1ποΈ How to Study
Six sittings. This chapter mixes story (experiments), machinery (processes) and numbers (dozens of them). Learn the experiments as stories, the processes as flowcharts and drill the numbers separately.
Day 1 β DNA structure & packaging
Polynucleotide chemistry, Watson-Crick features (pitch, bp/turn), Chargaff, nucleosome and chromatin. Memorise every number.
Day 2 β The three experiments
Griffith 1928 β Avery-MacLeod-McCarty 1944 β Hershey-Chase 1952. Then DNA vs RNA as genetic material + RNA world.
Day 3 β Replication
Meselson-Stahl generation-by-generation, Taylor's Vicia faba, enzymes, replication fork, continuous vs discontinuous strands.
Day 4 β Transcription & genetic code
Transcription unit, template vs coding strand, bacterial vs eukaryotic transcription, hnRNA processing, all 6 code features.
Day 5 β Translation & lac operon
tRNA, charging, ribosome, UTRs; then i-z-y-a genes, inducer logic, negative regulation.
Day 6 β HGP + Fingerprinting + Drill
HGP goals, salient features (numbers!), Sanger/BAC/YAC; VNTR steps of fingerprinting; then Traps and A-R drill.
2π Topic Map
| Block | What it contains | NEET priority |
|---|---|---|
| 5.1 Β· The DNA | Polynucleotide chain, double-helix features, central dogma, nucleosome & chromatin packaging | Highest |
| 5.2 Β· Search for genetic material | Griffith, Avery-MacLeod-McCarty, Hershey-Chase; DNA vs RNA criteria | Highest β asked every year in some form |
| 5.3 Β· RNA world | RNA first genetic material; catalyst but unstable; DNA evolved from RNA | Medium |
| 5.4 Β· Replication | Semiconservative; Meselson-Stahl; enzymes; fork; origin of replication | Highest |
| 5.5 Β· Transcription | Transcription unit; template/coding strands; gene, cistron, exon/intron; 3 RNA polymerases; splicing/capping/tailing | Highest |
| 5.6 Β· Genetic code | Gamow, Khorana, Nirenberg, Ochoa; 6 salient features; frameshift mutations; tRNA adapter | Highest |
| 5.7 Β· Translation | Charging of tRNA, ribosome as ribozyme, UTRs, initiation β elongation β termination | High |
| 5.8 Β· Lac operon | i-z-y-a genes, inducer, repressor-operator, negative regulation | Highest |
| 5.9 Β· HGP | Goals, methodology (ESTs, annotation, BAC/YAC, Sanger), salient features | High β number questions |
| 5.10 Β· DNA fingerprinting | Repetitive/satellite DNA, polymorphism, VNTR, 6 steps, applications | High |
3π Line by Line β the whole chapter in the simplest words
Every important NCERT line, then what it really means in plain language.
𧬠5.1 Β· The DNA β structure
"DNA is a long polymer of deoxyribonucleotides... ΟΓ174 has 5386 nucleotides, Bacteriophage lambda has 48502 base pairs, Escherichia coli has 4.6 Γ 10βΆ bp, and haploid content of human DNA is 3.3 Γ 10βΉ bp."
DNA length is counted in nucleotides or base pairs, and the count is an identity card: ΟΓ174 (5386 nt β single number, single-stranded!), lambda (48,502 bp), E. coli (4.6 million bp), human haploid (3.3 billion bp). These four numbers appear directly in NEET options.
"A nucleotide has three components β a nitrogenous base, a pentose sugar... and a phosphate group... base + sugar via N-glycosidic linkage = nucleoside; + phosphate at 5'-OH via phosphoester linkage = nucleotide. Two nucleotides are linked through 3'-5' phosphodiester linkage."
Build-up ladder: base + sugar = nucleoside (adenosine, guanosine, cytidine, uridine...). Add phosphate = nucleotide. Chain them with 3'β5' phosphodiester bonds. The chain has a free phosphate at the 5' end and a free 3'-OH at the 3' end; the sugar-phosphate is the backbone, bases project inward. RNA differs by a 2'-OH on ribose and uracil instead of thymine (thymine = 5-methyl uracil).
"DNA as an acidic substance present in nucleus was first identified by Friedrich Meischer in 1869. He named it as 'Nuclein'... in 1953 James Watson and Francis Crick, based on the X-ray diffraction data produced by Maurice Wilkins and Rosalind Franklin, proposed... the Double Helix model... based on the observation of Erwin Chargaff that... A:T and G:C ratios are constant and equal one."
Timeline: Meischer 1869 "nuclein" β Chargaff: A=T, G=C β Wilkins & Franklin: X-ray photos β Watson & Crick 1953: double helix. Complementarity means knowing one strand predicts the other β and hints at how DNA copies itself.
"Salient features of the Double-helix: two chains with anti-parallel polarity; A pairs T with two H-bonds, G pairs C with three; purine always opposite pyrimidine; coiled right-handed; pitch 3.4 nm with roughly 10 bp per turn; distance between base pairs ~0.34 nm; base stacking confers stability."
The exam skeleton: antiparallel (5'β3' against 3'β5'), A=T (2 bonds), Gβ‘C (3 bonds), purine-opposite-pyrimidine keeps the width constant, right-handed helix, 3.4 nm pitch, 10 bp/turn, 0.34 nm between bases. Stability comes from H-bonds PLUS base stacking. Crick then framed the Central Dogma: DNA β RNA β Protein (reversed in some viruses β reverse transcription).
