CBSE Class 12 Biology Question Paper 2024 (Set 2- 57/5/2) Available- Download solution PDF with Answer Key

Solution: The disorder depicted in the pedigree chart follows an autosomal recessive inheritance pattern. This can be inferred from the following observations:
1. The trait skips generations, which is typical for recessive traits.
2. Both males and females are equally affected, indicating an autosomal pattern.
3. The trait is expressed only when an individual inherits two copies of the recessive allele, one from each parent.

Solution:
(a)
• X: Inlet for the slurry (dung mixed with water).
• Y: Gas outlet for biogas collection.
• Z: Outlet for residual slurry (used as manure).

(b) Dung is preferred for biogas production because:
• It contains methanogenic bacteria essential for biogas production.
• It is readily available and provides a continuous supply of organic material for fermentation.

Solution:
DNA follows Chargaff's rule, which states:
• Adenine (A) pairs with Thymine (T), and their percentages are equal.
• Cytosine (C) pairs with Guanine (G), and their percentages are equal.

• If A = 31%, then T = 31%. Thus, A + T = 62%.
• The remaining percentage (100% - 62% = 38%) is shared equally between C and G. Therefore, C = 19%.

Solution (a):
Farmers prefer apomictic seeds because:
• Apomictic seeds maintain hybrid vigor over generations without the need for repeated hybridization.
• These seeds save time and cost as they do not require manual intervention for propagation.

Solution (b):
Advantage: Amniocentesis helps detect genetic disorders in the fetus, enabling early diagnosis.
Disadvantage: The procedure carries a small risk of miscarriage or complications.

Solution:
(a)
• Growth curve ‘A' represents exponential growth, which occurs when resources are unlimited and environmental constraints are absent.
• Growth curve ‘B' represents logistic growth, which occurs when resources are limited, leading to population stabilization at carrying capacity.

(b) 'K' in the graph represents the carrying capacity of the environment, which is the maximum population size that can be sustained indefinitely with available resources.

Solution (a):
The restriction enzyme that recognizes the given sequence is EcoRI. Such sequences are referred to as palindromic sequences, which read the same forward and backward on complementary strands.

Solution (b):
The arrows indicate the sites where the restriction enzyme EcoRI cuts the DNA. The result obtained thereafter is the formation of sticky ends, which are single-stranded overhangs that facilitate the joining of DNA fragments during genetic engineering.

Solution:
(a) Differences between grazing food chain and detritus food chain:
• The grazing food chain begins with green plants (producers) and herbivores, while the detritus food chain starts with decomposed organic matter.
• Energy flow in the grazing food chain is based on photosynthesis, whereas in the detritus food chain it depends on detritus breakdown by decomposers.

(b) The detritus food chain is connected to the grazing food chain because decomposers and detritivores recycle nutrients into the soil, which are then absorbed by plants in the grazing food chain. Example: Earthworms (detritivores) break down organic matter, enriching the soil for plant growth.

Solution:
(a) The number of chromosomes in:
• Zygote: 10 chromosomes (formed by fusion of haploid male and female gametes, each with 10 chromosomes).
• Endosperm: 30 chromosomes (formed by the fusion of two polar nuclei and one male gamete, each contributing 10 chromosomes).

(b) Process:
• Zygote Formation (Syngamy): A haploid male gamete fuses with a haploid female egg cell to form a diploid zygote.
• Endosperm Formation (Double Fertilization): One male gamete fuses with two polar nuclei in the embryo sac, forming a triploid (3n) endosperm.

Solution:
(a) A cell must be made 'competent' to allow the uptake of foreign DNA in biotechnology experiments. Calcium ions help by:
• Creating pores in the cell membrane.
• Neutralizing the negative charge on DNA and the cell membrane, facilitating DNA entry into the cell.

(b) The 'biolistic gun' or gene gun is used to introduce foreign DNA into cells by bombarding them with high-velocity microprojectiles coated with DNA. This method is especially useful for plant cells with rigid cell walls.

Solution:
(a) DNA replication occurs within a replication fork because:
• DNA polymerase can synthesize new strands only in the 5' to 3' direction.
• Replication proceeds progressively as the DNA helix unwinds at the fork.

(b) DNA replication is:
• Continuous on the leading strand: Synthesized in the 5' to 3' direction towards the replication fork.
• Discontinuous on the lagging strand: Synthesized in fragments (Okazaki fragments) away from the replication fork, which are later joined by DNA ligase.

Solution:
(a) Dominance: A dominant allele masks the expression of the recessive allele.
Example: In pea plants, tall (T) is dominant over dwarf (t).

(b) Incomplete Dominance: The heterozygous phenotype is intermediate between the two homozygous phenotypes.
Example: In snapdragons, crossing a red flower (RR) with a white flower (rr) produces pink flowers (Rr).

(c) Co-dominance: Both alleles are expressed simultaneously in the heterozygote.
Example: In human blood groups, the AB blood type shows co-dominance, as both A and B alleles are expressed.

Solution (a):
Tropical regions harbour more species than temperate regions due to:
• Stable Climate: Tropical regions have a consistent climate, allowing year-round reproduction and growth.
• Longer Evolutionary Time: These regions have remained undisturbed for millions of years, promoting species diversification.
• High Productivity: High temperature and sunlight lead to greater plant growth, which supports more species.

Solution (b):
(i) An ecological pyramid represents the trophic structure and function of an ecosystem, illustrating the flow of energy, biomass, or number of organisms across different trophic levels.

(ii)
• Upright Pyramid: The energy pyramid is always upright as energy decreases with each trophic level. Example: Grassland ecosystem.
• Inverted Pyramid: The biomass pyramid in aquatic ecosystems is inverted as the biomass of phytoplankton (producers) is smaller than that of zooplankton (primary consumers).

Solution:
The completed table is as follows:

Solution:
(a)
• (i) In humans, *Plasmodium* enters in the form of sporozoites, which are injected by the female *Anopheles* mosquito during a bite.
• (ii) In the female *Anopheles* body, *Plasmodium* enters in the form of gametocytes, which are taken up with the blood meal from an infected person.

(b) The symptoms of malaria do not appear immediately after the bite because the parasite undergoes an incubation period inside the human liver and blood cells. Symptoms appear 10-14 days later when the parasite multiplies and ruptures red blood cells, releasing toxins. This results in fever, chills, and sweating.

OR

In the female *Anopheles* mosquito, after sucking blood from an infected person:
• The gametocytes of *Plasmodium* develop into sporozoites in the mosquito's gut.
• The sporozoites migrate to the salivary glands of the mosquito, making it ready to infect another human during its next bite.

(c) A mosquito species other than *Anopheles* is *Aedes aegypti*, which carries the pathogen for dengue fever.

Solution:
(a) The hormones responsible for the menstrual cycle at puberty are:
• FSH (Follicle Stimulating Hormone): Secreted by the anterior pituitary gland, stimulates follicle development.
• LH (Luteinizing Hormone): Secreted by the anterior pituitary gland, triggers ovulation and corpus luteum formation.
• Estrogen and Progesterone: Secreted by the ovaries, regulate the uterine lining during the cycle.

(b) Implantation should occur during the secretory phase (days 15-28) of the menstrual cycle. During this phase, the uterine lining (endometrium) is thick and vascularized due to the action of progesterone, creating a suitable environment for embryo implantation.

(c) The hormone responsible for the proliferative phase is Estrogen, secreted by the developing ovarian follicles. It stimulates the regeneration and thickening of the uterine lining (endometrium) after menstruation.

OR

If the released ovum is not fertilized, the corpus luteum degenerates, leading to a drop in progesterone levels. This causes the uterine lining (endometrium) to break down and shed, resulting in menstruation.

Solution (a):
(i) The type of natural selection depicted is directional selection, as one extreme phenotype (dark-winged moth) is favoured over the other (white-winged moth).

(ii) After industrialisation, soot darkened tree trunks. Dark-winged moths were camouflaged and escaped predation, while white-winged moths were easily spotted by predators. This is an example of natural selection.

(iii) Anthropogenic actions like pesticide usage or antibiotic resistance drive evolution. Example: Overuse of antibiotics has led to the evolution of drug-resistant bacteria.

Solution (b):
(i) Hershey and Chase used 35S to label protein and 32P to label DNA to distinguish which molecule entered bacterial cells during viral infection.

(ii)
• Blending: Removed phage particles attached to the bacterial surface.
• Centrifugation: Separated heavier infected bacterial cells from lighter viral components.

(iii) The conclusion was that DNA is the genetic material, as only 32P (DNA) was found inside the bacterial cells.

Solution (a):
(i) Events of Spermatogenesis:
• Multiplication Phase: Spermatogonia (diploid cells) divide mitotically to form primary spermatocytes.
• Growth Phase: Primary spermatocytes grow in size.
• Maturation Phase: Primary spermatocytes undergo meiosis I to form secondary spermatocytes (haploid), which further undergo meiosis II to form spermatids.
• Differentiation Phase: Spermatids differentiate into mature spermatozoa (sperms).

(ii) Role of Hormones in Spermatogenesis:
• FSH (Follicle-Stimulating Hormone): Stimulates spermatogonia to undergo mitosis and supports Sertoli cells.
• LH (Luteinizing Hormone): Stimulates Leydig cells to secrete testosterone.
• Testosterone: Regulates spermatogenesis and promotes the differentiation of spermatids into spermatozoa.

Solution (b):
(i) Development of Megaspore Mother Cell:

(ii) Difference Between Geitonogamy and Xenogamy:
• Geitonogamy: Transfer of pollen grains from the anther of one flower to the stigma of another flower on the same plant.
• Xenogamy: Transfer of pollen grains from the anther of one flower to the stigma of a flower on a different plant.

(iii) Flowers that are invariably autogamous are cleistogamous flowers.

Solution (a):
(i) A schematic diagram of the cloning vector pBR 322 is shown below:

(ii) The 'rop' gene in pBR 322 encodes proteins involved in maintaining the copy number of the plasmid.
(iii) If a cloning vector does not have a selectable marker, it would be difficult to differentiate between transformed and non-transformed cells, complicating the identification of successful clones.
(iv) Insertional inactivation is preferred because it allows direct identification of recombinant colonies. It inactivates a selectable marker gene (e.g., antibiotic resistance), so only recombinant cells are easily identified.

Solution (b):
(i) The nematode that infects the roots of tobacco plants is *Meloidogyne incognita*.
(ii) The vector used to introduce nematode-specific genes into the host plant is *Agrobacterium tumefaciens*.
(iii) Sense and anti-sense RNAs are complementary RNA strands. When expressed together, they form a double-stranded RNA that silences the target nematode gene through RNA interference (RNAi).
(iv) The parasite could not survive in the transgenic tobacco plant because the RNAi mechanism silences the nematode-specific genes, preventing its growth and reproduction.