List of top Biology Questions asked in CBSE CLASS XII

(a) Write the full form of BOD.
(b) Define BOD. Explain how it is a measure of the organic matter present in the water body.
During a medical investigation, an infant was found to possess an extra copy of chromosome no. 21. Identify the disorder the child is suffering from. Describe the symptoms the child is likely to develop later in life.
Explain the neuroendocrine mechanism involved in the process of parturition in a human female leading to the expulsion of the baby out of the uterus through the birth canal.
Correctly depict (also indicate the trophic level) and describe the ecological pyramid of number with 32 birds dependent on 20 insects feeding on one banyan tree.
Explain how the interaction between a fig tree and its tight one-to-one relationship with the pollinator species of wasp is one of the best examples of mutualism.
The basic scheme of the essential steps involved in the process of recombinant DNA technology is summarised below in the form of a flow diagram. Study the given flow diagram and answer the questions that follow: \begin{center} Step-1 \quad Plasmid DNA (cut using Restriction Enzyme EcoR I) \quad + \quad Foreign DNA (cut using Restriction Enzyme EcoR I)
$\downarrow$
DNA ligase
$\downarrow$
Step-2 \quad Recombinant DNA molecule
$\downarrow$
Step-3 \quad Transfer of recombinant DNA molecule to the host cell
$\downarrow$
Step-4 \quad Replication of recombinant DNA molecule in the host cell \end{center} (a) Name the specific enzyme that might have been used to make the multiple copies of foreign DNA before undergoing Step-1 of the process.
(b) How does the use of restriction enzyme EcoR I in Step-1 facilitate the action of DNA ligase to form the recombinant DNA molecule? Explain.
(c) Name the most commonly used host in the above process.
Name any two VDs which might occur in a human female. State any two complications in a female if it is left untreated.
Explain what is meant by the term amniocentesis. How is this technique misused in India?

Given below are the diagrammatic representations of the replicating fork of DNA in E. coli. Study the diagrams and answer the questions that follow.

(a) Which one of the three diagrams (i), (ii) or (iii) is the correct representation of the replicating fork of DNA replication? Explain your answer.
(b) Name the enzyme used in E. coli to join the newly synthesised fragments of DNA.

Other than public awareness and counselling, enlist four measures taken up by NACO, WHO and other NGOs to prevent the spread of HIV infection in the society.
Give an account of the generalised structure of an antibody molecule produced by B-lymphocytes in response to the pathogen.
Assertion (A): Cu-T, Cu-7 and LNG-20 are the most widely used copper-releasing IUDs.
Reason (R): Cu-ions in IUDs effectively suppress sperm motility and the fertilising capacity of sperms.
Assertion (A): Gene pairs present on the same chromosome may be tightly linked or loosely linked.
Reason (R): Frequency of recombination between gene pairs on different chromosomes as a measure of the distance between genes can be used for ‘mapping’ their position on the chromosomes.
Assertion (A): Biogas plants are more often built in rural areas.
Reason (R): The excreta or gobar of cattle is rich in Methanobacterium.
Assertion (A): To generate only a part of the plant from a cell is totipotency.
Reason (R): Suitable special nutrient media and sterile conditions are required in ‘in vitro’ conditions for the division of cells in explants.
The correct depiction of the centrifugation step of the experiment conducted by Alfred Hershey and Martha Chase on using radioactive labelled phages to prove that DNA is the genetic material is:
Large scale industrial production of Butyric acid for human welfare is done using the microbe:
What would happen if a gene encoding a polypeptide of 50 amino acids, $25^th$ Codon (UAU) is mutated to “UAA”?
During a monohybrid cross involving a tall pea plant with a dwarf pea plant, the offspring populations were tall and dwarf in equal ratio. Find out the genotype of parent pea plants.
Isolation of DNA from a fungal cell can be achieved by using:

Which of the following combinations is a correct example of convergent evolution in Australian marsupials and placental mammals?

Select the statements that are true for pollination mechanism in flowering plants from the given options:
Colostrum secreted by the mother’s mammary glands in a human female during the initial days of lactation is rich in antibody:

Which of the following is not an example of aneuploidy?

During the process of transcription, after binding to a promoter, RNA polymerase catalyses and makes the bases in the template strand of DNA available for base pairing, with the bases of: