Q.A protoxin is:

Step 1 — How the Bt toxin actually works (this is the whole question).

Bacillus thuringiensis forms protein crystals containing the Cry protein, which exists as an inactive protoxin. When an insect eats it:

1. The insect's midgut is alkaline ($\text{pH} > 9.5$), which solubilises the crystal.
2. The protoxin is then converted into the active toxin.
3. The active toxin binds to specific receptors on the midgut epithelium, punching pores in the cells, which swell, lyse and kill the insect.

Every one of these three steps is insect-specific. That is exactly why the toxin is safe for humans and other animals — our stomachs are acidic, so the crystal is never solubilised, and we lack the midgut receptor.

Step 2 — Classify the four organisms.

Step 1 — The three layers of a blood-vessel wall

Both arteries and veins are built of the same three coats, from inside out:

Step 2 — Apply the pressure argument

Arteries receive blood ejected from the ventricles under high, pulsatile pressure. To absorb and smooth that surge, they need a thick, muscular, elastic tunica media. …

Step 1 — Recall the architecture of a myofibril

Each muscle fibre contains many myofibrils. A myofibril shows alternating light and dark bands:

• I-band (light) — contains only thin (actin) filaments; it is bisected by the Z-line.
• A-band (dark) — contains the thick (myosin) filaments; its centre carries the lighter H-zone, bisected by the M-line.

Step 2 — Define the functional (contractile) unit

The sarcomere is the portion of the myofibril between two successive Z-lines. It contains one full A-band flanked by two half I-bands. This is the smallest repeating assembly that contains everything needed to contract: anchored thin filaments (Z-line), overlapping thick filaments (A-band) and the cross-bridge machinery.

Step 3 — Why the sliding-filament model makes the Z-line the boundary

During contraction, myosin heads form cross-bridges and pull the actin filaments towards the centre of the A-band. The thin filaments are attached to the Z-lines, so the Z-lines are dragged inward:

Because the shortening is measured between the Z-lines, that span is by definition the functional unit. …

Step 1 — What MTP is

MTP = Medical Termination of Pregnancy, the intentional/voluntary termination of a pregnancy before full term. It is not a contraceptive method — it is a remedial measure.

Step 2 — The legislation

The Government of India legalised MTP in 1971, but attached strict regulations to it. The reason was twofold:

1. A very large number of unsafe, illegal abortions were being performed, endangering women's lives; and
2. The procedure was liable to misuse — most notoriously for illegal female foeticide after sex determination of the foetus.

Step 3 — The conditions attached

• MTP is considered relatively safe up to 12 weeks (first trimester); terminations after 12 weeks up to 20 weeks are much riskier and require stricter clearance (in the NCERT text, the approval of two doctors).
• MTPs are legally allowed on grounds such as risk to the mother's life/health, contraceptive failure, foetal abnormality, or pregnancy from rape. …

Step 1 — Classify the four enzymes by what they physically do to DNA.

• Nucleases cut phosphodiester bonds. They come in two kinds:
  • Exonucleases remove nucleotides from the ends of the DNA.
  • Endonucleases make cuts at specific positions within the DNA.
• Ligases join two DNA fragments by sealing the phosphodiester backbone.
• Polymerases synthesise new DNA on a template.

Step 2 — Ask what "cleave a plasmid" requires.

A plasmid is a small, circular, extra-chromosomal DNA molecule. A circle has no free ends. Therefore:

• An exonuclease literally cannot start on an intact circular plasmid — there is no terminus to attack. ✗ (C)
• To linearise/open the plasmid so that a foreign gene can be inserted, the cut must be made inside the molecule → an endonuclease. ✓ (B)

Step 3 — The specific enzyme used in practice.

In rDNA technology the enzyme used is a restriction endonuclease. Each such enzyme:

1. inspects the DNA and binds at a specific palindromic nucleotide sequence (e.g. EcoRI: $5'\text{-GAATTC-}3' / 3'\text{-CTTAAG-}5'$),
2. cuts each strand a little away from the centre, between the same two bases on both strands, …

Step 1 — Identify the molecule that treats emphysema.

Among the products obtainable by genetic engineering, the textbook list of human proteins reads: insulin (diabetes), $\alpha$-1 antitrypsin (emphysema), factor VIII (haemophilia), $\alpha$-interferon (viral infections/cancer) and $\alpha$-lactalbumin (a milk protein). The molecule paired with emphysema is unambiguously $\alpha$-1 antitrypsin. That leaves options (A) and (C) — both name antitrypsin — and the question then turns on how it is described.

Step 2 — The biology of why it works.

