Fundamentals
The core facts every aspirant should own — each a titled nugget with a real-world story, the concept in plain words, and a memory trick. Works even when the internet doesn't.
300 fundamentals
“Crossing a pure tall pea with a pure dwarf pea yields F2 seeds in a 3 tall :1 dwarf pattern, like counting red vs white beans.”
In a monohybrid cross Aa × Aa, F2 genotypes appear in 1 AA : 2 Aa : 1 aa (1:2:1) and phenotypes in 3 dominant : 1 recessive (3:1). Trap: confusing the 3:1 phenotypic ratio with the 9:3:3:1 dihybrid ratio.
Memory trick: 3 tall, 1 dwarf → 3:1; 1-2-1 genotype.
“A sperm carrying a Y chromosome is like a tiny messenger delivering the ‘boy’ ticket to the egg.”
Human sex depends on sex chromosomes: XX = female, XY = male. The egg always supplies an X; the sperm supplies either X (girl) or Y (boy). Trap: thinking maternal chromosomes decide sex—only the sperm’s chromosome matters.
Memory trick: Egg = X always; sperm X→girl, Y→boy.
“DNA’s double helix is a twisted ladder where each rung pairs A-T or G-C, just like matching teeth on a zipper.”
The two antiparallel strands coil into a right-handed helix; complementary bases pair via hydrogen bonds (A-T two, G-C three), giving uniform width and stability. Trap: forgetting the strands run opposite directions (5′→3′ vs 3′→5′).
Memory trick: A-T, G-C, always.
“Copying a rope by using the original as a scaffold gives two ropes each with one old and one new strand.”
In semi-conservative replication each daughter DNA retains one parental strand while synthesizing a new complementary strand. Meselson-Stahl’s N15-density gradient after one generation showed hybrid DNA, confirming this model. Trap: avoid the conservative model where the original stays intact.
Memory trick: Half old, half new.
“DNA writes a recipe, RNA copies it, and the ribosome cooks the protein dish.”
The central dogma states genetic information flows DNA → RNA → protein. Transcription makes mRNA; translation builds polypeptide chains. Exceptions like reverse transcription are not part of the basic flow. Trap: don’t assume RNA is the primary genetic material in all cells.
Memory trick: Transcribe then translate
“The three-letter word on mRNA tells the ribosome which Lego block (amino acid) to add.”
A codon is a sequence of three nucleotides in mRNA that specifies one amino acid or a stop signal during translation. The ribosome reads codons in a fixed, non-overlapping frame starting at the start codon AUG. Any shift (frameshift) changes all downstream amino acids, a common trap in questions.
Memory trick: 3 bases → 1 amino acid (triplet code)
“A beetle with a green shell blends into leafy grass, so it lives longer than the brown one.”
Natural selection favors individuals whose traits increase survival and reproductive success, shifting allele frequencies toward those advantageous traits. It acts on phenotypes, not on the idea of ‘strength’. Trap: assuming the strongest always wins; the key is who reproduces more.
Memory trick: Survive → reproduce → spread genes.
“Human forearm and bat wing bones are homologous, while the wings of a bat and a butterfly are analogous.”
Homologous organs originate from a common ancestor but may serve different functions; analogous organs perform similar functions but evolved independently. The similarity in function does not imply shared ancestry. Trap: mistaking analogous similarity for homology.
Memory trick: Homologous = same blueprint; analogous = same job.
“When you cut your finger, the immediate swelling and redness is your innate defense kicking in before any specific antibodies appear.”
Innate immunity is non-specific, present at birth and acts within minutes via barriers, phagocytes, inflammation. Acquired immunity develops after exposure, is antigen-specific, mediated by lymphocytes and memory cells. Exam trap: don't confuse rapid innate response with “first-line” and forget that acquired includes both humoral and cell-mediated arms.
Memory trick: Innate = INSTANT, Acquired = AFTER exposure.
“A child receiving the measles vaccine creates his own antibodies, while a newborn drinking colostrum gets antibodies made by the mother.”
Active immunity arises when the host’s immune system produces antibodies after vaccination or infection; it is long-lasting and involves memory cells. Passive immunity provides ready-made antibodies from another source (maternal IgG, antivenom) and lasts only weeks. Trap: passive immunity gives immediate protection but no memory.
Memory trick: Active = your own; Passive = from others.
“A doctor prescribes amoxicillin for a throat infection caused by streptococcus, but it won’t help a common cold caused by a rhinovirus.”
Antibiotics target bacterial structures or processes (cell wall synthesis, protein synthesis) and are ineffective against viruses that lack these targets. Misuse leads to resistance. Trap: remember that viral infections need antivirals or supportive care, not antibiotics.
Memory trick: ABCs = Antibiotics B act on Bacteria, not Viruses.
“Like a courier meeting a package at the train station, the sperm meets the egg in the fallopian tube, not the uterus.”
Human fertilisation occurs in the ampulla of the fallopian tube. The tube’s cilia and secretions support sperm-egg fusion and transport the zygote to the uterus. A frequent mistake is marking the uterus as the site; remember only the tube provides the right environment.
Memory trick: Fertilisation = Fallopian tube (F = F).
“The placenta is like a subway tunnel linking the mother’s city to the baby’s station, swapping supplies and waste.”
