The Biological Basis of Life — UPSC Anthropology Optional Notes

1. The cell

Discovery: Robert Hooke named cells after seeing cork under a microscope (Micrographia, 1665); Antonie van Leeuwenhoek first observed living single cells in the 1670s.

Cell theory: Matthias Schleiden (plants, 1838) and Theodor Schwann (animals, 1839) proposed that all organisms are made of cells; Rudolf Virchow (1855) added that every cell arises from a pre-existing cell (omnis cellula e cellula).

Prokaryotic and eukaryotic cells: prokaryotes (bacteria) lack a nucleus and membrane-bound organelles; eukaryotes (plants, animals, humans) have a true nucleus and organelles.

Plasma membrane: the fluid mosaic model (Singer and Nicolson, 1972) describes a lipid bilayer with embedded proteins that controls what enters and leaves the cell.

Nucleus: bounded by a double nuclear membrane with pores; contains chromatin (DNA and histone proteins) and the nucleolus, where ribosomal RNA is made.

Organelles: mitochondria (energy as ATP, with their own DNA inherited from the mother), ribosomes (protein synthesis), endoplasmic reticulum (rough with ribosomes; smooth for lipids), Golgi apparatus (packaging and secretion), lysosomes (digestion), centrioles (spindle formation in animal cells).

Anthropological relevance: mitochondrial DNA, passed only through the mother, is used to trace human migrations, as in the mitochondrial Eve study (1987) and studies of Andaman Islanders.

2. DNA: discovery and structure

DNA as genetic material: Friedrich Miescher isolated nuclein (1869); Griffith's transformation experiment (1928), Avery, MacLeod and McCarty (1944) and Hershey and Chase (1952) showed that DNA carries hereditary information.

Chargaff's rules (1950): in DNA the amount of adenine equals thymine and guanine equals cytosine.

Double helix (1953): James Watson and Francis Crick proposed the model using X-ray diffraction data, especially Rosalind Franklin's Photo 51; Watson, Crick and Maurice Wilkins received the Nobel Prize in 1962 (Franklin had died in 1958).

Nucleotide: each unit has a deoxyribose sugar, a phosphate group and a nitrogenous base: purines (adenine, guanine) or pyrimidines (thymine, cytosine).

Structure of the B-form helix: two antiparallel strands (5' to 3' and 3' to 5'), sugar-phosphate backbone outside, bases inside; A pairs with T by two hydrogen bonds and G with C by three; diameter about 2 nm; about 10 base pairs and 3.4 nm per turn; right-handed with major and minor grooves.

RNA differences: RNA is usually single-stranded, has ribose sugar and uracil instead of thymine; types are mRNA, tRNA and rRNA.

Human genome: about 3.1 billion base pairs and about 20,000 protein-coding genes; the Human Genome Project was completed in 2003 and the first gap-free (telomere-to-telomere) sequence was published in 2022.

3. DNA replication

Semi-conservative replication: each new DNA molecule has one old and one new strand, proved by Matthew Meselson and Franklin Stahl (1958) using heavy nitrogen (N15) in bacteria.

Steps: (1) helicase unwinds the helix at the origin, forming a replication fork; (2) single-strand binding proteins keep strands apart and topoisomerase relieves twisting; (3) primase lays down a short RNA primer.

Elongation: DNA polymerase adds nucleotides only in the 5' to 3' direction; the leading strand is made continuously and the lagging strand in short Okazaki fragments (Reiji Okazaki, 1968) that DNA ligase joins.

Accuracy: proofreading by DNA polymerase and repair systems keep errors very low; uncorrected errors become mutations.

Telomeres: repetitive ends of chromosomes (TTAGGG in humans) shorten with each division; telomerase restores them in germ cells; telomere length is studied in ageing research.

4. Protein synthesis

Central dogma: Francis Crick (1958) stated that information flows from DNA to RNA to protein; reverse transcription in retroviruses such as HIV (Temin and Baltimore, 1970) is an exception.

Transcription: in the nucleus, RNA polymerase binds a promoter and copies the template strand into pre-mRNA in the 5' to 3' direction until a termination signal.

RNA processing: a 5' cap and a 3' poly-A tail are added and introns are removed by splicing, joining exons; alternative splicing lets one gene make several proteins.

Translation: in the cytoplasm, ribosomes read mRNA codons; tRNAs with matching anticodons bring amino acids; initiation at AUG (methionine), elongation with peptide bonds, termination at a stop codon (UAA, UAG, UGA).

Post-translational changes: the chain folds into its three-dimensional shape and may be modified; for example haemoglobin assembles two alpha and two beta globin chains with haem groups.

5. The genetic code

Triplet code: three bases form a codon; 4 bases give 64 codons, of which 61 code for 20 amino acids and 3 are stop codons.

Properties: universal (nearly the same in all organisms, with small exceptions such as mitochondria), degenerate (most amino acids have several codons), non-overlapping, commaless and unambiguous.

Deciphering: Marshall Nirenberg and Heinrich Matthaei (1961) showed that UUU codes for phenylalanine; Har Gobind Khorana, born in Raipur (Punjab, now in Pakistan), synthesised defined RNA sequences; Nirenberg, Khorana and Robert Holley shared the 1968 Nobel Prize.

Example linking to anthropology: in the beta-globin gene, the sixth codon GAG (glutamic acid) changed to GTG (valine) produces haemoglobin S, the sickle-cell allele common in malarial regions including parts of central India.

6. Significance and debates

Molecular anthropology: DNA sequences allow estimates of divergence times (molecular clock, Zuckerkandl and Pauling, 1962), ancestry and admixture.

Ancient DNA: methods developed by Svante Pääbo recover DNA from fossils, revealing Neanderthal and Denisovan admixture; the Rakhigarhi (Haryana) Harappan genome was published in 2019.

Genome India Project: India's programme to sequence 10,000 genomes from diverse populations, including tribal groups, announced its dataset in 2025; it raises issues of consent and data sharing that anthropologists stress.

Debate on junk DNA: only about 1 to 2 percent of the genome codes for proteins; how much of the rest is functional is debated (the ENCODE project's 2012 claim of 80 percent function was widely criticised).

7. Diagrams to draw

Animal cell: a circle with plasma membrane, nucleus with nucleolus, mitochondria, rough and smooth ER, Golgi, ribosomes, lysosomes and centrioles, all labelled.

DNA double helix: two spiral backbones labelled sugar and phosphate, rungs labelled A–T and G–C, arrows showing 5' to 3' and 3' to 5', and measurements 2 nm and 3.4 nm per turn.

Replication fork: a Y shape with helicase at the fork, leading strand continuous towards the fork, lagging strand as Okazaki fragments, primers and ligase marked.

Protein synthesis flowchart: DNA (nucleus), then transcription to pre-mRNA, then processing to mRNA, then exit through nuclear pore, then translation on ribosome with tRNA, then polypeptide.

8. Using it in the exam

10 marks (DNA structure or replication): scholars and years, structural features with numbers, a labelled diagram, one line of anthropological relevance.

15 to 20 marks (protein synthesis): central dogma, transcription, processing, translation, genetic code properties, flowchart, and the sickle-cell example.

Keywords: antiparallel, complementary base pairing, semi-conservative, Okazaki fragments, codon and anticodon, degeneracy, central dogma.

1. The gene

History: Mendel's factors (1866); Wilhelm Johannsen coined the word gene (1909) and distinguished genotype and phenotype; Thomas Hunt Morgan's Drosophila work (1910s) placed genes on chromosomes in a linear order.

One gene–one enzyme: George Beadle and Edward Tatum (1941), using Neurospora, later refined to one gene–one polypeptide.

Modern definition: a gene is a DNA sequence that is transcribed into a functional RNA or protein, including exons (coding), introns (non-coding) and regulatory regions such as promoters.

Allele, locus, homozygous, heterozygous: alleles are alternative forms of a gene at the same locus; a person with two identical alleles is homozygous (AA or aa) and with two different alleles heterozygous (Aa).

Gene types and interactions: structural and regulatory genes (operon model, Jacob and Monod, 1961); dominance, codominance (ABO blood group A and B), multiple alleles (ABO), polygenes (skin colour, stature) and pleiotropy (one gene, many effects, as in sickle-cell disease).

2. Gene (point) mutations

Mutation: a heritable change in DNA; the term was introduced by Hugo de Vries (1901); mutations in germ cells pass to offspring, somatic mutations (as in many cancers) do not.

Substitutions: transitions (purine to purine, pyrimidine to pyrimidine) and transversions (purine to pyrimidine or reverse).

Effects of substitutions: silent (same amino acid), missense (different amino acid, as in sickle-cell haemoglobin, glutamic acid to valine at position 6 of beta-globin) and nonsense (a stop codon, as in some beta-thalassaemias).

Frameshift mutations: insertion or deletion of bases not in multiples of three shifts the reading frame, usually producing a non-functional protein; the CCR5-delta32 deletion gives resistance to some HIV strains.

Causes (mutagens): spontaneous replication errors, radiation (H.J. Muller showed X-rays induce mutations in Drosophila, 1927; Nobel 1946), ultraviolet light (thymine dimers), chemicals such as mustard gas and some dyes.

Evolutionary role: most mutations are neutral or harmful, but mutation is the ultimate source of variation; examples of advantage include HbS heterozygotes resisting malaria and lactase persistence.

Indian relevance: sickle-cell disease and beta-thalassaemia are major public health problems among some tribal and caste groups; HbE is common in north-east India.

Mutation rate: each human is born with roughly 60 to 70 new point mutations (an approximate figure from whole-genome studies of parents and children), most in non-coding DNA; the number rises with the father's age.

Dynamic mutations: expansion of triplet repeats across generations, as in Huntington disease (CAG repeats) and fragile X syndrome (CGG repeats), causes anticipation, with earlier or more severe disease in later generations.

Mutation and drift in small populations: in small endogamous groups a rare mutation can reach high frequency by founder effect, a point relevant to India's many endogamous castes and tribes (Nakatsuka and colleagues, 2017, reported strong founder events in many Indian groups).

Back mutation and mutation pressure: a mutant allele can revert; when forward and backward rates are unequal, mutation alone slowly changes allele frequencies, though usually far more weakly than selection or drift.

3. Chromosomes

Discovery: Wilhelm Waldeyer coined the word chromosome (1888); the Sutton–Boveri chromosome theory of heredity (1902–03) linked chromosomes with Mendel's factors.

Human number: Joe Hin Tjio and Albert Levan established 46 in 1956 (earlier counts said 48); 44 autosomes and two sex chromosomes.

Structure: at metaphase each chromosome has two sister chromatids joined at the centromere; arms are p (short) and q (long); telomeres cap the ends; DNA is wound around histone octamers as nucleosomes.

Classification by centromere: metacentric (middle, for example chromosome 1), submetacentric, acrocentric (near one end, chromosomes 13, 14, 15, 21 and 22) and telocentric (at the end, not found in humans).

Karyotype: an ordered arrangement of chromosomes by size and centromere position; the Denver classification (1960) grouped human chromosomes into A to G groups; G-banding with Giemsa stain identifies each chromosome.

Sex chromosomes and sex chromatin: the Y carries the SRY gene that triggers male development; Murray Barr and Ewart Bertram (1949) found the Barr body (an inactive X) in female cells; Mary Lyon's hypothesis (1961) explains random X inactivation.

4. Chromosomal aberrations

Structural (1) deletion: loss of a segment, for example cri-du-chat syndrome (deletion of part of 5p) with a cat-like cry and intellectual disability.

