# Molecular Basis of Inheritance β€” Part A (NEET Biology Class 12): DNA Structure, Packaging & Replication Source: https://tracknation.in/story/molecular-basis-of-inheritance-part-a-neet-biology-class-12-dna-structure-packaging-replication (TrackNation β€” free UPSC notes with PYQs) > NEET Class 12 Molecular Basis of Inheritance, Part A, mined from NCERT: DNA structure (Meischer, Watson-Crick, Wilkins-Franklin, Chargaff's rule) with all five salient features of the double helix and their exact numbers, the central dogma, DNA packaging (nucleoid, histone octamer, nucleosome, euchromatin vs heterochromatin), the three landmark experiments proving DNA is the genetic material (Griffith, Avery-MacLeod-McCarty, Hershey-Chase) in a comparison table, DNA vs RNA as genetic material, t ## DNA Structure & the Double Helix 🎯 **NEET priority: Highest-yield.** *7–9 questions a year across both parts. Part A covers DNA structure and the classic experiments β€” the **Hershey-Chase and Meselson-Stahl experiments** and the **exact double-helix numbers** (3.4 nm pitch, 10 bp/turn, 0.34 nm per bp) are asked almost every year.* ### DNA structure β€” the exact numbers NEET asks - DNA was first identified as an acidic substance in the nucleus by **Friedrich Meischer (1869)**, who named it **'Nuclein'**. In **1953**, **James Watson and Francis Crick** proposed the **Double Helix model**, based on the **X-ray diffraction data of Maurice Wilkins and Rosalind Franklin** and on **Erwin Chargaff's** rule that in double-stranded DNA, the ratios A:T and G:C are each constant and **equal to one**. - A **nucleotide** = nitrogenous base + pentose sugar + phosphate. **Purines: adenine, guanine. Pyrimidines: cytosine, uracil, thymine.** Base + sugar is joined by an **N-glycosidic linkage** (making a nucleoside); the phosphate joins the 5' carbon by a **phosphoester linkage**; two nucleotides join by a **3'-5' phosphodiester linkage**. ### The five salient features of the double helix | **Feature** | **Detail** | | Two chains | Sugar-phosphate backbone outside; bases project inside | | Anti-parallel polarity | One chain runs 5'β†’3', the other 3'β†’5' | | Base pairing | A=T via 2 hydrogen bonds; G≑C via 3 hydrogen bonds. A purine always pairs with a pyrimidine, keeping strand distance uniform | | Right-handed coil | Pitch = 3.4 nm; roughly 10 base pairs per turn; so distance between adjacent bp β‰ˆ 0.34 nm | | Base stacking | Each base-pair plane stacks over the next β€” this, PLUS the H-bonds, gives the helix its stability | **Crick's Central Dogma:** genetic information flows **DNA β†’ RNA β†’ Protein**. In some viruses the flow is reversed (RNA β†’ DNA) β€” that process is **reverse transcription**. ### Genome sizes worth remembering - Bacteriophage φ×174: **5386 nucleotides**. Bacteriophage lambda: **48502 bp**. *E. coli*: **4.6 Γ— 10⁢ bp**. Haploid human DNA: **3.3 Γ— 10⁹ bp**. ## DNA Packaging & the Search for Genetic Material ### Packaging of the DNA helix - A typical mammalian cell's DNA (6.6 Γ— 10⁹ bp Γ— 0.34 nm/bp) would stretch about **2.2 metres** β€” far larger than the nucleus (~10⁻⁢ m). So it must be packaged. - In **prokaryotes** (e.g. *E. coli*), the negatively charged DNA is held by positively charged proteins in a region called the **nucleoid**, organised as large loops. - In **eukaryotes**, positively charged basic proteins called **histones** β€” rich in the basic amino acids **lysine and arginine** β€” form a **histone octamer** (8 molecules). The negatively charged DNA wraps around it to form a **nucleosome**, containing **200 bp** of DNA. Under EM, nucleosomes appear as a **'beads-on-string'** structure. - Beads-on-string β†’ chromatin fibres β†’ coiled and condensed at **metaphase** into chromosomes. Higher-order packaging needs **Non-histone Chromosomal (NHC) proteins**. - **Euchromatin** is loosely packed, stains light, and is **transcriptionally