The placenta attaches the chorionic villi to the uterine wall, enabling nutrient, gas and waste exchange and secreting hCG, progesterone and estrogen. It is the only temporary fetal organ. Exam trap: confusing it with the umbilical cord, which only carries blood.
Memory trick: Placenta = 'Place' for exchange and hormones.
“Like inserting a new app into a smartphone, scientists paste a gene into a plasmid vector and let bacteria run it.”
Recombinant DNA involves cutting a gene of interest, ligating it into a cloning vector, and introducing the vector into a host cell for expression. The host replicates the plasmid, producing the protein in large amounts. Ensure the gene is oriented correctly and the vector has a suitable promoter.
Memory trick: Gene + vector + host = G-V-H
“Like a pair of scissors that only open on a specific pattern of stitches, restriction enzymes slice DNA at defined sequences.”
Restriction enzymes are proteins that bind short DNA motifs (4-8 bp) and cut both strands at or near those motifs. Most Type II enzymes recognize palindromic sequences, but Type IIs (e.g., FokI) recognize non-palindromes and cut downstream. In wrong buffers or high glycerol they show ‘star activity’, cleaving sites that resemble the target. Exam trap: not every enzyme cuts only the exact palindrome.
Memory trick: Palindromes → Type II; non-palindrome → Type IIs (FokI cuts outside).
“PCR is like a photocopier for DNA, repeatedly copying a tiny fragment into millions of pages using primers and a heat-stable polymerase.”
PCR amplifies a target DNA segment through repeated cycles of denaturation, primer annealing, and extension by Taq polymerase. Each cycle roughly doubles the product, giving exponential growth. Critical: primers must flank the region and have compatible melting temperatures.
Memory trick: Denature-Anneal-Extend = DAE cycle
“A campfire heats a pot: only about 10 % of the fire’s energy actually warms the water, the rest is lost as heat.”
When energy moves from one trophic level to the next, only ~10 % is retained as biomass; 90 % is lost as heat, work or waste. This 10 % rule applies between consumer levels, not to the sun-to-plant capture, which is much lower. In exam answers keep the 10 % only for successive trophic transfers.
Memory trick: 10 % rule: each step keeps 1/10, loses 9/10.
“The compost heap in your kitchen garden is a bustling city of bacteria and fungi turning banana peels into soil.”
Decomposers (bacteria and fungi) secrete enzymes that break down dead organic matter into inorganic nutrients. These nutrients are then available for plant uptake, completing the nutrient loop. Do not confuse them with detritivores that ingest material without external digestion.
Memory trick: Decomposers = D for Digest, release nutrients.
“A rainforest’s many tree species act like a choir; if one voice drops, the harmony often stays, yet some songs need that unique voice.”
Biodiversity = genetic, species and ecosystem variety. Higher diversity generally gives greater stability because several species can perform similar roles, but redundancy is not universal—some functions rely on a single species. Do not equate more species automatically with higher productivity or assume any loss is fully compensated.
Memory trick: GSE = Biodiversity: Genes, Species, Ecosystems
“Sorting library books into five sections—reference, magazines, textbooks, picture books, novels—mirrors Whittaker’s five kingdoms that group life by core traits.”
Whittaker’s five-kingdom system groups organisms by cell type (prokaryote/eukaryote), nutrition (autotrophic/heterotrophic) and body organization. Monera = prokaryotes; Protista = unicellular eukaryotes; Fungi = heterotrophic absorbers; Plantae = photosynthetic multicellular; Animalia = ingestive heterotrophs. Remember Monera is only bacteria and cyanobacteria.
Memory trick: M-P-F-P-A
“A virus is like a USB drive that can’t run programs alone; it must plug into a computer (host cell) to copy its data.”
Viruses are non-cellular particles consisting of DNA or RNA surrounded by a protein capsid, sometimes with a lipid envelope. They are obligate intracellular parasites: replication occurs only inside a suitable host cell using host machinery. Trap: not all viruses have envelopes; classification hinges on nucleic-acid type and capsid symmetry.
Memory trick: DNA/RNA + protein = virus; needs host to live
“Your yogurt’s live cultures are billions of friendly bacteria thriving in milk.”
Bacteria are single-celled prokaryotes without a nucleus and with a murein cell wall. They reproduce by binary fission and include beneficial types (gut flora, nitrogen-fixing) and pathogens. Beware the trap of mixing bacteria with archaea—both are prokaryotes, but only bacteria appear in this classification.
Memory trick: Bacteria = Both beneficial & bad
“Just like a person’s first and last name uniquely identifies them, a species gets a genus (surname) and specific epithet (given name).”
A binomial name has two Latin words: the genus (capitalised) and the specific epithet (lowercase), both italicised. The genus groups closely related species, while the epithet distinguishes the individual species. Exam pitfall: do not write the authority or abbreviate the genus unless it has been introduced earlier.
Memory trick: Genus big, species small
“A pine cone drops its seed bare, while an apple drops a seed wrapped in sweet fruit.”
Gymnosperms bear naked seeds in cones; angiosperms bear seeds enclosed in an ovary that becomes fruit. Angiosperms have flowers, double fertilisation, and a triploid endosperm, while gymnosperms lack flowers and endosperm. Trap: assuming all seed plants have double fertilisation – it occurs only in angiosperms.
Memory trick: Gymno = naked seed; Angio = fruit-enclosed seed.