Structural (2) duplication and inversion: duplication repeats a segment and supplies raw material for new genes (for example the globin gene family); inversion reverses a segment and can suppress crossing over.

Structural (3) translocation: exchange of segments between non-homologous chromosomes; the Philadelphia chromosome t(9;22) causes chronic myeloid leukaemia; Robertsonian translocation joins two acrocentrics and can cause familial Down syndrome.

Numerical, aneuploidy: gain or loss of single chromosomes through non-disjunction; Down syndrome (trisomy 21, 47 chromosomes; described by John Langdon Down, 1866, cause found by Jérôme Lejeune, 1959), Edwards syndrome (trisomy 18), Patau syndrome (trisomy 13).

Sex chromosome aneuploidies: Turner syndrome (45,X: short stature, infertile female, no Barr body), Klinefelter syndrome (47,XXY: tall, infertile male with one Barr body), triple X (47,XXX) and 47,XYY.

Polyploidy: whole extra sets of chromosomes (triploid, tetraploid), common in plants (for example wheat) and a route to sympatric speciation, but lethal in humans except in rare mosaics.

Maternal age effect: the risk of trisomy 21 rises with maternal age, roughly 1 in 1,500 births at age 20 to about 1 in 100 at age 40 (approximate figures), because oocytes stay arrested in meiosis for decades.

5. Evolution of chromosomes and debates

Human–ape difference: great apes have 48 chromosomes; human chromosome 2 arose from the head-to-head fusion of two ape chromosomes, shown by remnant telomere sequences and a second inactive centromere.

Y chromosome in anthropology: passed from father to son, the Y is used with mtDNA to trace paternal and maternal lineages; Indian studies show different patterns of male and female ancestry in caste and tribal groups (interpretations are debated).

Ethics: genetic testing and screening (for example premarital sickle-cell screening and genetic cards under the National Sickle Cell Anaemia Elimination Mission, 2023) raise issues of stigma, consent and privacy.

6. Diagrams to draw

Chromosome structure: an X-shaped metaphase chromosome with sister chromatids, centromere, p and q arms and telomeres labelled.

Types of chromosome: four sketches showing metacentric, submetacentric, acrocentric and telocentric positions of the centromere.

Structural aberrations: a bar with segments ABCDEFG; deletion ABDEFG; duplication ABCCDEFG; inversion ADCBEFG; translocation with segments swapped onto a second bar MNOP.

Karyotype of Down syndrome: 23 numbered pairs with three copies at position 21, written as 47,XX,+21 or 47,XY,+21.

7. Using it in the exam

10 marks (mutation): definition with de Vries, types (gene and chromosomal) with one example each, mutagens, evolutionary role.

15 to 20 marks (chromosomal aberrations): structural and numerical types, a table of syndromes with karyotype formulas, the non-disjunction mechanism, and relevance to genetic counselling.

Keywords: locus, allele, missense, frameshift, aneuploidy, non-disjunction, karyotype, Barr body, Lyon hypothesis.

1. The cell cycle

Phases: interphase (G1 growth, S phase when DNA is replicated, G2 preparation) followed by the M phase (mitosis and cytokinesis); non-dividing cells such as mature neurons enter G0.

Checkpoints: at G1/S, G2/M and the spindle checkpoint, proteins such as cyclins, cyclin-dependent kinases and p53 control division; failure leads to uncontrolled division (cancer).

History: Walther Flemming described and named mitosis (1882); Oscar Hertwig and others described meiosis in the 1870s–80s, and August Weismann predicted a reduction division.

Terms: diploid (2n = 46 in humans) and haploid (n = 23); homologous chromosomes are pairs, one from each parent; sister chromatids are identical copies joined at the centromere.

2. Mitosis

Prophase: chromatin condenses into visible chromosomes with two chromatids; the nucleolus disappears; centrioles move to opposite poles and the spindle forms.

