → RNA). The DNA copy produced from RNA is called cDNA (complementary DNA). It was discovered by Howard Temin and David Baltimore (independently) in 1970 — they shared the Nobel Prize in Physiology/Medicine 1975 (shared with Renato Dulbecco).
🧠 Why is it called "Reverse"?
Normal flow (Central Dogma): DNA → RNA (transcription). Reverse transcription goes backwards: RNA → DNA. It's like rewriting a photocopy back into an original master document. This violates the "original" Central Dogma (which did not account for this possibility). Crick later updated the Central Dogma to include reverse transcription as a "special transfer."
🦠 HIV — The Classic Example
HIV (Human Immunodeficiency Virus) is a retrovirus — its genome is RNA, not DNA.
HIV replication cycle:
1. HIV enters CD4+ T-cell (immune cell)
2. Reverse transcriptase converts HIV RNA → cDNA
3. Integrase enzyme inserts cDNA into host cell's chromosomal DNA → Provirus
4. Host cell now permanently carries HIV genes
5. When provirus is activated → host transcribes it → new HIV RNA + proteins → new HIV viruses assembled
Why HIV is hard to cure: The provirus hides inside chromosomal DNA for years — immune system cannot see it; ART (antiretroviral therapy) suppresses HIV but cannot eliminate provirus from all cells.
ART drugs target: Reverse transcriptase (e.g. zidovudine/AZT), Integrase, Protease, Fusion inhibitors.
HIV replication cycle:
1. HIV enters CD4+ T-cell (immune cell)
2. Reverse transcriptase converts HIV RNA → cDNA
3. Integrase enzyme inserts cDNA into host cell's chromosomal DNA → Provirus
4. Host cell now permanently carries HIV genes
5. When provirus is activated → host transcribes it → new HIV RNA + proteins → new HIV viruses assembled
Why HIV is hard to cure: The provirus hides inside chromosomal DNA for years — immune system cannot see it; ART (antiretroviral therapy) suppresses HIV but cannot eliminate provirus from all cells.
ART drugs target: Reverse transcriptase (e.g. zidovudine/AZT), Integrase, Protease, Fusion inhibitors.
🔬 RT-PCR — COVID-19 Diagnosis
Reverse Transcription PCR (RT-PCR) uses reverse transcriptase to convert RNA → cDNA, then PCR amplifies the cDNA for detection. Used to diagnose RNA virus infections:
• COVID-19: SARS-CoV-2 RNA detected in nasal/throat swabs
• Influenza, HIV viral load measurement
• Gene expression studies (measure mRNA levels)
The "gold standard" test for COVID-19 diagnosis during the pandemic.
• COVID-19: SARS-CoV-2 RNA detected in nasal/throat swabs
• Influenza, HIV viral load measurement
• Gene expression studies (measure mRNA levels)
The "gold standard" test for COVID-19 diagnosis during the pandemic.
🧪 cDNA in Biotechnology
cDNA (complementary DNA made from mRNA via reverse transcriptase) is used in:
• Gene cloning: cDNA contains only exons (introns already spliced out of mRNA) → easier to express in bacteria (which lack splicing machinery)
• cDNA libraries: collection of all cDNAs from a cell — represents all expressed genes
• Making human proteins in bacteria: Human insulin gene (cDNA from insulin mRNA) cloned into E. coli
• Gene cloning: cDNA contains only exons (introns already spliced out of mRNA) → easier to express in bacteria (which lack splicing machinery)
• cDNA libraries: collection of all cDNAs from a cell — represents all expressed genes
• Making human proteins in bacteria: Human insulin gene (cDNA from insulin mRNA) cloned into E. coli
🔑
Genetic Code — Rules for Reading mRNA
Triplet codon · 64 codons · 20 amino acids · Degenerate · Universal · AUG · Stop codons
📖 Definition
The Genetic Code is the set of rules by which the nucleotide sequence of mRNA is translated into the amino acid sequence of a protein. The code consists of codons — triplets of three nucleotides, each specifying one amino acid (or start/stop signal). Cracked by Marshall Nirenberg, Har Gobind Khorana, and Robert Holley (Nobel Prize in Physiology/Medicine 1968).
