CBSE Class 12 Biology Question Paper 2024 (Set 3- 57/5/3) Available- Download Solution PDF with Answer Key

Step 1: Understanding the gene distribution among chromosomes.
- Chromosome 1 has the highest number of genes, as it is the largest human chromosome.
- Chromosome Y has the least number of genes, as it primarily carries genes related to male sex determination and reproduction.

Step 2: Conclusion.
The correct answer is \( \textbf{(C)} \), as Chromosome 1 has the highest and Chromosome Y has the least number of genes. Quick Tip: Remember: Chromosome 1 is the largest and most gene-rich, while Chromosome Y is the smallest and least gene-dense.

Step 1: Understanding sex determination in birds.
- In birds, the sex of the offspring is determined by females, not males, as females are heterogametic (ZW), while males are homogametic (ZZ).
- Thus, the assertion (A) is incorrect.

Step 2: Analyzing the reason.
- The reason (R) correctly states that males are homogametic while females are heterogametic in birds.

Step 3: Conclusion.
Since the assertion is false and the reason is true, the correct answer is \( \textbf{(D)} \). Quick Tip: In birds: - Males (ZZ) are homogametic. - Females (ZW) are heterogametic and determine the offspring's sex.

Step 1: Analyzing the assertion (A).
- Biodiversity hotspots are indeed regions of high species richness and endemism, so the assertion is true.

Step 2: Analyzing the reason (R).
- The total number of biodiversity hotspots worldwide is 36, not 22. India is home to 4 biodiversity hotspots (the Himalayas, Indo-Burma, Western Ghats-Sri Lanka, and Sundaland). Hence, the reason is false.

Step 3: Conclusion.
Since the assertion is true but the reason is false, the correct answer is \( \textbf{(C)} \). Quick Tip: Currently, there are 36 biodiversity hotspots in the world, with four located in India: Himalayas, Indo-Burma, Western Ghats-Sri Lanka, and Sundaland.

Step 1: Analyzing the assertion (A).
- AIDS (Acquired Immunodeficiency Syndrome) is indeed caused by the HIV (Human Immunodeficiency Virus). Hence, the assertion is true.

Step 2: Analyzing the reason (R).
- HIV is a retrovirus with RNA, not DNA, as its genetic material. Therefore, the reason is false.

Step 3: Conclusion.
Since the assertion is true but the reason is false, the correct answer is \( \textbf{(C)} \). Quick Tip: HIV is a retrovirus with RNA as its genetic material, not DNA.

Step 1: Analyzing the assertion (A).
- Single-stranded DNA or RNA tagged with a radioactive molecule is indeed called a probe. Probes are used in molecular diagnosis to detect specific DNA sequences. Hence, the assertion is true.

Step 2: Analyzing the reason (R).
- A probe searches for and hybridizes with its complementary DNA sequence within a sample, confirming the assertion's purpose. The reason is true and correctly explains the assertion.

Step 3: Conclusion.
Both the assertion and the reason are true, and the reason explains the assertion. The correct answer is \( \textbf{(A)} \). Quick Tip: Probes in molecular biology: - Single-stranded DNA or RNA tagged with a radioactive or fluorescent molecule. - Used to detect specific DNA/RNA sequences by hybridization.

DNA follows Chargaff’s rule, which states:

The percentage of adenine (A) equals the percentage of thymine (T), and the percentage of cytosine (C) equals the percentage of guanine (G).
Since adenine constitutes 31%, thymine also constitutes 31%. Together, A + T = 62%.
The remaining 38% is divided equally between cytosine and guanine. Thus, cytosine constitutes 19%. Quick Tip: Chargaff’s Rule: A = T and C = G. Use this to calculate the percentage of any base in DNA when the percentage of one base is known.

(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. Quick Tip: - Exponential growth: Unlimited resources, no constraints.
- Logistic growth: Limited resources, stabilizes at carrying capacity (K).
- Carrying capacity (K): Maximum sustainable population size.

N/A Quick Tip: Remember: - EcoRI recognizes the sequence 5' – GAATTC – 3'. - Palindromic sequences are the same on complementary strands when read in opposite directions. - Sticky ends are crucial for recombination in genetic engineering.

