List of top Biology Questions asked in CBSE CLASS XII

Regulation of lac operon by repressor is referred to as:

Explain how the addition of lactose in the medium regulates the switching on of the lac operon in bacteria.

Read the following passage and answer the questions that follow. According to evolutionary theory, every evolutionary change involves the substitution of a new gene for the old one and the new allele arises from the old one. Continuous accumulation of changes in the DNA coding for proteins leads to evolutionary differences. The chemical composition of DNA is basically the same in all living beings, except for differences in the sequence of nitrogenous bases. Given below are percentage relative similarities between human DNA and DNA of other vertebrates:

(a) What is the term used for the substitution of a new gene for the old one and the new allele arising from the old one during evolutionary process?

(b) Which one of the following holds true for the data provided in the above table?

• [(A)] Greater the evolutionary distance, greater are the differences in the nitrogenous bases.
• [(B)] Lesser the evolutionary distance, greater are the differences in the nitrogenous bases.
• [(C)] Greater the evolutionary distance, lesser are the differences in the nitrogenous bases.
• [(D)] Lesser the evolutionary distance, lesser are the differences in the nitrogenous bases.

(c) [(i)] To which category of evolution (divergent or convergent) does the following relationship belong? Justify your answer.
Human and Rhesus Monkey 
OR

[(ii)] Differentiate between Convergent and Divergent evolution.

Answer the following questions:

(a) [(i)] Explain how some strains of Bacillus thuringiensis produce proteins that kill certain insects such as lepidopterans but do not kill the Bacillus.

[(ii)] How is the above mechanism exploited for the production of Bt cotton plant by biotechnologists?

(b) [(i)] Explain how the amplification of gene of interest is done using PCR.

[(ii)] State two applications of the desired amplified fragment of DNA.

Read the following passage and answer the questions that follow. Prevention is the frontline response to drug use. Effective interventions address the underlying conditions contributing to drug use, such as a lack of connection to family or community, instability, insecurity, trauma, mental health issues, etc. When addressed, these factors can effectively prevent the initiation of drug use and the progression to drug use disorders. Study the few key figures of drug use given below and answer the questions that follow.

(a) What do you infer from the figures in Table No. 1 about the people with drug use disorders, 2022 (in million)? State any two of your observations.

(b) How are Hepatitis C and HIV related to drug use disorders by people, as shown in Table No. 2? State the correlation between the two.

(c)

• [(i)] Give the scientific name of (p) shown in Table No. 1. 
  OR
• [(ii)] Give the scientific name of (q) shown in Table No. 1.

Answer the following questions:

[(i)] Explain the structure of a mature embryo sac of a typical flowering plant.

[(ii)] How is triple fusion achieved in these plants?

OR

[(i)] Describe the changes in the ovary and the uterus as induced by the changes in the level of pituitary and ovarian hormones during menstrual cycle in a human female.

Answer the following questions with respect to the sex determining mechanism observed in honey bee.

• [(a)] Name the type of sex determination system observed in honey bee.
• [(b)] Fill in the blanks (i), (ii) and (iii) in the given question.

• [(c)] What will be the sex and chromosome number of the progeny formed from the unfertilised eggs of honey bee?

(a) Name and explain the role of inner and middle walls of the human female uterus.
(b) Write the location and function of fimbriae in human female.

Flowering plants with hermaphrodite flowers have developed many reproductive strategies to ensure cross-pollination. Study the given outbreeding devices adopted by certain flowering plants and answer the questions that follow.

Note : All plants belong to the same species. No pollen tube growth/inhibition of pollen germination on stigma. Pollen germination on stigma.

• [(a)] Name and define the outbreeding device described in the above table.
• [(b)] Explain what would have been the disadvantage to the plant in the absence of the given strategy.

Study the given molecular structure of double-stranded polynucleotide chain of DNA and answer the questions that follow.

(a) How many phosphodiester bonds are present in the given double-stranded polynucleotide chain? 
(b) How many base pairs are there in each helical turn of double helix structure of DNA? Also write the distance between a base pair in a helix. 
(c) In addition to H-bonds, what confers additional stability to the helical structure of DNA?

Student to attempt either option (A) or (B):
(A) How is the interaction between Ophrys and its specific bee pollinator one of the best examples of co-evolution? Explain. 
OR 
(B) Arrange the given important steps of decomposition in their correct order of occurrence in the breakdown of complex organic matter and explain the fourth step in the process.

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.

(a) What is the technical term used for Step 4 in the above process?
(b) Which of the given two combinations of restriction enzyme should be used in Step 1? Justify your answer. 
(i) EcoR I to cut the plasmid and Hind III to cut the alien DNA. 
(ii) EcoR I to cut both the plasmid and alien DNA.