NEET 2024 — Botany

2,4-D and other synthetic auxins kill dicot weeds but leave mature monocots such as grasses unaffected, which is why they are used to keep lawns weed-free.

Lecithin is a phosphoglyceride — a glycerol backbone carrying two fatty acids and a phosphate-linked choline. It is the standard NCERT example of a phospholipid found in cell membranes.

Alternative forms of a gene are alleles (III). Crossing $F_1$ with the homozygous recessive parent is a test cross (IV); with either parent, a back cross (I). The number of chromosome sets is ploidy (II).

Vallisneria flowers are small and nectarless (A false); water lilies are pollinated by insects, not water (B true). Water-pollinated species protect pollen with a mucilaginous coat, some have ribbon-like pollen, and in some the pollen is carried passively under water. So B, C, D and E.

The IUCN maintains the Red List, the global inventory of threatened and endangered species.

A DNA fragment that carries only the gene of interest has no origin of replication, so it cannot multiply on its own (A, D and E are wrong). It may integrate into the host genome and then be copied and inherited with the host DNA — B and C.

The Calvin cycle (the 'dark' reaction) needs $\mathrm{C}{\mathrm{O}}_2$ as the substrate and the ATP and NADPH produced by the light reaction. It does not directly need light or chlorophyll.

Taking threatened species out of their habitat into a protected setting — zoos, botanical gardens, seed banks, cryopreservation — is ex-situ conservation. In-situ conservation protects them in their own habitat. (Of the options offered, 'biodiversity conservation' is the accepted answer.)

Bt toxins are indeed insect-group specific and encoded by cry genes (Statement I true). The inactive protoxin is activated by the alkaline pH of the insect gut, not acidic — Statement II is false.

A transcription unit runs, in the 5′→3′ direction of the coding strand, from a promoter through the structural gene to a terminator. Repressors, operators and inducers belong to regulation, not to the definition of the unit.

Label C points to a guard cell. Guard cells have thin outer walls and thick inner walls (facing the pore); this unequal thickening lets them bow outward when turgid and open the stoma.

Hind II, the first restriction endonuclease discovered, always cuts at a specific six-base-pair recognition sequence.

In both drawings the ovary sits at the centre of a cup-shaped thalamus with the sepals, petals and stamens arising from its rim at the same level as the ovary — the perigynous condition (half-inferior ovary), as in plum and rose. Neither shows the parts fused around the ovary (epigynous) or arising below it (hypogynous).

Datura flowers can be divided into equal halves by any radial plane — actinomorphic. Cassia, Pisum and Sesbania (all with irregular corollas) are zygomorphic.

Fungi are classified on the morphology of the mycelium, the mode of spore formation and the fruiting body. Mode of nutrition is not a criterion — all fungi are heterotrophic.

In the logistic equation $\frac{dN}{dt}=rN\left(\frac{K-N}{K}\right)$, $K$ is the carrying capacity — the maximum population the habitat can support; $r$ is the intrinsic rate of natural increase.

The law of dominance covers: characters are controlled by factors (D); factors occur in pairs (C); in a dissimilar pair one factor is dominant, the other recessive (A); so only one parental character shows in the $F_1$ of a monohybrid cross (E). Statement B describes incomplete/co-dominance, not dominance. So A, C, D and E.

Rhizopus is the bread mould (III), Ustilago the smut fungus (II), Puccinia the rust fungus (IV) and Agaricus the mushroom (I).

Malonate closely resembles succinate, the substrate, and competes for the active site of succinic dehydrogenase — the textbook example of competitive inhibition.

Mature parenchyma cells regaining the ability to divide and form interfascicular cambium is dedifferentiation; the cambium later redifferentiating into secondary tissues is redifferentiation.

Snapdragon flower colour shows incomplete dominance: pink is the heterozygote (Rr). Pink × red (Rr × RR) gives 1 RR : 1 Rr — red and pink plants only, no white.

To reveal a dominant plant's genotype, cross it with the homozygous recessive (bb) — a test cross. Any white-seeded progeny proves the parent is Bb.

Clostridium butylicum gives butyric acid (III); Saccharomyces cerevisiae, ethanol (I); Trichoderma polysporum, cyclosporin-A (IV); Streptococcus, streptokinase (II).

Each $\mathrm{C}{\mathrm{O}}_2$ fixed in the Calvin cycle costs 3 ATP and 2 NADPH — 18 ATP and 12 NADPH per hexose.

The capacity of any plant cell to regenerate a whole plant is totipotency, the basis of tissue culture.

The tropics have had a long undisturbed evolutionary time (A), receive more solar energy that supports higher productivity (C), and have constant, predictable environments that promote niche specialisation (D, E). Tropical environments are less seasonal, so B is false.