"6.6 Γ 10βΉ bp Γ 0.34 Γ 10β»βΉ m/bp... comes out to be approximately 2.2 metres. A length far greater than the dimension of a typical nucleus (approximately 10β»βΆ m)."
A single cell's DNA, stretched out, is 2.2 metres β stuffed into a nucleus a millionth of a metre wide. (Note the calculation uses the diploid 6.6 Γ 10βΉ bp.) Hence packaging.
"In prokaryotes... DNA (being negatively charged) is held with some proteins (that have positive charges) in a region termed as 'nucleoid'... In eukaryotes... positively charged, basic proteins called histones... rich in lysine and arginine... organised to form a unit of eight molecules called histone octamer. The negatively charged DNA is wrapped around the positively charged histone octamer to form a structure called nucleosome... A typical nucleosome contains 200 bp of DNA helix."
Prokaryotes: DNA in loops = nucleoid. Eukaryotes: DNA (β) wraps around a histone octamer (+, rich in lysine & arginine) making a nucleosome (~200 bp). Nucleosomes repeat like beads-on-string (chromatin, visible under EM), which coils into chromatin fibres β chromosomes at metaphase, helped by Non-Histone Chromosomal (NHC) proteins. Loose + light-staining = euchromatin (transcriptionally ACTIVE); dense + dark = heterochromatin (inactive).
π 5.2 Β· The search for genetic material
"In 1928, Frederick Griffith, in a series of experiments with Streptococcus pneumoniae... S strain bacteria have a mucous (polysaccharide) coat, while R strain does not... heat-killed S injected into mice did not kill them. When he injected a mixture of heat-killed S and live R bacteria, the mice died... some 'transforming principle', transferred from the heat-killed S strain, had enabled the R strain to synthesise a smooth polysaccharide coat and become virulent."
Griffith's four injections: live S β dies; live R β lives; heat-killed S β lives; heat-killed S + live R β DIES, and live S recovered from the corpse! Something from the dead S bacteria "transformed" R into S. He proved transformation happens but NOT what the transforming substance was.
"Oswald Avery, Colin MacLeod and Maclyn McCarty (1933-44)... discovered that DNA alone from S bacteria caused R bacteria to become transformed... proteases and RNases did not affect transformation... Digestion with DNase did inhibit transformation... They concluded that DNA is the hereditary material, but not all biologists were convinced."
Avery's team purified each biochemical from heat-killed S and tested it. Protein-digesters and RNA-digesters: transformation still works. DNase: transformation stops. So DNA is the transforming principle β but the world stayed sceptical (protein was the favourite candidate).
"The unequivocal proof that DNA is the genetic material came from the experiments of Alfred Hershey and Martha Chase (1952)... grew some viruses on radioactive phosphorus and some on radioactive sulfur... Bacteria which was infected with viruses that had radioactive DNA were radioactive... DNA is therefore the genetic material that is passed from virus to bacteria."
The blender experiment: DNA contains phosphorus (Β³Β²P) but no sulfur; protein contains sulfur (Β³β΅S) but no phosphorus. Label phages both ways β infect E. coli β blender (shake off coats) β centrifuge (separate). Only Β³Β²P showed up inside bacteria β what entered was DNA. Case closed, unequivocally.
"A molecule that can act as a genetic material must fulfill: (i) generate its replica; (ii) chemically and structurally stable; (iii) scope for slow changes (mutation); (iv) able to express itself as 'Mendelian characters'."
The four job criteria. Both DNA and RNA can replicate (proteins fail at step one). But RNA's 2'-OH makes it labile and easily degradable, and RNA is catalytic β reactive. Thymine (instead of uracil) adds even more stability to DNA. RNA mutates faster (why RNA viruses evolve fast); RNA expresses directly, DNA needs RNA. Verdict: DNA better for storage, RNA better for transmission.
"RNA was the first genetic material... essential life processes evolved around RNA... RNA used to act as a genetic material as well as a catalyst... DNA has evolved from RNA with chemical modifications that make it more stable."
The RNA World: life started with RNA doing both jobs (information + catalysis). Being reactive, it was unstable β so evolution built the tougher archive, DNA, from RNA.
π 5.4 Β· Replication
"'It has not escaped our notice that the specific pairing we have postulated immediately suggests a possible copying mechanism for the genetic material' (Watson and Crick, 1953)... each DNA molecule would have one parental and one newly synthesised strand β semiconservative DNA replication."
The most famous understatement in biology: complementary strands = a built-in photocopier. Each daughter DNA keeps ONE old strand and ONE new β semiconservative.
"Matthew Meselson and Franklin Stahl performed the following experiment in 1958: grew E. coli in ΒΉβ΅NHβCl for many generations... distinguished from normal DNA by centrifugation in a cesium chloride (CsCl) density gradient. (ΒΉβ΅N is not a radioactive isotope)... after one generation (20 minutes) hybrid density; after another generation equal amounts of hybrid and light DNA."
Heavy nitrogen (ΒΉβ΅N β heavy, NOT radioactive!) labels all DNA. Switch to normal ΒΉβ΄N: Gen I (20 min) β ALL hybrid. Gen II (40 min) β 50% hybrid : 50% light. Gen III (60 min) β 25% hybrid : 75% light; at 80 min β 12.5% : 87.5% (1:7). Only semiconservative replication predicts exactly this. Taylor (1958) proved the same in chromosomes of Vicia faba using radioactive thymidine.