Prometaphase: the nuclear envelope breaks down and spindle fibres attach to kinetochores at the centromeres.

Metaphase: chromosomes line up at the equatorial (metaphase) plate; this stage is used for karyotyping, often arrested with colchicine.

Anaphase: centromeres split and sister chromatids move to opposite poles.

Telophase and cytokinesis: chromosomes decondense, nuclear envelopes and nucleoli re-form; in animal cells a cleavage furrow divides the cytoplasm, giving two identical diploid daughter cells.

Significance of mitosis: growth, repair and replacement of cells (for example skin and blood cells), asexual reproduction in some organisms, and maintenance of the chromosome number.

3. Meiosis I (reduction division)

Prophase I, leptotene: chromosomes become visible as long thin threads.

Zygotene: homologous chromosomes pair (synapsis) through the synaptonemal complex, forming bivalents.

Pachytene: each bivalent shows four chromatids (a tetrad) and crossing over takes place between non-sister chromatids, exchanging segments.

Diplotene and diakinesis: homologues begin to separate but stay joined at chiasmata (visible sites of crossing over); in diakinesis chromosomes condense fully and the nuclear envelope breaks down; human oocytes arrest at diplotene from before birth until ovulation.

Metaphase I to telophase I: bivalents align at the plate with random orientation (independent assortment); in anaphase I homologous chromosomes (not chromatids) separate; two haploid cells result, each chromosome still with two chromatids.

4. Meiosis II (equational division)

Process: similar to mitosis without DNA replication; in anaphase II sister chromatids separate.

Result: four haploid cells, each genetically different because of crossing over and independent assortment.

Variation in numbers: independent assortment alone gives 2 to the power 23, about 8.4 million chromosome combinations per gamete in humans, before counting crossing over.

Comparison with meiosis I: meiosis I is reductional (2n to n) and meiosis II equational (chromatid number halves, chromosome number stays n); there is no S phase between the two divisions, only a short interkinesis.

Recombination hotspots: crossovers cluster at particular sites, many set by the PRDM9 protein, whose variants differ between human populations; on average there are roughly 1 to 3 crossovers per chromosome pair in each meiosis.

5. Gametogenesis

Unequal division in females: cytoplasm is divided unequally so that the ovum keeps most nutrients and organelles, including mitochondria, which is why mitochondrial DNA is inherited from the mother.

Numbers: a female foetus has several million (about 6 to 7 million) oocytes at about five months of gestation, about one to two million at birth and only about 400 to 500 are ovulated in a lifetime (approximate figures).

Paternal age: because spermatogonia keep dividing by mitosis throughout life, older fathers pass on more new point mutations, while older mothers face more chromosomal non-disjunction.

Spermatogenesis: begins at puberty in the seminiferous tubules; one primary spermatocyte gives four functional sperm; continuous through adult life.

Oogenesis: begins before birth; primary oocytes arrest in prophase I; at each ovulation meiosis I completes, giving a secondary oocyte and a first polar body; meiosis II completes only after fertilisation, giving one ovum and polar bodies.

Fertilisation: fusion of haploid sperm and ovum restores the diploid number (46) in the zygote, which then divides by mitosis.

6. Errors and significance

Non-disjunction: failure of homologues (meiosis I) or chromatids (meiosis II) to separate gives gametes with n+1 or n-1 chromosomes, leading to trisomy or monosomy; most cases of Down syndrome arise from maternal meiosis I errors.

Crossing over and genetic maps: Alfred Sturtevant (1913) used crossing-over frequencies to build the first genetic map; recombination frequency measures distance between linked genes (1 percent = 1 centimorgan).

Evolutionary significance: meiosis and sexual reproduction create variation each generation, helping populations adapt; this is a key input in the synthetic theory.

Debate on sex: why sex evolved despite its costs is debated; the Red Queen hypothesis (Leigh Van Valen, 1973; developed by W.D. Hamilton) argues that recombination helps hosts keep up with parasites.