🔢 Key Numbers
• 4 bases (A, U, C, G) × 3 positions = 4³ = 64 codons total
• 20 amino acids in nature (not 64) → code is degenerate (multiple codons per amino acid)
• 61 codons code for amino acids
• 3 stop codons: UAA, UAG, UGA — signal end of protein
• 1 start codon: AUG (codes for Methionine — every protein starts with Met)
• Tryptophan (Trp) and Methionine (Met) = only amino acids with a single codon each (UGG and AUG respectively)
• 20 amino acids in nature (not 64) → code is degenerate (multiple codons per amino acid)
• 61 codons code for amino acids
• 3 stop codons: UAA, UAG, UGA — signal end of protein
• 1 start codon: AUG (codes for Methionine — every protein starts with Met)
• Tryptophan (Trp) and Methionine (Met) = only amino acids with a single codon each (UGG and AUG respectively)
📋 Properties of the Genetic Code
1. Triplet: 3 nucleotides = 1 codon = 1 amino acid
2. Degenerate/Redundant: Multiple codons → same amino acid. E.g. Leucine has 6 codons (CUU, CUC, CUA, CUG, UUA, UUG). Reduces harm from mutations.
3. Unambiguous: Each codon codes for only ONE specific amino acid (not multiple) — no ambiguity
4. Universal: Same codon = same amino acid in almost all organisms (bacteria to humans). Evidence of common ancestry. Exceptions: mitochondria (UGA = Trp, not stop)
5. Non-overlapping: Each nucleotide belongs to only one codon
6. Commaless: No gaps between codons — read continuously
2. Degenerate/Redundant: Multiple codons → same amino acid. E.g. Leucine has 6 codons (CUU, CUC, CUA, CUG, UUA, UUG). Reduces harm from mutations.
3. Unambiguous: Each codon codes for only ONE specific amino acid (not multiple) — no ambiguity
4. Universal: Same codon = same amino acid in almost all organisms (bacteria to humans). Evidence of common ancestry. Exceptions: mitochondria (UGA = Trp, not stop)
5. Non-overlapping: Each nucleotide belongs to only one codon
6. Commaless: No gaps between codons — read continuously
🔑 Important Codons to Remember
Start codon: AUG → Methionine (Met, M). Every protein synthesis begins here. Also acts as signal in mRNA to begin reading frame.
Stop codons (3): UAA · UAG · UGA
No tRNA matches these → ribosome releases polypeptide. Memory: "U Are Annoying / U Are Gone / U Go Away"
No tRNA matches these → ribosome releases polypeptide. Memory: "U Are Annoying / U Are Gone / U Go Away"
Single-codon amino acids:
Methionine (Met): only AUG
Tryptophan (Trp): only UGG
Methionine (Met): only AUG
Tryptophan (Trp): only UGG
Most codons for one amino acid:
Leucine (Leu): 6 codons
Serine (Ser): 6 codons
Arginine (Arg): 6 codons
Leucine (Leu): 6 codons
Serine (Ser): 6 codons
Arginine (Arg): 6 codons
Wobble hypothesis (Crick): The third base of a codon (3' position) is less strictly paired — one tRNA can recognise multiple codons. Explains degeneracy with fewer tRNA types than codons.
⚠
Exceptions & Extensions to Central Dogma High Yield
Reverse transcription · RNA replication · Prions · RNA editing · RNAi · CRISPR
🔄 The Central Dogma — Original vs Extended
Original (Crick, 1958)
General transfers:
DNA → DNA (Replication)
DNA → RNA (Transcription)
RNA → Protein (Translation)
Crick also allowed RNA → RNA and RNA → DNA as theoretically possible "special transfers."
DNA → DNA (Replication)
DNA → RNA (Transcription)
RNA → Protein (Translation)
Crick also allowed RNA → RNA and RNA → DNA as theoretically possible "special transfers."
Extended (after 1970)
Special transfers confirmed:
RNA → DNA (Reverse transcription — HIV, retroviruses)
RNA → RNA (RNA replication — RNA viruses like Influenza, Polio)
Prion exception:
Protein → Protein (misfolded protein converts normal protein — no nucleic acid involved) — Prusiner Nobel 1997
RNA → DNA (Reverse transcription — HIV, retroviruses)
RNA → RNA (RNA replication — RNA viruses like Influenza, Polio)
Prion exception:
Protein → Protein (misfolded protein converts normal protein — no nucleic acid involved) — Prusiner Nobel 1997
| Transfer Type | Direction | Enzyme/Agent | Example | Category |
|---|---|---|---|---|
| Replication | DNA → DNA | DNA polymerase | All cell division | General (always occurs) |
| Transcription | DNA → RNA | RNA polymerase | All gene expression | General (always occurs) |
| Translation | RNA → Protein | Ribosome + tRNA | All protein synthesis | General (always occurs) |
| Reverse Transcription | RNA → DNA | Reverse transcriptase | HIV, HTLV, Hepatitis B | Special (retroviruses) |
| RNA Replication | RNA → RNA | RNA-dependent RNA polymerase (RdRp) | Influenza, SARS-CoV-2, Polio | Special (RNA viruses) |
| Prion transfer | Protein → Protein | None (conformational templating) | Mad Cow Disease, CJD, Kuru | Special (exception to dogma) |
📜
PYQs & Practice MCQs
UPSC 2019 RNAi · Practice Q from text · Transcription location · Reverse transcription
📜 UPSC Prelims 2019 — RNA Interference
PYQ 2019
Q. 'RNA interference (RNAi)' technology has gained popularity in the last few years. Why?
- It is used in developing gene silencing therapies.
- It can be used in developing therapies for the treatment of cancer.
- It can be used to develop hormone replacement therapies.
- It can be used to produce crop plants that are resistant to viral pathogens.
- (a) 1, 2 and 4 ✓
- (b) 2 and 3
- (c) 1 and 3
- (d) 1 and 4 only
Statement 1 CORRECT — Gene silencing: RNAi is a natural cellular mechanism using small RNA molecules (siRNA and miRNA) to silence specific genes by degrading their mRNA before it can be translated. siRNA (small interfering RNA): binds complementary mRNA → RISC (RNA-Induced Silencing Complex) cleaves and degrades the mRNA → gene "silenced." Used therapeutically — first siRNA drug: Onpattro (patisiran, 2018) for TTR amyloidosis.
Statement 2 CORRECT — Cancer therapy: Many cancers are driven by overexpression of specific genes (oncogenes). RNAi can silence these oncogenes — e.g., siRNA against BCL-2 (apoptosis inhibitor in cancer), HER2 (in breast cancer), KRAS (in pancreatic cancer). Clinical trials underway for several RNAi-based cancer treatments.
Statement 3 WRONG — Hormone replacement: RNAi silences/degrades mRNA — it does not produce or replace hormones. Hormone replacement therapy involves administering hormones (estrogen, progesterone, insulin). RNAi could potentially silence a gene that overproduces a hormone, but it does not provide hormone replacement. This is why statement 3 is excluded.
Statement 4 CORRECT — Virus-resistant crops: Plants can be engineered with RNAi to target viral RNA. When a virus infects the plant, the plant's RNAi machinery recognises viral RNA sequences → degrades viral RNA → virus cannot replicate → crop is protected. Examples: Papaya Ringspot Virus-resistant papaya (Hawaii), RNA viruses attacking wheat/rice/potato. India's Bt brinjal controversy included RNAi-based pest resistance research. Nobel 2006 (Fire & Mello) for discovering RNAi.
Statement 2 CORRECT — Cancer therapy: Many cancers are driven by overexpression of specific genes (oncogenes). RNAi can silence these oncogenes — e.g., siRNA against BCL-2 (apoptosis inhibitor in cancer), HER2 (in breast cancer), KRAS (in pancreatic cancer). Clinical trials underway for several RNAi-based cancer treatments.