"The main enzyme is referred to as DNA-dependent DNA polymerase... E. coli... completes the process of replication within 18 minutes; the average rate of polymerisation has to be approximately 2000 bp per second... Deoxyribonucleoside triphosphates serve dual purposes: substrates AND energy."
The copier enzyme reads DNA to make DNA β fast (~2000 bp/s, whole E. coli genome in 18 min) and accurate (mistakes = mutations). The building blocks (dNTPs) also pay the energy bill via their two terminal high-energy phosphates.
"The replication occur within a small opening of the DNA helix, referred to as replication fork... polymerases catalyse polymerisation only in one direction, 5'β3'... on one strand (template 3'β5') replication is continuous, while on the other (template 5'β3') it is discontinuous... fragments are later joined by DNA ligase."
The helix opens only at a small fork (full unzipping costs too much energy). The enzyme writes only 5'β3', so against the 3'β5' template it copies continuously; against the other it makes fragments that DNA ligase stitches. Replication starts only at the origin of replication (why cloning vectors must carry one), happens in S-phase in eukaryotes, and a replication-without-division accident gives polyploidy.
π 5.5 Β· Transcription
"The process of copying genetic information from one strand of the DNA into RNA is termed as transcription... adenosine now forms base pair with uracil... only a segment of DNA and only one of the strands is copied."
Transcription = DNA β RNA, one segment, ONE strand. Why not both? (1) Two different RNAs β two different proteins from one gene = chaos. (2) Two complementary RNAs would zip into double-stranded RNA β no translation. Both reasons are quotable NEET answers.
"A transcription unit is defined primarily by three regions: a Promoter, the Structural gene, and a Terminator... the strand with polarity 3'β5' acts as template strand; the other strand (5'β3'), with sequence same as RNA (except thymine at the place of uracil), is displaced and called coding strand. Strangely, this strand does not code for anything."
Three parts: promoter (RNA polymerase's parking spot, at the 5'/upstream end), structural gene, terminator (3'/downstream). The template strand (3'β5') is actually read; the coding strand (5'β3') just matches the RNA's sequence and codes for nothing β NCERT itself calls this "strange". The promoter's position defines which strand is which.
"A gene is defined as the functional unit of inheritance... a cistron as a segment of DNA coding for a polypeptide... monocistronic (mostly in eukaryotes) or polycistronic (mostly in bacteria)... In eukaryotes the genes are split: exons (appear in mature RNA) interrupted by introns (do not appear in mature or processed RNA)."
Gene = functional unit; cistron = one-polypeptide segment. Eukaryotes: one gene-one mRNA (monocistronic) but split into exons (kept) and introns (cut out). Bacteria: several genes per mRNA (polycistronic). tRNA/rRNA sequences are also genes even though no protein comes from them.
"In bacteria... single DNA-dependent RNA polymerase... associates transiently with initiation-factor (Ο) and termination-factor (Ο)... Since mRNA does not require any processing and there is no separation of cytosol and nucleus, transcription and translation can be coupled in bacteria."
Bacteria: ONE RNA polymerase does all RNA; it borrows sigma (Ο) to start and rho (Ο) to stop β by itself it only elongates. And because there's no nucleus, ribosomes start translating an mRNA before it is even finished β coupled transcription-translation.
"In eukaryotes: RNA polymerase I transcribes rRNAs (28S, 18S, and 5.8S); RNA polymerase III transcribes tRNA, 5s rRNA, and snRNAs; RNA polymerase II transcribes the precursor of mRNA, the heterogeneous nuclear RNA (hnRNA)... hnRNA undergoes splicing (introns removed), capping (methyl guanosine triphosphate at 5'-end) and tailing (200-300 adenylate residues at 3'-end, template independent)."
Three polymerases with division of labour: Pol I β rRNA (28S/18S/5.8S), Pol II β hnRNA (pre-mRNA), Pol III β tRNA + 5S rRNA + snRNA. The raw transcript is processed: splicing cuts introns, capping adds methyl-guanosine-triphosphate at 5', tailing adds a 200β300 adenylate poly-A tail (no template!). Only then does mRNA leave the nucleus. Splicing echoes the ancient RNA world.
π€ 5.6 Β· The genetic code
"George Gamow, a physicist, argued that since there are only 4 bases and if they have to code for 20 amino acids, the code should be made up of three nucleotides... 4Β³ = 64 codons... Har Gobind Khorana's chemical method synthesised RNAs with defined combinations; Marshall Nirenberg's cell-free system helped the code be deciphered; Severo Ochoa's enzyme (polynucleotide phosphorylase) polymerised RNA in a template-independent manner."
Four contributors, four roles: Gamow β maths says triplet (4Β²=16 too few, 4Β³=64 enough). Khorana β made designer RNAs (homo- & copolymers). Nirenberg β cell-free protein synthesis cracked the meanings. Ochoa β enzyme to build RNA without template.
"Salient features: the codon is triplet β 61 code for amino acids and 3 are stop codons; the code is degenerate; read in a contiguous fashion without punctuations; nearly universal (exceptions in mitochondrial codons and some protozoans); AUG has dual functions β codes for Methionine and acts as initiator; UAA, UAG, UGA are stop codons."
The six features to recite: triplet (61 + 3 stop), degenerate (one amino acid, several codons), commaless, nearly universal (mitochondria & some protozoans excepted), AUG = Met + start, stops = UAA, UAG, UGA. Corollary: reading amino acids β RNA is ambiguous BECAUSE of degeneracy.
