CBSE Class 12 Biology Question Paper 2024 (Set 3- 57/1/3) Available- Download Solution PDF with Answer Key

Solution: The Venn diagram illustrates the classification of seeds into dicotyledonous (P), endospermic (Q), and monocotyledonous (R). Based on the characteristics:

• P (Dicotyledonous): Bean is a dicot seed, which has two cotyledons.
• Q (Endospermic seed): Castor seed is an endospermic dicot seed, where the endosperm persists in the mature seed.
• R (Monocotyledonous): Maize is a monocot seed, having only one cotyledon.

Solution: In a polynucleotide chain, the dotted line (‘—') represents the bond between the nitrogenous base (B) and the sugar (S). This bond is known as an N-glycosidic linkage. It connects the 1' carbon of the pentose sugar to the nitrogen atom of the nitrogenous base.

Other bonds in the polynucleotide structure: Phosphodiester bond: Links the sugar molecules of adjacent nucleotides. Hydrogen bond: Occurs between complementary nitrogenous bases in double-stranded DNA. Peptide bond: Found in proteins, not in polynucleotides.

Solution: Lactobacillus is a heterotrophic bacterium that converts lactose in milk into lactic acid, setting the milk into curd.

Solution: Transgenic mice are used to test the safety of the polio vaccine because they can be genetically modified to mimic human susceptibility to the polio virus.

Solution: Ernst Haeckel proposed the "Biogenetic Law,” which states that embryonic development (ontogeny) recapitulates the evolutionary history (phylogeny) of an organism.

Solution: Human Chorionic Gonadotropin (hCG) is secreted by the placenta and maintains pregnancy by supporting the corpus luteum, ensuring the continuous production of progesterone.

Solution: During an allergic reaction, histamine is released by mast cells and basophils, causing symptoms like swelling, itching, and inflammation.

Solution: In the case of 44 + XXY (Klinefelter syndrome), the individual is typically male but may have some characteristics of females, such as reduced fertility and mild developmental issues, but not overall feminine development.

Solution: Hind II is a restriction enzyme that recognizes the palindromic sequence of six base pairs (5'-AAGCTT-3') and cuts between the A and the G.

Solution: During days 10 to 17 of the menstrual cycle, ovulation occurs and the woman is most fertile, making it the period of high risk for conception if abstinence is not followed.

Solution: Stanley Cohen and Herbert Boyer are famous for their pioneering work in molecular genetics, particularly for the development of the first recombinant DNA (rDNA) technology. In their landmark experiment, they used the bacterium Salmonella typhimurium as part of their work. The process involved taking DNA from two different sources and combining them into a single molecule of recombinant DNA. The genetic material was introduced into Salmonella typhimurium, allowing for the transfer and expression of the recombinant DNA.

Here is a breakdown of the options:
- Option (A) Agrobacterium tumefaciens: While Agrobacterium tumefaciens is widely used in genetic engineering, particularly for introducing genes into plant cells, it was not involved in the construction of the first rDNA by Cohen and Boyer.
- Option (B) Mycobacterium sp.: Mycobacterium species, such as Mycobacterium tuberculosis, are known for causing diseases like tuberculosis. These bacteria were not used by Cohen and Boyer in their rDNA work.
- Option (C) E. coli: Although Escherichia coli (E. coli) is commonly used in modern recombinant DNA technology as a host for cloning, Cohen and Boyer initially worked with Salmonella typhimurium for their rDNA construction.
- Option (D) Salmonella typhimurium: Correct.

Solution: Fish are often secondary consumers (feeding on zooplankton) and primary consumers (feeding on phytoplankton), occupying more than one trophic level.

Solution: The assertion is correct as GEAC (Genetic Engineering Approval Committee) evaluates the safety and environmental impact of GM crops in India. However, the reason is false because genetic modification can affect ecosystems in various ways, including unintended crossbreeding and ecological changes.

Solution: Gene flow introduces new genetic variations by mixing alleles between populations. The random introduction and removal of alleles do indeed affect allele frequencies, supporting the assertion.

Solution: The assertion is true as adoption is legally permitted in India. However, the reason is false because emotional, religious, and social factors can often act as barriers in adoption processes.

Solution: Agrobacterium tumefaciens is not a pathogen of monocot plants but primarily infects dicot plants. The reason is correct as it delivers T-DNA, causing tumor formation in infected plants.

Solution: In the given pyramid of biomass:

• Level 'A' could be occupied by Producers (PC) such as plants, which have the highest biomass in most ecosystems.
• Level 'B' could be occupied by Primary Consumers (PP) like herbivores, which feed on plants.

This type of pyramid is known as an Inverted Pyramid of Biomass, and it is typically found in Aquatic Ecosystems where the biomass of producers (like phytoplankton) is lower than that of primary consumers (like zooplankton or small fish).

