Top 50 NEET Biology
Questions & Answers

Productivity is the rate of energy production per unit area per unit time, so it must include both area (m⁻²) and time (yr⁻¹). Options B and C lack the time component, and D uses volume instead of area — neither fits ecosystem energy flow measurement.

Menarche marks the onset of the very first menstrual cycle, typically between ages 11–13, signalling the start of reproductive maturity in females. Menopause is the permanent cessation of menstruation, and diapause is a state of suspended development seen in some animals — not humans.

This is Mendel's classic dihybrid cross. When F₁ hybrids (RrYy) are self-crossed, Mendel's Law of Independent Assortment applies, generating 4 phenotypic classes — 9 round yellow : 3 round green : 3 wrinkled yellow : 1 wrinkled green. The 9:7 ratio appears only when there is epistasis.

Spindle fibres (made of tubulin protein) attach to the kinetochores at centromeres of chromosomes and physically pull sister chromatids toward opposite poles during anaphase. They ensure each daughter cell receives an equal, accurate set of chromosomes.

The megaspore mother cell undergoes 1 meiotic division to produce 4 haploid megaspores (3 degenerate). The functional megaspore then undergoes 3 successive mitotic divisions to produce the 7-celled, 8-nucleate embryo sac (the mature female gametophyte).

The antigen-binding site (paratope) is located at the variable (V) region at the N-terminal end of both heavy and light chains — not the C-terminal. The C-terminal carries the constant regions. An antibody has a Y-shaped structure with two identical antigen-binding sites.

Tapetum cells are indeed multinucleate with dense cytoplasm — this is true per NCERT. However, the reason given (that multiple nuclei increase nourishing efficiency) is not the accepted NCERT explanation. Tapetum is the innermost layer of the anther wall and provides nutrition to developing microspores, but the multinucleate condition is not explained as the reason for this.

Gemmae are small, multicellular, green vegetative propagules found in special cups (gemma cups) on the thallus of liverworts like Marchantia. When they detach and land in suitable conditions, they germinate directly into new plants — a classic example of vegetative/asexual reproduction in bryophytes.

George Gamow, a physicist-turned-biologist, mathematically proposed in 1954 that codons must consist of 3 nucleotides, since \(4^3 = 64\) combinations are enough to encode all 20 amino acids (whereas \(4^2 = 16\) is insufficient). Francis Crick later experimentally proved the triplet nature of the code.

Sweet potato (modified root) and potato (modified stem) perform the same function (food storage) but have different structural origins — this is analogy. When unrelated structures converge to serve the same function due to similar environmental pressures, it is called convergent evolution. Homology implies similar origin but different functions (e.g., forelimbs of vertebrates).

Histone proteins are positively charged due to their abundance of basic amino acids — lysine (Lys) and arginine (Arg). This positive charge enables strong electrostatic attraction with the negatively charged phosphate backbone of DNA, allowing 147 bp of DNA to wrap around each histone octamer to form a nucleosome.

The Verhulst-Pearl logistic growth equation is \[ \frac{dN}{dt} = rN \cdot \frac{K - N}{K} \] Here, \(r\) = intrinsic rate of natural increase, \(N\) = current population size, \(K\) = carrying capacity. As \(N \to K\), the term \((K-N)/K \to 0\), slowing growth and producing the characteristic S-shaped (sigmoid) curve.

RuBisCO (Ribulose-1,5-bisphosphate carboxylase/oxygenase) is the key enzyme of the Calvin cycle. It catalyses the fixation of CO₂ onto RuBP (a 5-carbon compound), producing two molecules of 3-PGA (3-carbon). It is the most abundant protein on Earth. It also has oxygenase activity (leading to photorespiration in C3 plants), but its primary role is carboxylation.

An enzyme consists of a protein portion called the apoenzyme and a non-protein component called the cofactor (which can be a metal ion or organic molecule). Together they form the holoenzyme (fully active enzyme). Coenzymes are loosely bound organic cofactors (e.g., NAD⁺, FAD), and prosthetic groups are tightly/permanently bound cofactors.

In prokaryotes, transcription terminates by two mechanisms — Rho-dependent (requires the Rho protein, which unwinds RNA-DNA hybrid) and Rho-independent (intrinsic termination via hairpin loops). Sigma (σ) factor is needed for promoter recognition and initiation — not termination. Rho is an ATP-dependent RNA-DNA helicase.

ADH (vasopressin) and oxytocin are both synthesised by neurosecretory cells in the hypothalamus, then transported via axons to the posterior pituitary, where they are stored and released. ACTH, LH, FSH and other tropic hormones are both synthesised AND secreted by the anterior pituitary.

Insulin is a polypeptide hormone made of two chains (A and B) linked by disulfide bonds. If taken orally, digestive enzymes (proteases like pepsin and trypsin) in the gastrointestinal tract would break it down into amino acids before it reaches the bloodstream — rendering it ineffective. Hence insulin must be administered subcutaneously (injection).

RNAi is a gene-silencing mechanism in which complementary double-stranded RNA (dsRNA) triggers sequence-specific degradation of the corresponding mRNA. The dsRNA is processed into small interfering RNAs (siRNAs) by the enzyme Dicer. This is used in agriculture (e.g., nematode-resistant tobacco) and forms the basis of many gene therapy approaches.

Catalase contains an iron-containing haem group as its prosthetic group. It catalyses the decomposition of hydrogen peroxide into water and oxygen: \(2H_2O_2 \rightarrow 2H_2O + O_2\). Carbonic anhydrase contains zinc, RuBisCO uses Mg²⁺, and succinate dehydrogenase uses FAD as its cofactor.

In 1982, Eli Lilly became the first company to market recombinant human insulin, commercially called Humulin. The human insulin gene was cloned and expressed in E. coli bacteria. The A and B chains were separately produced and then combined in vitro to form functional insulin, replacing animal-derived insulin and reducing allergic reactions.

Malonate is structurally similar to succinate (both are dicarboxylic acids). It competes with succinate for the active site of succinic dehydrogenase, blocking the normal substrate from binding. This is the textbook definition of competitive inhibition — the inhibitor can be overcome by increasing substrate concentration. This inhibition is reversible.

Bulliform cells (motor cells) are large, colourless, bubble-like epidermal cells found on the upper surface of monocot leaves (e.g., grasses). During water stress, they lose turgor and collapse, causing the leaf to curl inward, which reduces the exposed surface area and thereby decreases water loss through transpiration — an adaptation to drought.

Dedifferentiation is the process by which already differentiated, mature cells regain the capacity to divide (meristematic activity). Mature parenchyma cells of the medullary rays become interfascicular cambium to enable secondary growth. This is different from redifferentiation, which is when dedifferentiated cells differentiate again into new structures.

For each CO₂ fixed, the Calvin cycle consumes 3 ATP and 2 NADPH. For 1 glucose (6 CO₂ fixed), the total is 18 ATP + 12 NADPH. The ATP is used in the phosphoglycerate kinase and RuBP regeneration steps, while NADPH reduces 1,3-bisphosphoglycerate to glyceraldehyde-3-phosphate (G3P).

Totipotency is the inherent potential of a single plant cell to divide, differentiate, and develop into a complete plant organism. First demonstrated by F.C. Steward using carrot phloem cells. It forms the theoretical basis of plant tissue culture and micropropagation. Every cell carries the full genome — totipotency is its complete expression.

During metaphase, all chromosomes are maximally condensed, aligned at the equatorial plate (metaphase plate), and spindle fibres from both poles are fully attached to kinetochores. This is why metaphase is the ideal stage for karyotyping. Separation of chromatids happens in anaphase, after spindle tension pulls them apart.

