CLASS 12 BIOLOGY
CHAPTER 5
Molecular Basis of Inheritance
Conceptual • NCERT-Focused • NEET-Oriented
What is this chapter really about?
This chapter answers five questions in sequence: What is the genetic material made of? How was DNA proved to be that material? How does DNA copy itself? How does DNA's information become protein? And how do we exploit differences in DNA sequence to identify individuals? Read it as one continuous information pipeline, not as isolated topics.
| MASTER FLOW DNA STRUCTURE (double helix) → PROOF THAT DNA IS THE GENETIC MATERIAL ↓ REPLICATIONreplicationThe process by which DNA makes an identical copy of itself. (DNA → DNA, semiconservative) ↓ TRANSCRIPTIONtranscriptionThe synthesis of RNA using a DNA template. (DNA → RNARNARNA is usually single-stranded and uses ribose sugar and uracil instead of deoxyribose and thymine.) ↓ GENETIC CODE (rules that connect nucleotides to amino acids) ↓ TRANSLATIONtranslationThe synthesis of a polypeptide using the codon sequence of mRNA. (RNARNARNA is usually single-stranded and uses ribose sugar and uracil instead of deoxyribose and thymine. → PROTEIN) ↓ REGULATION OF GENE EXPRESSION (switching genes on/off) ↓ HUMAN GENOME PROJECT + DNA FINGERPRINTINGDNA fingerprintingIdentification based on highly variable repetitive DNA regions in an individual's genome. (applications of sequence knowledge) |
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THE DNA — STRUCTURE OF THE MOLECULE
DNA is a long polymer of deoxyribonucleotides. Its length is defined by the number of nucleotides (or base pairs) it contains — this is itself characteristic of an organism (E. coli: 4.6 × 10⁶ bp; haploid human DNA: 3.3 × 10⁹ bp).
Structure of the Polynucleotide Chain
A nucleotidenucleotideThe basic unit of DNA or RNA, consisting of a base, sugar, and phosphate group. has three components: a nitrogenous base, a pentose sugar, and a phosphate group. A base joined to the sugar (without phosphate) is a nucleosidenucleosideA nitrogenous base joined to a pentose sugar, without a phosphate group.; adding a phosphate group makes it a nucleotidenucleotideThe basic unit of DNA or RNA, consisting of a base, sugar, and phosphate group..
| Feature | DNA | RNARNARNA is usually single-stranded and uses ribose sugar and uracil instead of deoxyribose and thymine. |
|---|---|---|
| Sugar | Deoxyribose | Ribose (has extra 2'-OH group) |
| Pyrimidines | Cytosine, Thymine | Cytosine, Uracil (in place of thymine) |
| Purines | Adenine, Guanine | Adenine, Guanine (same as DNA) |
| Strandedness | Usually double-stranded | Usually single-stranded |
| ⚠️ NEET TRAP Cytosine is common to BOTH DNA and RNARNARNA is usually single-stranded and uses ribose sugar and uracil instead of deoxyribose and thymine.. Only thymine (DNA) vs uracil (RNARNARNA is usually single-stranded and uses ribose sugar and uracil instead of deoxyribose and thymine.) actually differs between the two — do not list cytosine as a DNA-only or RNARNARNA is usually single-stranded and uses ribose sugar and uracil instead of deoxyribose and thymine.-only base. |
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Two nucleotides join through a 3'-5' phosphodiester linkage to form a dinucleotide, and repeating this builds a polynucleotide chain with a free 5'-phosphate at one end and a free 3'-OH at the other — giving the chain a fixed direction (polarity).
The Double Helix
DNA as an acidic nuclear substance was first identified by Friedrich Meischer (1869), who called it 'Nuclein'. It took until 1953 for James Watson and Francis Crick, using X-ray diffraction data from Maurice Wilkins and Rosalind Franklin, to propose the Double Helix model — built on Erwin Chargaff's observation that A:T and G:C ratios are always constant and equal to one.
Made of two polynucleotide chains with a sugar-phosphate backbone on the outside and bases projecting inward.
The two chains are anti-parallel: one runs 5'→3', the other 3'→5'.
Bases pair through hydrogen bonds — Adenine=Thymine (2 H-bonds), Guanine≡Cytosine (3 H-bonds) — always a purine opposite a pyrimidine, keeping strand distance constant.
The two chains are coiled right-handed; helix pitch is 3.4 nm with ~10 bp per turn, so the rise per bp is ~0.34 nm.
Base pairs stack on one another, which — in addition to H-bonding — stabilises the helix.
| ⚠️ NEET TRAP A–T pairs by 2 hydrogen bonds; G–C pairs by 3. More G-C content means a more thermally stable DNA — a frequently tested inference, not stated as a number but expected as reasoning. |
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| 🔥 HIGH-YIELD Helix pitch = 3.4 nm with ~10 bp/turn → rise per base pair ≈ 0.34 nm. These three numbers (3.4 nm, 10 bp, 0.34 nm) are directly interconvertible and a classic calculation-based NEET question. |
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| CENTRAL DOGMA (Francis Crick) DNA —replicationreplicationThe process by which DNA makes an identical copy of itself.→ DNA ↓ DNA —transcriptiontranscriptionThe synthesis of RNA using a DNA template.→ mRNA ↓ mRNA —translationtranslationThe synthesis of a polypeptide using the codon sequence of mRNA.→ Protein |
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| 📖 NCERT FOCUS In some viruses the information flow reverses — RNARNARNA is usually single-stranded and uses ribose sugar and uracil instead of deoxyribose and thymine. to DNA — a process given a simple name: reverse transcriptiontranscriptionThe synthesis of RNA using a DNA template.. This exception is directly referenced by NCERT as a discussion prompt. |
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Packaging of DNA Helix
The full length of DNA in a mammalian cell (~2.2 metres) is far greater than the nucleus (~10⁻⁶ m), so it must be tightly packaged.
| Organism type | Packaging |
|---|---|
| Prokaryotes (e.g. E. coli) | No true nucleus; DNA (negatively charged) held with positively charged proteins in a region called the nucleoid, organised in large loops |
| Eukaryotes | DNA wrapped around a histone octamer (8 positively charged histone molecules, rich in lysine and arginine) to form a nucleosomenucleosomeDNA wrapped around a histone octamer; the basic unit of chromatin packaging.; nucleosomes form 'beads-on-string' chromatin, further coiled and condensed into chromosomes |
| 🔥 HIGH-YIELD A typical nucleosomenucleosomeDNA wrapped around a histone octamer; the basic unit of chromatin packaging. contains 200 bp of DNA wrapped around a histone octamer. H1 histone sits outside the octamer core and helps in higher-order packaging — a fine distinction NEET sometimes tests. |
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| Chromatin type | Packing | Staining | Activity |
|---|---|---|---|
| Euchromatin | Loosely packed | Stains light | Transcriptionally active |
| Heterochromatin | Densely packed | Stains dark | Transcriptionally inactive |
THE SEARCH FOR GENETIC MATERIAL
Griffith's Transforming Principle (1928)
Frederick Griffith worked with Streptococcus pneumoniae: the S strain (smooth, polysaccharide coat, virulent) kills mice; the R strain (rough, no coat) does not.
| GRIFFITH'S EXPERIMENT FLOW S strain (live) → injected into mice → Mice die ↓ R strain (live) → injected into mice → Mice live ↓ S strain (heat-killed) → injected into mice → Mice live ↓ S strain (heat-killed) + R strain (live) → injected into mice → Mice DIE (live S bacteria recovered) |
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| ⚠️ NEET TRAP Griffith showed that SOME 'transforming principle' from heat-killed S strain converted live R into virulent S — but he did NOT identify the biochemical nature of that principle. That came later, from Avery, MacLeod and McCarty. |
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Biochemical Characterisation (Avery, MacLeod, McCarty, 1933–44)
They purified biochemicals (proteins, DNA, RNARNARNA is usually single-stranded and uses ribose sugar and uracil instead of deoxyribose and thymine.) from heat-killed S cells and tested each for transforming ability. Protein-digesting enzymes (proteases) and RNARNARNA is usually single-stranded and uses ribose sugar and uracil instead of deoxyribose and thymine.-digesting enzymes (RNases) did NOT stop transformation; only DNase digestion inhibited transformation — proving DNA, not protein or RNARNARNA is usually single-stranded and uses ribose sugar and uracil instead of deoxyribose and thymine., was the transforming principle.
The Hershey–Chase Experiment (1952)
Working with bacteriophages, Hershey and Chase grew viruses with radioactive phosphorus (³²P) (labels DNA, since protein lacks phosphorus) or radioactive sulfur (³⁵S) (labels protein, since DNA lacks sulfur).
| HERSHEY–CHASE FLOW Radioactive phage infects E. coli ↓ Blending removes viral coats from bacterial surface ↓ Centrifugation separates bacteria from viral debris ↓ ³²P (DNA-labelled) phages → radioactivity found INSIDE bacteria ↓ ³⁵S (protein-labelled) phages → radioactivity found in the supernatant, NOT inside bacteria |
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| 🔥 HIGH-YIELD Hershey–Chase gave the unequivocal proof that DNA (not protein) is the genetic material passed from virus to bacterium — this is the single most quoted experiment for this claim. |
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Why DNA Over RNARNARNA is usually single-stranded and uses ribose sugar and uracil instead of deoxyribose and thymine.? Properties of the Genetic Material
A molecule qualifying as genetic material must: (i) replicate, (ii) be chemically/structurally stable, (iii) allow slow mutations for evolution, and (iv) be able to express itself as Mendelian characters.
