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

Q20. What are linked genes? Diagrammatically represent a cross between a white eyed female and red eyed male drosophila.
Ans20. Genes, which tend to move together; that is, appear to be ‘linked’ together occur on the same chromosome and are called ‘linked genes’. A pair of genes may be identified as linked if their recombination frequency in a test-cross is lower than the expected value of 50 per cent.

Q21. How does an excess of tryptophan cause ‘Switching off’ of the ‘tryptophan operon’?
Ans21. Tryptophan Operon consists of five genes coding for five enzymes catalyzing the synthesis of tryptophan and thus constitute an anabolic pathway. The regulator R gene produces a protein, referred to as Apoepressor, which itself is unable to bind to operator O unless its Co-repressor Tryptophan is present. When Tryptophan is present, the functional repressor forms and binds to the operator. This binding prevents the transcription of the operon and production of tryptophan.

Q22. If abiotic origin of life is in progress on a planet other than earth, what should be the condition there? Explain.

Ans22. If abiotic origin of life is in progress on a planet other than the earth, the conditions there should be same as the one prevalent on the primitive earth. Thus the following conditions would be required :
Solids, liquids, and gases coexist. The atmosphere should be reducing.

• Hydrogen, nitrogen, water vapour, carbon dioxide, methane, and ammonia should be present.

• Sources of energy such as thermal energy, radiation, and lightning should be present.
  

Q23. Trace the formation of Ovum from oogonium.
Ans23. Ova form in the ovary. The maturing Graffian follicle contains a diploid (2n) Primary Oocyte. It undergoes meiosis I to produce two haploid cells : a larger one, the secondary oocyte and a small Polar body. During meiosis I of the primary oocyte, the Graffian follicle comes on the ovarian surface.
Immediately after the meiosis I the follicle ruptures and ejects the secondary oocyte near the open end of the Fallopian tube. As the secondary oocyte moves down the Fallopian tube, the oocyte starts the meiosis II, though it does not complete unless the sperm enters the secondary oocyte. Meiosis
II gives a haploid mature ovum and a second polar body.

Q24. Describe the role of red blood cells in the transport of oxygen and carbon dioxide by blood.
Ans24. Blood transports oxygen with the help of haemoglobin in RBCs and carbon dioxide with the help of cytoplasm of red blood cells. Oxygen diffuses into the RBCs and combines loosely with the Fe²⁺ ions of hemoglobin to form oxyhaemoglobin. A lower PO^₂ and higher PCO_² in the tissues causes dissociation of oxyhaemoglobin to deoxyhaemoglobin (reduced haemoglobin)
and molecular oxygen.
Carbon dioxide diffuses into RBCs and reacts with water in the cytoplasm, to form Carbonic acid :
CO₂ + H₂O = H₂CO₃, The reaction is catalysed by zinc containing Carbonic Anhydrase.
In RBCs carbonic acid forms bicarbonate :
H₂CO₃ H⁺+HCO₃^-
Some of the bicarbonate remains in the RBCs but most of it comes out in plasma. Some of the CO₂ reacts with globin of deoxyhaemoglobin to form Carbaminohaemoglobin in RBCs.

Q25. Draw a diagram to show the internal structure of the human heart. Label six parts in all including at least three valves.
Ans25.

Q26. What is metastasis? List any four danger signals of cancer.
Ans26. A malignant tumour does not remain confined to the original tissue. Cancerous cells are carried by the blood and lymph to other parts of the body, where they multiply to produce secondary malignant growths. The spread of cancerous cells to distant sites is termed Metastasis.

Some danger signals of cancer are :
A lump or hard area in the breast.
A change in a wart or mole.                                                          jump to top
A persistent change in digestive and bowel habits.
A persistent cough or hoarseness.

Q27. By providing one example for each category, classify the angiospermic flowers based on the position of ovary.
Ans27. There are three type of flowers in angiosperm, based on the disposition of floral parts with respect to ovary. They are :
Hypogynous flowers : In such flowers the ovary is superior that is, the ovary is situated on the torus above all the other floral parts. Examples are: Musturd and Tomato.
Perigynous flowers : In such flowers the thalamus forms a cup-shaped structure around the ovary and bears sepals, petals, and stamens. The ovary appears sunken in these cases, for example in rose.
Epigynous flowers : In such flowers the thalamus is not only cup-shaped but is fused with the ovary such that the other floral parts arise on the top of the ovary. The ovary in these cases is inferior. Examples are: Cucumber, Apple.

Q28. Describe the process of DNA replication.
Ans28. 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 catalyzes 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 29Specify how C4 photosynthetic pathway increases CO₂ concentration in bundle sheathcells of sugarcane. How much is C₄ pathway more energy-expensive than C₃ pathway?
Ans29. C₄pathway is the one in which carbon dioxide is fixed in the mesophyll cells and the initial product is a 4-Carbon compound unlike the Calvin cycle whgich is referred to as C₃ pathway because the first product after CO₂ fixing is a 3-carbon compound.

The carbon dioxide acceptor in C₄ pathway is a 3-Carbon compound, Phosphoenolpyruvate (PEP) and it forms 4-carbon compound oxaloaceticacid (OAA) by accepting CO₂ in outer mesophyll cells. OAA is converted to malic acid, which is transported to cells surrounding the vascular bundle, the Bundle sheath cells. Here malic acid is converted to pyruvic acid and CO₂ is released in the cytoplasm. Thus the concentration of CO₂ increases in the bundle
sheath cells.

The pyruvic acid generated in the bundle sheath cells is transferred back to the mesophyll. It is converted to PEP by the expenditure of an ATP molecule. But this conversion generates AMP (and not ADP, as is normal) causing requirement of two additional ATPs to regenerate AMP. The total additional requirement of ATPs per six CO₂ molecules is thus 12 ATPs in comparison to C₃ pathway.

The C₄ pathway is thus more energy-expensive than C₃ pathway. The C₃ pathway requires 18 ATPs for synthesis of one molecule of glucose as against C₄ pathway which requires 30 ATPs.

Q30. What is a synapse? How is the nerve impulse transmitted across a synapse?
Ans30. Synapse is the junction between two neurons, across which the impulse has to pass from one neuron to the next. Synapses are formed between the axon terminals of one neuron and the dendrites, the cell body or even sometimes the axon of one or more neurons. There is no actual continuity between neurons at the synapse. The axon of a neuron divides near its
termination into many branches, each branch losing its myelin sheath before termination.
These axon terminals of a neuron then end in expanded feet-like forms on the dendrites or the cell body of another neuron; there is a narrow fluid-filled space, called Synaptic cleft, separating the membranes of the two neurons at the synapse. The axon terminal contains many membrane-bound vesicles, called Synaptic Vesicle, in its cytoplasm. Within these vesicles, chemical substances, such as adrenaline and acetylcholine remain stored.

When a nerve impulse passes the axon terminal, its synaptic vesicles release their stored chemicals to the synaptic cleft. These diffuse through the cleft to reach the membrane of the next neuron, and stimulating the latter. This causes the nerve impulse to be transmitted along the next neuron.