📖 Chapter BN06 · NDA Class 11–12 Level🎯 NDA Level : High Priority
Genetics and Evolution is consistently one of the most tested Biology chapters in NDA. Mendel's laws with Punnett square ratios, DNA structure (Watson-Crick double helix), the central dogma (DNA→RNA→Protein), and Darwin's theory of natural selection with evidence are all directly examined. This chapter rewards students who understand a few clear diagrams and memorise key ratios and organ-pair examples.
📌 What to expect in NDA (based on 2022–2025 pattern): (1) Mendel's three laws — statement, cross ratios, test cross concept; (2) Monohybrid cross (3:1 phenotype; 1:2:1 genotype) and dihybrid cross (9:3:3:1); (3) DNA structure — bases, strands, hydrogen bonds, antiparallel, double helix; (4) RNA types — mRNA, tRNA, rRNA functions; transcription and translation basics; (5) Central dogma — direction of information flow (DNA → RNA → Protein); (6) Darwin's natural selection — five key points; evidence for evolution (fossils, homologous, analogous, vestigial organs).
Topics at a Glance
① Mendel's Laws & Crosses
Law of Dominance, Segregation, Independent Assortment; monohybrid & dihybrid
② DNA, RNA & Central Dogma
DNA structure, replication, RNA types, transcription, translation
Pea plant experiments (1856–1863); published 1866; rediscovered 1900 by De Vries, Correns, Tschermak
Gregor Johann Mendel (1822–1884), an Austrian monk, conducted hybridisation experiments on garden pea (Pisum sativum) for 8 years and studied 7 pairs of contrasting traits. His work, ignored during his lifetime, was rediscovered in 1900 and became the foundation of modern genetics.
🌸 Why Pea Plants?
Short generation time; easy to grow
Naturally self-pollinating — pure lines easily maintained
Easily cross-pollinated manually
7 pairs of well-defined, contrasting characters
Large number of offspring per cross
📚 7 Contrasting Traits Studied
Seed shape: Round (D) vs Wrinkled (r)
Seed colour: Yellow (D) vs Green (r)
Pod shape: Inflated (D) vs Constricted (r)
Pod colour: Green (D) vs Yellow (r)
Flower colour: Violet (D) vs White (r)
Flower position: Axial (D) vs Terminal (r)
Plant height: Tall (D) vs Dwarf (r) [D = dominant]
Key terms:Dominant allele = expressed in both homozygous and heterozygous condition. Recessive allele = expressed only in homozygous recessive condition. Genotype = genetic makeup (e.g. TT, Tt, tt). Phenotype = observable trait (Tall or Dwarf). Homozygous = both alleles same (TT or tt). Heterozygous = different alleles (Tt).
1.2
Three Laws of Mendel
Know the statement + the experiment that proved each law + any exception
① Law of Dominance
In a cross between two pure-breeding varieties, the character that appears in F1 is dominant; the suppressed one is recessive
When TT x tt → all F1 = Tt (Tall) — tallness dominates
The two alleles of a gene pair separate (segregate) during gamete formation and only one allele enters each gamete
Also called Law of Purity of Gametes
Basis: separation of homologous chromosomes in Meiosis I
Proven by monohybrid cross F2 ratio 3:1
This law is universal — no exceptions
③ Law of Independent Assortment
Alleles of different genes assort independently into gametes during gamete formation
Proven by dihybrid cross F2 ratio 9:3:3:1
Basis: independent orientation of non-homologous chromosomes in Meiosis I
Exception: linked genes on same chromosome do NOT assort independently
1.3
Monohybrid Cross & Punnett Square
Tall (TT) x Dwarf (tt) — the most fundamental cross; produces 3:1 phenotype ratio in F2
Fig. 1 — Monohybrid cross: Tall (TT) x Dwarf (tt). F1 = all Tt (Tall; dominant). F1 x F1 → F2 Punnett square gives phenotype ratio 3 Tall : 1 Dwarf and genotype ratio 1 TT : 2 Tt : 1 tt. Mendel's Law of Segregation is demonstrated here.
🔄 Key Ratios in Mendel's Crosses (Must Memorise)
Monohybrid F2 Phenotype: 3:1
Monohybrid F2 Genotype: 1:2:1
Dihybrid F2 Phenotype: 9:3:3:1
Test Cross (Tt x tt) = 1:1
Back Cross: F1 x either parent
🧠 Test Cross vs Back Cross: Test cross: crossing unknown genotype with homozygous recessive (tt) to determine if unknown is TT or Tt. If all offspring are Tall → parent was TT. If 1:1 Tall:Dwarf → parent was Tt. Back cross: F1 x either parent (could be dominant or recessive). If F1 x recessive parent, it IS a test cross. If F1 x dominant parent, it is back cross but NOT a test cross.
1.4
Dihybrid Cross (9:3:3:1)
Two pairs of traits; proves Law of Independent Assortment; produces 4 gamete types from F1
Fig. 2 — Dihybrid cross: Round Yellow (RRYY) x Wrinkled Green (rryy). F1 = all RrYy (Round Yellow). F1 x F1 → F2 = 16 combinations in a 4x4 Punnett square, giving the 9:3:3:1 phenotype ratio. Two new combinations (Round Green and Wrinkled Yellow) appear — demonstrating Mendel's Law of Independent Assortment.
Neither allele is completely dominant → blending in F1
Classic example: Snapdragon (Antirrhinum) — Red (RR) x White (rr) → F1 = Pink (Rr)
F2 ratio: 1 Red : 2 Pink : 1 White (phenotype = genotype ratio = 1:2:1)
Also in: Mirabilis jalapa (4 o'clock plant), Andalusian fowl
🟠 Codominance
Both alleles expressed simultaneously in heterozygote — no blending
Classic example: ABO Blood Group (I₀) — I₀I₀ = A; I₀Iᵢ = AB (both A & B antigens expressed)
MN blood group: M and N both expressed in Mᴳ individuals
Sickle cell: Hbᵃ and Hbᴸ both seen in sickle cell trait carriers
⚠️ Sex-Linked Inheritance
Genes located on sex chromosomes (X or Y)
X-linked recessive: more common in males (XY — only one X)
Examples: Colour blindness, Haemophilia A (both X-linked recessive)
Affected mother (carrier XᴻX) → 50% affected sons
Haemophilia A was called "Royal disease" in European royalty (Queen Victoria)
📌 ABO Blood Group Genetics (NDA High-Frequency):
I₀ (dominant) > Iᵢ (dominant) > i (recessive) — three alleles, two expressed simultaneously Blood Group A: I₀I₀ or I₀i | B: IᵢIᵢ or Iᵢi | AB: I₀Iᵢ (codominant; both antigens) | O: ii (recessive; no antigen)
AB is the universal recipient; O is the universal donor — this is because O has no A/B antigens; AB has no anti-A/B antibodies.
PYQTopic-Wise PYQs — Mendel's Laws
Q1. In Mendel's monohybrid cross (Tt x Tt), the F2 phenotype ratio is:
A. 1:2:1
B. 9:3:3:1
C. 3:1
D. 1:1
Answer: C — 3:1. The F2 phenotype ratio (observable traits) in a monohybrid cross is 3 Dominant : 1 Recessive. The genotype ratio (genetic makeup) is 1:2:1 (1 TT : 2 Tt : 1 tt). The 9:3:3:1 ratio is the F2 phenotype ratio in a dihybrid cross. The 1:1 ratio is the result of a test cross.
Q2. The F2 phenotype ratio in a dihybrid cross (RrYy x RrYy) is:
A. 3:1
B. 1:2:1
C. 9:3:3:1
D. 1:1:1:1
Answer: C — 9:3:3:1. In a dihybrid cross, 16 genotypic combinations are produced (4x4 Punnett square). The phenotype ratio is 9 (both dominant) : 3 (first dominant only) : 3 (second dominant only) : 1 (both recessive). This proves Mendel's Law of Independent Assortment.
Q3. In snapdragon (Antirrhinum), Red (RR) x White (rr) gives Pink F1. This is an example of:
A. Codominance
B. Incomplete dominance
C. Epistasis
D. Complete dominance
Answer: B — Incomplete dominance. In incomplete dominance, neither allele completely masks the other, resulting in an intermediate (blending) phenotype in heterozygotes. Here, Red (RR) x White (rr) → Pink (Rr) in F1. F2 ratio = 1 Red : 2 Pink : 1 White (phenotype same as genotype ratio). In codominance, both traits are expressed simultaneously without blending (e.g. AB blood group).