"A change of single base pair in the gene for beta globin chain results in the change of amino acid residue glutamate to valine... sickle cell anaemia... RAM HAS RED CAP... Insertion or deletion of one or two bases changes the reading frame... frameshift mutations. Insertion or deletion of three or its multiple bases... reading frame remains unaltered."
Point mutation showcase: Glu β Val in beta-globin = sickle cell anaemia. The RAM HAS RED CAP game: insert/delete 1 or 2 letters β everything downstream is garbage (frameshift); insert/delete 3 (or multiples) β one word gained/lost, rest of the sentence fine.
"Crick postulated the presence of an adapter molecule that would on one hand read the code and on other hand would bind to specific amino acids... tRNA, then called sRNA... has an anticodon loop and an amino acid acceptor end... specific for each amino acid... initiator tRNA... no tRNAs for stop codons... secondary structure looks like a clover-leaf; actual structure is a compact inverted L."
tRNA = the translator: anticodon loop reads the mRNA; acceptor end carries the amino acid. One tRNA per amino acid, a special initiator tRNA, and NO tRNA for stop codons. On paper: clover-leaf. In 3-D: inverted L.
π 5.7 Β· Translation
"Amino acids are activated in the presence of ATP and linked to their cognate tRNA β a process commonly called as charging of tRNA or aminoacylation... The ribosome consists of structural RNAs and about 80 different proteins... two subunits... The ribosome also acts as a catalyst (23S rRNA in bacteria is the enzyme β ribozyme) for the formation of peptide bond."
Step 1: charge each tRNA with its amino acid (ATP needed). Step 2: the ribosome (2 subunits; ~80 proteins + rRNAs) clamps on mRNA at AUG β recognised only by the initiator tRNA β and moves codon by codon; its large subunit holds two tRNA sites. The peptide-bond-forming enzyme is not a protein but RNA: 23S rRNA β a ribozyme. A release factor ends it at the stop codon. Flanking the coding stretch are UTRs (5' before AUG, 3' after stop) needed for efficient translation.
ποΈ 5.8 Β· The lac operon
"The elucidation of the lac operon was a result of a close association between a geneticist, Francois Jacob and a biochemist, Jacque Monod... a polycistronic structural gene is regulated by a common promoter and regulatory genes... referred to as operon... lac operon, trp operon, ara operon, his operon, val operon."
Jacob (geneticist) + Monod (biochemist) = first transcriptionally regulated system ever explained. An operon = genes packaged with their own switch β common in bacteria.
"The lac operon consists of one regulatory gene (the i gene β the term i is derived from the word inhibitor) and three structural genes (z, y, and a). The z gene codes for beta-galactosidase... hydrolysis of lactose into galactose and glucose. The y gene codes for permease... The a gene encodes a transacetylase."
Four genes: i = inhibitor (NOT inducer!) making the repressor; z = Ξ²-galactosidase (splits lactose); y = permease (lets lactose in); a = transacetylase. All three products serve one pathway β lactose metabolism.
"Lactose is the substrate for the enzyme beta-galactosidase and it regulates switching on and off of the operon. Hence, it is termed as inducer... The repressor is synthesised all-the-time (constitutively) from the i gene... binds to the operator... prevents RNA polymerase from transcribing... In the presence of an inducer, such as lactose or allolactose, the repressor is inactivated... This allows RNA polymerase access to the promoter."
Default: repressor (made constantly) sits on the operator β operon OFF. Lactose enters (a basal trickle of permease always exists!) β binds the repressor β repressor lets go β operon ON. When lactose is used up, the repressor re-binds β the operon shuts itself down. Regulation of enzyme synthesis by its substrate. Repressor-based control = negative regulation. Glucose or galactose cannot induce.
π§ 5.9 Β· Human Genome Project
"A very ambitious project of sequencing human genome was launched in the year 1990... 3 Γ 10βΉ bp at US $3 per bp β 9 billion US dollars... 3300 books of 1000 pages of 1000 letters each... closely associated with the rapid development of Bioinformatics... a 13-year project coordinated by the U.S. Department of Energy and the National Institute of Health; Wellcome Trust (U.K.) became a major partner... completed in 2003."
HGP: 1990 β 2003, 13 years, ~$9 billion, data = 3300 fat books, birthing Bioinformatics. Goals: all ~20,000β25,000 genes, all 3 billion bp, databases, better analysis tools, tech transfer to industry, and ELSI (ethical-legal-social issues). Model organisms also sequenced: bacteria, yeast, C. elegans, Drosophila, rice, Arabidopsis.
"One approach focused on identifying all the genes that are expressed as RNA (Expressed Sequence Tags β ESTs). The other took the blind approach of sequencing the whole genome and later assigning functions (Sequence Annotation)... cloned in BAC (bacterial artificial chromosomes) and YAC (yeast artificial chromosomes)... sequenced using automated sequencers on the principle of Frederick Sanger... The sequence of chromosome 1 was completed only in May 2006."
Two roads: ESTs (only expressed genes) vs whole-genome + annotation. Chop DNA β clone in BAC/YAC vectors β sequence by Sanger's method β computers align the overlaps. Chromosome 1 β the last of the 24 (22 autosomes + X + Y) β finished May 2006. Genetic/physical maps used restriction-site polymorphism and microsatellites.