ACTIVE**. **Heterochromatin** is densely packed, stains dark, and is **INACTIVE**. This is a repeat NEET question β€” don't invert it. ### The search for the genetic material β€” three landmark experiments | **Experiment** | **Who & when** | **What was done** | **Conclusion** | | Transforming principle | Frederick Griffith, 1928 | Streptococcus pneumoniae: S strain (smooth, mucous coat, virulent) killed mice; R strain (rough) did not. Heat-killed S alone was harmless β€” but heat-killed S + live R KILLED the mice, and live S was recovered | R strain was 'transformed' by some transforming principle from heat-killed S. But its biochemical nature was NOT identified | | Biochemical characterisation | Avery, MacLeod & McCarty, 1933–44 | Purified proteins, DNA and RNA from heat-killed S cells and tested which transformed live R cells. Proteases and RNases did NOT stop transformation; DNase DID | DNA alone caused transformation β€” so DNA is the hereditary material (though not all biologists were convinced) | | Unequivocal proof | Alfred Hershey & Martha Chase, 1952 | Grew bacteriophages on radioactive Β³Β²P (labels DNA β€” protein has no phosphorus) or radioactive ³⁡S (labels protein β€” DNA has no sulphur). Infected E. coli, blended off the viral coats, centrifuged | Bacteria infected by Β³Β²P phages WERE radioactive; those infected by ³⁡S phages were NOT. Therefore DNA, not protein, entered the bacteria β€” DNA is the genetic material | ## DNA vs RNA & the RNA World ### Properties of genetic material β€” why DNA beats RNA - A genetic material must be able to (1) **replicate**, (2) be **chemically and structurally stable**, (3) allow **slow mutation** for evolution, and (4) **express itself as Mendelian characters**. - **Why DNA is more stable:** RNA has a reactive **2'-OH group** at every nucleotide, making it labile and easily degradable; RNA is also **catalytic and therefore reactive**. The presence of **thymine instead of uracil** confers further stability to DNA. Being double-stranded with a complementary strand, DNA also evolved **repair** mechanisms. - **Where RNA wins:** RNA can **directly code** for protein synthesis; DNA depends on RNA. RNA also **mutates faster** β€” which is why **RNA viruses evolve faster** and have shorter life spans. Conclusion: **DNA is better for STORAGE, RNA is better for TRANSMISSION** of genetic information. - **RNA is the genetic material** in some viruses β€” e.g. **Tobacco Mosaic Virus (TMV)** and **QB bacteriophage**. ### RNA World - **RNA was the FIRST genetic material.** Essential life processes (metabolism, translation, splicing) evolved around RNA, which acted as **both genetic material AND catalyst**. But being catalytic made it reactive and unstable β€” so **DNA evolved FROM RNA** with chemical modifications for greater stability. ## DNA Replication & Why This Matters for NEET ### DNA Replication β€” semiconservative - Watson and Crick immediately proposed that each strand acts as a template, so each daughter DNA has **one parental and one newly synthesised strand** β€” **semiconservative replication**. ### The Meselson–Stahl experiment (1958) β€” the proof | **Step** | **What happened** | **Result** | | Grow in ¹⁡N | E. coli grown many generations in ¹⁡NHβ‚„Cl (heavy nitrogen β€” NOT radioactive, separated by density only) | All DNA is 'heavy'; distinguishable by CsCl density-gradient centrifugation | | Transfer to ¹⁴N, 20 min (I generation) | Cells moved to normal ¹⁴NHβ‚„Cl medium; E. coli divides every 20 minutes | All DNA had HYBRID (intermediate) density β€” proving each new molecule has one old + one new strand | | 40 min (II generation) | Allowed one more division | Equal amounts of HYBRID DNA and 'LIGHT' DNA | Taylor and colleagues (1958) confirmed the same in ***Vicia faba* (faba beans)** using radioactive thymidine, proving chromosomal DNA also replicates semiconservatively. ### The replication machinery - The main enzyme is **DNA-dependent DNA polymerase**. In *E. coli* (4.6 Γ— 10⁢ bp), replication finishes in about **18 minutes** β€” an average rate of roughly **2000 bp per second**, and with very high accuracy (errors would be mutations). - **Deoxyribonucleoside triphosphates serve a dual purpose** β€” they are both the **substrates** AND the **energy source** (their two terminal phosphates are high-energy, like ATP). - Because separating the whole helix would cost too much energy, replication happens inside a small opening β€” the **replication fork**. DNA polymerase works **only in the 5'β†’3' direction**. So on the template with **3'β†’5'** polarity replication is **continuous**, while on the **5'β†’3'** template it is **discontinuous** β€” those fragments are joined by **DNA ligase**. - Replication starts only at a definite **origin of replication** β€” which is exactly why a **vector** is needed in recombinant DNA work (the vector supplies the origin). In eukaryotes, DNA replication occurs in the **S phase**, and failure of cell division after replication causes **polyploidy**. ### Why this matters for NEET - High-value one-liners: Meischer named it **Nuclein (1869)**; Chargaff's ratio **A/T = G/C = 1**; pitch **3.4 nm**, **10 bp/turn**, **0.34 nm/bp**; A=T has **2 H-bonds**, G≑C has **3**; nucleosome = **200 bp** on a **histone octamer**; Hershey-Chase used **Β³Β²P for DNA, ³⁡S for protein**; Meselson-Stahl used **¹⁡N** and **CsCl**. - **Trap:** ¹⁡N in Meselson-Stahl is a **heavy but NOT radioactive** isotope β€” separated purely by density. Hershey-Chase, by contrast, used genuinely **radioactive** isotopes. Also, Griffith identified the transforming principle's **existence**, but Avery-MacLeod-McCarty identified its **chemical nature** β€” don't credit Griffith with proving DNA is the genetic material. ## Test Yourself: MCQs, PYQs & Active Recall Answer these, then close the article and do an Active Recall. Reveal each answer only after you commit to one. ## Practice Questions **Q1.** DNA was first identified as an acidic substance in the nucleus and named 'Nuclein' by: - (a) Erwin Chargaff - (b) Rosalind Franklin - (c) Watson and Crick - (d) Friedrich Meischer **Answer: (d)** β€” Friedrich Meischer identified DNA in 1869 and named it Nuclein. **Q2.** The X-ray diffraction data used by Watson and Crick was produced by: - (a) Chargaff and Meischer - (b) Hershey and Chase - (c) Maurice Wilkins and Rosalind Franklin - (d) Meselson and Stahl **Answer: (c)** β€” Watson and Crick built their 1953 double-helix model on X-ray diffraction data from Maurice Wilkins and Rosalind Franklin. **Q3.** According to Chargaff's rule, in double-stranded DNA: - (a) A/T = G/C = 1 - (b) A/T = G/C = 2 - (c) Purines exceed pyrimidines - (d) A + T = G + C always **Answer: (a)** β€” Chargaff observed that the ratios of adenine to thymine and guanine to cytosine are each constant and equal to one. **Q4.** The number of hydrogen bonds between guanine and cytosine is: - (a) Four - (b) One - (c) Two - (d) Three **Answer: (d)** β€” G pairs with C via three hydrogen bonds; A pairs with T via two. **Q5.** The pitch of the DNA double helix and the number of base pairs per turn are: - (a) 34 nm and 10 bp - (b) 3.4 nm and 10 bp - (c) 3.4 nm and 20 bp - (d) 0.34 nm and 10 bp **Answer: (b)** β€” The pitch is 3.4 nm with roughly 10 base pairs per turn, giving about 0.34 nm between adjacent base pairs. **Q6.** Two nucleotides in a polynucleotide chain are linked by a: - (a) 3'-5' phosphodiester linkage - (b) Peptide bond - (c) N-glycosidic linkage - (d) Phosphoester linkage **Answer: (a)** β€” A 3'-5' phosphodiester linkage joins two nucleotides; N-glycosidic joins base to sugar and phosphoester joins phosphate to sugar. **Q7.** A typical nucleosome contains how many base pairs of DNA? - (a) 1000 bp - (b) 200 bp - (c) 100 bp - (d) 300 bp **Answer: (b)** β€” A typical nucleosome has 200 bp of DNA wrapped around a histone octamer. **Q8.