List of top Questions asked in CBSE CLASS XII- 2025

Given below is a flower with its characteristic features specialised for the most common type of abiotic pollination.

Answer the following questions based on the above diagram:
(a) Name the mode of abiotic pollination that will be adopted by the given plant species in the above picture.
(b) State the need of exposed large feathery stigmas for the flower.
(c) What will be the two important adaptations in the pollen grains of the flowers pollinated by the above mode of pollination?
(d)What could be the probable reason for the petals being small and non-green?

Explain the biological treatment of primary effluent when passed into the large aeration tanks in a sewage treatment plant (STP).

Gregor Mendel conducted hybridisation experiments in garden pea for seven years and proposed the law of inheritance.
Why was he successful in his hybridisation experiments? Give two reasons.
State the law of independent assortment as proposed by Mendel after his dihybrid crosses.

Explain the process of formation of placenta in a human female after the implantation of the blastocyst in the endometrium of the uterus.

Correctly depict (also indicate the trophic level) and describe the ecological pyramid of biomass in sea with 40 standing crop of phytoplankton supporting 90 standing crop of zooplankton which further supports 120 small fishes.

Explain how the interaction between sea anemone and clownfish is one of the best examples of commensalism in nature.

The basic scheme of the essential steps involved in the process of recombinant DNA technology is summarized below in the form of a flow diagram. Study the given flow diagram and answer the questions that follow.

(a) Name the enzyme used in Step-1 to join the cut plasmid and alien DNA.
(b) State the technical term used for Step-3.
(c) Justify the use of the same Restriction Enzyme EcoRI to cut both the vector DNA and the alien DNA.

Name any two STIs which might occur in a human female. State its two early symptoms.

(i)What are interferons? Explain their role in providing immunity to a person.
(ii) Which category of innate immunity defence barrier can interferons be classified into?

Explain how the immunity of a person is affected if there is atrophy (degeneration) of the thymus gland at an early stage of life.

Assertion (A): ABO blood grouping in humans is an example of multiple allelism.
Reason (R): More than two genes in a population govern the same character in ABO blood grouping in humans.

Assertion (A): ‘Saheli’, an oral contraceptive inhibits ovulation and increases phagocytosis of sperms.
Reason (R): It is a non-steroidal preparation.

Assertion (A): A person infected with malaria suffers from chill and high fever, recurring every three or four days.
Reason (R): The parasite attacks the RBC resulting in their rupture and release of haemozoin.

Assertion (A): The meristems are grown ‘in vitro’ to obtain virus-free plants from an infected plant.
Reason (R): If the plant is infected with a virus, the roots and the stems are free of virus.

In a pea plant (Pisum sativum) inflated pod shape is dominant over constricted pod shape. The expected ratio of phenotypes of the offspring in a cross between both the parents with heterozygous inflated pod shape will be:

Bioactive molecule Cyclosporin A used for human welfare is derived from:

The correct depiction of the experiment performed by Matthew Meselson and Franklin Stahl to prove that DNA replicates semi-conservatively on separation of DNA by centrifugation after 40 minutes is:

The technique for the early detection of a disease based on the principle of antigen-antibody interaction is:

Use the given information to select the amino acid attached to the 3′ end of tRNA during the process of translation, if the coding strand of the structural gene being transcribed has the nucleotide sequence TAC.

About 15 mya during human evolution, the primates which used to walk like gorillas and chimpanzees were:

Select the statements that are true for a typical dicotyledonous embryo from the given options.

In which of the following human diseases does the body's self-defence mechanism attack self-cells?

The substrate used during DNA replication by the enzyme DNA-dependent DNA polymerase is:

Given below are a few statements with reference to the accessory ducts of the human male reproductive system:

[(i)] The seminiferous tubules of the testes open into rete testis then into the vas deferens.
[(ii)] The vasa efferentia leave the testes and open into the epididymis.
[(iii)] The epididymis leads to vas deferens that ascends into the abdomen.
[(iv)] The vas deferens receives a duct from the prostrate gland and opens into the urethra as ejaculatory duct.
[(v)] The urethra originates from the urinary bladder and extends through the penis to its external opening, urethral meatus.

Choose the option with all true statements from the given options:

In its extended 'beads-on-string' form of chromatin, the 'beads' in the string represent: