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

Solution: Plants in which both male and female flowers are borne on the same plant are called monoecious plants. Maize is a monoecious plant. Geitonogamy occurs when pollen grains are transferred between flowers on the same plant. Xenogamy occurs when pollen is transferred between flowers of different plants. Maize shows both these modes of pollination.

Solution: Human Chorionic Gonadotropin (hCG) is a hormone secreted by the placenta. It supports pregnancy by maintaining the corpus luteum and the production of progesterone during early pregnancy. Relaxin is produced in the later stages of pregnancy to relax the pelvic muscles. Oxytocin is responsible for uterine contractions during labor. Human Placental Lactogen supports fetal growth and metabolism but does not directly maintain pregnancy.

Solution: Periodic abstinence involves avoiding sexual intercourse during the fertile period of the menstrual cycle to prevent pregnancy. The fertile period typically falls around ovulation, which occurs approximately on Day 14 in a 28-day cycle. To account for variations in ovulation, abstinence is advised from Day 10 to 17 when the chances of fertilization are highest.

Solution: (A) 45 + XX: Refers to Down Syndrome, characterized by physical features like broad palms and creased palms.
(B) 44 + XXY: Incorrect. This karyotype corresponds to Klinefelter Syndrome, where males exhibit partial masculine development, not feminine development.
(C) 44 + XO: Refers to Turner Syndrome, where females are sterile due to rudimentary ovaries.
(D) 44 + XY: Refers to normal males.

Solution: To determine the minimum number of base combinations required to code for 96 different amino acids, we can use the formula for the number of possible combinations:
Number of combinations = (Number of bases)^{Number of bases per codon}

For the given case: We know that there are 12 different types of bases in the DNA. We need to determine how many bases per codon are required to create at least 96 different amino acids.
Let n be the number of bases required per codon. The number of possible codons will be 12ⁿ, and we want this to be at least 96.
12ⁿ > 96

Now we calculate the powers of 12:
12¹ = 12
12² = 144

Thus, 12² = 144 is greater than 96, so the minimum number of bases per codon required is 2.

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: Mast cells play a key role in allergic reactions. They release histamine and other chemicals during allergic responses, leading to symptoms like itching, swelling, and redness. Diseases like typhoid, ascariasis, or AIDS are not related to mast cell activation.

Solution: Lactobacillus is a heterotrophic bacterium as it relies on organic substances (milk) for energy. It converts lactose in milk to lactic acid, setting the milk into curd.

Solution: Transgenic mice are genetically modified to express human genes. They are widely used for testing the safety and efficacy of vaccines, including the polio vaccine.

Solution: Hind II recognizes specific palindromic sequences in DNA consisting of 6 base pairs and cleaves precisely at this point.

Solution: Golden rice is genetically modified to biosynthesize beta-carotene, a precursor of vitamin A, in the endosperm, addressing vitamin A deficiency.

Solution:
• Gross primary productivity (GPP): Total energy captured by autotrophs.
• Secondary productivity: Biomass formed by consumers from the organic matter consumed.

Solution: The laws of India permit legal adoption, making it one of the best options for childless couples. However, emotional, religious, and social factors often act as deterrents to the adoption of children, especially in conservative settings.

Solution: Linked genes are inherited together because they are located on the same chromosome. They do not follow Mendel's law of independent assortment, which is why the dihybrid F₂ ratio 9:3:3:1 is not observed.

Solution: Agrobacterium tumefaciens is primarily a pathogen of dicot plants, not monocots. However, it does deliver T-DNA into the host plant genome, causing the formation of crown gall tumors.

Solution: The Genetic Engineering Appraisal Committee (GEAC) evaluates and monitors genetically modified organisms (GMOs) for research and release. However, genetic modification can significantly affect ecosystems when GMOs are introduced due to possible crossbreeding or ecological imbalance.

Solution:
• Pericarp (A): The fruit wall derived from the ovary wall.
• Scutellum (B): A specialized cotyledon found in monocot seeds (grasses).
• Embryonal axis (C): Includes the plumule (shoot) and radicle (root).
• Remains of nucellus (D): Called the perisperm in mature seeds.

Solution:
Naturalist: Jean-Baptiste Lamarck

Solution: Lamarck explained evolution through the theory of Use and Disuse and Inheritance of Acquired Characteristics. He suggested that giraffes evolved long necks by stretching to reach tall trees for food. This acquired trait was passed on to subsequent generations.

Solution:
Basic Steps in ART Programme:
1. Retrieval of eggs from the female and collection of sperm from the male.
2. Fertilization of the egg and sperm outside the body in a laboratory dish (in vitro fertilization).
3. Development of the fertilized egg (embryo) in a controlled environment.
4. Transfer of the embryo into the uterus for implantation and pregnancy.

Reason for Test Tube Baby Programme: The term “test tube baby" refers to the process of fertilization occurring outside the human body, typically in a laboratory setup, rather than a test tube.

Solution:
(i) The doctor injected anti-venom serum. This contains pre-formed antibodies that neutralize the snake venom, providing immediate relief and preventing further damage.
(ii) The immunity provided by this injection is passive immunity, as the antibodies are directly administered rather than being produced by the body.

Solution: Organic farmers discourage complete eradication of insect pests because:
1. Ecological Balance: Insects play a role in the food chain and biodiversity. Their complete eradication can disrupt the balance.
2. Natural Pest Control: Some pests attract their predators, which naturally control pest populations, reducing the need for chemical interventions.

Solution:
• Level A: Zooplankton (Primary Consumers)
• Level B: Phytoplankton (Primary Producers)

Type of Pyramid: Inverted Pyramid of Biomass.

Ecosystem: Found in an aquatic ecosystem.

In such ecosystems, the biomass of primary producers (phytoplankton) is lower than that of primary consumers (zooplankton) at any given time. This is because phytoplankton reproduce rapidly and are consumed quickly by zooplankton.

Solution:
Mode of Action:
1. Contraceptive pills contain synthetic hormones (estrogen and progesterone) that prevent ovulation by inhibiting the secretion of FSH and LH from the pituitary gland.
2. They thicken cervical mucus, making it difficult for sperm to enter the uterus.
3. They alter the endometrial lining, preventing implantation of a fertilized egg.

Schedule for Effective Outcome:
• Pills should be taken daily for 21 days starting from the 5th day of the menstrual cycle, followed by a 7-day gap during which withdrawal bleeding occurs.

Solution:
• Type of DNA: Short Tandem Repeats (STRs) or Variable Number Tandem Repeats (VNTRs).
• Characteristics:
1. They are non-coding, repetitive sequences of DNA.
2. They show high polymorphism, making them unique to each individual (except identical twins).

Solution:
1. Forensic Science: Used for identification in criminal investigations.
2. Paternity Testing: Determines biological relationships.

Solution:
Significance of Miller's Experiment:
• Demonstrated that organic molecules like amino acids could be synthesized abiotically under conditions resembling early Earth's atmosphere.
• Supported the theory that life originated from simple organic compounds.
• Scientists: The experiment was based on the hypothesis of Alexander Oparin and J.B.S. Haldane.

Solution: Meteorite analysis revealed the presence of simple organic compounds, such as amino acids, on extraterrestrial objects. This suggests that organic molecules can form naturally, supporting the idea of abiotic synthesis.

Solution:
• Pneumonia symptoms: A include breathing difficulty, cough, and chest pain.
• Typhoid causative organism: B is Salmonella typhi.
• Common cold symptoms: C include nasal congestion, sore throat, and cough.
• Ringworm causative organism: D is Trichophyton.
• Ascariasis symptoms: E include internal bleeding, muscular pain, and anemia.
• F: Disease caused by Entamoeba histolytica is Amoebiasis.

Solution: Colour blindness is an X-linked recessive disorder. Males inherit the X chromosome from their mother, while females inherit one X chromosome from each parent. The daughter inherits the X^c chromosome from her father and X^C (normal) from her mother, making her a carrier. The son cannot inherit X^c from the father as he receives only the Y chromosome from him.

Cross:
Father (X^cY) × Mother (X^CX^C)
Gametes: X^c, Y (Father) and X^C, X^C (Mother)

Offspring:
Daughter (X^CX^c) (Carrier)
Son (X^CY) (Normal).

Hence, the son and daughter cannot inherit colour blindness from the father as it is X-linked.

Solution: Transgenic animals are those that have had foreign genes deliberately inserted into their genome. These genes are introduced to study gene functions, improve livestock traits, or produce biologically important substances.

Solution: The first transgenic cow is Rosie. Importance: Rosie produced milk enriched with the human protein alpha-lactalbumin, making it nutritionally more balanced for infants.

Solution: (i) EcoRI:
1. "E" = Genus (Escherichia).
2. "co" = Species (coli).
3. "R" = Strain (RY13).
4. "I" = First identified enzyme from this strain.

(ii)

Solution: The orchid Ophrys ensures pollination by mimicking the appearance and scent of a female bee. Male bees, attracted to the orchid, attempt to mate with it (pseudocopulation) and inadvertently carry pollen to other flowers.

Solution: Co-evolution is the process where two or more species influence each other's evolution. In the case of Ophrys and its specific bee species, the orchid evolved to mimic the female bee, while the bee adapted to the orchid's mimicry.

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 male gametophyte in angiosperms develops within the pollen grain:
1. Microspore Formation: Each microspore undergoes mitotic division to form a small generative cell and a larger vegetative cell.
2. Mature Pollen Grain: The generative cell divides mitotically to produce two male gametes, resulting in a three-celled structure (two male gametes + one vegetative cell).

Solution:

Solution:

Solution: Primary follicles are formed during the fetal stage of development in a female.