Lecithin (phosphatidylcholine) is a phospholipid composed of glycerol, two fatty acids, a phosphate group, and a choline head group. It is a major structural component of all cell membranes, forms the fluid bilayer described in the Singer-Nicolson model, and is also found in high quantities in egg yolk and brain tissue.

A transcription unit has three essential components: (1) Promoter — where RNA polymerase binds to initiate transcription; (2) Structural gene — the coding sequence transcribed into RNA; (3) Terminator — signals where transcription ends. An operon (like Lac operon) additionally has an operator, but that is a regulatory element, not a basic transcription unit component.

Bacillus thuringiensis produces protoxin crystals (Cry proteins) encoded by cry genes — these are indeed group-specific (e.g., cryIAc targets bollworms, cryIIAb targets armyworms). However, Statement II is wrong: Bt toxin is activated by the alkaline pH of the insect midgut (not acidic), which solubilises the crystal and releases the active toxin that binds to gut epithelium.

Gibberellins (especially GA₃) dramatically promote internode elongation in stem. In sugarcane, spraying with gibberellin increases the length of internodes, which increases the total cane biomass and sugar yield by up to 20 tonnes per acre. They also break dormancy, promote seed germination, and induce flowering in long-day plants.

Glucagon is a peptide hormone (29 amino acids) secreted by α-cells of the pancreas — it raises blood glucose by stimulating glycogenolysis. Progesterone, cortisol, and testosterone are all steroid hormones derived from cholesterol. Steroids are lipid-soluble and act via nuclear receptors, while peptide hormones act via cell surface receptors.

Penguins (birds) and dolphins (mammals) evolved flippers from entirely different ancestral forelimb structures, yet they look and function similarly due to the same aquatic environment. This is convergent evolution — unrelated lineages independently evolving similar traits. These structures are analogous (similar function, different origin), not homologous.

Hardy-Weinberg equilibrium is maintained when: there is random mating, no mutation, no gene flow, no genetic drift, no natural selection, and a large population. A constant gene pool is the assumption of the equilibrium itself — it is the condition, not a disrupting force. Factors that disturb H-W equilibrium include mutation, migration, genetic drift, natural selection, and non-random mating.

In the alveoli, conditions favour oxyhaemoglobin formation: high pO₂ (~104 mmHg), low pCO₂ (~40 mmHg), low H⁺ concentration (alkaline pH ~7.4), and lower temperature. High CO₂, high H⁺ (Bohr effect), and high temperature shift the oxygen-dissociation curve to the right, favouring O₂ release in tissues — not loading in lungs.

The Ti (Tumour-inducing) plasmid of Agrobacterium tumefaciens contains T-DNA (transfer DNA), which integrates into the host plant genome and causes crown gall disease. Scientists disarmed the Ti plasmid (removed tumour-causing genes) and use it as a natural vector to deliver foreign genes of interest into plant cells — making it the most important tool in plant genetic engineering.

In mosses, spores germinate to form the protonema, which is the first and filamentous stage of the gametophyte. This is directly and correctly explained by the reason: spores are produced in the sporophyte capsule, and upon germination form protonema. The protonema then differentiates into the leafy gametophore — the second stage of the gametophyte.

After double fertilisation: Synergids are haploid (n) — they guide the pollen tube. The Zygote is diploid (2n) — formed by fusion of male gamete with the egg cell. The Primary Endosperm Nucleus (PEN) is triploid (3n) — formed by fusion of the second male gamete with two polar nuclei, developing into endosperm.

Ethylene (the only gaseous plant hormone) promotes rapid internode elongation in deep water rice when submerged — this keeps the leaves above the water surface, allowing photosynthesis and gas exchange. This is a unique case where ethylene promotes growth rather than its typical roles of fruit ripening, leaf abscission, and senescence.

The "Evil Quartet" are the four main drivers of biodiversity loss: habitat loss/fragmentation, over-exploitation, alien species invasion, and co-extinctions. Of these, habitat destruction is the single biggest cause globally — tropical deforestation, wetland destruction, and coral reef degradation collectively threaten more species than all other factors combined.

