CBSE Set Qa2 Q Sample Test Papers For Class 12th for students online

Q.21 How would non-secretion of hydrochloric acid in our stomach affect food digestion ? Explain ?
Ans21. Hydrochloric acid is responsible for the following :
(i) Activates pepsinogen, prorenin and other protease proenzymes.
(ii) Denatures food proteins and facilitates action of pepsin on them.
(iii) Stops the action of salivary amylase.
(iv) Disinfect the ingested food.
In the absence of HCl in the gastric juice no protein digestion would occur and only carbohydrate digestion would continue.

Q.22 A farmer adds Azotobactor culture in the soil before sowing maize. How does it increase the yield of maize.
Ans22. Azotobacter is a free living non-symbiotic soil-bacteria, which can fix atmospheric nitrogen. The fixed nitrogen becomes available to cereal crops. The inoculation of maize seeds with Azotobacter will ensure simultaneous growth of Azotobacter in the field and will provide better nitrogen availability, better yields and saving of mineral nitrogen fertiliser's.

Q.23 Give one example of co-dominance in human genetics.
Ans23. The ABO system of blood in humans is an example of both multiple allelism and codominance. The gene exists as three alleles: I^A, I^B, and i. Both I^A and I^B are dominant over i. Both the dominant alleles confer AB phenotype when present together.

Q.24 Draw a labelled diagram of the longitudinal section of a pistil showing pollen germination.
Ans24.



Q.25 Describe the process of DNA replication.
Ans25. The DNA replication occurs in the following steps :

Origin : Replication starts from specific positions in the DNA helix, called ‘Origin’. When the replication starts the DNA strands uncoil to expose the template surface for synthesis of daughter strands on them. Helicase enzymes help in unwinding the helix and Topoisomerases break and reseal one strand of DNA to avoid supercoiling. The double stranded DNA unwinds as
a Y-shaped structure called ‘Replication fork’.

Primer formation : A short stretch of RNA is formed on the DNA template, which serves as the primer for the DNA synthesis. Primase catalyses the synthesis of RNA on the existing DNA.

DNA synthesis : DNA polymerase enzyme adds the building blocks to the primer in a sequence as influenced by the template in 5’ to 3’ direction. On the parental 5’-3’ strand daughter strand forms continuously in 5’ – 3’ direction; but on the parental 3’ – 5’ strand daughter-strand forms discontinuously in short stretches of 5’ – 3’ fragments, called ‘Okazaki Fragments’. These
fragments are then joined together. The continuous daughter-strand is called ‘Leading strand’, and the discontinuous daughter-strand is called ‘Lagging strand’.

DNA Polymerase has proof-reading capacity and goes-back to any wrong base and removes it to put new base in its position before it moves to a new base in 5’ – 3’ direction.
As the new strands grow from the fork and the replication proceeds, the point of divergence at the replication fork moves away from origin.


Q.26 If abiotic origin of life is in process on a planet other than the earth, what would be the conditions then ? Explain them.
Ans26. If the abiotic origin of life has to take place on any planet :
(i) Hydrogen, nitrogen, water vapour, carbon dioxide, methane, and ammonia must be present in its environment.
(ii) Sources of energy such as thermal energy, radiation, and lightening should be available.
(iii) Atmosphere must be non-reducing.

Q.27 Explain the structure of a mammalian heart.
Ans27. A mammalian heart is a four chambered structure : two Atria (auricles), and two Ventricles. The sinus venosus has totally fused with the right auricle and does not form a separate chamber. The right and left halves of the heart are completely partitioned off. The Interatrial Septum separates right and left atria. Interventricular septum separates the two ventricles. The great veins which return blood from body parts open in the right atrium and are called Superior and Inferior Vana Cavae. Pulmonary veins pool oxygenated blood from lungs in left atrium. Atrioventricular valves guard openings between respective auricles and ventricles. Right AV valve is a Tricuspid valve and the left AV valve is a Bicuspid (Mitral) valve.
The right ventricle opens into the pulmonary artery, which carries blood for oxygenation to the lungs. Left ventricle opens into the Aorta , which supplies blood to all the body parts. Semilunar valves guard openings of these great arteries.



Q.28 Give a labelled diagram of the sectional view of the duct system and the ovary of the reproductive organs of the human female.
Ans28.


Q.29 Describe the mode of infection, symptoms and preventive measure of Diphtheria, Tetanus and Measles.
Ans29.

Q.30 Describe two important functions each of the elements P, B and S in green plants and also write the deficiency symptoms of any two of them.
Ans30.

Q.31 Describe how our brain gets a continuous supply of oxygen from the atmosphere.
Ans31. Respiratory gas exchange and the blood circulation provide a continuous supply of oxygen from the atmosphere to the brain. Inhalation draws in fresh air from the atmosphere into lungs through nostrils and via nasopharynx, and trachea. In the alveoli of the lungs gaseous exchange occurs between blood and the inhaled air. The high PO₂ and low PCO₂ in the lungs cause diffusion of O₂ into the RBCs where it combines loosely with the Fe²⁺ ions of haemoglobin to form oxyhaemoglobin. When the oxygenated blood reaches in the brain tissue, the lower PO₂ and higher PCO₂ there causes dissociation of oxyhemoglobin to deoxyhemoglobin (reduced haemoglobin) and molecular oxygen.
The brain tissue picks up O₂ and releases CO₂ to blood.


Q.32 In an experiment a phenotypic ratio 3 : 3 : 1 : 1 was obtained in the offspring on crossing Yellow sed-Tall stem Yy Tt variety of pea plant with yellow seed-dwarf stem variety. Determine the accuracy of this data by punnet squire.
Ans32.