Solution:
1. At Ovulation: Surge in LH (Luteinizing Hormone) leads to the rupture of the Graafian follicle and release of the ovum.
2. Post-Ovulation: The ruptured follicle transforms into the corpus luteum, which secretes progesterone. Progesterone prepares the endometrium for implantation.
3. If Fertilization Does Not Occur: The corpus luteum degenerates, leading to a drop in progesterone levels. The endometrium breaks down, resulting in menstruation.

Solution:
• Scientist: Frederick Griffith

• Experiment:
1. He worked on Streptococcus pneumoniae (S strain = virulent, R strain = non-virulent).
2. Heat-killed S strain mixed with live R strain transformed R into a virulent S strain.

• Conclusion: A "transforming principle" (later identified as DNA) is responsible for genetic stability and transfer of traits.

Solution:
1. Case 1: Homozygous (TTVV)
• Cross: TTVV × TTVV
• Genotypic ratio: 100% TTVV
• Phenotypic ratio: 100% Tall Violet

2. Case 2: Heterozygous (TtVv)
• Cross: TtVv × TtVv
• Genotypic ratio: 1:2:1 (TT:Tt:tt) for height, 1:2:1 (VV:Vv:vv) for color.
• Phenotypic ratio: 9:3:3:1 (Tall violet: Tall white: Dwarf violet: Dwarf white).

Solution: The property present in normal cells but lost in cancer cells is contact inhibition. Contact inhibition refers to the process where normal cells stop dividing when they come into contact with neighboring cells. This mechanism ensures that cells grow in an organized manner and maintain proper tissue structure. It prevents overgrowth and helps to regulate tissue size. In cancer cells, this property is lost, and they continue to divide uncontrollably, even when they come into contact with other cells. This uncontrolled division results in tumor formation and cancer progression.

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

1. Oncogenes: Oncogenes are mutated forms of normal genes called proto-oncogenes. Proto-oncogenes normally promote cell growth and division. However, when mutated or overexpressed, they become oncogenes, leading to uncontrolled cell division and potentially causing cancer. Example: The Ras gene, when mutated, continuously signals the cell to divide, even when not needed, leading to uncontrolled cell proliferation.

2. Tumor Suppressor Genes: Tumor suppressor genes are responsible for inhibiting cell growth and promoting the repair of damaged DNA or apoptosis (programmed cell death). If these genes are mutated or inactivated, cells can bypass growth control mechanisms and continue to divide. Example: The p53 gene is a critical tumor suppressor gene. When it is mutated, it fails to prevent the division of cells with damaged DNA, which can lead to cancer development.

Solution:
1. Biopsy and Histopathology: A biopsy involves removing a small sample of tissue from a suspected tumor or abnormal growth. The tissue is then examined under a microscope through histopathology. The role of biopsy and histopathology is essential for diagnosing cancer. They help identify the presence of cancerous cells, determine the type of cancer, and assess its stage and grade. This also provides critical information on the malignancy of the tumor and helps plan appropriate treatment options.

2. Magnetic Resonance Imaging (MRI): MRI is a non-invasive imaging technique that uses magnetic fields and radio waves to produce detailed images of the inside of the body. It is especially useful in detecting soft tissue abnormalities such as brain, spinal cord, and breast cancers. The role of MRI in cancer diagnosis includes identifying and locating tumors, determining their size, and assessing whether cancer has spread to other parts of the body (metastasis). It is also used to monitor treatment progress and plan surgeries or radiation therapy.

Solution: Primary effluent is passed into large aeration tanks to increase the oxygen supply to the effluent. This helps in the growth of aerobic microorganisms that break down the organic matter in the sewage, converting it into simpler substances. The process of aeration encourages the growth of these microorganisms, which helps in reducing the biochemical oxygen demand (BOD) of the water, making it less polluting.

Solution: The sediment formed is called sludge. It consists of organic and inorganic matter that settles at the bottom of the settling tank. The significance of sludge is that it contains a high concentration of organic matter, which can be further processed to extract biogas or treated for use as fertilizer. Proper disposal of sludge is crucial for preventing environmental pollution.

Solution: The final step involves the treatment of the effluent in the settling tank, where the remaining suspended particles, including microorganisms, are allowed to settle down. This is followed by the removal of the clear water (supernatant), which is relatively free from pollutants. This treated water is then released into water bodies. Further treatment may be performed, depending on the quality of the effluent.

Solution:
• Rhizobium (Kingdom: Bacteria): Rhizobium is a nitrogen-fixing bacterium that forms a symbiotic relationship with leguminous plants. It helps in fixing atmospheric nitrogen into a form that the plants can use for growth, thereby acting as a natural fertilizer.

• Azotobacter (Kingdom: Bacteria): Azotobacter is a free-living nitrogen-fixing bacterium found in soil. It helps in fixing nitrogen from the atmosphere and making it available to plants, improving soil fertility. Unlike Rhizobium, Azotobacter does not form symbiotic relationships with plants but independently fixes nitrogen.