"Salient features: 3164.7 million bp; average gene 3000 bases β largest human gene dystrophin at 2.4 million bases; ~30,000 genes; 99.9% of bases identical in all humans; functions unknown for over 50% of genes; less than 2% codes for proteins; repeated sequences form a very large portion; chromosome 1 has most genes (2968), Y has fewest (231); ~1.4 million SNP locations."
The number sheet NEET quizzes: 3164.7 Mbp Β· average gene 3000 b Β· biggest gene dystrophin 2.4 Mb Β· ~30,000 genes (way below the 80,000β1,40,000 guesses) Β· humans 99.9% identical Β· <2% codes protein Β· >50% of genes function-unknown Β· chr 1 = 2968 genes, Y = 231 Β· 1.4 million SNPs ("snips").
π 5.10 Β· DNA Fingerprinting
"DNA fingerprinting involves identifying differences in some specific regions in DNA sequence called as repetitive DNA... These repetitive DNA are separated from bulk genomic DNA as different peaks during density gradient centrifugation. The bulk DNA forms a major peak and the other small peaks are referred to as satellite DNA... classified into micro-satellites, mini-satellites etc... show high degree of polymorphism."
Repetitive DNA spins out as small "satellite" peaks beside the main DNA peak in density-gradient centrifugation. These stretches don't code for proteins but differ wildly between people (polymorphism) β the raw material of fingerprinting. Every tissue of one person shows the SAME polymorphism, and it's inherited β forensics + paternity testing.
"Polymorphism (variation at genetic level) arises due to mutations... an allelic variation is a DNA polymorphism if more than one variant (allele) at a locus occurs with a frequency greater than 0.01... The probability of such variation in non-coding DNA would be higher."
Definition with a number: allele frequency > 0.01 = polymorphism. Non-coding DNA accumulates more of them (mutations there don't hurt reproduction). Germ-cell mutations spread through generations β the engine of variability, evolution and speciation.
"The technique of DNA Fingerprinting was initially developed by Alec Jeffreys. He used a satellite DNA as probe... Variable Number of Tandem Repeats (VNTR)... involved Southern blot hybridisation using radiolabelled VNTR as a probe."
Alec Jeffreys + VNTR (a mini-satellite; size 0.1β20 kb). The six steps in order: (1) isolate DNA β (2) cut with restriction endonucleases β (3) electrophoresis β (4) Southern blotting onto nitrocellulose/nylon β (5) hybridise with radiolabelled VNTR probe β (6) autoradiography. Result: a band pattern unique to each person β except monozygotic twins. PCR now makes it work from a single cell.
4π§ Concepts that decide questions
1 Β· Template vs coding strand most tested
Template = 3'β5', actually read. Coding = 5'β3', same sequence as mRNA (T for U), codes for nothing. All transcription-unit references use the coding strand.
2 Β· The three experiments, one ladder
Griffith: transformation happens. Avery-MacLeod-McCarty: the transforming principle is DNA (biochemical proof). Hershey-Chase: DNA enters the host β unequivocal proof. Know which claim belongs to which.
3 Β· Meselson-Stahl fractions
Gen I: 100% hybrid. Gen II: Β½ hybrid Β½ light. Gen n: hybrid fraction = 2/2βΏ. 80 min = Gen IV... careful β 80 min = 4 generations (20 min each): hybrid 2/16 = 1/8, light 7/8. NCERT asks 80 min: answer 1:3 hybrid:light at Gen III? No β 80/20 = 4 generations: 2 hybrid molecules of 16 total.
4 Β· Polymerase divisions of labour
Bacteria: ONE RNA polymerase + Ο (start) + Ο (stop). Eukaryotes: Pol I β rRNA (28S, 18S, 5.8S); Pol II β hnRNA/mRNA; Pol III β tRNA, 5S rRNA, snRNA. "5S goes with Pol III, 5.8S with Pol I" is the classic trick.
5 Β· hnRNA processing trio
Splicing (introns out), Capping (5', methyl guanosine triphosphate), Tailing (3', 200β300 A's, template-independent). Order of ends matters in options.
6 Β· Lac operon gene map
i β repressor (constitutive). z β Ξ²-galactosidase. y β permease. a β transacetylase. Inducer = lactose/allolactose. i is from "inhibitor". Negative regulation = repressor-mediated.
7 Β· Ribozyme sightings
23S rRNA (bacteria) catalyses peptide-bond formation β an RNA enzyme. Pairs with the RNA-world logic: catalysis evolved around RNA.
8 Β· Why DNA over RNA for storage
No 2'-OH (less reactive), double-stranded with repair, thymine instead of uracil (extra stability). RNA wins for transmission and speed of evolution (viruses).