Applied relevance: knowledge of meiosis underpins prenatal diagnosis (amniocentesis, chorionic villus sampling), genetic counselling for older mothers and assisted reproduction.

7. Diagrams to draw

Cell cycle wheel: a circle divided into G1, S, G2 and M, with G0 as a side exit from G1.

Mitosis stages: four cells for prophase, metaphase, anaphase and telophase, using a cell with 2n = 4 (two pairs, one shaded for maternal, one plain for paternal).

Meiosis flow: one 2n = 4 cell, then bivalents with a crossover in prophase I, then two haploid cells after meiosis I, then four haploid cells after meiosis II, each with a different mix of shaded and plain segments.

Non-disjunction: one chromosome pair failing to separate in meiosis I, giving two gametes with an extra copy and two with none; fertilisation of an n+1 gamete gives trisomy.

8. Using it in the exam

10 marks: define mitosis or meiosis, list stages with one line each, and give significance; one diagram.

15 to 20 marks (compare and significance): a comparison table, stage diagrams, sub-stages of prophase I, gametogenesis, non-disjunction and evolutionary importance.

Keywords: diploid, haploid, synapsis, bivalent, chiasma, crossing over, independent assortment, non-disjunction, polar body.

1. The Cell, DNA and Protein Synthesis

Example 1 (UPSC 2026, 10 marks): Distinguishing features of nuclear and mitochondrial DNA

Paper and question: Paper I, Q5(b)

Approach: A short note: compare nuclear and mitochondrial DNA on location, structure, size, copy number, inheritance, recombination and mutation rate (a compact comparison works well), then state their uses in anthropology.

Location and structure: nuclear DNA lies in the nucleus as 46 linear chromosomes wrapped around histone proteins; mitochondrial DNA (mtDNA) lies in the mitochondria as a small, circular molecule without histones, much like a bacterial genome.

Size and content: the nuclear genome has about 3.2 billion base pairs and roughly 20,000 protein-coding genes; mtDNA has about 16,569 base pairs and 37 genes (13 for proteins of energy production, 22 for transfer RNAs and 2 for ribosomal RNAs), with little non-coding DNA except the control region (D-loop).

Copy number: two copies of each nuclear chromosome per cell, against hundreds to thousands of mtDNA copies.

Inheritance: nuclear DNA comes from both parents and recombines every generation; mtDNA is inherited only through the mother and does not recombine, and it mutates faster.

Uses: these features make mtDNA a molecular clock for tracing maternal lineages, as in the mitochondrial Eve study (Cann, Stoneking and Wilson, 1987); nuclear DNA gives the full ancestry, as shown by Neanderthal admixture in non-Africans (2010).

Example 2 (UPSC 2023, 10 marks): Gene expression

Paper and question: Paper I, Q1(e)

Approach: A short note: define gene expression, explain its two steps (transcription and translation) and its regulation, with an example and its anthropological relevance.

Gene expression is the process by which the information in a gene is used to make a functional product, usually a protein, following the central dogma stated by Francis Crick (1958): DNA to RNA to protein.

Transcription: in the nucleus, RNA polymerase copies one strand of DNA into messenger RNA; non-coding introns are spliced out and the exons joined. Translation: on ribosomes in the cytoplasm, each three-base codon of the mRNA is read and transfer RNAs bring the matching amino acids, which are linked into a polypeptide.

Regulation: every cell has the same genes, but only some are switched on, through promoters, enhancers and transcription factors, the lac operon model of Jacob and Monod (1961) in bacteria, and epigenetic marks such as DNA methylation.

Examples: lactase is normally switched off after weaning, but in lactase-persistent populations a regulatory variant keeps it on in adults; foetal haemoglobin is replaced by adult haemoglobin after birth.

Changes in regulation, not only in genes, help explain human-chimpanzee differences and gene-environment interaction.