Q4. Haemophilia is more common in males than females because it is:
A. Y-linked recessive
B. Autosomal dominant
C. X-linked recessive
D. Autosomal recessive
Answer: C — X-linked recessive. Males have only one X chromosome (XY), so a single recessive allele on their X causes haemophilia (XᴻY). Females need two copies (XᴻXᴻ) to be affected; one copy makes them a carrier (XᴻX). This is why haemophilia is far more common in males. Carrier mothers pass it to 50% of their sons.
TRICKY🧐 Genetics Traps
⚠️ "Mendel's Law of Independent Assortment applies to all genes." True or False?
False. The Law of Independent Assortment applies only to genes located on different chromosomes (non-homologous). Genes on the same chromosome are said to be linked and tend to be inherited together (they do NOT assort independently). This is called genetic linkage, discovered by Morgan. The frequency of recombination between linked genes is used to construct genetic maps. NDA may ask: "Linked genes violate which Mendelian law?" → Law of Independent Assortment.
⚠️ "In a test cross, the dominant parent is crossed with the homozygous dominant." True or False?
False. A test cross crosses the organism of unknown genotype with a homozygous RECESSIVE (e.g. tt). The purpose is to determine whether the unknown is homozygous dominant (TT) or heterozygous (Tt). If all offspring are tall → parent was TT. If 1:1 tall : dwarf → parent was Tt. Crossing with homozygous dominant (TT) would give all dominant offspring regardless of the unknown's genotype — making it useless for determining genotype.
2. DNA, RNA & Central Dogma
2.1
Structure of DNA — Watson-Crick Double Helix Model (1953)
Nobel Prize 1962 (Watson, Crick, Wilkins); Rosalind Franklin's X-ray data was crucial
Fig. 3 — Watson-Crick DNA Double Helix (1953). Two antiparallel sugar-phosphate backbones (blue = 5'→3'; red = 3'→5') wind around each other. Nitrogenous bases pair specifically: A=T (2 hydrogen bonds) and G≡C (3 hydrogen bonds). Purines (A,G) always pair with Pyrimidines (T,C). G-C rich DNA is more thermally stable.
🔄 DNA vs RNA — Key Differences
Sugar: DNA = Deoxyribose; RNA = Ribose (has an –OH at 2' carbon)
Thymine (T): in DNA only; RNA has Uracil (U) instead
Strands: DNA = double-stranded (dsDNA); RNA = single-stranded (ssRNA)
Stability: DNA more stable (deoxyribose + no –OH at 2'); RNA less stable
Location: DNA = nucleus (mainly); RNA = nucleus + cytoplasm
Function: DNA = genetic blueprint; RNA = protein synthesis
🔷 Three Types of RNA
mRNA (messenger RNA): carries genetic code from DNA (in nucleus) to ribosome (in cytoplasm); contains codons (3-base code); longest; least abundant
tRNA (transfer RNA): clover-leaf structure; carries specific amino acid to ribosome; has anticodon loop; shortest RNA (~80 nucleotides)
rRNA (ribosomal RNA): structural component of ribosomes; most abundant RNA (~80% of total cellular RNA); combines with proteins to form ribosomes
🌳 DNA Replication
Mode: Semi-conservative (proven by Meselson-Stahl, 1958 using ⁺N/⁻N isotopes)
Each daughter DNA has 1 old strand + 1 new strand
Occurs in: S phase of cell cycle
Key enzyme: DNA polymerase (adds nucleotides in 5'→3' direction)
Helicase: unwinds double helix at replication fork
Primase: lays RNA primer to start synthesis
DNA Ligase: joins Okazaki fragments (lagging strand)
▶️ Central Dogma of Molecular Biology
Proposed by Francis Crick (1958)
Direction: DNA → RNA → Protein
Transcription: DNA → mRNA (in nucleus; by RNA polymerase)
Translation: mRNA → Protein (at ribosome; by ribosomes + tRNA)
Reverse transcription: RNA → DNA (in retroviruses like HIV; by reverse transcriptase)
Codon: 3-nucleotide code on mRNA for 1 amino acid (64 total codons)
AUG = Start codon; UAA, UAG, UGA = Stop codons
Fig. 4 — Central Dogma of Molecular Biology (Crick, 1958). Normal flow: DNA → mRNA (Transcription by RNA polymerase in nucleus) → Protein (Translation at ribosomes in cytoplasm). Self-replication of DNA (S phase). Dashed line: Reverse transcription (RNA → DNA), unique to retroviruses like HIV, catalysed by reverse transcriptase.
