List of top Biology Questions asked in CBSE CLASS XII

Read the following passage and answer the questions that follow: 
“Mosquitoes are drastically affecting human health in almost all developing tropical countries. Different species of mosquitoes cause fatal diseases, leading to loss of life or reduced productivity. Consequently, the health index of these countries suffers.” 
(a) Name the form in which Plasmodium gains entry into (1) human body (2) the female {Anopheles} body. 

Identify a, b, c, d, e, and f in the table given below:
\[ \begin{array}{|c|c|c|c|} \hline \textbf{Sl. No.} & \textbf{Organism} & \textbf{Bioactive Molecule} & \textbf{Use} \\ \hline 1 & \textit{Monascus purpureus} & \text{Statins} & \text{Lowers blood cholesterol} \\ 2 & \textit{Streptomyces} & \text{Antibiotic} & \text{Treats infections} \\ 3 & \textit{Trichoderma polysporum} & \text{Cyclosporin A} & \text{Immunosuppressant} \\ 4 & \textit{Penicillium notatum} & \text{Penicillin} & \text{Kills bacteria} \\ 5 & \textit{Aspergillus niger} & \text{Citric acid} & \text{Food additive} \\ 6 & \textit{Lactobacillus} & \text{Lactic acid} & \text{Used in food preservation} \\ \hline \end{array} \]
(b) Mention one advantage and disadvantage of amniocentesis.
(a) Why do farmers prefer apomictic seeds to hybrid seeds?

If adenine is 31% in DNA, calculate the percentage of cytosine. Explain briefly. 
 

(b) Why is dung preferred for the production of biogas?
(a) Identify ‘X’, ‘Y’, and ‘Z’.
Assertion (A): AIDS is a syndrome caused by HIV.
Reason (R): HIV is a virus that damages the immune system with AIDS as its genetic material.
Assertion (A): In molecular diagnosis, single-stranded DNA or RNA tagged with radioactive molecule is called a probe.
Reason (R): A probe always searches and hybridizes with its complementary DNA in a clone of cells.
Assertion (A): Communities that comprise of more species tend to be more stable.
Reason (R): A higher number of species results in less year-to-year variation in total biomass.

Study the pedigree chart of a family showing the inheritance pattern of a certain disorder. Select the option that correctly identifies the nature of the trait depicted in the pedigree chart: 

Interferons are proteins secreted by:
A person with trisomy of 21\textsuperscript{st chromosome shows:}
(i) Furrowed tongue
(ii) Characteristic palm crease
(iii) Rudimentary ovaries
(iv) Gynaecomastia
Select the correct option from the choices given below:
If the sequence of nitrogen bases of the coding strand in a transcription unit is 5'–ATGAATG–3', the sequence of bases in its RNA transcript would be:
During biological treatment of sewage, the masses of bacteria held together by fungal filaments to form mesh-like structures are called:
Which one of the following is not a characteristic feature of “humus” that is formed during decomposition of detritus?
Which one of the following enzymes a fungal cell should be treated with to get the DNA along with other macromolecules released from it?
Match the following genes of the lac operon listed in column 'A' with their respective products listed in column 'B':
\[ \begin{array}{|c|c|} \hline \textbf{A: Gene} & \textbf{B: Products} \\ \hline \text{a. 'i' gene} & \text{(i) β-galactosidase} \\ \text{b. 'z' gene} & \text{(ii) lac permease} \\ \text{c. 'a' gene} & \text{(iii) repressor} \\ \text{d. 'y' gene} & \text{(iv) transacetylase} \\ \hline \end{array} \] Select the correct option:

Identify the most appropriate technique depicted in the above diagram: 

Identify the correct labellings in the figure of a fertilised embryo sac of an angiosperm given below: 

Explain the events which occur within a female Anopheles mosquito after it has sucked blood from a malaria patient.

(c) Name a species of mosquito other than female Anopheles and the disease, for which it carries the pathogen.

Natural selection operates in different ways in nature. 
(a) (i) Identify the type of natural selection depicted in the graph above. 
 

(i) Draw a schematic diagram of the cloning vector pBR322 and label:
(1) BamHI site 
(2) Gene for ampicillin resistance 
(3) Ori 
(4) Rop gene 

(iv) Why could parasite not survive in a transgenic tobacco plant?