Emphysema is the progressive destruction of the alveolar walls of the lung. The enzyme neutrophil elastase digests elastin in lung tissue; $\alpha$-1 antitrypsin is a protease inhibitor — a plasma protein — that neutralises this elastase and thereby protects the alveoli. A hereditary deficiency of $\alpha$-1 antitrypsin leaves the elastase unopposed, the alveolar walls are destroyed, and emphysema results. Administering the (recombinant/purified) protein restores the missing inhibitor. Note that it works by inhibiting an enzyme, which is exactly what a protease-inhibitor protein does — it does not act on a receptor as a signalling molecule, so it cannot be a hormone.

Step 3 — Distinguish (A) from (C) — the actual discriminator. …

Step 1 — Define the two terms

• Exon — the expressed sequence; it appears in the mature mRNA and is translated.
• Intron — the intervening sequence; it is transcribed but removed during splicing and never appears in mature mRNA.

Step 2 — What "monocistronic but split" means

• Monocistronic → one eukaryotic mRNA codes for one polypeptide (unlike prokaryotic polycistronic mRNA, e.g. the lac operon, which codes for several).
• Split → the coding information of that one gene is not in one continuous block; it is interrupted.

Step 3 — The processing that follows

The hnRNA additionally receives capping (methyl guanosine triphosphate at the 5′ end) and tailing (poly-A at the 3′ end) before it is exported for translation.

Step 4 — Eliminate the other options …

Step 1 — The historical sequence of virus discovery

The discovery of viruses came from work on tobacco mosaic disease, and each worker in the options contributed a different step:

Step 2 — Match the term to the worker …

The question asks for a plant that is both a source of a drug and native to America. The key is to connect each plant to its geographical origin and the drug it yields. Many medicinal and psychoactive plants have been spread globally by humans, but their native ranges are specific. America here refers to the Americas (North and South America).

Let’s examine each option:

1. Papaver somniferum (opium poppy) – This is the source of morphine, codeine, and heroin. It is native to the eastern Mediterranean region and western Asia, not America. It was introduced elsewhere later.

2. Erythroxylum coca – This shrub is the source of cocaine. It is native to the eastern slopes of the Andes in South America (Peru, Bolivia, Colombia). Indigenous peoples have used its leaves for centuries. This matches the requirement perfectly.

3. Cannabis sativa – Source of marijuana (THC) and hemp fiber. Its native range is debated but is generally considered to be Central Asia or the Indian subcontinent. It is not native to the Americas.

4. Atropa belladonna (deadly nightshade) – Source of atropine and other tropane alkaloids. It is native to Europe, North Africa, and western Asia. Not native to America. …

Step 1 — Read the parts of the axoneme.

A cilium/flagellum is a 9 + 2 array enclosed by the plasma membrane:

• 9 peripheral doublets — pairs of microtubules arranged in a ring.
• 2 central microtubules — the central pair, wrapped in a proteinaceous central sheath.

Step 2 — Distinguish the two kinds of cross-link (this is the whole question).

The key here is to understand what "advantage" means in the context of genetically modified crops. An advantage is a benefit — something that improves farming, the environment, or human well-being. The question tests whether you can spot the one statement that describes a disadvantage or a false claim.

Let’s examine each option carefully.

1. Option (A): Reduces the reliance on chemical pesticides.

   This is a well-known advantage. Many GM crops are engineered to be pest-resistant (e.g., Bt cotton produces a protein toxic to certain insects). This means farmers need to spray fewer chemical pesticides, which saves money and reduces environmental harm. So this is an advantage.

2. Option (B): Increases the post harvest losses.

   "Post harvest losses" refer to the spoilage or damage of crops after they are harvested — during storage, transport, or handling. GM crops are often modified to have longer shelf life, resist bruising, or tolerate better storage conditions (e.g., Flavr Savr tomato). So they decrease post harvest losses, not increase them. This statement is the opposite of an advantage — it is a disadvantage. Therefore, this is the correct answer.

3. Option (C): Increases efficiency of mineral usage in plants.

   Some GM crops are designed to use minerals (like nitrogen or phosphorus) more efficiently. This means they can grow well even in poorer soils, reducing the need for fertilizers. That is clearly an advantage — it lowers costs and environmental pollution. …

Step 1 — What defines a retrovirus.

A retrovirus is an RNA virus that reverses the usual flow of information: instead of DNA → RNA, it goes RNA → DNA, using the enzyme reverse transcriptase. HIV is the standard example.

Step 2 — The replication cycle (as in NCERT).

1. The virus attaches to a host cell (in HIV, a helper T-lymphocyte / macrophage) and its envelope fuses with the cell membrane.
2. The viral RNA genome enters the host cell. ← this is the step the question asks about
3. Inside the cell, reverse transcriptase copies the viral RNA into complementary DNA (cDNA).
4. That viral DNA is integrated into the host cell's DNA (as a provirus).
5. The infected cell then transcribes and translates viral genes, producing new virus particles.

Step 3 — Eliminate the distractors.

• (A) Viral protein: capsid/envelope proteins are not what is delivered as the genome. …