Biology (Theory)

In eukaryotes, negatively charged DNA wraps around a positively charged histone octamer to form a nucleosome (about 200 bp of DNA per nucleosome).
Under the electron microscope, chromatin looks like a string with beads: each bead is a nucleosome and the thread between beads is linker DNA.
Common mistake: choosing 'histone proteins' – histones are only the core of the bead; the bead itself is DNA + histone octamer = nucleosome.

Correct path of sperms: seminiferous tubules → rete testis → vasa efferentia → epididymis → vas deferens → ejaculatory duct → urethra.
(i) is false: the rete testis opens into the vasa efferentia, not directly into the vas deferens.
(iv) is false: the vas deferens receives the duct of the seminal vesicle (not the prostate) and opens into the urethra as the ejaculatory duct.
(ii), (iii) and (v) are correct.

DNA polymerase adds deoxyribonucleoside triphosphates (dATP, dGTP, dCTP, dTTP).
They are called nucleoside triphosphates because the three phosphates are attached to a nucleoside (base + deoxyribose); these dNTPs act both as substrate and as the energy source (two phosphates are released as pyrophosphate).
Ribo- forms are used by RNA polymerase, not DNA polymerase.

The trait skips the parents (both normal) and appears in a son → recessive.
Only males are affected and the trait passes from a normal mother (carrier) to her son, and again from the normal daughter (carrier) to the grandson – the typical criss-cross pattern of an X-linked gene.
Mother = XᴴXʰ (carrier), father = XᴴY → son XʰY affected; daughter XᴴXʰ carrier → her son XʰY affected.
So haemophilia here is X-linked (sex-linked) recessive.

When the immune system fails to distinguish self from non-self and attacks self-cells, the disease is called an autoimmune disease.
Rheumatoid arthritis is an autoimmune disease (the joints are attacked).
Thalassemia and phenylketonuria are Mendelian (genetic) disorders; filariasis is caused by the worm Wuchereria.

A dicot embryo has an embryonal axis and two cotyledons.
Above the cotyledons = epicotyl (ends in plumule); below = hypocotyl (ends in radicle with root cap).
(i) is false – scutellum is the single cotyledon of monocots.
(iii) is false – the part below the cotyledons is the hypocotyl; coleorhiza is the radicle sheath of monocots.
So (ii) and (iv) are true.

About 15 mya, Dryopithecus and Ramapithecus existed; they were hairy and walked like gorillas and chimpanzees (Ramapithecus was more man-like, Dryopithecus more ape-like).
Australopithecines appeared about 2 mya; Homo habilis, H. erectus, Neanderthals and H. sapiens are much later.

The mRNA has the same sequence as the coding strand, with U in place of T.
Coding strand TAC → mRNA codon UAC.
UAC codes for tyrosine, so the charged tRNA (anticodon AUG) carries tyrosine at its 3′ end.
Common mistake: writing the complement of the coding strand (AUG → methionine). The complement rule applies to the template strand, not the coding strand.

ELISA (Enzyme Linked Immuno-Sorbent Assay) works on the antigen–antibody interaction: it detects the antigens of the pathogen or the antibodies made against it.
PCR also gives early diagnosis but works by amplifying nucleic acid, not by antigen–antibody binding. RNAi is gene silencing; ESTs are used in genome sequencing.

E. coli divides every 20 minutes, so 40 minutes = 2 generations in ¹⁴N medium.
After generation 1 (20 min): all DNA is hybrid ¹⁵N/¹⁴N → one intermediate band.
After generation 2 (40 min): each hybrid molecule gives one hybrid and one fully light molecule → equal amounts of hybrid and light DNA → two bands (intermediate and light).
A heavy band never reappears, so (B) and (C) are wrong; a single hybrid band (D) is the 20-minute result.

Cyclosporin A, an immunosuppressive drug used in organ-transplant patients, is produced by the fungus Trichoderma polysporum.
Others: Propionibacterium sharmanii – large holes in Swiss cheese; Monascus purpureus – statins (blood-cholesterol lowering); Aspergillus niger – citric acid.

Let I = inflated (dominant), i = constricted.
Cross: Ii × Ii → 1 II : 2 Ii : 1 ii.
II and Ii are inflated (3), ii is constricted (1).
Phenotypic ratio = 3 inflated : 1 constricted (the genotypic ratio is 1 : 2 : 1).

Virus-free plants are indeed obtained from infected plants by meristem culture, so A is true.
But it is the meristem (apical and axillary) that stays free of the virus, not the whole root and stem; the virus spreads through the rest of the plant. So R is false.
Hence option (C).

In the human host, Plasmodium multiplies inside RBCs.
When the RBCs burst, a toxic substance haemozoin is released, which causes the chill and high fever.
This rupture happens in cycles, so the fever recurs every 3–4 days.
So A is true, R is true, and R explains A.

Saheli is a once-a-week, non-steroidal oral pill developed by CDRI, Lucknow – so R is true.
Increasing phagocytosis of sperms inside the uterus is the action of IUDs (non-medicated/copper IUDs), not of oral pills. Pills act by inhibiting ovulation/implantation and altering cervical mucus.
So A is false.

ABO blood groups are controlled by ONE gene (I) with three alleles – Iᴬ, Iᴮ and i – so A is true.
Multiple allelism means more than two ALLELES of a single gene in a population, not more than two genes. R wrongly says 'genes', so R is false.

The thymus is a primary lymphoid organ where immature lymphocytes differentiate into antigen-sensitive T-lymphocytes.
If it degenerates early, T-cells will not develop and mature.
Without T-cells, cell-mediated immunity (graft rejection, killing of infected cells) fails and B-cells get less help, so the person's overall immunity decreases and infections become frequent.

(i) Interferons are proteins released by cells that have been infected by a virus.
They act on neighbouring non-infected cells and protect them from further viral infection.
(ii) Interferons are cytokine barriers – one of the four barriers of innate immunity (physical, physiological, cellular, cytokine).

The genetic code is triplet: every 3 bases = 1 codon = 1 amino acid; a stop codon (UAA, UAG, UGA) ends the chain.
(a) 22 bases read from base 1: CUC GCU UGC CGA UCA AGG GUU + A (left over). 7 complete codons, none is a stop → 7 amino acids.
(b) 23 bases, read from base 2 (22 bases used): UGG CAC UCA GUC CUU AAU GGC + G (left over). 7 complete codons, no stop → 7 amino acids.
Common mistake: reading frame (b) from base 1 – GUG GCA CUC AGU CCU UAA would hit the stop codon UAA.

MTP (medical termination of pregnancy) is the intentional or voluntary ending of a pregnancy before full term (induced abortion).
It was legalised (MTP Act 1971, amended 2017) with strict conditions to check illegal, unsafe abortions and to get rid of unwanted pregnancies – due to casual unprotected sex, failure of a contraceptive, rape – or when continuing the pregnancy would harm or even be fatal to the mother, the foetus or both.

STIs (any two): gonorrhoea, syphilis, genital herpes, chlamydiasis, genital warts, trichomoniasis, hepatitis-B, HIV/AIDS.
Early symptoms (any two): itching in the genital region, abnormal fluid discharge, slight pain, swelling or redness in the genital region.
Note: early symptoms are mild, so females often ignore them, which can lead to complications later.

(a) DNA ligase joins the cut ends (seals the sugar-phosphate backbone).
(b) Introducing recombinant DNA into a bacterial host cell is called transformation.
(c) EcoRI cuts at its palindrome GAATTC, between G and A, leaving the same single-stranded overhangs (sticky ends) on both DNAs. Because they are complementary, the ends of vector and alien DNA pair by hydrogen bonds and DNA ligase can join them. A different enzyme would give non-matching ends.

Commensalism (+, 0): one species benefits and the other is neither harmed nor benefited.
The clownfish lives among the stinging tentacles of the sea anemone, which protect it from predators – the clownfish benefits.
The sea anemone gets no clear benefit and is not harmed from the clownfish.

Drawing: three stacked bars widening upwards –
Bottom (narrowest): phytoplankton 40 – primary producers (first trophic level).
Middle: zooplankton 90 – primary consumers (second trophic level).
Top (widest): small fishes 120 – secondary consumers (third trophic level).
Description: biomass increases at higher levels, so the pyramid of biomass in the sea is inverted. This happens because phytoplankton are small, multiply very fast and are eaten quickly, so their standing crop at any moment is low even though productivity is high.