Statement 3 WRONG — Hormone replacement: RNAi silences/degrades mRNA — it does not produce or replace hormones. Hormone replacement therapy involves administering hormones (estrogen, progesterone, insulin). RNAi could potentially silence a gene that overproduces a hormone, but it does not provide hormone replacement. This is why statement 3 is excluded.
Statement 4 CORRECT — Virus-resistant crops: Plants can be engineered with RNAi to target viral RNA. When a virus infects the plant, the plant's RNAi machinery recognises viral RNA sequences → degrades viral RNA → virus cannot replicate → crop is protected. Examples: Papaya Ringspot Virus-resistant papaya (Hawaii), RNA viruses attacking wheat/rice/potato. India's Bt brinjal controversy included RNAi-based pest resistance research. Nobel 2006 (Fire & Mello) for discovering RNAi.
📜 Practice Question — Central Dogma Statements
Practice Q
Q. Consider the following statements:
- In eukaryotic organisms, transcription and translation occur in the cytoplasm.
- The process of translation is carried out by ribosomes.
- Reverse transcription is the flow of information from DNA to RNA.
- (a) Only one ✓
- (b) Only two
- (c) All three
- (d) None
Statement 1 WRONG: In eukaryotes, transcription occurs in the NUCLEUS (where DNA is located). The mRNA is then processed and exported to the cytoplasm where translation occurs. Both transcription AND translation in cytoplasm is true only for prokaryotes (bacteria), which have no nucleus.
Statement 2 CORRECT: Translation is carried out by ribosomes — molecular machines in the cytoplasm. Ribosomes consist of rRNA + ribosomal proteins, forming small (30S/40S) and large (50S/60S) subunits. They read mRNA codons and catalyse peptide bond formation between amino acids, assembling proteins.
Statement 3 WRONG: Reverse transcription is RNA → DNA (the reverse of normal transcription). Normal transcription = DNA → RNA. Reverse transcription = RNA → DNA (using reverse transcriptase enzyme). This is how HIV and other retroviruses convert their RNA genome into DNA to integrate into the host cell's chromosomes. The name "reverse" transcription specifically means it is the reverse of the DNA → RNA direction.
Statement 2 CORRECT: Translation is carried out by ribosomes — molecular machines in the cytoplasm. Ribosomes consist of rRNA + ribosomal proteins, forming small (30S/40S) and large (50S/60S) subunits. They read mRNA codons and catalyse peptide bond formation between amino acids, assembling proteins.
Statement 3 WRONG: Reverse transcription is RNA → DNA (the reverse of normal transcription). Normal transcription = DNA → RNA. Reverse transcription = RNA → DNA (using reverse transcriptase enzyme). This is how HIV and other retroviruses convert their RNA genome into DNA to integrate into the host cell's chromosomes. The name "reverse" transcription specifically means it is the reverse of the DNA → RNA direction.
🧪 Practice MCQs — Central Dogma (Click to attempt)
Q1. Which of the following correctly distinguishes transcription from translation in eukaryotic cells?
- (a) Transcription occurs in cytoplasm; translation occurs in nucleus
- (b) Both transcription and translation occur in the nucleus in eukaryotes
- (c) Transcription occurs in nucleus (DNA → mRNA); translation occurs in cytoplasm at ribosomes (mRNA → protein). In prokaryotes, both occur in cytoplasm simultaneously.
- (d) Transcription and translation are the same process; they differ only in the type of enzyme used
Transcription (DNA → RNA): Occurs in the nucleus in eukaryotes. RNA polymerase reads the DNA template strand and synthesises mRNA. The mRNA then undergoes processing (5' cap, poly-A tail, intron splicing) and is exported from the nucleus to the cytoplasm via nuclear pores. Translation (mRNA → Protein): Occurs in the cytoplasm at ribosomes. The ribosome reads mRNA codons, tRNA brings amino acids, peptide bonds form the polypeptide chain. In prokaryotes (no nucleus): transcription and translation both occur in the cytoplasm, and translation can begin on an mRNA molecule even before transcription is complete (coupled transcription-translation). This is one of the most frequently tested distinctions in UPSC molecular biology questions.