PYQTopic-Wise PYQs — DNA, RNA & Central Dogma
Q5. The complementary base pairing in DNA is:
A. A-G and T-C
B. A-T and G-C
C. A-U and G-C
D. A-C and T-G
Answer: B — A-T and G-C. In DNA: Adenine (A) pairs with Thymine (T) via 2 hydrogen bonds; Guanine (G) pairs with Cytosine (C) via 3 hydrogen bonds. In RNA, Thymine is replaced by Uracil (U), so A-U pairs. A-C and T-G pairings would violate the purine-pyrimidine rule (both A and G are purines; T and C are pyrimidines).
Q6. The mode of DNA replication proven by the Meselson-Stahl experiment is:
A. Conservative
B. Dispersive
C. Semi-conservative
D. Random
Answer: C — Semi-conservative. In 1958, Meselson and Stahl used heavy (⁻N) and light (⁺N) isotopes of nitrogen to label DNA. After one round of replication, all DNA had intermediate density (one old + one new strand), proving semi-conservative replication. Each daughter DNA retains one original strand and synthesises one new complementary strand.
Q7. Which type of RNA carries amino acids to the ribosome during translation?
A. mRNA
B. rRNA
C. tRNA
D. snRNA
Answer: C — tRNA (transfer RNA). tRNA has a cloverleaf secondary structure with an anticodon loop (complementary to mRNA codon) and a 3' CCA tail that carries the specific amino acid. It acts as an adaptor between the codon (mRNA) and the amino acid. mRNA carries the genetic message; rRNA is structural component of ribosomes.
Q8. In DNA, if Adenine = 30%, what is the percentage of Guanine?
A. 30%
B. 20%
C. 70%
D. 50%
Answer: B — 20%. By Chargaff's rules: A = T and G = C. If A = 30%, then T = 30%. Total A+T = 60%, so G+C = 40%, meaning G = C = 20% each. NDA frequently tests Chargaff's rules: A+G = T+C (purines = pyrimidines = 50%). If you know any one base percentage, you can calculate all others.
TRICKY🧐 DNA/RNA Traps
⚠️ "Thymine is found in both DNA and RNA." True or False?
False. Thymine (T) is found ONLY in DNA. RNA contains Uracil (U) instead of Thymine. The difference: Uracil lacks the methyl group (–CH₃) at position 5 that Thymine has. So: DNA bases = A, T, G, C; RNA bases = A, U, G, C. This is one of NDA's most repeated molecular biology MCQ distinctions. Thymine is also called 5-methyluracil.
⚠️ "HIV is a DNA virus." True or False?
False. HIV (Human Immunodeficiency Virus) is an RNA virus — specifically a retrovirus. It carries RNA as its genetic material and uses reverse transcriptase to convert RNA → DNA inside host cells. The DNA then integrates into the host genome. This violates the normal central dogma (DNA → RNA) and is the basis for antiretroviral drugs like AZT (which inhibit reverse transcriptase).
