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

Solution:
- Apomixis is a unique process where embryos form without fertilization.
- In species like Mango, apomictic reproduction occurs, allowing seeds to develop from somatic or gametophytic cells.
- This ensures genetic consistency as the offspring are genetically identical to the parent plant.

Solution:
- The theory of Panspermia suggests that life is distributed across the universe via celestial objects like comets or meteoroids.
- According to this hypothesis, life on Earth may have originated from spores or microscopic life forms that traveled through space and reached Earth.
- These life-bearing entities could survive harsh cosmic conditions and initiated life on Earth upon finding suitable conditions.

Solution:
- In apomictic reproduction, embryos may arise from nucellus (somatic cells) or gametophytic tissue.
- Nucellus cells are somatic and hence diploid (2n). Thus, apomictic embryos from nucellus cells have a ploidy of 2n.
- Antipodal cells are part of the female gametophyte, which is haploid (n) due to their origin from meiotic division.
- Therefore, the ploidy of embryos is 2n for nucellus-derived and n for antipodal-derived embryos.

Solution:
- Adenine (A) pairs with Thymine (T) via 2 hydrogen bonds, while Guanine (G) pairs with Cytosine (C) via 3 hydrogen bonds.
- Given: Total nucleotides = 3000, Guanine (G) = 160.
- Cytosine (C) = 160 (as G pairs with C).
- Remaining nucleotides form A-T pairs:
A + T = Total nucleotides - (G + C) = 3000 - (160 + 160) = 2680.
- Thus, the number of bases forming double hydrogen bonds is 2680.

Solution:
- During industrialization, tree trunks darkened due to soot, favoring the camouflage of dark-winged moths.
- As a result, dark-winged moths were less visible to predators and their numbers increased.
- In contrast, white-winged moths became more visible to predators, leading to a decline in their population.
- This phenomenon demonstrates natural selection driven by environmental changes.

Solution:
- Down's syndrome is caused by trisomy of chromosome 21, meaning there are three copies of this chromosome instead of two.
- The disorder is characterized by:
- Short stature and small head size.
- A furrowed tongue and partially open mouth.
- Cognitive impairments and developmental delays.
- This condition arises due to nondisjunction during meiosis.

Solution:
- Snapdragon plants exhibit incomplete dominance, where the heterozygous genotype results in an intermediate phenotype.
- A pink flowered plant has the genotype Rr, while a white flowered plant has the genotype rr.
- The cross is represented as:
Rr x rr → Gametes: R, r and r, r
F1 Progeny: 50%Rr(Pink) and 50%rr(White).

Solution:
- Haemophilus influenzae is a bacterial pathogen responsible for pneumonia, an infection of the alveoli in the lungs.
- Symptoms include: - Fever, chills, and cough.
- Cyanosis, which causes bluish lips and fingernails due to reduced oxygen levels.
- Additional symptoms like headache and difficulty breathing.

Solution:
- Stanley Cohen and Herbert Boyer pioneered recombinant DNA technology by linking the antibiotic resistance gene to the plasmid vector.
- DNA ligase was the enzyme that joined the DNA fragments by forming phosphodiester bonds between the sugar and phosphate groups.
- This experiment laid the foundation for genetic engineering and the creation of recombinant DNA molecules.

Solution:
- In the first generation, all DNA will be hybrid (one strand ¹⁴N, one strand ¹⁵N).
- With successive generations in the ¹⁵N medium, the proportion of hybrid DNA decreases, while heavy DNA (¹⁵N) increases.
- After six generations, the DNA bands will show a ratio of 1:31 for hybrid to heavy DNA.
- This result is due to the exponential replication of ¹⁵N-labeled DNA in the absence of ¹⁴N.

Solution:
- In an ideal environment with unlimited resources, population growth follows an exponential pattern.
- The population size increases at a constant rate, represented mathematically as: Nt = N₀e^rt
where Nt is the population at time t, N0 is the initial population, and r is the intrinsic growth rate.
- This results in a J-shaped curve, where the population doubles at regular intervals.
- Such conditions rarely occur in nature due to limiting factors like food, space, and predation.

Solution:
- PCR (Polymerase Chain Reaction) is used to amplify a specific segment of DNA.
- Primers are short nucleotide sequences designed to bind to opposite strands of the DNA template in a complementary manner.
- The binding occurs at the 3' ends of the primers, allowing DNA polymerase to extend the sequence in the 5' to 3' direction.
- This ensures amplification of the target DNA region between the primer binding sites.

Solution:
- The zygote in angiosperms develops into a heart-shaped embryo before transitioning into the proembryo stage. This is a key stage of embryogenesis.
- The zygote is indeed located at the micropylar end of the embryo sac, but this does not explain the heart-shaped stage.
- The heart-shaped embryo results from cell differentiation and tissue patterning during embryogenesis.
- Thus, both statement (A) is false but (R) is true

Solution:
- In a stirred-tank bioreactor, the stirrer facilitates the even distribution of oxygen and nutrients throughout the culture medium.
- The flat base of a bioreactor provides structural stability, but it has no role in oxygen mixing.
- Thus, while the assertion (A) is correct, the reason (R) is incorrect.

Solution:
- Eukaryotic primary transcripts are processed to remove non-functional regions called introns. This process is called splicing.
- The primary transcript consists of both exons (functional coding regions) and introns (non-functional sequences).
- Splicing ensures that only exons remain in the mature mRNA, which is then translated into proteins.
- The assertion (A) is correct as splicing removes introns, and the reason (R) correctly explains the presence of both exons and introns in the primary transcript.

Solution:
- Chronic alcohol consumption damages liver cells, leading to cirrhosis, a condition where the liver is scarred and its function is impaired.
- While alcohol addiction may cause mental and financial distress to families, this is unrelated to the physiological process of cirrhosis development.
- Thus, both the assertion and reason are correct, but the reason is not the correct explanation of the assertion.

Solution:
- Amniocentesis is a prenatal diagnostic technique used to detect genetic disorders and chromosomal abnormalities in the fetus.
- However, it is banned in many countries due to its misuse for determining the sex of the fetus, which may lead to:
1. Female feticide, causing a decline in the female-to-male ratio.
2. Ethical concerns regarding the deliberate termination of pregnancies based on the fetus's sex.
3. Social and cultural issues promoting gender discrimination.

Solution:
- Set-up B. DNA fragments are negatively charged and migrate toward the positive electrode (anode) under an electric field. Set-up B correctly places the positive electrode opposite the sample, ensuring proper separation.

Solution:
- Band II contains longer DNA fragments. Larger molecules move more slowly through the gel matrix compared to smaller fragments, causing them to remain closer to the loading well.

Solution:
- After substitution, the sequence changes to: 5'-UCAUU AACCC AGACCUUCUU AAAAGGA-3'.
- The 5th codon changes from UCU (Serine) to CCU (Proline). The total number of codons encoding amino acids remains the same.
- Amino acids = Total codons – Stop codon = 8.

Solution:
- The sequence changes to: 5′-UCAUU AAUCC AGAUCUUCUU AAAAGGA-3'.
- This causes the fourth codon to become UAA, a premature stop codon, halting translation.
- The polypeptide chain will be shorter as translation stops earlier.

Solution:
- Humus is a dark, organic material formed by the decomposition of plant and animal matter.
Key features include:
1. Rich in nutrients: Humus provides essential nutrients for plant growth.
2. Improves soil fertility: Enhances water retention and microbial activity.
3. Resistant to decomposition: Remains in the soil for a long time, supporting ecosystem stability.

Solution: The graph shows a logarithmic relationship, with species richness increasing with area.

Solution: S = cA^z, where:
• S = Species richness.
• A = Area.
• c = Constant.
• z = Slope of the line (logarithmic).

Solution:
- A vaccine is a biological preparation that provides active immunity against specific diseases.
- It contains inactivated or weakened pathogens/antigens that stimulate the immune system to:
1. Recognize and neutralize pathogens upon future exposure.
2. Produce memory cells for long-term immunity.

Solution:
The transverse section (T.S.) of a mature anther includes the following wall layers:
• Epidermis: The outermost layer that provides mechanical protection.
• Endothecium: Helps in the dehiscence of the anther to release pollen grains.
• Tapetum: Nourishes developing pollen grains by supplying nutrients and enzymes.

Solution:
Given: Recessive trait frequency (q²) = 9% = 0.09.
q² = 0.09 => q = √0.09 = 0.3.
p + q = 1 => p = 1 – 0.3 = 0.7.
Homozygous dominant (AA) frequency: p² = (0.7)² = 0.49 = 49%.
Heterozygous dominant (Aa) frequency: 2pq = 2(0.7)(0.3) = 0.42 = 42%.

Solution:
The mechanism is Natural Selection. Brown snakes had better camouflage in the desert environment, leading to higher survival and reproductive success, whereas grey snakes were more visible to predators. Over generations, this selection led to an increased frequency of the brown allele.

Solution:
• Cow-dung contains methanogenic bacteria that are highly efficient in producing biogas.
• It is readily available in rural areas and environmentally sustainable as a resource.

Solution:
Unspent slurry is used as a nutrient-rich organic fertilizer, improving soil fertility and supporting sustainable agriculture.

Solution:
• (i) Myocardial infarction: Streptokinase (source: *Streptococcus*).
• (ii) High blood cholesterol level: Statins (source: *Monascus purpureus*).
• (iii) Organ transplantation: Cyclosporine A (source: *Trichoderma polysporum*).

Solution:
(a) When the process is initiated.
• Spermatogenesis: Begins at puberty.
• Oogenesis: Begins during fetal development.

(b) Number of functional gametes produced per primary spermatocyte/oocyte.
• Spermatogenesis: Produces four functional sperm from one primary spermatocyte.
• Oogenesis: Produces one ovum and two polar bodies from one primary oocyte.

(c) Specific site at which meiosis II is completed.
• Spermatogenesis: Completed in the seminiferous tubules of the testes.
• Oogenesis: Completed in the fallopian tubes after fertilization.

Solution:
(a) Cross: Violet, axial × White, axial
Genotype of parents: *VvAa* × *vvAa*
Explanation:
• 6/16 Violet, axial: *VvAa*.
• 2/16 White, terminal: *vvAa*.

(b) Cross: Violet, axial × White, terminal
Genotype of parents: *VvAa* × *vvaa*
Explanation:
• 1/4 Violet, axial: *VvAa*.
• 1/4 Violet, terminal: *Vvaa*.
• 1/4 White, axial: *vvAa*.
• 1/4 White, terminal: *vvaa*.

(c) Cross: Violet, axial × Violet, axial
Genotype of parents: *VvAa* × *VvAa*
Explanation:
• 3/4 Violet, axial: *V_Aa*.
• 1/4 White, axial: *vvAa*.

Solution:
Roles of predation:
• Maintains species diversity: Predators control prey populations, preventing any one species from dominating. Example: Tigers preying on herbivores in forests.
• Regulates ecosystem balance: Predators prevent overgrazing by herbivores, maintaining a balance between plant and animal populations. Example: Wolves controlling deer populations in Yellowstone National Park.
• Promotes natural selection: Predation drives evolutionary adaptations in prey species to develop defense mechanisms. Example: Camouflage in stick insects and moths.

Solution:
*Bacillus thuringiensis* (*Bt*)

Solution:
GM cotton incorporates a gene from *Bacillus thuringiensis* to resist insect attacks:
• The *Bt* gene produces a toxin that becomes active in the alkaline gut of bollworms.
• The toxin binds to gut receptors, causing cell lysis, leading to the death of the insect.

Solution:
The sequence contains three *Alul* recognition sites. DNA fragments formed:
5'C-C-G-T-A-G-C-T-A-T-C-A-G-C-T-G-G 3'
3'G-G-C-A-T-C-G-A-T-A-G-T-C-G-A-C-C 5'
• 5'-CCGG-3'
• 5'-ATCCTG-3'
• 5'-CGAT-3'

Solution:
*BamHI* is preferred because it generates sticky ends, which facilitate the joining of DNA fragments to form recombinant DNA.

Solution:
Restriction enzymes cut DNA at specific recognition sites, which are typically palindromic sequences. These sequences enable the enzyme to cleave at precise locations, generating sticky or blunt ends.
• The specific term used for these sequences is recognition sequence.

Solution:
*EcoRI* recognises the palindromic DNA sequence 5'-GAATTC-3' and cleaves it between G and A, producing sticky ends.

Solution:
The broad base of Rwanda's age pyramid indicates:
• High birth rate: A significant proportion of the population is in the younger age groups (0-14 years).
• Rapid population growth: As a developing nation, Rwanda has a higher fertility rate with more children being born than individuals dying.

Solution:
Sweden's age pyramid indicates:
• Stable population: The population growth rate is low due to effective family planning and healthcare.
• Low birth and death rates: Nearly equal proportions of individuals in all age groups indicate balanced demographic growth.

Solution:
Stationary age pyramid.

Solution:
Expanding age pyramid.

Solution:
Codominance occurs when both alleles in a heterozygous state are fully expressed without blending.
• Example: ABO blood group inheritance.
• The ABO blood group system involves three alleles: I^A, I^B, and i.
• The I^A and I^B alleles are codominant, while i is recessive.
• A person with the genotype I^AI^B will express both alleles equally, resulting in the AB blood group.

Solution:
The *lac operon* is switched ‘on' in the presence of lactose:
• Lactose acts as an inducer by binding to the repressor protein, changing its conformation.
• The altered repressor cannot bind to the operator region of the operon.
• This allows RNA polymerase to bind to the promoter region and transcribe the structural genes (*lacZ*, *lacY*, *lacA*).
• The enzymes produced metabolize lactose, enabling its utilization as an energy source.

Solution:
The *lac operon* is negatively regulated because:
• In the absence of lactose, the repressor protein binds to the operator region, preventing RNA polymerase from transcribing the structural genes.
• This ensures that the operon is inactive when lactose is unavailable, conserving energy.

Solution:
The life cycle of *Plasmodium*:
• Sporozoites enter the human bloodstream through a mosquito bite and travel to the liver.
• In liver cells, sporozoites multiply to form merozoites, which are released into the bloodstream.
• Merozoites infect red blood cells, causing them to rupture. This leads to malaria symptoms like fever and chills.
• Some merozoites develop into gametocytes, which circulate in the blood.
• When a mosquito bites an infected person, it ingests gametocytes, continuing the cycle.

Solution:
• Gametogenesis: Gametocytes develop into male and female gametes inside the mosquito.
• Sporozoite Formation: The zygote develops into sporozoites, which migrate to the salivary glands, ready to infect the next host.

Solution:
• Malignant Tumor:
- Malignant tumors are cancerous and invade nearby tissues, causing significant damage.
- They have the ability to metastasize, spreading to other parts of the body via blood or lymphatic systems.

• Benign Tumor:
- Benign tumors are non-cancerous and grow at a slower rate, staying confined to their original location.
- They do not invade surrounding tissues or metastasize to distant parts of the body.

Solution:
Three widely used diagnostic techniques for cancer detection are:
• Biopsy: Involves taking a small sample of tissue from the affected area, which is then analyzed under a microscope to check for cancerous cells.
• Imaging Techniques:
- X-rays: Effective for identifying abnormalities in bones or certain organs.
- MRI (*Magnetic Resonance Imaging*): Produces detailed cross-sectional images of soft tissues, helping to locate and assess tumors.
• Blood Tests: Used to detect tumor markers, which are specific proteins or substances produced by cancer cells.

Solution:
Flowering plants have evolved the following mechanisms to promote cross-pollination:
• Dichogamy: The male and female reproductive parts of a flower mature at different times to prevent self-pollination. For example, protandry (male matures first) in sunflower.
• Self-incompatibility: A genetic mechanism prevents the germination of self-pollen on the stigma, ensuring cross-pollination. Example: *Brassica* species.
• Herkogamy: Structural adaptation in which male and female organs are spatially separated, making self-pollination unlikely. Example: *Hibiscus*.
• Monoecy and Dioecy: In monoecious plants, male and female flowers are present on the same plant (e.g., maize), while in dioecious plants, they occur on separate plants (e.g., papaya).

Solution:
Plants discourage self-pollination because it leads to inbreeding depression, which reduces genetic variation and adaptability, making the population more susceptible to diseases and environmental changes.

Solution:
(i) Follicular phase/proliferative phase
• Ovarian events: A primary follicle develops into a mature Graafian follicle in the ovary.
• Uterine events: The endometrium of the uterus regenerates and thickens in preparation for possible implantation.
• Hormones:
- FSH (Follicle Stimulating Hormone): Promotes follicular growth and maturation.
- Estrogen: Secreted by the growing follicle, aiding in endometrial repair and stimulating LH secretion.

(ii) Luteal phase/secretory phase
• Ovarian events: The ruptured follicle transforms into the corpus luteum, which secretes progesterone.
• Uterine events: The endometrium thickens further, becoming more vascularized and glandular to prepare for embryo implantation.
• Hormones:
- Progesterone: Maintains the thickened endometrium and inhibits FSH and LH secretion.
- Estrogen: Supports the uterine lining.

(iii) Menstrual phase
• Ovarian events: If fertilization does not occur, the corpus luteum degenerates into the corpus albicans.
• Uterine events: The thickened endometrium breaks down, leading to menstrual bleeding.
• Hormones:
- Progesterone and Estrogen: Their levels decline, causing the breakdown of the uterine lining.
- FSH secretion increases: This initiates the next cycle by stimulating the growth of new follicles.