List of top Questions asked in CBSE CLASS XII- 2025

The RNA triplet UAG acts as a stop codon which terminates the synthesis of a polypeptide. Below is given a strand of DNA which codes for four amino acids.
In which position will an insertion of a thymine nucleotide result in the termination of translation?
DNA strand:
3′ T C C A C C C G A T G C 5′

Answer the following questions:

(a)

  1. What is a test cross? How can we find out the zygosity of a plant by the use of test cross? 
  2. Why are human females rarely haemophilic? Explain with the help of a cross. Under what conditions can a haemophilic female be born? How do haemophilic patients suffer? 

OR 
(b) Name the two events ‘a’ and ‘c’ shown in the diagram below. 

 

  1. Mention where in the cell these events take place. 
  2. Explain the changes taking place during event ‘b’ and mention why these are needed.
The base sequence in one of the strands of DNA is: ‘TAGCATGAT’
  • Give the base sequence of its complementary strand.
  • How are these base pairs held together in DNA molecule?
  • Explain the base complementarity rule. Name the scientist who framed this rule.
Name the process involved in the production of nematode-resistant tobacco plants, using genetic engineering. Explain the strategy adopted to develop such plants.
Answer the following questions:
  • Write the scientific name of the fruit fly.
  • Why did Morgan prefer to work with fruit flies for his experiments? State two reasons.
Student to attempt either option (A) or (B): (A) Biotechnology has helped farmers to get pest-resistant cotton plants. Explain the technique adopted along with its mode of action.
OR
(B) Explain any two molecular diagnostic techniques that help to detect pathogens from suspected patient.
Assertion (A): Human male is referred to as heterogametic.
Reason (R): The male produces millions of sperms of the same kind.

Study the table given below showing the relationship between organism, its trophic level and kind of food chain it belongs to.

Name the microbe that is used as biocontrol agent for controlling pest butterfly caterpillars:
A dwarf pea plant was treated with gibberellic acid and it becomes as tall as a tall pea plant. If these treated pea plants are crossed with pure tall pea plants, the phenotypic ratio in $F_1$ generation will be:
During the process of oogenesis in human female, the primary oocyte results in the formation of which of the following?
Choose the correct option.
pBR which is frequently used as a vector for cloning gene in E. coli is a/an:
In Bohr model of hydrogen atom, an electron is revolving in second orbit. Find the value of:
(i) angular momentum of electron,
(ii) radius of the orbit, and
(iii) kinetic energy of electron.
Take radius of first orbit of hydrogen atom as 0.5 \AA.
A concave mirror has a radius of curvature 20 cm. Calculate the distance of an object from the mirror so as to form an image of magnification -2. Also, find the location of the image.
Differentiate between ‘nuclear fission’ and ‘nuclear fusion’. Briefly discuss one example of each.
The wavelength of a photon is equal to the wavelength associated with an electron. Both will have the same value of:
A particle of mass \( m \) and charge \( q \) moving with velocity \( \vec{v} = v \hat{i} \) is subjected to a uniform electric field \( \vec{E} = E \hat{j} \). The particle will initially have a tendency to move in a circle of radius:
Describe briefly the Geiger-Marsden scattering experiment. Depict the graph showing the variation of the number of scattered particles detected with the scattering angle. How did this graph lead to the discovery of the nucleus?
A ray of light is incident at an angle \( i \) on a parallel sided glass slab of thickness \( d \) and gets refracted into the slab at angle \( r \). Draw a ray diagram to show its path as it emerges out of the slab. Hence, obtain an expression for the lateral shift of the ray. Under what condition will the shift be minimum?
Define the term 'drift velocity' of conduction electrons in a conductor.
A ray of light is incident on face AB of a prism ABC with angle of prism \( A \) and emerges out from face AC. The prism is set in the position of minimum deviation with angle of deviation \( \delta \). Find: \begin{enumerate} \item the angle of incidence and \item the angle of refraction on face AB. \end{enumerate}
A straight conductor is carrying a current of 2 A in the \( +x \) direction along it. A uniform magnetic field \( \vec{B} = (0.6 \hat{j} + 0.8 \hat{k}) \) T is switched on in the region. The force acting on 10 cm length of the conductor is:
An electric field \( \vec{E} = (10x + 5) \hat{i} \, \text{N/C} \) exists in a region in which a cube of side \( L \) is kept as shown in the figure. Here \( x \) and \( L \) are in metres. Calculate the net flux through the cube.
An electric field E⃗ = (10x + 5)ˆi N/C
A charge \( Q \) is distributed over two concentric hollow spheres of radii \( r \) and \( R \) (\( R>r \)) such that their surface charge densities are equal. Find: \begin{enumerate} \item the electric field, and \item the potential at their common center. \end{enumerate}
Using Huygens’ principle, explain the refraction of a plane wavefront, propagating in air, at a plane interface between air and glass. Hence verify Snell’s law.