3. Evolution
3.1
Darwin's Theory of Natural Selection
Published in On the Origin of Species (1859); Alfred Russel Wallace independently proposed same idea
🌟 Five Key Postulates of Natural Selection
1. Overproduction: organisms produce far more offspring than can possibly survive
2. Variation: individuals in a population differ in heritable traits
3. Struggle for Existence: competition for limited resources (food, space, mates)
4. Survival of the Fittest: individuals with favourable variations survive and reproduce better ("fit" = suited to environment, not physically strong)
5. Inheritance: favourable traits are passed to offspring; over generations, the population changes
📚 Before Darwin — Historical Context
Lamarck (1809): proposed inheritance of acquired characteristics (e.g. giraffe neck stretching); now known to be incorrect
Darwin (1859): natural selection; based on Galapagos observations and Malthus's essay on population
Wallace: independently proposed natural selection; wrote to Darwin (1858) → joint presentation
Neodarwinism / Modern Synthesis: combines Darwin's natural selection with Mendelian genetics and mutation theory
Hugo de Vries: proposed Mutation Theory — new species arise by mutations, not gradual change
📌 Darwin's Classic Examples (NDA Historical Questions): Galapagos finches: 13 species from one ancestor; beaks adapted to different food sources (evidence of adaptive radiation) Industrial melanism (Peppered moth):Biston betularia — before industrialisation, pale moths survived on light bark; after soot darkened trees, dark (melanic) moths became more common (natural selection in action; direct observable evidence) Antibiotic resistance in bacteria: modern living example of natural selection (bacteria with resistance genes survive antibiotic treatment and multiply)
3.2
Evidence for Evolution
Six categories — homologous vs analogous organs is the most tested NDA distinction in this section
🛎 Fossil Evidence
e.g. Archaeopteryx, Dimetrodon, Trilobite
Fossils = preserved remains of past organisms in rocks (sedimentary)
Show gradual changes over time (geological time scale)
Archaeopteryx: transitional fossil between reptiles and birds (had feathers + reptile teeth + claws on wings)
Older fossils are simpler; younger ones more complex
Radiometric dating (radiocarbon) dates fossils
🌿 Homologous Organs
Forelimbs of humans, whales, bats, horses
Same basic structure (same bones: humerus, radius, ulna) but different functions
Evidence of divergent evolution from common ancestor
Mammals' forelimb: Human arm (grasping), Whale flipper (swimming), Bat wing (flying), Horse leg (running) — same bones, different use
Thorn (stem modification) and tendril of Bougainvillea = homologous (both modified stems)
⚜ Analogous Organs
Wings of bat vs wings of butterfly vs wings of birds
Different basic structure but same function
Evidence of convergent evolution (unrelated organisms evolving similar solutions)
Bat wing (modified forelimb), Bird wing (modified forelimb with feathers), Insect wing (exoskeletal extension) — all for flying but structurally different
Thorn (stem) vs Tendril of Cucumber (leaf modification) = analogous (same function: protection/support; different origin)
🐱 Embryological Evidence
Ernst Haeckel (1866) — "Biogenetic Law"
Embryos of different vertebrates look remarkably similar in early stages (all have pharyngeal slits, tail, similar body plan)
"Ontogeny recapitulates phylogeny" — Haeckel's biogenetic law (oversimplified but shows common ancestry)
Human embryo shows gill pouches, tail — evidence of evolutionary relationship with fish and other vertebrates
🔮 Biochemical Evidence
DNA/protein sequences, cytochrome c
Closely related organisms share similar DNA sequences and protein structures
Cytochrome c (respiratory protein): nearly identical in humans and chimpanzees (differs by only 1 amino acid)
Chimp DNA 98-99% similar to human DNA
All living organisms use same genetic code (64 codons) — universal code = common ancestry
🌍 Vestigial Organs
Appendix, coccyx, wisdom teeth, ear muscles in humans
Reduced, non-functional structures that were functional in ancestors
Appendix in humans: vestigial (cellulose-digesting organ in herbivorous ancestors)
Coccyx: vestigial tail vertebrae in humans
Nictitating membrane: vestigial third eyelid (functional in birds and reptiles)
Plica semilunaris: vestigial structure in human eye
Fig. 5 — Homologous organs (left): same underlying bone structure (Humerus, Radius, Ulna) in human arm, whale flipper, and bat wing — different functions but same structural plan = Divergent Evolution from common ancestor. Analogous organs (right): bird wing, insect wing, bat wing all serve flying but have completely different structural origins = Convergent Evolution (independent, parallel solutions).
🧠 Homologous vs Analogous — Memory Trick: HOMologous = HOM-e (same structure, born from same ancestor) but different function — divergent evolution ANAlogous = ANA-logy (like analogy = same function, different origin) — convergent evolution
Classic NDA pair to know: Thorn (stem modification in Bougainvillea) & Tendril of Bougainvillea = Homologous; Thorn (stem) & Tendril of Cucumber (leaf modification) = Analogous
PYQTopic-Wise PYQs — Evolution
Q9. Homologous organs are evidence of:
A. Convergent evolution
B. Parallel evolution
C. Divergent evolution
D. Co-evolution
Answer: C — Divergent evolution. Homologous organs have the same basic structure (derived from the same ancestral structure) but perform different functions in different organisms. This shows they diverged from a common ancestor but adapted to different environments. Example: Human arm, bat wing, whale flipper — same bones, different functions. Analogous organs show convergent evolution.
