Top 20 Must-Do Questions
for NEET & JEE 2025

A peptide bond is a covalent bond formed between the carboxyl group (−COOH) of one amino acid and the amino group (−NH₂) of the next, with the elimination of one water molecule (condensation/dehydration reaction).

• Dipeptide = 2 amino acids, 1 peptide bond
• Polypeptide = many amino acids, multiple peptide bonds
• Primary structure of proteins is determined by the sequence of amino acids held by peptide bonds
• Peptide bonds have partial double bond character → rigid and planar

DNA replication is semi-conservative (proven by Meselson & Stahl experiment). Each strand acts as a template for a new strand. It occurs in S-phase of cell cycle.

Key Enzymes:
• Helicase — unwinds and separates DNA strands at replication fork
• Primase — synthesizes RNA primer (3'−OH group required)
• DNA Pol III — adds nucleotides 5'→3' (main replication enzyme in prokaryotes)
• DNA Pol I — removes RNA primer, fills gaps
• DNA Ligase — joins Okazaki fragments on lagging strand
• SSB proteins — stabilize single strands

Light reactions occur in the thylakoid membranes of chloroplasts. Two photosystems are involved: PS I (P700) and PS II (P680).

Non-cyclic Photophosphorylation (Z-scheme):
PS II absorbs light → water splitting (photolysis) → O₂ released → electrons pass through ETC → PS I energizes electrons → NADP⁺ reduced to NADPH → ATP synthesized by chemiosmosis

Cyclic Photophosphorylation:
Only PS I involved. Electrons cycle back → only ATP produced, NO NADPH, NO O₂. Occurs when NADP⁺ is not available.

The cardiac cycle is the sequence of events in one complete heartbeat (~0.8 sec at 72 bpm):
• Atrial systole (0.1 sec): Atria contract, push blood into ventricles
• Ventricular systole (0.3 sec): Ventricles contract, blood pushed to lungs and body
• Diastole (0.4 sec): Both chambers relax, heart fills again

SA Node (Sino-atrial node): Located in right atrium; the natural pacemaker of the heart. Generates impulses at 72/min. Impulses travel to AV node → Bundle of His → Purkinje fibers → ventricular contraction.

Ecological succession is the sequential change in species composition of a community over time in a given area, leading to a stable climax community.

Primary Succession: Occurs on bare, sterile substrate where no community previously existed (e.g., bare rock, new volcanic island). Very slow process. Pioneer species: lichens → mosses → grasses → shrubs → forest.

Secondary Succession: Occurs on land where a community previously existed but was disrupted (e.g., after forest fire, flood, abandoned farmland). Much faster — soil already present. Pioneer species: grasses and weeds.

Oogenesis: Formation of ova (egg cells) in the ovary. Begins before birth (fetal life) and completes after fertilization.
Oogonium → Primary oocyte (meiosis I paused at prophase I) → After puberty at each cycle → Secondary oocyte + 1st polar body → After fertilization → Ovum + 2nd polar body

Comparison with Spermatogenesis:
• Spermatogenesis: occurs in testes; produces 4 equal spermatids from one spermatocyte
• Oogenesis: occurs in ovary; produces 1 ovum + 3 polar bodies (unequal cytokinesis)
• Spermatogenesis is continuous; Oogenesis is cyclic (menstrual cycle)
• Sperm are motile; eggs are non-motile and large

An operon is a cluster of structural genes under the control of a single promoter and operator, proposed by Jacob and Monod (1961).

Lac Operon (E. coli): Controls lactose metabolism.
Components: Promoter (P) + Operator (O) + Structural genes (lacZ, lacY, lacA)

When lactose is absent: Repressor protein binds operator → blocks RNA polymerase → genes OFF.
When lactose is present: Lactose → allolactose (inducer) → binds repressor → repressor changes shape → cannot bind operator → RNA polymerase transcribes genes → β-galactosidase, permease, transacetylase produced → lactose metabolized.