Q2. The genetic code is described as "degenerate." Which of the following BEST explains what this means?
- (a) The genetic code has become corrupted over evolutionary time and no longer accurately specifies amino acids
- (b) Multiple different codons can code for the same amino acid — e.g., Leucine is coded by six different codons (CUU, CUC, CUA, CUG, UUA, UUG) — meaning a single mutation in the third position of a codon often does not change the amino acid produced (silent/synonymous mutation)
- (c) Each amino acid is coded by exactly one codon, and each codon codes for exactly one amino acid — making the code non-degenerate and perfectly one-to-one
- (d) The genetic code only works in degenerate (diseased) cells; healthy cells use a different coding system
Degeneracy (redundancy) of the genetic code means multiple codons specify the same amino acid. Of the 64 possible codons: 61 code for the 20 amino acids, 3 are stop codons (UAA, UAG, UGA). Since 61 codons must specify only 20 amino acids, most amino acids have more than one codon. Examples: Leucine (6 codons), Serine (6), Arginine (6), Glycine (4), Alanine (4), Valine (4). Only Methionine (AUG = start) and Tryptophan (UGG) have just one codon each. Biological significance of degeneracy: (1) Mutation buffering: Most mutations in the third position of a codon are "silent" — they change the codon but not the amino acid (synonymous/silent mutation). Example: GUU and GUC both code for Valine. A mutation from GUU→GUC produces the same protein. (2) The Wobble Hypothesis (Crick, 1966) explains how fewer tRNA types than codons can serve all 61 codons — the third codon base pairs less strictly with the tRNA anticodon, allowing one tRNA to read multiple codons.
Q3. Reverse Transcriptase is the key enzyme in HIV's replication. Why is it an important drug target for HIV treatment?
- (a) Reverse transcriptase is targeted because it is the enzyme that assembles new HIV proteins — blocking it prevents the virus from making structural proteins needed for new viral particles
- (b) Reverse transcriptase is targeted because it digests the host cell's DNA, and blocking it prevents damage to the host genome
- (c) Reverse transcriptase synthesises the lipid envelope of new HIV particles — blocking it prevents HIV from escaping the host cell
- (d) Reverse transcriptase converts HIV's RNA genome into DNA (cDNA), which then integrates into the host's chromosomal DNA as a provirus — blocking reverse transcriptase prevents this conversion, so HIV cannot establish a permanent infection in the host's genome
HIV (Human Immunodeficiency Virus) is a retrovirus — its genome consists of two copies of single-stranded RNA. HIV replication requires: (1) Reverse Transcriptase: converts HIV RNA → cDNA → double-stranded DNA. (2) Integrase: inserts this viral DNA into host chromosomal DNA → Provirus (permanent). (3) RNA Polymerase II (host's own): transcribes provirus DNA → HIV mRNA + genomic RNA. (4) Protease: cleaves viral protein precursors into functional proteins. (5) Assembly and Budding: new HIV particles assembled and released. Why Reverse Transcriptase is a critical drug target: If RT is blocked, HIV RNA cannot become DNA → cannot integrate into host genome → HIV cannot establish persistent infection. NRTIs (Nucleoside Reverse Transcriptase Inhibitors): Zidovudine (AZT) — first anti-HIV drug (1987). Tenofovir, Emtricitabine. They mimic nucleotides but lack the 3'-OH group → chain termination. NNRTIs (Non-Nucleoside RTIs): Efavirenz, Nevirapine — bind to RT directly and block it. Modern ART (Antiretroviral Therapy) combines 3+ drugs (including RT inhibitors + integrase inhibitors + protease inhibitors) → suppresses HIV to undetectable levels. "Undetectable = Untransmittable" (U=U). India context: India has the third-largest HIV epidemic globally. National AIDS Control Programme (NACP) provides free ART. India is a major generic ART manufacturer for low-income countries.