5ποΈ Tables β memorise cold
| Scientist(s) | Year | Contribution |
|---|---|---|
| Friedrich Meischer | 1869 | Identified DNA as acidic substance in nucleus β "Nuclein" |
| Frederick Griffith | 1928 | Transformation in Streptococcus pneumoniae (S & R strains) |
| Avery, MacLeod, McCarty | 1933β44 | Biochemical proof: transforming principle is DNA (DNase test) |
| Hershey & Chase | 1952 | Β³Β²P/Β³β΅S blender experiment β unequivocal proof DNA is genetic material |
| Watson & Crick | 1953 | Double helix (X-ray data of Wilkins & Franklin; Chargaff's rules) |
| Meselson & Stahl | 1958 | Semiconservative replication in E. coli (ΒΉβ΅N/CsCl gradient) |
| Taylor et al. | 1958 | Semiconservative replication in Vicia faba chromosomes (radioactive thymidine) |
| George Gamow | β | Argued the code must be triplet (4Β³ = 64) |
| Har Gobind Khorana | β | Chemical synthesis of defined RNAs (homo/copolymers) |
| Marshall Nirenberg | β | Cell-free protein synthesis system β code deciphered |
| Severo Ochoa | β | Polynucleotide phosphorylase β template-independent RNA synthesis |
| Francis Crick | β | Central dogma; postulated the adapter (tRNA) |
| Jacob & Monod | β | Lac operon β first transcriptionally regulated system |
| Frederick Sanger | β | Sequencing method used in HGP (also protein sequencing) |
| Alec Jeffreys | β | DNA fingerprinting using VNTR probes |
| T.O. Diener | 1971 | (Ch 2 link) viroids β free RNA pathogens |
| RNA polymerase (eukaryotes) | Transcribes |
|---|---|
| Pol I | rRNAs β 28S, 18S, 5.8S |
| Pol II | hnRNA (precursor of mRNA) |
| Pol III | tRNA, 5S rRNA, snRNAs |
| Lac operon gene | Product | Job |
|---|---|---|
| i | Repressor | Binds operator, blocks transcription (constitutive; "i" = inhibitor) |
| z | Ξ²-galactosidase | Hydrolyses lactose β glucose + galactose |
| y | Permease | Increases permeability to Ξ²-galactosides |
| a | Transacetylase | Acetylation role in lactose metabolism |
6π’ Numbers Bank β every figure NEET can quote
| Number | Meaning |
|---|---|
| 5386 nucleotides | ΟΓ174 bacteriophage (single-stranded) |
| 48502 bp | Bacteriophage lambda |
| 4.6 Γ 10βΆ bp | E. coli genome (replicated in 18 min β ~2000 bp/s) |
| 3.3 Γ 10βΉ bp | Human HAPLOID DNA content (diploid 6.6 Γ 10βΉ) |
| 2.2 m | Length of DNA in a mammalian cell (6.6 Γ 10βΉ Γ 0.34 nm) |
| 3.4 nm / 10 bp / 0.34 nm | Helix pitch / bp per turn / bp spacing |
| 2 and 3 | H-bonds: A=T two, Gβ‘C three |
| 8 Β· 200 bp | Histone octamer molecules Β· DNA per nucleosome |
| 61 + 3 = 64 | Sense codons + stop codons |
| 200β300 | Adenylate residues in poly-A tail |
| ~80 | Different proteins in a ribosome |
| 1990β2003 (13 yr) | Human Genome Project; ~$9 billion; chr 1 finished May 2006 |
| 3164.7 million bp | Human genome size (HGP) |
| 3000 b / 2.4 Mb | Average gene / dystrophin (largest) |
| ~30,000 Β· 99.9% Β· <2% | Genes Β· identity between humans Β· protein-coding fraction |
| 2968 / 231 | Genes on chromosome 1 (most) / Y (fewest) |
| 1.4 million | SNP locations |
| 0.1β20 kb | VNTR size range |
| > 0.01 | Allele frequency threshold for DNA polymorphism |
7β‘ Shortcuts
S1 Β· Chargaff arithmetic in 5 seconds.
A=T and G=C. Given C% β G=C, A=T=(100β2C)/2. C=20% β G=20%, A=T=30% each.
S2 Β· mRNA = coding strand with U.
Any "write the mRNA" question: copy the coding strand, swap TβU. For the template strand, take the complement.
S3 Β· Meselson-Stahl fractions formula.
After n generations: hybrid = 2/2βΏ of molecules, light = rest. n=2 β 50:50; n=3 β 25:75; n=4 (80 min) β 12.5:87.5.
S4 Β· Frameshift test.
Insertions/deletions in multiples of 3 β frame safe. Anything else β frameshift from that point.
S5 Β· "Who proved what" one-liners.
Griffith = transformation Β· Avery = DNA is transforming principle Β· Hershey-Chase = unequivocal proof Β· Meselson-Stahl = semiconservative. Match by verb.
S6 Β· Radioactive or not?
Β³Β²P and Β³β΅S (Hershey-Chase) = radioactive. ΒΉβ΅N (Meselson-Stahl) = heavy but NOT radioactive. Radioactive thymidine (Taylor) = radioactive. This exact distinction is a repeated PYQ.
8β οΈ Traps
T1 Β· "ΒΉβ΅N is radioactive" β FALSE.
It is a heavy isotope, separated by density (CsCl gradient), not by radioactivity.
T2 Β· The coding strand codes for nothing.
The template strand is read; the coding strand merely matches the mRNA. NCERT calls the naming "strange" β questions exploit it.
T3 Β· Human DNA: 3.3 Γ 10βΉ is HAPLOID.
The 2.2 m calculation uses 6.6 Γ 10βΉ (diploid). Mixing the two is the classic error.
T4 Β· i gene β inducer gene.
"i" is from inhibitor; it makes the repressor. The inducer is lactose (allolactose).
T5 Β· Glucose/galactose cannot induce lac operon.
Only lactose/allolactose. And the operon shuts down when lactose is exhausted.
T6 Β· 5S vs 5.8S rRNA.
5.8S β Pol I (with 28S, 18S). 5S β Pol III (with tRNA, snRNA).