Solution: Louis Pasteur's experiment, known as the "swan-neck flask experiment,” was conducted to disprove the theory of spontaneous generation. In this experiment, Pasteur used a flask with a long curved neck (swan-neck) to allow air but prevent contaminants from entering the flask.

He boiled the nutrient broth in the flask to kill any existing microorganisms. The swan neck prevented dust and microbes from entering, and no microbial growth was observed in the broth. However, if the neck was broken, allowing dust and microorganisms to enter, microbial growth occurred.

Conclusion: This experiment concluded that microorganisms do not spontaneously arise from non-living matter but are introduced from the environment. It supported the theory of biogenesis.

Solution:

• A: Pericarp – The outer covering of the fruit, often referred to as the fruit wall.
• B: Cotyledon in seed of grass family – The seed leaves, which are important in providing energy during germination.
• C: Embryonal axis – The region of the seed that connects the embryo with the seed and helps in the development of the plant.
• D: Remains of nucellus in a seed – The remnant of the tissue that provides nutrition to the developing embryo in the seed.

Solution: Cheese varieties differ due to the microorganisms used in their fermentation, affecting the texture, flavor, and taste. Examples include:

• Swiss Cheese: The bacterium Propionibacterium shermanii produces holes and imparts a nutty flavor to Swiss cheese.
• Blue Cheese: Bacteria such as Penicillium roqueforti cause the blue veins in blue cheese and contribute to its distinct, sharp flavor.

Solution:

• Papaver somniferum: A: Morphine, which is a depressant and slows down body functions.
• Cannabis sativa: B: THC (Tetrahydrocannabinol), which affects the nervous system and causes altered perception.
• Erythroxylum coca: C: Cocaine, which stimulates the central nervous system, causes addiction, and can lead to heart issues.

Solution: The basic steps followed in the Assisted Reproductive Technologies (ART) programme are:

• Step 1: Hormonal stimulation is provided to the female to induce the production of multiple ova.
• Step 2: The eggs (ova) are retrieved from the ovaries and fertilized with sperm in a laboratory setting.
• Step 3: The fertilized egg (zygote) undergoes early embryonic development in vitro (outside the body).
• Step 4: The resulting embryo is then implanted into the uterus for further development.

It is known as the test tube baby programme because fertilization and early embryonic development occur outside the female body in a laboratory setting, commonly in a petri dish or test tube.

Solution: S.L. Miller's experiment was significant because it simulated the conditions of early Earth and demonstrated that organic molecules, including amino acids, could be synthesized from inorganic compounds under primitive conditions. This experiment supported the idea of abiogenesis, the origin of life from non-living matter.

The hypothesis prompting the experiment was proposed by Alexander Oparin and J.B.S. Haldane, who suggested that life originated from simple molecules under reducing conditions.

Solution: Meteorite analysis has shown that many organic compounds, including amino acids, are present in meteorites. This supports the hypothesis that organic molecules could have formed in space and later arrived on Earth, potentially aiding in the origin of life.

Solution: The picture shows a cellular structure at a very high magnification. It is likely a microscopic image viewed under a Transmission Electron Microscope (TEM).

Solution: The dark spots in the image represent ribosomes or dense areas of the cell that are involved in protein synthesis. These structures appear darker in electron microscopy because they absorb more electrons due to their dense, compact structure.

Solution: In humans, color blindness is a sex-linked recessive trait carried on the X chromosome. For a male to be color blind, he must inherit the X chromosome carrying the color blindness allele from his mother (because males have only one X chromosome). A female would need to inherit two copies of the color blindness allele (one from each parent) to be color blind.

Cross:

• Father (color blind): X^cY
• Mother (normal vision): X^NX^N

Father: X^cY Mother: X^NX^N

Possible offspring:

• Daughter: X^NX^c (Carrier, not color blind)
• Son: X^cY (Color blind)

Thus, the daughter is a carrier (X^NX^c) and does not have the disease, while the son inherits color blindness from his mother (X^cY), not from his father.

Conclusion: The son inherited the disease from the mother, not the father. The daughter, being a carrier, did not inherit the disease from the father but can pass the gene on to her offspring.

Solution: In the diagram:

• A represents the Lymph nodes.
• B represents the Lymphatic vessels.

The role of each in the immune system is as follows:

Lymph nodes (A): Lymph nodes are small, bean-shaped structures that filter lymph (a fluid that circulates in the lymphatic system) and trap foreign particles like bacteria, viruses, and cancer cells. They contain immune cells, such as lymphocytes, that help in recognizing and attacking harmful pathogens.

Lymphatic vessels (B): These vessels are part of the lymphatic system, which transports lymph, immune cells, and waste products throughout the body. The lymphatic vessels also transport the filtered lymph back to the bloodstream, thus playing an important role in immune surveillance and response.