⚡ Quick Revision — Central Dogma
| Topic | Key Facts |
|---|---|
| Central Dogma | DNA → RNA → Protein. Proposed by Francis Crick, 1958. Describes flow of genetic information. Replication (DNA→DNA) + Transcription (DNA→RNA) + Translation (RNA→Protein). |
| Replication | Semi-conservative (Meselson & Stahl, 1958). DNA polymerase synthesises new strand (reads 3'→5', builds 5'→3'). DNA helicase unwinds. DNA ligase joins Okazaki fragments. Each daughter DNA has 1 old + 1 new strand. |
| Transcription | DNA → mRNA. Enzyme: RNA polymerase. Template strand (3'→5') → mRNA (5'→3'). In eukaryotes: NUCLEUS. In prokaryotes: cytoplasm. Three stages: Initiation (binds promoter) → Elongation → Termination. Eukaryote processing: 5' cap + poly-A tail + splicing (introns removed, exons joined). |
| Translation | mRNA → Protein. At ribosomes in CYTOPLASM (both prokaryotes and eukaryotes). tRNA brings amino acids (anticodon matches mRNA codon). Stages: Initiation (AUG) → Elongation (A-site, P-site, E-site) → Termination (stop codon: UAA/UAG/UGA). Energy from ATP and GTP. |
| Prokaryote vs Eukaryote | Prokaryote: transcription + translation both in cytoplasm; simultaneous (coupled). Eukaryote: transcription in NUCLEUS, translation in CYTOPLASM; sequential, not simultaneous. Prokaryote mRNA is polycistronic; eukaryote is monocistronic. |
| Genetic Code | Triplet codons. 4³ = 64 codons. 20 amino acids. Degenerate (multiple codons per amino acid). Unambiguous (one codon = one amino acid). Universal (same in almost all organisms). Start: AUG (Met). Stop: UAA, UAG, UGA. Single codon: Met (AUG) and Trp (UGG). Cracked by Nirenberg & Khorana (Nobel 1968). |
| Reverse Transcription | RNA → DNA (enzyme: Reverse Transcriptase). Retroviruses (HIV, HTLV). HIV: RNA → cDNA (RT) → integrates as Provirus → permanent. Discovered by Temin & Baltimore (Nobel 1975). RT-PCR uses this for COVID-19 diagnosis. ART drugs (AZT, Tenofovir) target RT. cDNA used in biotechnology (gene cloning, libraries). |
| Extensions to Central Dogma | RNA → RNA: RNA replication (Influenza, SARS-CoV-2 using RdRp). RNA → DNA: reverse transcription (retroviruses). Protein → Protein: prions (misfolding without nucleic acid). These are "special transfers" — not routine cellular processes. |
| RNAi (UPSC 2019) | RNA Interference — small RNAs (siRNA, miRNA) silence genes by degrading mRNA. Nobel 2006 (Fire & Mello). Uses: gene silencing therapies (Onpattro for amyloidosis), cancer treatment, virus-resistant crops (papaya, wheat). NOT for hormone replacement. |
🚨 5 UPSC Traps — Central Dogma:
Trap 1 — "In eukaryotes, transcription and translation both occur in cytoplasm" → WRONG! In eukaryotes, TRANSCRIPTION occurs in the NUCLEUS; TRANSLATION in the CYTOPLASM. Only in prokaryotes (bacteria, which have no nucleus) do both transcription and translation occur in the cytoplasm. This is the #1 most-tested factual error in UPSC molecular biology — Statement 1 in the practice question directly tests this.
Trap 2 — "Reverse transcription is DNA → RNA" → WRONG! (direction reversed) Reverse transcription goes from RNA → DNA. It is "reverse" because it is the opposite of normal transcription (DNA → RNA). The enzyme is reverse transcriptase. HIV uses this to convert its RNA genome into DNA which then integrates into host chromosomal DNA. The name "reverse" always means the direction is reversed relative to the normal flow.
