CBSE Class 12 Board Exam Biology Syllabus

State Mendel's Law of Segregation.

The two alleles of a gene separate (segregate) during gamete formation so that each gamete receives only one allele of each gene.

State Mendel's Law of Independent Assortment.

During gamete formation, alleles of different genes assort independently of one another (applies to genes on different chromosomes).

What phenotypic ratio is obtained in the F2 generation of a dihybrid cross, and what is the monohybrid F2 ratio?

Dihybrid F2 phenotypic ratio is 9:3:3:1; monohybrid F2 phenotypic ratio is 3:1 (genotypic 1:2:1).

What is incomplete dominance, with an example?

Incomplete dominance is when the heterozygote shows an intermediate phenotype between the two homozygotes, e.g., pink flowers in snapdragon (Antirrhinum) from a red x white cross.

Explain co-dominance using the human ABO blood group example.

Co-dominance is when both alleles express fully in the heterozygote. In blood group AB, both I^A and I^B alleles are expressed, producing both A and B antigens.

What is pleiotropy, with an example?

Pleiotropy is when a single gene affects multiple phenotypic traits, e.g., phenylketonuria or sickle-cell anemia, where one gene affects several characteristics.

How is sex determined in humans (XY type) and in birds (ZW type)?

In humans, females are XX and males are XY (male is heterogametic). In birds, females are ZW and males are ZZ (female is heterogametic).

What is the genetic basis and inheritance pattern of haemophilia?

Haemophilia is an X-linked recessive disorder; it is more common in males, who need only one defective allele, while females must be homozygous to be affected.

Name the chromosomal disorders Down's syndrome, Klinefelter's syndrome, and Turner's syndrome with their chromosomal constitution.

Down's syndrome: trisomy of chromosome 21 (47, 21+). Klinefelter's syndrome: 47, XXY. Turner's syndrome: 45, X0.

In Griffith's transforming principle experiment, what was demonstrated?

Heat-killed virulent (S) bacteria could transform live non-virulent (R) bacteria into the virulent form, indicating a 'transforming principle'—later shown to be DNA.

What did the Hershey-Chase experiment prove, and what did they use to label DNA and protein?

It proved DNA, not protein, is the genetic material. DNA was labeled with radioactive phosphorus (32P) and protein with radioactive sulfur (35S).

State Chargaff's rule for DNA base composition.

In DNA, the amount of adenine equals thymine (A=T) and guanine equals cytosine (G=C); thus purines equal pyrimidines (A+G = T+C).

What kind of DNA replication is described as 'semiconservative', as proven by Meselson and Stahl?

Semiconservative replication means each new DNA double helix contains one parental (old) strand and one newly synthesized strand.

Differentiate the leading strand from the lagging strand during DNA replication.

The leading strand is synthesized continuously in the 5'→3' direction toward the replication fork. The lagging strand is synthesized discontinuously as Okazaki fragments (away from the fork).

What is the role of RNA polymerase, and which strand serves as the template during transcription?

RNA polymerase catalyzes RNA synthesis using the template (3'→5') strand of DNA; the RNA produced is complementary and antiparallel to the template.

What are introns and exons, and what is splicing?

Exons are coding sequences expressed in mature mRNA; introns are non-coding intervening sequences. Splicing is the removal of introns and joining of exons during RNA processing in eukaryotes.

List the salient features of the genetic code.

The genetic code is triplet (codon), nearly universal, degenerate (multiple codons per amino acid), unambiguous and specific, non-overlapping, and commaless; AUG is the initiator codon; UAA, UAG, UGA are stop codons.

In the lac operon, what happens in the presence of lactose (the inducer)?

Lactose (allolactose) binds the repressor, inactivating it, so it cannot bind the operator; RNA polymerase transcribes the structural genes (z, y, a), and the operon is switched on.

What is the function of tRNA in translation?

tRNA acts as an adapter molecule: it reads codons on mRNA via its anticodon and carries the corresponding amino acid to the ribosome for polypeptide synthesis.

What were the major goals/outcomes of the Human Genome Project (HGP)?

To sequence all ~3 billion base pairs of human DNA and identify all genes (~20,000–25,000), store information in databases, and develop tools for data analysis; it advanced genomics and bioinformatics.

What is DNA fingerprinting based on, and name one application.

DNA fingerprinting is based on variations (polymorphisms) in repetitive DNA sequences called VNTRs (Variable Number of Tandem Repeats), detected via Southern blotting/probes. Applications: forensic identification, paternity testing.

State Darwin's theory of evolution by natural selection in one line.

Individuals with favorable heritable variations survive and reproduce more successfully ('survival of the fittest'), so these variations accumulate over generations leading to evolution.

What is the Hardy-Weinberg principle, and write its equation.

In an ideal population, allele frequencies remain constant across generations (genetic equilibrium). Equation: p² + 2pq + q² = 1, where p + q = 1.

List the five factors that can disturb Hardy-Weinberg equilibrium (cause evolution).

Gene flow (migration), genetic drift, mutation, genetic recombination, and natural selection.

What is adaptive radiation, with an example?

Adaptive radiation is the evolution of diverse species from a common ancestor in a new geographic area, e.g., Darwin's finches in the Galapagos and Australian marsupials.

Distinguish homologous from analogous organs.

Homologous organs have similar structure/origin but different functions (divergent evolution), e.g., forelimbs of mammals. Analogous organs have different structure/origin but similar function (convergent evolution), e.g., wings of butterfly and bird.

What did the Miller-Urey experiment demonstrate about the origin of life?

By passing electric discharge through a mixture of CH4, NH3, H2, and water vapor (simulating early Earth), they formed amino acids, supporting chemical/abiogenic origin of life (Oparin-Haldane hypothesis).

Differentiate innate immunity from acquired immunity.

Innate immunity is non-specific, present from birth, provides immediate defense (barriers, phagocytes). Acquired immunity is pathogen-specific, develops after exposure, has memory, and is mediated by B-lymphocytes (humoral) and T-lymphocytes (cell-mediated).

Differentiate active immunity from passive immunity.

Active immunity: the body produces its own antibodies after exposure to antigen/vaccine (slow, long-lasting). Passive immunity: ready-made antibodies are given from outside (fast, short-lived), e.g., colostrum, anti-tetanus serum.

Which cells does HIV primarily attack, and what is the consequence?

HIV attacks helper T-lymphocytes (CD4+ cells), drastically reducing their number, which weakens immunity and leads to AIDS, making the patient susceptible to opportunistic infections.

Name the causative organisms of malaria, typhoid, and pneumonia.

Malaria: Plasmodium (e.g., P. vivax, P. falciparum), a protozoan. Typhoid: Salmonella typhi (bacterium). Pneumonia: Streptococcus pneumoniae / Haemophilus influenzae (bacteria).

What is the difference between benign and malignant tumors?

Benign tumors remain localized, do not spread, and are generally harmless. Malignant tumors grow uncontrollably, invade surrounding tissues, and spread to other sites (metastasis).

Name the microorganisms used to produce curd, bread/idli, and ethanol.

Curd: Lactobacillus (lactic acid bacteria). Bread/idli dough (fermentation, CO2): Saccharomyces cerevisiae (baker's yeast). Ethanol: Saccharomyces cerevisiae (brewer's yeast).

From which microorganisms are the antibiotic penicillin and the cholesterol-lowering statins obtained?

Penicillin is obtained from the fungus Penicillium notatum (chrysogenum); statins are produced by the yeast Monascus purpureus.

What are biofertilizers, and name examples of nitrogen-fixing organisms used as biofertilizers.

Biofertilizers are organisms that enrich soil nutrients. Examples: Rhizobium (symbiotic with legumes), Azospirillum and Azotobacter (free-living), and cyanobacteria (e.g., Anabaena, Nostoc); also mycorrhizal fungi (Glomus).

What is the role of methanogens (e.g., Methanobacterium) in biogas production?

Methanogens are anaerobic bacteria (found in cattle dung/rumen and sludge) that produce methane (biogas) by breaking down organic matter/cellulose.

Define a food chain and distinguish grazing food chain from detritus food chain.

A food chain is the transfer of energy through successive trophic levels. A grazing food chain (GFC) starts with producers (green plants). A detritus food chain (DFC) starts with dead organic matter (detritus) consumed by detritivores/decomposers.

State the 10 percent law of energy transfer in ecosystems.

Only about 10% of the energy stored at one trophic level is transferred to the next trophic level; the rest is lost mainly as heat (Lindeman's 10% law).

Why is an ecological pyramid of energy always upright?

Because energy decreases at each successive trophic level (only ~10% passes on), so producers always have the most energy and top consumers the least—it can never be inverted.

What is ecological succession, and differentiate primary from secondary succession.

Ecological succession is the gradual, orderly change in community composition over time toward a climax. Primary succession begins on bare/lifeless areas (e.g., bare rock). Secondary succession occurs where a community was destroyed but soil remains (faster).

According to the species-area relationship, what does the slope (z value) typically range for small areas?

For relatively small areas, the slope of the regression (z value) of the species-area relationship is remarkably similar, ranging between 0.1 and 0.2 (regardless of taxonomic group or region).

What is biodiversity, and name its three levels.

Biodiversity is the variety of life forms. Its three levels are genetic diversity, species diversity, and ecological (ecosystem) diversity.

Differentiate in-situ from ex-situ conservation of biodiversity with examples.

In-situ conservation protects species in their natural habitat (e.g., national parks, biosphere reserves, sanctuaries). Ex-situ conservation protects species outside their habitat (e.g., zoos, botanical gardens, seed banks, cryopreservation).

What are the two core techniques on which modern biotechnology is based?

Genetic engineering (recombinant DNA technology, gene cloning, gene transfer) and maintenance of sterile/aseptic microbial/cell culture conditions for large-scale production of biotech products.

What is the function of restriction endonucleases in genetic engineering?

Restriction endonucleases are 'molecular scissors' that cut DNA at specific recognition sequences (palindromic sites), producing fragments often with sticky ends used in recombinant DNA technology.

What is the role of DNA ligase in recombinant DNA technology?

DNA ligase is 'molecular glue' that joins DNA fragments by forming phosphodiester bonds, e.g., sealing the foreign DNA into the vector.

What are the three basic requirements (features) of a good cloning vector?

An origin of replication (ori), a selectable marker (e.g., antibiotic resistance gene), and recognition/cloning sites (unique restriction sites); ideally a small size and high copy number.

Why is calcium chloride (and heat shock) used in transformation of bacteria?

Treating bacterial cells with divalent cations like CaCl2 makes the cell membrane more permeable ('competent'), and a brief heat shock (42°C) allows recombinant DNA to enter the host cell.

What are the three main steps of one cycle of the Polymerase Chain Reaction (PCR)?

Denaturation (separating DNA strands by heat ~94°C), annealing (primers bind to template), and extension (Taq polymerase synthesizes new strands at ~72°C).

Why is Taq polymerase used in PCR?

Taq polymerase, isolated from the thermophilic bacterium Thermus aquaticus, is heat-stable and remains active at the high temperatures used during denaturation, so it need not be added each cycle.

What is gel electrophoresis used for, and in which direction does DNA move?

Gel electrophoresis separates DNA/RNA fragments by size. Since DNA is negatively charged, it moves toward the positive electrode (anode); smaller fragments move faster/farther through the agarose gel.

What is Bt cotton, and which gene gives it insect resistance?

Bt cotton is a transgenic crop carrying cry genes (e.g., cryIAc, cryIIAb) from Bacillus thuringiensis; these produce Bt toxin that kills bollworm/insect pests.

How does RNA interference (RNAi) protect tobacco plants from nematode (Meloidogyne incognita) infestation?

By introducing nematode-specific genes using Agrobacterium vectors, the plant produces both sense and antisense RNA that form double-stranded RNA; this silences the nematode's specific mRNA (RNAi), so the parasite cannot survive.

What is the significance of humanized/recombinant insulin (Humulin), and why was earlier animal insulin problematic?

Recombinant human insulin (made by inserting A and B chain genes into E. coli, then joining the chains) avoids allergic reactions; insulin from slaughtered animals sometimes caused allergy or immune reactions in patients.

What is gene therapy, and give the first example of its clinical use.

Gene therapy is the correction of a genetic defect by inserting a normal/functional gene into a patient's cells. The first clinical use (1990) treated a girl with ADA (adenosine deaminase) deficiency.

What is molecular diagnosis using PCR/ELISA used for in biotechnology applications?

PCR detects very low amounts of pathogen DNA/RNA for early diagnosis (e.g., HIV, mutations). ELISA detects infection by detecting antigens or antibodies, enabling early disease diagnosis before symptoms appear.

What is biopiracy, and why was the Indian patent on basmati rice / turmeric significant?

Biopiracy is the unauthorized use and patenting of bio-resources or traditional knowledge by organizations/companies without proper authorization or compensation. The basmati and turmeric cases highlighted the need to protect indigenous traditional knowledge.