| Property | DNA | RNARNARNA is usually single-stranded and uses ribose sugar and uracil instead of deoxyribose and thymine. |
|---|---|---|
| Stability | More stable (no reactive 2'-OH group) | Labile, easily degraded (reactive 2'-OH group) |
| MutationmutationA heritable change in the nucleotide sequence of genetic material. rate | Lower (more stable) | Higher — RNARNARNA is usually single-stranded and uses ribose sugar and uracil instead of deoxyribose and thymine. viruses mutate and evolve faster |
| Can replicate? | Yes | Yes |
| Can directly code for protein? | No — depends on RNARNARNA is usually single-stranded and uses ribose sugar and uracil instead of deoxyribose and thymine. | Yes — directly translatable |
| Preferred role | Storage of genetic information | Transmission/expression of information |
| ⚠️ NEET TRAP Thymine (in DNA) instead of uracil also adds chemical stability. RNARNARNA is usually single-stranded and uses ribose sugar and uracil instead of deoxyribose and thymine.'s extra 2'-OH group is what makes it reactive and easily degraded — this structural detail is the actual reason DNA outcompetes RNARNARNA is usually single-stranded and uses ribose sugar and uracil instead of deoxyribose and thymine. for long-term storage. |
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RNARNARNA is usually single-stranded and uses ribose sugar and uracil instead of deoxyribose and thymine. WORLD
Evidence suggests RNARNARNA is usually single-stranded and uses ribose sugar and uracil instead of deoxyribose and thymine. was the first genetic material — essential life processes (metabolism, translationtranslationThe synthesis of a polypeptide using the codon sequence of mRNA., splicing) evolved around RNARNARNA is usually single-stranded and uses ribose sugar and uracil instead of deoxyribose and thymine., and RNARNARNA is usually single-stranded and uses ribose sugar and uracil instead of deoxyribose and thymine. could act as both genetic material and catalyst. Because RNARNARNA is usually single-stranded and uses ribose sugar and uracil instead of deoxyribose and thymine.-as-catalyst is reactive and unstable, DNA evolved from RNARNARNA is usually single-stranded and uses ribose sugar and uracil instead of deoxyribose and thymine. with chemical modifications for greater stability; being double-stranded and complementary, DNA further resists change through repair mechanisms.
REPLICATIONreplicationThe process by which DNA makes an identical copy of itself.
Watson and Crick immediately recognised that base pairing itself suggests a copying mechanism: the two strands separate, and each acts as a template for a new complementary strand. Each resulting DNA molecule has one parental and one new strand — this is semiconservative replicationsemiconservative replicationDNA replication in which each daughter molecule contains one parental and one newly made strand..
Meselson–Stahl Experiment (1958) — The Proof
| MESELSON–STAHL FLOW E. coli grown in ¹⁵NH₄Cl (heavy nitrogen) for many generations → all DNA is heavy (¹⁵N-¹⁵N) ↓ Cells transferred to normal ¹⁴NH₄Cl medium ↓ After 1 generation (20 min): DNA is entirely hybrid density (¹⁴N-¹⁵N) ↓ After 2 generations (40 min): DNA is 50% hybrid (¹⁴N-¹⁵N), 50% light (¹⁴N-¹⁴N) ↓ Densities separated and measured by CsCl density-gradient centrifugation |
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| ⚠️ NEET TRAP ¹⁵N is a heavy but NON-radioactive isotope — it is separated from ¹⁴N purely by density (centrifugation), not by radioactivity. Confusing this with a radioactive-tracer method is a common error. |
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| 📖 NCERT FOCUS Taylor and colleagues (1958) repeated an analogous experiment using radioactive thymidine on Vicia faba (faba bean), confirming that chromosomal DNA also replicates semiconservatively in a eukaryote. |
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Enzymes and the ReplicationreplicationThe process by which DNA makes an identical copy of itself. Fork
DNA-dependent DNA polymerase uses a DNA template to polymerise deoxynucleotides — extremely fast (E. coli: ~2000 bp/second) and highly accurate. Deoxyribonucleoside triphosphates serve a dual role: substrates AND energy source (their terminal phosphates are high-energy, like ATP).
Because DNA polymerase works only 5'→3', and the two template strands run antiparallel, replicationreplicationThe process by which DNA makes an identical copy of itself. proceeds differently on each strand at the replicationreplicationThe process by which DNA makes an identical copy of itself. fork:
| Strand | Template polarity | Synthesis type |
|---|---|---|
| Leading strand | 3'→5' template | Continuous |
| Lagging strand | 5'→3' template | Discontinuous (fragments later joined by DNA ligase) |
| 🔥 HIGH-YIELD DNA ligase joins the discontinuously synthesised fragments on the lagging strand — a frequently tested enzyme-function pairing. |
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| 📖 NCERT FOCUS ReplicationreplicationThe process by which DNA makes an identical copy of itself. does not start randomly — it begins at a definite region, the origin of replicationreplicationThe process by which DNA makes an identical copy of itself.. This is why vectors used in recombinant DNA technology must provide an origin of replicationreplicationThe process by which DNA makes an identical copy of itself. to be propagated. In eukaryotes, replicationreplicationThe process by which DNA makes an identical copy of itself. occurs during the S-phase of the cell cycle. |
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TRANSCRIPTIONtranscriptionThe synthesis of RNA using a DNA template.
TranscriptiontranscriptionThe synthesis of RNA using a DNA template. copies genetic information from one strand of DNA into RNARNARNA is usually single-stranded and uses ribose sugar and uracil instead of deoxyribose and thymine., governed by complementarity (adenine pairs with uracil, not thymine, in the new RNARNARNA is usually single-stranded and uses ribose sugar and uracil instead of deoxyribose and thymine.). Unlike replicationreplicationThe process by which DNA makes an identical copy of itself., only a segment of DNA and only one strand is copied.
| 📖 NCERT FOCUS If both DNA strands were transcribed, they would produce two different RNARNARNA is usually single-stranded and uses ribose sugar and uracil instead of deoxyribose and thymine. sequences (complementary ≠ identical) coding for two different proteins from one DNA segment, AND the two RNAs — being complementary to each other — would form double-stranded RNARNARNA is usually single-stranded and uses ribose sugar and uracil instead of deoxyribose and thymine. that cannot be translated. Both reasons make single-strand transcriptiontranscriptionThe synthesis of RNA using a DNA template. necessary. |
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The TranscriptiontranscriptionThe synthesis of RNA using a DNA template. Unit
Defined by three regions: a promoter, the structural gene, and a terminator.
| Strand | Polarity | Role |
|---|---|---|
| Template strand | 3'→5' | Copied by RNARNARNA is usually single-stranded and uses ribose sugar and uracil instead of deoxyribose and thymine. polymerase to synthesise RNARNARNA is usually single-stranded and uses ribose sugar and uracil instead of deoxyribose and thymine. |
| Coding strand | 5'→3' | Same sequence as RNARNARNA is usually single-stranded and uses ribose sugar and uracil instead of deoxyribose and thymine. (except T instead of U); displaced during transcriptiontranscriptionThe synthesis of RNA using a DNA template., does NOT itself code for anything |
| ⚠️ NEET TRAP The 'coding strand' is named for having the same sequence as the RNARNARNA is usually single-stranded and uses ribose sugar and uracil instead of deoxyribose and thymine. product, but it is the TEMPLATE strand that is actually read and copied by RNARNARNA is usually single-stranded and uses ribose sugar and uracil instead of deoxyribose and thymine. polymerase. This naming irony is explicitly flagged by NCERT and is a favourite trick question. |
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The promoter lies towards the 5'-end (upstream) of the structural gene (referenced to the coding strand) and provides the RNARNARNA is usually single-stranded and uses ribose sugar and uracil instead of deoxyribose and thymine. polymerase binding site; it is the promoter's presence that fixes which strand is template and which is coding. The terminator lies towards the 3'-end (downstream) and usually ends transcriptiontranscriptionThe synthesis of RNA using a DNA template..
TranscriptiontranscriptionThe synthesis of RNA using a DNA template. Unit vs Gene
A cistron is a DNA segment coding for one polypeptide. A structural gene may be monocistronic (mostly eukaryotes) or polycistronic (mostly bacteria). In eukaryotes, genes are split — coding sequences (exons, which appear in mature RNARNARNA is usually single-stranded and uses ribose sugar and uracil instead of deoxyribose and thymine.) are interrupted by non-coding introns (removed before the RNARNARNA is usually single-stranded and uses ribose sugar and uracil instead of deoxyribose and thymine. is mature).
Types of RNARNARNA is usually single-stranded and uses ribose sugar and uracil instead of deoxyribose and thymine. and the TranscriptiontranscriptionThe synthesis of RNA using a DNA template. Process
| RNARNARNA is usually single-stranded and uses ribose sugar and uracil instead of deoxyribose and thymine. type | Role |
|---|---|
| mRNA (messenger) | Provides the template read during translationtranslationThe synthesis of a polypeptide using the codon sequence of mRNA. |
| tRNA (transfer) | Brings amino acids and reads the genetic codeThe genetic codeThe genetic code is the set of rules by which nucleotide triplets (codons) specify amino acids during translation. |
| rRNA (ribosomal) | Structural and catalytic role in translationtranslationThe synthesis of a polypeptide using the codon sequence of mRNA. |
In bacteria, a single RNARNARNA is usually single-stranded and uses ribose sugar and uracil instead of deoxyribose and thymine. polymerase transcribes all RNARNARNA is usually single-stranded and uses ribose sugar and uracil instead of deoxyribose and thymine. types. It cannot itself initiate or terminate — it associates transiently with an initiation factor (σ, sigma) and a termination factor (ρ, rho) for those steps, catalysing only elongation itself.
| 🔥 HIGH-YIELD Because bacteria lack a nuclear membrane, transcriptiontranscriptionThe synthesis of RNA using a DNA template. and translationtranslationThe synthesis of a polypeptide using the codon sequence of mRNA. happen in the same compartment and can be COUPLED — translationtranslationThe synthesis of a polypeptide using the codon sequence of mRNA. can begin before transcriptiontranscriptionThe synthesis of RNA using a DNA template. of the mRNA finishes. This is impossible in eukaryotes, where the nucleus separates the two processes. |
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In eukaryotes, three nuclear RNARNARNA is usually single-stranded and uses ribose sugar and uracil instead of deoxyribose and thymine. polymerases divide the labour:
| Enzyme | Transcribes |
|---|---|
| RNARNARNA is usually single-stranded and uses ribose sugar and uracil instead of deoxyribose and thymine. polymerase I | rRNAs (28S, 18S, 5.8S) |
| RNARNARNA is usually single-stranded and uses ribose sugar and uracil instead of deoxyribose and thymine. polymerase II | hnRNA (precursor of mRNA) |
| RNARNARNA is usually single-stranded and uses ribose sugar and uracil instead of deoxyribose and thymine. polymerase III | tRNA, 5S rRNA, snRNAs |
| ⚠️ NEET TRAP RNARNARNA is usually single-stranded and uses ribose sugar and uracil instead of deoxyribose and thymine. polymerase II makes hnRNA (the mRNA precursor), NOT mature mRNA directly. Confusing which polymerase makes which RNARNARNA is usually single-stranded and uses ribose sugar and uracil instead of deoxyribose and thymine. is one of the most reliably tested facts in this chapter. |
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| hnRNA → mRNA PROCESSING FLOW hnRNA (contains both exons and introns, non-functional) ↓ Splicing — introns removed, exons joined in order ↓ Capping — methyl guanosine triphosphate cap added at 5'-end ↓ Tailing — 200–300 adenylate residues added at 3'-end (template-independent) ↓ Mature mRNA → transported out of the nucleus for translationtranslationThe synthesis of a polypeptide using the codon sequence of mRNA. |
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| 📖 NCERT FOCUS The split-gene arrangement and splicing are considered evidence of an ancient RNARNARNA is usually single-stranded and uses ribose sugar and uracil instead of deoxyribose and thymine.-world legacy retained in modern eukaryotic genomes. |
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GENETIC CODE
TranslationtranslationThe synthesis of a polypeptide using the codon sequence of mRNA. needs to convert a nucleotidenucleotideThe basic unit of DNA or RNA, consisting of a base, sugar, and phosphate group. sequence into an amino acid sequence, but no complementarity exists between nucleotides and amino acids — a code was needed. George Gamow reasoned that since 4 bases must code for 20 amino acids, the code must use combinations of bases; a doublet code (4²=16) is insufficient, so he proposed a triplet code (4³=64 codons) — more than enough.
Har Gobind Khorana synthesised RNAs with defined base combinations; Marshall Nirenberg's cell-free protein-synthesising system helped decode the codons; Severo Ochoa's enzyme (polynucleotide phosphorylase) helped polymerise RNARNARNA is usually single-stranded and uses ribose sugar and uracil instead of deoxyribose and thymine. with defined sequences without a template.
Salient Features of the Genetic CodeThe genetic codeThe genetic code is the set of rules by which nucleotide triplets (codons) specify amino acids during translation.
The codoncodonA sequence of three nucleotides on mRNA that specifies an amino acid or stop signal. is a triplet; 61 codons code for amino acids, 3 are stop codons (UAA, UAG, UGA).
The code is degenerate — most amino acids are coded by more than one codoncodonA sequence of three nucleotides on mRNA that specifies an amino acid or stop signal..
The codoncodonA sequence of three nucleotides on mRNA that specifies an amino acid or stop signal. is read contiguously in mRNA — no punctuation between codons.
The code is nearly universal (UUU = Phenylalanine from bacteria to humans), with exceptions in mitochondrial codons and some protozoans.
AUG has a dual role: it codes for Methionine AND acts as the initiator (start) codoncodonA sequence of three nucleotides on mRNA that specifies an amino acid or stop signal..
| 🔥 HIGH-YIELD AUG = start codoncodonA sequence of three nucleotides on mRNA that specifies an amino acid or stop signal. AND codes for methionine — a two-in-one fact frequently tested directly. UAA, UAG, UGA are the three stop codons; there is no tRNA for any of them. |
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Mutations and the Genetic CodeThe genetic codeThe genetic code is the set of rules by which nucleotide triplets (codons) specify amino acids during translation.