Trap 3 — "The genetic code has 64 codons for 20 amino acids, so each amino acid has exactly 3 codons" → WRONG! The distribution is NOT equal. Some amino acids have 1 codon (Met, Trp), some have 2, 3, 4, or even 6 codons (Leu, Ser, Arg have 6 each). 3 codons are stop codons (UAA, UAG, UGA) — no amino acid. This unequal distribution is "degeneracy" — and it is biologically advantageous because it buffers against mutations.
Trap 4 — "Ribosomes in mitochondria and chloroplasts are 80S like other eukaryotic ribosomes" → WRONG! Mitochondria and chloroplasts have 70S ribosomes (like bacteria) — NOT 80S like the rest of the eukaryotic cell. This is one of the key pieces of evidence for the Endosymbiotic Theory — that mitochondria and chloroplasts evolved from ancient bacteria. 70S ribosomes in these organelles means some antibiotics (e.g. chloramphenicol, streptomycin that target 70S) can potentially affect mitochondrial protein synthesis at high doses.
Trap 5 — "AUG is only the start codon and always signals the start of a new protein" → MISLEADING! AUG is the start codon AND codes for Methionine. In the middle of a coding sequence, AUG simply codes for Methionine (internal Met residue in protein) — it does not restart translation. AUG is only the start codon when it appears in the correct context (Kozak sequence in eukaryotes; Shine-Dalgarno sequence in prokaryotes). Also: every protein begins with Methionine, but the initiating Met is often cleaved off after translation — so the final protein may not have Met at its N-terminus.
Trap 1 — "In eukaryotes, transcription and translation both occur in cytoplasm" → WRONG! In eukaryotes, TRANSCRIPTION occurs in the NUCLEUS; TRANSLATION in the CYTOPLASM. Only in prokaryotes (bacteria, which have no nucleus) do both transcription and translation occur in the cytoplasm. This is the #1 most-tested factual error in UPSC molecular biology — Statement 1 in the practice question directly tests this.
Trap 2 — "Reverse transcription is DNA → RNA" → WRONG! (direction reversed) Reverse transcription goes from RNA → DNA. It is "reverse" because it is the opposite of normal transcription (DNA → RNA). The enzyme is reverse transcriptase. HIV uses this to convert its RNA genome into DNA which then integrates into host chromosomal DNA. The name "reverse" always means the direction is reversed relative to the normal flow.
Trap 3 — "The genetic code has 64 codons for 20 amino acids, so each amino acid has exactly 3 codons" → WRONG! The distribution is NOT equal. Some amino acids have 1 codon (Met, Trp), some have 2, 3, 4, or even 6 codons (Leu, Ser, Arg have 6 each). 3 codons are stop codons (UAA, UAG, UGA) — no amino acid. This unequal distribution is "degeneracy" — and it is biologically advantageous because it buffers against mutations.
Trap 4 — "Ribosomes in mitochondria and chloroplasts are 80S like other eukaryotic ribosomes" → WRONG! Mitochondria and chloroplasts have 70S ribosomes (like bacteria) — NOT 80S like the rest of the eukaryotic cell. This is one of the key pieces of evidence for the Endosymbiotic Theory — that mitochondria and chloroplasts evolved from ancient bacteria. 70S ribosomes in these organelles means some antibiotics (e.g. chloramphenicol, streptomycin that target 70S) can potentially affect mitochondrial protein synthesis at high doses.
Trap 5 — "AUG is only the start codon and always signals the start of a new protein" → MISLEADING! AUG is the start codon AND codes for Methionine. In the middle of a coding sequence, AUG simply codes for Methionine (internal Met residue in protein) — it does not restart translation. AUG is only the start codon when it appears in the correct context (Kozak sequence in eukaryotes; Shine-Dalgarno sequence in prokaryotes). Also: every protein begins with Methionine, but the initiating Met is often cleaved off after translation — so the final protein may not have Met at its N-terminus.