T7 Β· Histones are basic (positive), DNA acidic (negative).
Rich in lysine and arginine. Reverse-charged options are wrong.
T8 Β· Euchromatin is ACTIVE.
Loose, light-staining, transcriptionally active. Heterochromatin: dense, dark, inactive.
T9 Β· No tRNA for stop codons.
A release factor, not a tRNA, reads the stop. "Stop tRNA" options are fake.
T10 Β· Ribosome's peptide-bond catalyst is rRNA.
23S rRNA (ribozyme) β not a ribosomal protein.
T11 Β· Tailing is template-INDEPENDENT.
200β300 A's are added without any template; capping uses methyl guanosine triphosphate at the 5' end (not 3').
T12 Β· Splicing happens to hnRNA, in the nucleus.
Bacterial mRNA needs NO processing β that's why coupled transcription-translation is possible there.
T13 Β· Fingerprinting bands differ for everyone EXCEPT identical twins.
Monozygotic twins share the pattern; even siblings do not.
T14 Β· Sickle cell: Glu β Val (beta-globin).
Single base change, point mutation. Direction reversed in options (Val β Glu) is wrong.
9𧡠Mnemonics
M1 Β· Stop codons β "U Are Away, U Are Gone, U Go Away"
UAA, UAG, UGA.
M2 Β· Purines vs pyrimidines β "Pure As Gold" / "CUT the py"
Purines: Adenine, Guanine. Pyrimidines: Cytosine, Uracil, Thymine.
M3 Β· Eukaryotic polymerases β "1 R, 2 M, 3 T"
Pol I β rRNA, Pol II β mRNA (hnRNA), Pol III β tRNA (+5S, snRNA).
M4 Β· Lac operon genes β "i zya" β "I Zap Your Acetyl"
i (repressor) β z (Ξ²-gal) β y (permease) β a (transacetylase), in map order.
M5 Β· Code crackers β "Gamow Guessed, Khorana Cooked, Nirenberg Named, Ochoa Oiled"
Triplet logic Β· synthetic RNAs Β· cell-free deciphering Β· template-free RNA enzyme.
M6 Β· Fingerprinting steps β "I Dig Every Bit, Hybridise, Autograph"
Isolation β Digestion β Electrophoresis β Blotting β Hybridisation β Autoradiography.
M7 Β· Ο starts, Ο stops.
Sigma = start factor, rho = release/termination factor in bacterial transcription.
10π¨ Exceptions β the odd ones out NEET loves
E1 Β· RNA as genetic material.
TMV and QB bacteriophage β DNA is the rule, these are the exceptions.
E2 Β· Central dogma reversed.
Some viruses flow RNA β DNA (reverse transcription β retroviruses).
E3 Β· Code universality has exceptions.
Mitochondrial codons and some protozoans.
E4 Β· AUG moonlights.
Codes methionine AND initiates β one codon, two jobs.
E5 Β· An enzyme made of RNA.
23S rRNA ribozyme β peptide bond formation; catalysis is not a protein monopoly.
E6 Β· A gene without protein.
tRNA and rRNA genes code for RNA that is never translated.
E7 Β· Repetitive DNA codes for nothing yet decides identity.
Satellite DNA/VNTR β non-coding but the basis of fingerprinting and mapping.
E8 Β· Basal lac expression is always on.
A low level of permease must pre-exist β otherwise lactose could never enter to induce the operon. Favourite assertion-reason twist.
11π₯ Most-Asked in NEET
| Asked pattern | Frequency | Ready answer |
|---|---|---|
| Hershey-Chase isotopes and logic | Almost every year | Β³Β²P β DNA entered; Β³β΅S β protein stayed out |
| Meselson-Stahl generation fractions | High | Gen II = 50% hybrid + 50% light; use 2/2βΏ |
| Genetic code features | High | Triplet, degenerate, commaless, nearly universal, AUG dual, 3 stops |
| Lac operon component match | High | i-repressor, z-Ξ²-gal, y-permease, a-transacetylase; inducer = lactose |
| Eukaryotic RNA polymerase match | High | I-rRNA, II-hnRNA, III-tRNA/5S/snRNA |
| Nucleosome numbers | Medium | Octamer of 8 histones, ~200 bp, lysine+arginine rich |
| HGP salient-feature numbers | Medium | 3164.7 Mbp, 30,000 genes, <2% coding, chr1=2968, Y=231, SNPs 1.4 M |
| Fingerprinting steps/order | Medium | Isolate β digest β electrophorese β blot β hybridise (VNTR) β autoradiograph |
| Chargaff calculation | Medium | A=T, G=C; percentages sum to 100 |
12π― AssertionβReason Drill
Mark: (a) both true, R explains A Β· (b) both true, R doesn't explain A Β· (c) A true, R false Β· (d) A false, R true.
A: In the Hershey-Chase experiment, bacteria infected by Β³β΅S-labelled phages were not radioactive.
R: DNA contains sulfur but proteins do not.
Answer
(c) β A true, R false (it's the reverse: proteins contain sulfur, DNA does not).
A: DNA is a better genetic material than RNA for storage of information.
R: The 2'-OH group in RNA makes it labile and easily degradable.
Answer
(a) β both true, R explains A.
A: Both strands of DNA are copied during transcription.
R: Two simultaneously produced complementary RNAs would form double-stranded RNA and block translation.
Answer
(d) β A false (only one strand is copied), R true (and it is one reason why).
A: The lac operon is expressed at a low basal level even before lactose is added.