A classic point mutationmutationA heritable change in the nucleotide sequence of genetic material. example: a single base change in the beta-globin gene changes the amino acid glutamate to valine, causing sickle cell anemia.
| ⚠️ NEET TRAP Insertion or deletion of ONE or TWO bases shifts the reading frame from that point onward — a frameshift mutationmutationA heritable change in the nucleotide sequence of genetic material.. Insertion or deletion of THREE bases (or a multiple of three) adds/removes whole codons (amino acids) but leaves the reading frame downstream UNCHANGED. Getting this '1 or 2 vs 3' distinction backwards is a common error. |
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tRNA — The Adapter Molecule
Francis Crick predicted the need for an adapter molecule to link the code to amino acids, since amino acids cannot read nucleotidenucleotideThe basic unit of DNA or RNA, consisting of a base, sugar, and phosphate group. sequences directly. tRNA (once called sRNA) fills this role: it has an anticodonanticodonA three-base sequence on tRNA complementary to an mRNA codon. loop (bases complementary to the codoncodonA sequence of three nucleotides on mRNA that specifies an amino acid or stop signal.) and an amino acid acceptor end. tRNAs are specific to each amino acid; a special initiator tRNA starts translationtranslationThe synthesis of a polypeptide using the codon sequence of mRNA.; there is no tRNA for stop codons. Structurally, tRNA looks like a cloverleaf in 2D but an inverted L in its actual 3D shape.
TRANSLATIONtranslationThe synthesis of a polypeptide using the codon sequence of mRNA.
TranslationtranslationThe synthesis of a polypeptide using the codon sequence of mRNA. polymerises amino acids into a polypeptide, in the order dictated by mRNA codons, joined by peptide bonds. Amino acids are first activated (using ATP) and linked to their specific tRNA — charging of tRNA (aminoacylation).
The ribosome — made of structural RNAs and ~80 proteins, existing as large and small subunits — is the site of translationtranslationThe synthesis of a polypeptide using the codon sequence of mRNA.. It also acts as a catalyst for peptide bond formation via its rRNA (23S rRNA in bacteria is a ribozyme).
| TRANSLATIONtranslationThe synthesis of a polypeptide using the codon sequence of mRNA. FLOW Small ribosomal subunit binds mRNA at the start codoncodonA sequence of three nucleotides on mRNA that specifies an amino acid or stop signal. (AUG), recognised by initiator tRNA — INITIATION ↓ Charged tRNAs bind sequentially to codons via anticodonanticodonA three-base sequence on tRNA complementary to an mRNA codon. base-pairing; ribosome moves codoncodonA sequence of three nucleotides on mRNA that specifies an amino acid or stop signal. to codoncodonA sequence of three nucleotides on mRNA that specifies an amino acid or stop signal. — ELONGATION (peptide bonds form) ↓ Release factor binds the stop codoncodonA sequence of three nucleotides on mRNA that specifies an amino acid or stop signal. → completed polypeptide released — TERMINATION |
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| 📖 NCERT FOCUS A translational unit in mRNA is flanked by the start and stop codons, but mRNA also carries untranslated regions (UTRs) at both the 5'-end (before start codoncodonA sequence of three nucleotides on mRNA that specifies an amino acid or stop signal.) and 3'-end (after stop codoncodonA sequence of three nucleotides on mRNA that specifies an amino acid or stop signal.), needed for efficient translationtranslationThe synthesis of a polypeptide using the codon sequence of mRNA.. |
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REGULATION OF GENE EXPRESSION
Gene expression can be regulated at several levels in eukaryotes: transcriptional (forming the primary transcript), processing (splicing), transport of mRNA out of the nucleus, and translational level. In prokaryotes, transcriptional initiation is the predominant control point.
Regulatory proteins can act as activators (positive regulation) or repressors (negative regulation), often by interacting with a DNA sequence called the operator, adjacent to the promoter.
The Lac OperonoperonA group of genes regulated together under the control of one promoter and operator.
Elucidated by Francois Jacob and Jacque Monod, the lac operonoperonA group of genes regulated together under the control of one promoter and operator. is a polycistronic structural gene arrangement regulated by one promoter and regulatory sequences — the prototype operonoperonA group of genes regulated together under the control of one promoter and operator..
| Gene | Product | Function |
|---|---|---|
| i (regulatory gene) | Repressor protein | Binds the operator, blocks transcriptiontranscriptionThe synthesis of RNA using a DNA template. in the absence of lactose |
| z | β-galactosidase | Hydrolyses lactose into galactose + glucose |
| y | Permease | Increases cell permeability to β-galactosides |
| a | Transacetylase | Additional lactose-metabolism enzyme |
| ⚠️ NEET TRAP The 'i' in the i gene stands for INHIBITOR, not 'inducer' — NCERT explicitly flags this naming trap. The i gene itself codes for the repressor, which blocks the operonoperonA group of genes regulated together under the control of one promoter and operator.; it is lactose (the inducer) that inactivates the repressor. |
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| LAC OPERONoperonA group of genes regulated together under the control of one promoter and operator. REGULATION FLOW Without lactose: i gene → repressor protein → binds operator → blocks RNARNARNA is usually single-stranded and uses ribose sugar and uracil instead of deoxyribose and thymine. polymerase → operonoperonA group of genes regulated together under the control of one promoter and operator. OFF ↓ With lactose (inducer) present: lactose binds and inactivates the repressor → operator freed → RNARNARNA is usually single-stranded and uses ribose sugar and uracil instead of deoxyribose and thymine. polymerase transcribes z, y, a → operonoperonA group of genes regulated together under the control of one promoter and operator. ON |
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| 📖 NCERT FOCUS Glucose and galactose CANNOT act as inducers of the lac operonoperonA group of genes regulated together under the control of one promoter and operator. — only lactose (or allolactose) can. This is regulation of enzyme synthesis by its own substrate. |
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| 🔥 HIGH-YIELD Regulation of the lac operonoperonA group of genes regulated together under the control of one promoter and operator. by the repressor is called NEGATIVE regulation. (Positive regulation of the same operonoperonA group of genes regulated together under the control of one promoter and operator. exists too, but is beyond NCERT's discussion depth — NEET rarely tests it.) |
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HUMAN GENOME PROJECT
Launched in 1990, this 13-year 'mega project' (completed 2003) aimed to sequence every base pair of the human genome, coordinated by the US Department of Energy and NIH, with major contributions from the Wellcome Trust (UK), Japan, France, Germany, China and others.
Goals of HGP
Identify all ~20,000–25,000 genes in human DNA.
Determine the sequence of all 3 billion base pairs.
Store this information in databases; improve data-analysis tools.
Transfer related technologies to other sectors (e.g. industry).
Address Ethical, Legal, and Social Issues (ELSI) arising from the project.
Two sequencing approaches were used: Expressed Sequence Tags (ESTs), identifying genes expressed as RNARNARNA is usually single-stranded and uses ribose sugar and uracil instead of deoxyribose and thymine., and whole-genome shotgun sequencing followed by Sequence Annotation (assigning function afterwards). DNA fragments were cloned into BAC (bacterial artificial chromosomes) and YAC (yeast artificial chromosomes) vectors and sequenced using automated sequencers based on Frederick Sanger's method.
Salient Features of the Human Genome
Human genome ≈ 3164.7 million bp.
Average gene ≈ 3000 bases; the largest known human gene is dystrophin at 2.4 million bases.
Total genes estimated at ~30,000 — far lower than earlier estimates of 80,000–1,40,000.
99.9% of nucleotidenucleotideThe basic unit of DNA or RNA, consisting of a base, sugar, and phosphate group. bases are identical across all humans.
Functions are unknown for over 50% of discovered genes.
Less than 2% of the genome codes for proteins; repetitive sequences make up a very large portion.
Chromosome 1 has the most genes (2968); the Y chromosome has the fewest (231).
About 1.4 million SNP (single nucleotidenucleotideThe basic unit of DNA or RNA, consisting of a base, sugar, and phosphate group. polymorphism) locations have been identified.
| 🔥 HIGH-YIELD Only ~30,000 human genes, but <2% of the genome codes for protein, while 99.9% of bases are identical between any two humans — three of the most frequently quoted HGP statistics. |
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DNA FINGERPRINTINGDNA fingerprintingIdentification based on highly variable repetitive DNA regions in an individual's genome.
Since 99.9% of human DNA is identical between individuals, identification relies on the remaining variable regions. DNA fingerprintingDNA fingerprintingIdentification based on highly variable repetitive DNA regions in an individual's genome. compares individuals quickly using repetitive DNA — short sequences repeated many times, which separate from bulk genomic DNA as distinct peaks (satellite DNA) during density-gradient centrifugation.
Developed by Alec Jeffreys, using satellite DNA probes showing high polymorphism, called VNTR (Variable Number of Tandem Repeats) — a class of mini-satellite DNA.
| DNA FINGERPRINTINGDNA fingerprintingIdentification based on highly variable repetitive DNA regions in an individual's genome. TECHNIQUE FLOW Isolation of DNA ↓ Digestion by restriction endonucleases ↓ Separation of fragments by electrophoresis ↓ Blotting (transfer) onto nitrocellulose/nylon membrane ↓ Hybridisation using a labelled VNTR probe ↓ Detection of hybridised fragments by autoradiography → banding pattern |
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| 🔥 HIGH-YIELD VNTR copy number varies between individuals (except monozygotic/identical twins), producing a unique banding pattern — the basis of both forensic identification and paternity testing. PCR sensitivity now allows fingerprinting from a single cell. |
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| 📖 NCERT FOCUS DNA polymorphism (variation at the genetic level) arises from mutations and is more likely to persist, unselected, in non-coding DNA — this is why repetitive/satellite regions, not coding genes, are used for fingerprinting. |
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MUST-KNOW DIAGRAMS
⭐ MUST KNOW — DNA Double Helix / Double-Stranded Polynucleotide Chain
Structure → Label → Function: Label the sugar-phosphate backbone on the outside, bases projecting inward, and the A-T (2 H-bonds) / G-C (3 H-bonds) pairing between antiparallel strands. The function tested is always the constancy of base-pairing rules and the resulting uniform helix width.
⭐ MUST KNOW — NucleosomenucleosomeDNA wrapped around a histone octamer; the basic unit of chromatin packaging. / 'Beads-on-String'
Structure → Label → Function: Label the histone octamer (core), the wrapped DNA (~200 bp), and the H1 histone outside the core. The function tested is how negatively-charged DNA is compactly packaged around positively-charged histones.
⭐ MUST KNOW — The Replicating Fork
Structure → Label → Function: Label the parental template strands, the continuously synthesised (leading) strand, and the discontinuously synthesised (lagging) strand with its fragments. The function tested is why one strand is continuous and the other discontinuous — both stem from DNA polymerase's strict 5'→3' synthesis direction.
⭐ MUST KNOW — TranscriptiontranscriptionThe synthesis of RNA using a DNA template. Unit
Structure → Label → Function: Label promoter, structural gene, terminator, template strand (3'→5') and coding strand (5'→3'). The function tested is correctly identifying which strand is actually copied (template) versus which merely resembles the RNARNARNA is usually single-stranded and uses ribose sugar and uracil instead of deoxyribose and thymine. product (coding).
Understand the structure — The Lac OperonoperonA group of genes regulated together under the control of one promoter and operator.
Structure → Label → Function: Label the i gene, promoter (p), operator (o), and structural genes z-y-a, in both the presence and absence of the inducer. Conceptual understanding of the repressor–operator–inducer relationship matters more than reproducing exact operonoperonA group of genes regulated together under the control of one promoter and operator. layout.
LAST-MINUTE CONFUSION BUSTERS
| NucleosidenucleosideA nitrogenous base joined to a pentose sugar, without a phosphate group. base + sugar | NucleotidenucleotideThe basic unit of DNA or RNA, consisting of a base, sugar, and phosphate group. → base + sugar + phosphate |
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| Template strand actually copied by RNARNARNA is usually single-stranded and uses ribose sugar and uracil instead of deoxyribose and thymine. polymerase | Coding strand → same sequence as RNARNARNA is usually single-stranded and uses ribose sugar and uracil instead of deoxyribose and thymine., but NOT copied |
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| Exons appear in mature RNARNARNA is usually single-stranded and uses ribose sugar and uracil instead of deoxyribose and thymine. | Introns → removed during splicing |
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| Leading strand continuous synthesis | Lagging strand → discontinuous, joined by DNA ligase |
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| RNARNARNA is usually single-stranded and uses ribose sugar and uracil instead of deoxyribose and thymine. polymerase II makes hnRNA (mRNA precursor) | RNARNARNA is usually single-stranded and uses ribose sugar and uracil instead of deoxyribose and thymine. polymerase I → rRNA | RNARNARNA is usually single-stranded and uses ribose sugar and uracil instead of deoxyribose and thymine. polymerase III → tRNA, 5S rRNA |
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| Frameshift mutationmutationA heritable change in the nucleotide sequence of genetic material. insertion/deletion of 1 or 2 bases | In-frame mutationmutationA heritable change in the nucleotide sequence of genetic material. → insertion/deletion of 3 (or multiples of 3) bases |
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| i gene stands for Inhibitor, codes for the repressor | Inducer → lactose, inactivates the repressor |
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| Repetitive/Satellite DNA basis of DNA fingerprintingDNA fingerprintingIdentification based on highly variable repetitive DNA regions in an individual's genome. | Structural genes → basis of protein-coding function |
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| Griffith showed transformation occurs (didn't identify DNA) | Avery-MacLeod-McCarty → proved DNA is the transforming principle | Hershey-Chase → unequivocal proof DNA is genetic material |
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ACTIVE RECALL
Why does adenine always pair with thymine and guanine with cytosine, and how many hydrogen bonds does each involve?
Explain why the 'coding strand' does not actually code for anything during transcriptiontranscriptionThe synthesis of RNA using a DNA template..
Why is replicationreplicationThe process by which DNA makes an identical copy of itself. continuous on one strand and discontinuous on the other?
Which enzyme joins the Okazaki-type fragments on the lagging strand?
What is the difference in outcome between a frameshift mutationmutationA heritable change in the nucleotide sequence of genetic material. and an in-frame (multiple-of-three) mutationmutationA heritable change in the nucleotide sequence of genetic material.?
Why is AUG described as having a 'dual function'?
Explain, step by step, why lactose but not glucose can induce the lac operonoperonA group of genes regulated together under the control of one promoter and operator..
Why is the i gene name potentially misleading, and what does it actually code for?
Why is DNA fingerprintingDNA fingerprintingIdentification based on highly variable repetitive DNA regions in an individual's genome. done using repetitive/satellite DNA rather than coding genes?
State two reasons NCERT gives for why only one DNA strand is transcribed.