Solution: The orchid Ophrys ensures pollination by a specific species of bee through a mechanism called sexual deception. The flower of Ophrys mimics the appearance, scent, and texture of a female bee, attracting the male bee of a particular species. When the male bee attempts to mate with the flower, it brushes against the flower's anthers and picks up pollen, which is transferred to another flower, facilitating pollination.

Solution: Co-evolution refers to the reciprocal evolutionary change in interacting species. In the case of Ophrys and the specific bee species, both the orchid and the bee have evolved in response to each other. The orchid has evolved to mimic the female bee, attracting the male for pollination, while the male bee evolves to recognize the orchid as a mate, thus ensuring pollination. This mutual evolution benefits both species.

Solution: Contraceptive pills work primarily by:

• Inhibiting ovulation: The hormones in the pill prevent the release of eggs from the ovaries.
• Thickening cervical mucus: This makes it more difficult for sperm to enter the uterus.
• Changing the uterine lining: This prevents implantation of a fertilized egg.

Schedule: The pill is typically taken daily for 21 days, followed by a 7-day break during which menstruation occurs. A new pack is started after the break.

Solution: Transgenic animals are animals that have been genetically modified to carry a foreign gene, which is incorporated into their DNA. These genes can be from different species and are typically inserted to study gene function or to produce useful substances like pharmaceuticals.

Solution: The first transgenic cow was Rosie, genetically engineered to produce milk containing human protein. This was an important breakthrough for biopharmaceuticals, as it demonstrated the potential of animals to produce therapeutic proteins.

Solution: EcoRI is named using the following convention:

• E is the first letter of the genus Escherichia.
• co refers to the species coli.
• R indicates that it was isolated from strain R of Escherichia coli.
• I denotes that it is the first enzyme identified from this strain.

Solution: EcoRI cuts DNA at the specific recognition site 5'-GAATTC-3', producing sticky ends. The enzyme cuts between the G and the A on both strands of the DNA molecule, resulting in overhanging ends that can be used for further recombination or ligation.

EcoRI cuts between G and A:

5' G | AATTC 3'
3' CTTAA | G 5'

Sticky ends:

5'G AATTC 3'
3'CTTAA G 5'

Solution: Region III has maximum species richness due to:

1. Favourable climatic conditions such as tropical climate with high rainfall and temperature.
2. High resource availability and ecological niches supporting a large number of species.

Solution: Region I has minimum species richness because:

1. It likely represents a cold or arid climate, which is less suitable for supporting diverse life forms.
2. Limited resource availability and harsh environmental conditions restrict the number of species.

Solution: This uneven distribution of species diversity is referred to as Beta Diversity. It reflects the variation in species composition between different ecosystems or regions.

Solution:

1. Prokaryotes lack morphological diversity, making species identification challenging.
2. Many prokaryotic species cannot be cultured in laboratories, limiting data collection.
3. Molecular techniques like DNA sequencing are required to identify them, which is time-consuming and resource-intensive.

Solution: The transformed colony is the one that grows on Plate M (with ampicillin) but does not grow on Plate N (with both ampicillin and tetracycline).

Justification: The recombinant DNA disrupts the tetracycline resistance gene, making the bacteria sensitive to tetracycline, but retains ampicillin resistance, allowing growth on Plate M.

Solution: The sites are called selectable marker sites.

Role: These genes help in identifying and selecting transformed cells:

• Ampicillin resistance gene ensures growth on ampicillin-containing media.
• Tetracycline resistance gene helps distinguish recombinant bacteria (tetracycline-sensitive) from non-recombinant ones.

Solution:

1. Restriction endonucleases: Cut DNA at specific sequences.
2. DNA ligase: Joins DNA fragments to form recombinant DNA.

Solution: Insertional inactivation of the β-galactosidase gene prevents the production of the enzyme, resulting in white colonies (instead of blue) on media with X-gal. This helps identify recombinant bacteria.

Solution: The diagram shows the blastocyst, which is the stage of the embryo that gets implanted in the uterine lining during pregnancy. The blastocyst has the following structure:

• Inner cell mass (which will develop into the embryo).
• Trophoblast (outer layer that forms the placenta).
• Blastocoel (fluid-filled cavity).

Solution: After implantation, the embryo undergoes several developmental stages:

1. Placenta Formation: The trophoblast layer of the blastocyst grows and invades the uterine lining, forming the placenta, which allows nutrient and gas exchange between mother and embryo.
2. Formation of Three Germ Layers: The inner cell mass undergoes gastrulation, forming three germ layers:
   • Ectoderm: Forms the skin and nervous system.
   • Mesoderm: Forms muscles, bones, and the circulatory system.
   • Endoderm: Forms the digestive and respiratory systems.