Manganese (Mn) is an essential component of the Oxygen-Evolving Complex (OEC), also called the water-splitting complex, in Photosystem II. It undergoes cyclical oxidation states (Mn²⁺ → Mn⁴⁺) to catalyse the reaction: \(2H_2O \rightarrow 4H^+ + 4e^- + O_2\). Magnesium is the central atom of chlorophyll, while copper is found in plastocyanin (electron carrier).

In axile placentation, the placenta is on the central axis where the septa (partitions) of a multilocular (many-chambered) ovary meet. Classic examples: China rose (Hibiscus), Petunia, Lemon, Tomato. Marginal placentation: Pea; Parietal: Mustard, Cucumber; Free central: Dianthus, Primrose; Basal: Marigold.

The reaction centre of PS II is P680 (absorbs light at 680 nm) and the reaction centre of PS I is P700 (absorbs light at 700 nm). When P680 absorbs light, it ejects an electron that passes through the electron transport chain. Photolysis of water occurs at PS II to replenish the lost electron, releasing O₂ as a byproduct.

The S (Synthesis) phase is when DNA replication (semi-conservative) takes place. The DNA content doubles from 2C to 4C without any change in chromosome number. G₁ is the growth phase before DNA replication, G₂ is the preparation phase after replication, and M phase is mitosis itself — no new DNA is made during M phase.

Heterosporous pteridophytes produce two distinct types of spores: microspores (small, develop into male gametophyte) and megaspores (large, develop into female gametophyte). Selaginella and Salvinia are classic heterosporous pteridophytes. Lycopodium, Psilotum, Equisetum are homosporous. Heterospory is considered an evolutionary precursor to seed habit.

For complete synthesis of 1 glucose (C₆H₁₂O₆), 6 CO₂ must be fixed: \[ 6\,CO_2 + 18\,ATP + 12\,NADPH \rightarrow C_6H_{12}O_6 + 18\,ADP + 12\,NADP^+ \] The 18 ATP are used in the phosphorylation steps (PGA → BPGA and RuBP regeneration), while the 12 NADPH provide reducing power to convert BPGA to G3P.

Eukaryotes have three RNA polymerases: RNA Pol I transcribes rRNA (28S, 18S, 5.8S) in the nucleolus; RNA Pol II transcribes mRNA precursors (hnRNA) and some snRNAs; RNA Pol III transcribes small stable RNAs — tRNA, 5S rRNA, and snRNAs. These three polymerases are inhibited by α-amanitin at different concentrations.

Alfred Sturtevant (a student of Morgan) created the world's first genetic linkage map in 1913 using Drosophila. He realised that recombination frequency between two genes is proportional to their distance apart on the chromosome — 1% recombination = 1 centimorgan (cM). Morgan discovered linkage; Sturtevant quantified it.

Chemiosmosis (Mitchell's hypothesis) requires: (1) a selectively permeable membrane; (2) a proton pump (ETC complexes) that builds the gradient; (3) a proton gradient (H⁺ concentration difference across the membrane); (4) ATP synthase (Complex V) through which protons flow back, releasing energy used to synthesise ATP from ADP + Pi.

HIV (Human Immunodeficiency Virus) selectively infects and destroys CD4⁺ T-helper lymphocytes — the master regulators of immune response. HIV uses reverse transcriptase to convert its RNA genome into DNA, which integrates into the host cell's DNA (provirus). As CD4⁺ count drops below 200 cells/μL, the patient is diagnosed with AIDS, leaving them vulnerable to opportunistic infections.

Statement I is wrong: DNA in all organisms is negatively charged (due to phosphate groups). Prokaryotic DNA is associated with positively charged polyamines and basic proteins (not negatively charged ones). Statement II is correct: In eukaryotes, DNA wraps around positively charged histone octamers (rich in Lys and Arg) to form nucleosomes, enabling efficient packaging of DNA in the nucleus.