Q10. Archaeopteryx is considered a transitional fossil because it shows features of both:
A. Fish and Amphibians
B. Reptiles and Birds
C. Mammals and Reptiles
D. Amphibians and Reptiles
Answer: B — Reptiles and Birds. Archaeopteryx (Late Jurassic, ~150 million years ago) had bird features (feathers, wishbone) and reptile features (teeth, clawed wings, long bony tail). It is the classic example of a transitional or "missing link" fossil showing the evolutionary transition from reptiles to birds.
Q11. Industrial melanism in the peppered moth (Biston betularia) is an example of:
A. Mutation theory
B. Lamarckian evolution
C. Natural selection in action
D. Genetic drift
Answer: C — Natural selection in action. Before industrialisation, pale peppered moths were better camouflaged on light-coloured bark. After industrial pollution blackened tree bark, dark (melanic) moths became camouflaged while pale moths were eaten by predators. The frequency of dark moths increased — a direct, observable demonstration of Darwinian natural selection operating in real time.
Q12. The theory of inheritance of acquired characteristics was proposed by:
A. Charles Darwin
B. Gregor Mendel
C. Jean-Baptiste Lamarck
D. Hugo de Vries
Answer: C — Jean-Baptiste Lamarck. Lamarck proposed in 1809 that organisms can develop new structures through use or disuse during their lifetime, and these acquired traits are passed to offspring (e.g. giraffes stretching their necks → longer necks passed to offspring). This is now disproven. Darwin proposed natural selection; Mendel proposed laws of inheritance; de Vries proposed mutation theory.
TRICKY🧐 Evolution Traps
⚠️ "Wings of a bat and wings of a butterfly are homologous organs." True or False?
False — they are ANALOGOUS. Bat wings are modified forelimbs (skin membrane stretched between elongated finger bones). Butterfly wings are exoskeletal extensions (entirely different structural origin, no bones). Both function for flying, but their underlying structures are completely different — proving they evolved independently (convergent evolution). For organs to be homologous, they must share the same ancestral structural plan (same bones/tissues), just modified for different purposes.
⚠️ "Lamarck's theory of use and disuse has been proven correct." True or False?
False. Lamarck's theory of inheritance of acquired characteristics has been definitively disproven. Changes to phenotype (body) during an organism's lifetime (like a bodybuilder's muscles) are NOT passed to offspring because they involve no change in DNA sequence. DNA is in the nucleus and muscle protein changes don't alter germ cells (sperm/egg). However, Epigenetics shows that some environmental influences can affect gene expression across generations — but this is different from Lamarck's mechanism.
📄 Quick-Reference Fact Sheet — BN06
🌸 Mendel's Laws & Ratios
Law of Dominance: one allele masks the other
Law of Segregation: alleles separate in gamete formation (universal; no exceptions)
Law of Independent Assortment: genes on different chromosomes assort independently
Monohybrid F2 phenotype: 3:1 | Genotype: 1:2:1
Dihybrid F2 phenotype: 9:3:3:1
Test cross: unknown x homozygous recessive (tt)
🔸 Exceptions to Mendel's Laws
Incomplete dominance: blending — Snapdragon (Red x White → Pink); F2 = 1:2:1
Codominance: both expressed — ABO blood group (AB group = I₀Iᵢ)
Sex-linked: genes on X chromosome — Haemophilia, Colour blindness (more in males)
Linkage: genes on same chromosome — violate Independent Assortment
Blood group O = ii (recessive); Universal donor
Blood group AB = I₀Iᵢ (codominant); Universal recipient
🔄 DNA Structure Keys
Watson-Crick double helix (1953); Nobel 1962
A=T (2 H-bonds); G≡C (3 H-bonds); Chargaff's rule
DNA sugar: Deoxyribose | RNA sugar: Ribose
DNA unique base: Thymine (T) | RNA unique: Uracil (U)
Antiparallel: one strand 5'→3', other 3'→5'
Purines: A, G | Pyrimidines: T, C (U in RNA)
▶️ RNA & Central Dogma
mRNA: carries genetic code (codons); longest; from nucleus to cytoplasm
tRNA: cloverleaf; anticodon; carries amino acid to ribosome; shortest
rRNA: most abundant (80%); structural component of ribosomes
Central dogma: DNA → mRNA → Protein (Crick, 1958)
DNA replication: semi-conservative (Meselson-Stahl, 1958)
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