The nucleolus is the site of rRNA synthesis (III); the centriole has a cartwheel organisation (II); leucoplasts store nutrients (IV); the Golgi apparatus forms glycolipids and glycoproteins (I).

The embryo of the dicot seed consists of the embryonal axis with the plumule at one end and the radicle at the other. Label C marks the radicle, which grows out first at germination to form the root.

Spindle fibres attach to kinetochores during metaphase, when chromosomes align at the equatorial plate; they were assembled during prometaphase.

Chromosomes first become visible as thin threads in leptotene (I true); diplotene begins with dissolution of the synaptonemal complex, leaving chiasmata (II true).

Collenchyma is a living tissue (Statement I false). Vessels are characteristic of angiosperms and absent from gymnosperms, whose xylem has only tracheids (Statement II true).

The 'Evil Quartet' of biodiversity loss is habitat loss and fragmentation, over-exploitation, alien species invasion and co-extinction. Mutation and migration are not causes. So A, B and D.

Lactose enters E. coli through lac permease, the product of the y gene; $\beta -$galactosidase (z) then hydrolyses it inside the cell.

Bulliform (motor) cells in the upper epidermis of grass leaves lose turgor in dry conditions and make the leaf roll inward, reducing water loss.

Carboxypeptidase requires zinc as its cofactor (a metal ion bound at the active site); niacin and flavin are coenzyme precursors, haem the prosthetic group of catalase and peroxidase.

In Phaeophyceae (brown algae) asexual reproduction is by biflagellate zoospores (A); food is stored as mannitol or laminarin (C); pigments are chlorophyll a, c, carotenoids and xanthophyll (D); the wall is cellulose coated with algin (E). Sexual reproduction may be isogamous, anisogamous or oogamous, so B is false. Hence A, C, D and E.

Robert May estimated global species diversity at about 7 million (III); Humboldt described the species–area relationship (I); Ehrlich proposed the rivet-popper hypothesis (IV); Tilman ran long-term outdoor plot experiments (II).

In $C_3$ plants RuBisCO's oxygenase activity binds ${\mathrm{O}}_2$ and lowers $\mathrm{C}{\mathrm{O}}_2$ fixation — Statement I true. In $C_4$ plants photorespiration is avoided because bundle-sheath cells (where RuBisCO sits) are kept $\mathrm{C}{\mathrm{O}}_2$-rich; the statement's reversal of which cells photorespire is wrong — Statement II false.

Chloroplast DNA, like mitochondrial and bacterial DNA, is circular and double-stranded, without histones.

By the 10% law each trophic level passes on about a tenth of its production. From a producer NPP of $100x$, the second level takes in $10x$. The official key marks $10x$ — it treats the third level's gross intake as the energy handed on by the level below it. (Applying the 10% step once more, the third level's own NPP would be $x$; that is not what was marked.)

Somatic hybridisation fuses protoplasts (cells stripped of their walls) from two different varieties or species, e.g. the pomato.

The citric acid cycle runs in the mitochondrial matrix (II); glycolysis in the cytoplasm (I); the electron transport system sits in the inner membrane (IV); protons are pumped into the intermembrane space to build the gradient (III).

Griffith showed transformation (III); Jacob and Monod described the lac operon (IV); Khorana helped decipher the genetic code (I); Meselson and Stahl proved semi-conservative replication (II).

China rose has monoadelphous stamens (one bundle, IV); pea diadelphous (two bundles, II); Citrus polyadelphous (many bundles, I); lily's stamens are epiphyllous, attached to the perianth (III).

The figure shows a grass inflorescence with long, dangling stamens hanging well out of the florets — an adaptation of wind-pollinated flowers, which expose stamens and feathery stigmas to the air. Water-pollinated stamens have a mucilaginous coat, and cleistogamous flowers never open.

GLUT-4 transports glucose into cells (IV); insulin is a hormone (I); trypsin an enzyme (II); collagen the intercellular ground substance of connective tissue (III).

Succinyl-CoA → succinic acid is a substrate-level phosphorylation that releases CoA and makes GTP; no hydrogen is removed. The other three steps are dehydrogenations (oxidations).

Gibberellin sprayed on sugarcane increases internode length, so the stem — where sugar is stored — grows longer and yield rises.

Rose has a perigynous flower (II); pea shows marginal placentation (IV); cotton has twisted aestivation (I); mango is a drupe (III).

DNA-dependent DNA polymerase adds nucleotides only in the 5′→3′ direction — on the leading strand continuously and on the lagging strand as Okazaki fragments. The 3′→5′ direction is never synthesised.