1. After implantation, finger-like projections called chorionic villi grow out from the trophoblast.
2. The chorionic villi become surrounded by uterine tissue and maternal blood.
3. The chorionic villi and uterine tissue become interdigitated (interlocked) with each other.
4. Together they form the placenta, a structural and functional unit between the developing embryo (foetus) and the mother's body.
5. The placenta is connected to the embryo by the umbilical cord and supplies O₂ and nutrients and removes CO₂ and wastes.
6. It also acts as an endocrine tissue (hCG, hPL, oestrogens, progesterone).

(a) Any two:
– He chose true-breeding pea varieties.
– He studied traits with clear, contrasting forms (tall/dwarf etc.).
– He carried out artificial self- and cross-pollination carefully.
– He used large sample sizes and statistical analysis.
– He confirmed results over several generations.
(b) Law of independent assortment: when two pairs of traits are combined in a hybrid, the segregation of one pair of characters is independent of the other pair (this is why a 9 : 3 : 3 : 1 ratio appears in F₂).

(a) Bond (a) joins the 5′-OH of the first sugar to a phosphate – a phosphoester bond. Bond (b) links the 3′-C of one sugar to the 5′-C of the next through a phosphate – a 3′–5′ phosphodiester bond.
(b) Purines are adenine and guanine; the strand has A, T, G → 2 purines (A, G). T is a pyrimidine.
(c) The drawing must show:
– Three deoxyribose (pentagon) sugars, each with a base on C1′: adenine, thymine, guanine.
– A free phosphate at the 5′ end (on C5′ of the first sugar).
– Phosphodiester links (–O–P(=O)(O⁻)–O–) from C3′ of each sugar to C5′ of the next.
– A free OH on C3′ of the last sugar, with the 5′ and 3′ ends labelled.

1. Primary effluent is passed into large aeration tanks where it is constantly agitated mechanically and air is pumped in.
2. This allows vigorous growth of useful aerobic microbes into flocs – masses of bacteria associated with fungal filaments forming mesh-like structures.
3. While growing, these microbes consume the major part of the organic matter in the effluent.
4. This significantly reduces the BOD of the effluent. When BOD is reduced enough, the effluent is passed to a settling tank where flocs settle as activated sludge (part is returned as inoculum, rest goes to anaerobic sludge digesters).

(a) Anemophily – pollination by wind.
(b) Wind carries pollen randomly, so large feathery stigmas exposed to air give a big surface to trap air-borne pollen grains.
(c) Pollen grains are light and non-sticky so that wind can carry them; they are also produced in very large numbers.
(d) Wind-pollinated flowers do not need to attract insects, birds or bats, so bright, large petals are not needed; small petals also let wind flow freely to the anthers and stigma.

(a) Once Lantana was removed, native grasses and legumes could grow again. More fodder supports more herbivores (deer etc.), which means more prey for tigers, so the tiger population and the whole food chain recover; animals also stray less into villages, reducing human–wildlife conflict.
(b) Lantana is an alien invasive species. It spreads fast and does not let native plants grow around it, so the 25 native grass and legume species decline. This reduces the herbivore population, which in turn causes the tiger population to decline – alien species invasion is one of the 'evil quartet' causes of biodiversity loss.

Advantage (1 mark): high-yielding, disease-resistant varieties with better irrigation and fertilisers greatly increased food production (especially wheat and rice), making the country self-sufficient in food grains and able to support a larger population.
Disadvantages (any two, 1 mark each):
• Heavy use of fertilisers and pesticides polluted soil and water over time (and can cause biomagnification).
• Irrigation and other crop-management practices are expensive.
• Agrochemicals are costly for small farmers, and crops become dependent on them.
• Excess irrigation raised soil salinity, and chemicals lowered the soil's natural fertility.

(a) Haemoglobin is the same (homologous) protein inherited from a common ancestor and changed along different lines, so both are divergent evolution: (i) divergent, (ii) divergent.
(b) Similar haemoglobin in humans, frog and lamprey shows that they share a common ancestry.
(c) Fewer amino acid differences mean a more recent common ancestor. Macaque differs from humans by only 8 amino acids while lamprey differs by 125, so the macaque is much more closely related to humans.