** Histones are rich in which basic amino acid residues? - (a) Serine and threonine - (b) Lysine and arginine - (c) Valine and leucine - (d) Glycine and alanine **Answer: (b)** β€” Histones are rich in lysine and arginine, whose positively charged side chains bind the negatively charged DNA. **Q9.** Transcriptionally active chromatin is: - (a) Both equally - (b) Heterochromatin, densely packed - (c) Neither - (d) Euchromatin, loosely packed **Answer: (d)** β€” Euchromatin is loosely packed, stains light and is transcriptionally active; heterochromatin is densely packed and inactive. **Q10.** In Griffith's experiment, mice died when injected with: - (a) Heat-killed S strain alone - (b) Live R strain alone - (c) Heat-killed S strain plus live R strain - (d) Live R strain plus heat-killed R strain **Answer: (c)** β€” Heat-killed S + live R killed the mice, and live S bacteria were recovered β€” showing R had been transformed. **Q11.** Avery, MacLeod and McCarty showed that transformation was prevented by digestion with: - (a) Protease - (b) RNase - (c) DNase - (d) Lipase **Answer: (c)** β€” Proteases and RNases did not affect transformation, but DNase did β€” proving DNA is the transforming principle. **Q12.** In the Hershey-Chase experiment, radioactive sulphur was used to label: - (a) DNA - (b) The bacterial cell wall - (c) Both DNA and protein - (d) Protein **Answer: (d)** β€” DNA contains no sulphur, so 35S labelled only protein; 32P labelled only DNA since protein has no phosphorus. **Q13.** Which of the following makes RNA less stable than DNA? - (a) Having a phosphate backbone - (b) The reactive 2'-OH group at every nucleotide - (c) Being double stranded - (d) Presence of thymine **Answer: (b)** β€” The reactive 2'-OH group present at every RNA nucleotide makes RNA labile and easily degradable. **Q14.** Semiconservative DNA replication means each daughter DNA molecule has: - (a) Two entirely new strands - (b) Two entirely parental strands - (c) One parental and one new strand - (d) A random mixture of fragments **Answer: (c)** β€” In semiconservative replication each daughter molecule retains one parental strand and one newly synthesised strand. **Q15.** In the Meselson-Stahl experiment, the heavy isotope used was: - (a) 14C - (b) 15N (non-radioactive) - (c) Radioactive 32P - (d) Radioactive 35S **Answer: (b)** β€” 15N is a heavy but NOT radioactive isotope of nitrogen, separated from 14N purely on the basis of density in a CsCl gradient. **Q16.** DNA polymerase catalyses polymerisation only in which direction? - (a) Direction varies by organism - (b) Both directions equally - (c) 3' to 5' - (d) 5' to 3' **Answer: (d)** β€” DNA-dependent DNA polymerase works only 5' to 3', which is why one strand is synthesised continuously and the other discontinuously. **Q17.** Discontinuously synthesised DNA fragments are joined by the enzyme: - (a) DNA polymerase - (b) Helicase - (c) DNA ligase - (d) DNase **Answer: (c)** β€” DNA ligase joins the discontinuously synthesised fragments on the lagging strand template. **Q18.** The new strand marked A is synthesised - (a) continuously, in the 5β€²β†’3β€² direction, towards the replication fork - (b) in short pieces, in the 3β€²β†’5β€² direction - (c) in short pieces, in the 5β€²β†’3β€² direction, away from the fork - (d) continuously, in the 3β€²β†’5β€² direction **Answer: (a)** β€” A is the leading strand: DNA polymerase can add nucleotides only to a 3β€²-OH end, so the strand whose template runs 3β€²β†’5β€² towards the fork is made continuously in the 5β€²β†’3β€² direction as the fork opens. **Q19.** The short fragments marked B are finally joined by the enzyme - (a) Primase - (b) Helicase - (c) DNA ligase - (d) DNA polymerase III **Answer: (c)** β€” B are Okazaki fragments of the lagging strand. After the RNA primers are replaced by DNA, DNA ligase seals the nicks to form a continuous strand. **Q20.