R: Without pre-existing permease, lactose could not enter the cell to induce the operon.
Answer
(a) β both true, R explains A.
A: Insertion of three bases in a structural gene causes a frameshift mutation.
R: Insertion or deletion of one or two bases changes the reading frame from that point.
Answer
(d) β A false (multiples of three preserve the frame, adding one codon), R true.
A: DNA fingerprinting patterns of monozygotic twins are identical.
R: Monozygotic twins arise from a single zygote and share the same DNA sequence.
Answer
(a) β both true, R explains A.
13βοΈ NCERT Exercises β Solved
Q1 Β· Group as nitrogenous bases and nucleosides: Adenine, Cytidine, Thymine, Guanosine, Uracil, Cytosine.
Answer
Bases: Adenine, Thymine, Uracil, Cytosine. Nucleosides (base + sugar): Cytidine, Guanosine. (Tip: nucleoside names end in -sine/-dine.)
Q2 Β· dsDNA has 20% cytosine. Percent adenine?
Answer
G = C = 20% β G+C = 40% β A+T = 60% β A = 30%.
Q3 Β· Complementary strand (5'β3') of 5'-ATGCATGCATGCATGCATGCATGCATGC-3'.
Answer
5'-GCATGCATGCATGCATGCATGCATGCAT-3' (complement read antiparallel, then written 5'β3').
Q4 Β· mRNA from coding strand 5'-ATGCATGCATGCATGCATGCATGCATGC-3'.
Answer
5'-AUGCAUGCAUGCAUGCAUGCAUGCAUGC-3' β same as coding strand with U for T.
Q5 Β· Which property of the double helix led to the semiconservative hypothesis?
Answer
Complementary base pairing between the two strands: each strand can act as a template, so separation and copying yields two daughters, each with one parental and one new strand.
Q6 Β· List nucleic acid polymerases by template and product.
Answer
(1) DNA-dependent DNA polymerase (replication). (2) DNA-dependent RNA polymerase (transcription). (3) RNA-dependent DNA polymerase (reverse transcriptase, some viruses). (4) RNA-dependent RNA polymerase (RNA-genome viruses).
Q7 Β· How did Hershey and Chase differentiate DNA from protein?
Answer
By element-specific radiolabels: DNA has phosphorus but no sulfur (Β³Β²P label); protein has sulfur but no phosphorus (Β³β΅S label). After infection, blending and centrifugation, only Β³Β²P appeared inside bacteria β DNA is the genetic material.
Q8 Β· Differentiate: (a) repetitive vs satellite DNA (b) mRNA vs tRNA (c) template vs coding strand.
Answer
(a) Repetitive DNA = all many-times-repeated sequences; satellite DNA = the repetitive fraction that separates as small peaks beside bulk DNA in density-gradient centrifugation (micro-, mini-satellites). (b) mRNA carries the template/codons for protein; tRNA is the adapter β anticodon loop reads the codon, acceptor end carries the amino acid. (c) Template strand (3'β5') is transcribed; coding strand (5'β3') matches the RNA sequence (T for U) and is not read.
Q9 Β· Two essential roles of ribosome in translation.
Answer
(1) Platform: binds mRNA and holds charged tRNAs in the two large-subunit sites so amino acids come close enough for bonding. (2) Catalyst: 23S rRNA (ribozyme) catalyses peptide-bond formation.
Q10 Β· Why does the lac operon shut down some time after lactose is added?
Answer
The operon's own enzymes consume the lactose (Ξ²-galactosidase splits it into glucose + galactose). Once the inducer is exhausted, the constitutively made repressor is no longer inactivated, binds the operator again, and transcription stops β substrate-controlled switch-off.
Q11 Β· One-line functions: (a) promoter (b) tRNA (c) exons.
Answer
(a) DNA sequence at the 5' end of a transcription unit where RNA polymerase binds to start transcription (also defines template/coding strands). (b) Adapter that reads the mRNA codon via its anticodon and delivers the corresponding amino acid. (c) Coding/expressed sequences of split genes that appear in mature processed RNA.
Q12 Β· Why is HGP called a mega project?
Answer
Scale on every axis: 3 Γ 10βΉ bp to sequence; ~$9 billion cost ($3/bp); data equal to 3300 books of 1000 pages; 13 years (1990β2003); international consortium (US DOE + NIH, Wellcome Trust, Japan, France, Germany, China); demanded new high-speed computation (bioinformatics).
Q13 Β· What is DNA fingerprinting? Applications.
Answer
A quick technique to compare individuals via polymorphism in repetitive (satellite/VNTR) DNA β Southern hybridisation of restriction-digested, electrophoresed DNA with a radiolabelled VNTR probe gives an individual-specific band pattern. Applications: forensic identification, paternity disputes, population and genetic-diversity studies.
Q14 Β· Briefly describe: transcription, polymorphism, translation, bioinformatics.
Answer
Transcription: copying a DNA segment's template strand into RNA by DNA-dependent RNA polymerase (promoter β structural gene β terminator). Polymorphism: inheritable variation at a locus (mutation-born) present at frequency >0.01 β commonest in non-coding DNA; basis of mapping and fingerprinting. Translation: ribosome-mediated polymerisation of amino acids into a polypeptide as per mRNA codons, via charged tRNAs; ends at stop codon with a release factor. Bioinformatics: the computational field (storage, retrieval, analysis of sequence data) that grew alongside the HGP.