(a) Opioids such as heroin are commonly taken intravenously; sharing contaminated needles/syringes transmits the hepatitis B virus, which damages the liver.
(b) Direct drug-related disease/death is one caused by the drug itself – mainly fatal overdose of opioids – rather than by an infection picked up indirectly.
(c) Opioids come from the poppy plant Papaver somniferum; they are extracted from its latex (milky juice of the unripe fruit/capsule). Morphine is extracted from this latex and heroin is made from morphine by acetylation.

(i) 1. The pollen tube enters one of the synergids through the filiform apparatus and releases two male gametes into its cytoplasm.
2. One male gamete moves to the egg cell and fuses with its nucleus – syngamy – forming a diploid zygote.
3. The other male gamete moves to the two polar nuclei in the central cell and fuses with them – triple fusion – forming the triploid primary endosperm nucleus (PEN); the central cell becomes the primary endosperm cell (PEC).
4. Since two fusions (syngamy + triple fusion) occur in one embryo sac, it is called double fertilisation.
(ii) The PEC develops into the endosperm (nutritive tissue); the zygote develops into the embryo. (Ovules become seeds and the ovary becomes the fruit.)

(i) Each testis has about 250 compartments called testicular lobules.
Each lobule contains one to three highly coiled seminiferous tubules, where sperms are produced.
The tubules are lined by two types of cells:
– Male germ cells (spermatogonia): undergo meiotic divisions that finally form sperms.
– Sertoli cells: provide nutrition to the germ cells.
(Outside the tubules, Leydig/interstitial cells secrete androgens.)
(ii) At puberty, increased GnRH from the hypothalamus acts on the anterior pituitary and stimulates secretion of two gonadotropins:
– LH, which acts on Leydig cells to secrete androgens that stimulate spermatogenesis;
– FSH, which acts on Sertoli cells to secrete factors that help spermiogenesis.

Strategy: RNA interference (RNAi).
1. RNAi is a natural method of cellular defence in all eukaryotes: a complementary double-stranded RNA binds to and prevents translation of a specific mRNA (silencing).
2. Using Agrobacterium vectors, nematode-specific genes were introduced into the tobacco (host) plant.
3. The DNA was introduced so that it produced both sense and anti-sense RNA in the host cells.
4. These two complementary RNAs formed double-stranded RNA (dsRNA), which initiated RNAi.
5. This silenced the specific mRNA of the nematode, so the parasite could not survive in the transgenic host expressing the interfering RNA – the plant was protected.

PCR makes many copies of a gene in vitro. Ingredients: template DNA, two sets of primers, dNTPs, thermostable DNA polymerase.
Each cycle has three steps:
1. Denaturation: the DNA is heated to a high temperature; hydrogen bonds break and the two strands separate.
2. Annealing: on cooling, two sets of primers (short, chemically synthesised oligonucleotides complementary to the ends of the target region) attach at the 3′ ends of the two template strands.
3. Extension: DNA polymerase extends the primers using the nucleotides provided and the DNA as template. The enzyme is Taq polymerase from Thermus aquaticus, which stays active during the high-temperature denaturation.
Repeating the cycle about 30 times amplifies the DNA segment about a billion times. The amplified fragment can then be ligated into a vector for cloning.

(i) With limited resources, competition sets a limit – the carrying capacity (K). The population first shows a lag phase, then phases of acceleration and deceleration, and finally an asymptote when population density reaches K.
(ii) Verhulst–Pearl logistic equation:
\[ \frac{dN}{dt} = rN\left(\frac{K-N}{K}\right) \]
N = population density at time t, r = intrinsic rate of natural increase, K = carrying capacity.
(iii) Logistic growth curve – an S-shaped (sigmoid) curve.
Graph: x-axis time (t), y-axis population density (N); the curve rises slowly, then steeply, then levels off along a horizontal line marked K.
It is considered more realistic than exponential growth because resources in nature are finite.

(i) Alexander von Humboldt observed that within a region, species richness increases with increasing explored area, but only up to a limit. For a wide variety of taxa (angiosperms, birds, bats, freshwater fishes) the curve of species richness against area is a rectangular hyperbola.
(ii) Graph: x-axis area (A), y-axis species richness (S); the curve rises steeply then flattens. On a log–log scale the relationship is a straight line.
(iii) \( \log S = \log C + Z \log A \)
S = species richness, A = area, Z = slope of the line (regression coefficient), C = Y-intercept.
Z is usually 0.1–0.2 for smaller areas but steeper (0.6–1.2) for very large areas such as whole continents.