Solution:

1. Placenta: The placenta provides nutrients and oxygen to the developing fetus, removes waste products, and produces hormones to support pregnancy.
2. Three Germ Layers: These layers give rise to all the organs and tissues of the body, forming the basis for the development of the fetus.

Solution: Viola and Oxalis are examples of plants that exhibit self-pollination, which ensures that seeds are formed even without pollinators. These plants have both male and female reproductive organs in the same flower, and pollen can fertilize the ovule within the same flower or between flowers of the same plant.

Solution:

1. Protection of the embryo: Seeds protect the embryo from harsh environmental conditions.
2. Dispersal: Seeds can be dispersed to new areas, helping the plant species spread.
3. Dormancy: Seeds can remain dormant for long periods, allowing them to germinate when conditions are favorable.
4. Genetic variation: Seeds result from fertilization, leading to genetic variation, which is important for evolution and adaptation.

Solution:

1. Zygote: The fertilized egg cell that develops into the embryo.
2. Endosperm: Provides nourishment to the developing embryo.
3. Integuments: Surround the embryo sac and later form the seed coat.
4. Micropyle: The opening through which the pollen tube enters to deliver sperm to the ovule.

Solution: The property that is present in normal cells but is lost in cancer cells is contact inhibition. In normal cells, when they come into contact with each other, they stop dividing. This prevents uncontrolled cell growth and maintains tissue structure. In cancer cells, contact inhibition is lost, leading to uncontrolled cell division and tumor formation.

Solution: The genes that may become cancerous under certain conditions are called oncogenes and tumor suppressor genes.

• Oncogenes: These are mutated forms of proto-oncogenes (normal genes that regulate cell growth). When they mutate, they cause excessive cell growth and division.
• Tumor suppressor genes: These genes regulate the repair of damaged DNA and control cell division. When these genes are inactivated or mutated, they fail to stop uncontrolled cell division, leading to cancer.

Solution:

1. Biopsy and Histopathology: A biopsy involves taking a tissue sample from the suspected area to examine under a microscope. Histopathology helps in identifying abnormal or cancerous cells in the tissue.
2. Magnetic Resonance Imaging (MRI): MRI uses magnetic fields and radio waves to create detailed images of internal organs. It is used to detect tumors, their size, and location, helping in cancer diagnosis.

Solution: The primary effluent is passed into large aeration tanks to introduce oxygen into the water. The oxygen promotes the growth of aerobic microorganisms that decompose organic matter, reducing the pollution load of the effluent.

Solution: The 'sediment' formed in the settling tanks is called sludge. It consists of organic matter, microorganisms, and debris that settle down after the aeration process. Sludge is important because it can be further processed to recover valuable materials or treated for safe disposal.

Solution: In the final step, the effluent in the settling tank undergoes a process where it is allowed to settle, and the remaining solid particles (sludge) are removed. The treated effluent is then passed through filtration to remove any remaining impurities before being released into water bodies.

Solution: Two organisms commonly used as biofertilizers are:

• Rhizobium (Kingdom: Bacteria): Rhizobium bacteria form symbiotic relationships with leguminous plants by fixing atmospheric nitrogen into a form that plants can use for growth.
• Mycorrhiza (Kingdom: Fungi): Mycorrhiza fungi form symbiotic relationships with plant roots, enhancing nutrient uptake, especially phosphorus, and improving plant growth.

Solution: The scientist who demonstrated the stability of genetic material through early experiments was Gregor Mendel. His famous experiments involved crossing pea plants to study inheritance patterns.

Experiment: Mendel performed monohybrid and dihybrid crosses using pea plants, focusing on traits such as plant height and flower color. In his monohybrid cross, he crossed a tall plant (T) with a short plant (t) and observed that the F1 generation had all tall plants. When F1 plants were self-pollinated, the F2 generation exhibited a 3:1 ratio of tall to short plants.

Conclusion: Mendel concluded that inheritance follows specific ratios (such as 3:1 for a monohybrid cross), and that genetic material is inherited in a stable manner, maintaining the ratio generation after generation. This laid the foundation for the laws of inheritance (Mendel's Laws).

Solution: Let the tallness (T) be dominant over shortness (t), and violet color (V) be dominant over white color (v). The given pea plant is tall with violet flowers, but its genotype is unknown.

Cross 1: Selfing Tall Violet (T? V?) - Assume the genotype of the plant is heterozygous for both traits: TtVv.

Parental Cross: TtVv × TtVv

The Punnett square for this cross gives the following genotypic ratio:

Genotypic ratio: 1 TTVV : 2TtVV : 2TTVv : 4TtVv : 1ttVV : 2ttVv : 1 Ttvv : 1 ttvv

Phenotypic ratio:

Tall, Violet: 15 : Short, Violet: 3