** The enzyme marked C, which unwinds the double helix at the fork, is - (a) helicase - (b) DNA ligase - (c) RNA polymerase - (d) topoisomerase I only **Answer: (a)** β€” Helicase breaks the hydrogen bonds between the base pairs and unwinds the DNA, creating the Y-shaped replication fork. Topoisomerases relieve the supercoiling ahead of it. **Q21.** The short green segments marked E at the start of each Okazaki fragment are - (a) introns - (b) telomeres - (c) Shine-Dalgarno sequences - (d) RNA primers **Answer: (d)** β€” DNA polymerase cannot start a chain from scratch, so primase makes a short RNA primer at the start of each fragment; later it is replaced by DNA. ### NEET Previous Year Questions (PYQs) *Real NEET previous-year questions on this chapter, with explanations in our own words.* **Q22.** Unequivocal proof that DNA is the genetic material came from the experiments of: (NEET PYQ) - (a) Hershey and Chase - (b) Griffith - (c) Avery, MacLeod and McCarty - (d) Meselson and Stahl **Answer: (a)** β€” Hershey and Chase (1952), using radioactively labelled bacteriophages, gave the unequivocal proof that DNA is the genetic material. **Q23.** Which of these viruses has RNA as its genetic material? (NEET PYQ) - (a) Bacteriophage φ×174 - (b) Tobacco Mosaic Virus - (c) Bacteriophage lambda - (d) Bacteriophage T2 **Answer: (b)** β€” Tobacco Mosaic Virus and QB bacteriophage are examples where RNA, not DNA, is the genetic material. **Q24.** In E. coli, replication of the entire genome is completed in approximately: (NEET PYQ) - (a) 18 minutes - (b) 2 minutes - (c) 20 hours - (d) 6 hours **Answer: (a)** β€” E. coli (4.6 x 10^6 bp) completes replication in about 18 minutes, at roughly 2000 bp per second. **Q25.** The 'beads-on-string' structure seen under the electron microscope refers to: (NEET PYQ) - (a) Ribosomes on mRNA - (b) Chromosomes at metaphase - (c) Nucleosomes in chromatin - (d) The replication fork **Answer: (c)** β€” Nucleosomes, which are the repeating units of chromatin, appear as a beads-on-string structure under EM. **Q26.** The haploid content of human DNA is approximately: (NEET PYQ) - (a) 4.6 x 10^6 bp - (b) 5386 nucleotides - (c) 3.3 x 10^9 bp - (d) 48502 bp **Answer: (c)** β€” Haploid human DNA content is about 3.3 x 10^9 base pairs; E. coli has 4.6 x 10^6 bp. **Q27.** In the Meselson-Stahl experiment, DNA extracted after 40 minutes (second generation) consisted of: (NEET PYQ) - (a) All light DNA - (b) Equal amounts of hybrid and light DNA - (c) All heavy DNA - (d) All hybrid DNA **Answer: (b)** β€” After the second generation (40 min), the DNA was composed of equal amounts of hybrid density DNA and light DNA. **Q28.** Deoxyribonucleoside triphosphates in replication act as: (NEET PYQ) - (a) Neither - (b) Substrates only - (c) Both substrates and energy source - (d) Energy source only **Answer: (c)** β€” They serve a dual purpose β€” acting as substrates and providing energy through their two terminal high-energy phosphates. ### Active Recall Prompt Write everything you can recall about **Molecular Basis of Inheritance (Part A)**, naming each part first: **DNA structure** (Meischer, Watson and Crick, Wilkins and Franklin, Chargaff's rule, the linkages, and the five salient features with their exact numbers); the **central dogma**; **DNA packaging** (nucleoid, histones, nucleosome, euchromatin vs heterochromatin); the **search for genetic material** (Griffith's transforming principle, Avery-MacLeod-McCarty, Hershey-Chase β€” what each showed); **properties of genetic material** (why DNA is more stable than RNA); the **RNA world**; and **DNA replication** (semiconservative, the Meselson-Stahl experiment, DNA polymerase, the replication fork, continuous vs discontinuous synthesis, DNA ligase, origin of replication). Begin each fact with its topic and end it with a full stop.