The following events occur during sewage treatment to reduce the Biochemical Oxygen Demand (BOD) of primary effluent:

The primary effluent is passed into a large aeration tank.
The effluent is constantly agitated mechanically, and air is pumped into it.
Vigorous growth of useful aerobic microbes (such as fungi and bacteria) occurs, forming flocs.
These microbes consume the major part of the organic matter present in the effluent, reducing the BOD. Quick Tip: Remember: BOD reduction is achieved in the aeration tank through the action of aerobic microbes that degrade organic matter in the primary effluent.

Farmers prefer apomictic seeds because:

They do not require repeated hybridization, saving time and cost.
They maintain the hybrid vigor (heterosis) over generations, as apomixis produces genetically identical offspring. Quick Tip: Remember: Apomictic seeds are cost-effective and maintain hybrid vigor across generations, making them favorable for farmers.

Advantage:

Amniocentesis can detect genetic abnormalities and chromosomal disorders in the fetus, aiding in early diagnosis and management.

Disadvantage:

It carries a small risk of complications such as miscarriage or infection. Quick Tip: Amniocentesis: Think of "genetic diagnosis" (advantage) vs. "risk of miscarriage" (disadvantage).

The processing of hnRNA into mRNA in eukaryotes involves the following steps:

Capping: A 7-methylguanosine cap is added to the 5' end of hnRNA to protect it from degradation and assist in ribosome binding during translation.
Tailing: A poly-A tail (a string of adenine nucleotides) is added to the 3' end of the hnRNA to enhance stability and facilitate export from the nucleus.
Splicing: Introns (non-coding regions) are removed, and exons (coding regions) are joined together to form a continuous sequence that codes for a protein.

This processing occurs in the nucleus of the eukaryotic cell. Quick Tip: Remember the three main steps in hnRNA processing: Capping, Tailing, and Splicing. These modifications occur in the nucleus and are essential for mRNA stability and functionality.

(a) Importance of Mother’s Milk:

Mother’s milk provides all essential nutrients required for the growth and development of the newborn.
It contains antibodies (such as IgA) that boost the immune system and protect the baby from infections.
Breast milk promotes healthy gut flora and reduces the risk of allergies and illnesses.


(b) Vaccine and Its Principle:

A vaccine is a biological preparation containing dead or weakened pathogens or their components.
The principle of vaccination is to stimulate the immune system to produce memory cells against a specific pathogen, enabling a faster and stronger response upon subsequent exposure. Quick Tip: Breastfeeding provides both nutritional and immunological benefits. Vaccines work on the principle of immunological memory.

(a) Reasons for Higher Biodiversity in Tropical Regions:

Favorable Climate: Tropical regions have a warm and stable climate throughout the year, which promotes the growth and reproduction of diverse organisms.
Higher Productivity: Abundant sunlight and rainfall increase primary productivity, supporting a variety of life forms.
Long Evolutionary Time: Tropical regions have been relatively undisturbed over geological timescales, allowing species to evolve and diversify.


(b) (i):
An ecological pyramid represents the flow of energy and the number of organisms at each trophic level in an ecosystem. It is a graphical representation that shows the relative amount of energy or biomass at each level, starting with producers at the base, followed by herbivores, carnivores, and top predators. The pyramid illustrates the decrease in energy and number of organisms as we move up through the trophic levels.




(b) (ii):
Ecological pyramids may take two forms:


Upright Ecological Pyramid: In this type, the number of organisms, biomass, or energy decreases as we move up from the producers to the apex consumers. It is the most common shape of ecological pyramids, where producers form the base and each subsequent level contains fewer organisms or less energy.
Example: A forest ecosystem is an example of an upright pyramid, where plants (producers) form the base, followed by herbivores (primary consumers), then carnivores (secondary consumers), and finally apex predators.

Inverted Ecological Pyramid: This occurs when the number of organisms or the biomass increases at higher trophic levels. This happens typically in aquatic ecosystems, where the biomass of primary producers (e.g., phytoplankton) is less than that of primary consumers (e.g., zooplankton). In these ecosystems, producers are consumed rapidly, and a large number of consumers are supported.
Example: In a pond or lake, the pyramid of biomass may be inverted, with more zooplankton than phytoplankton. Quick Tip: - Tropical regions have high biodiversity due to favorable conditions and longer evolutionary times. - In species-area relationships, larger areas generally support more species, but the rate of increase diminishes.

Reasons for Interest in Artificial Hybridisation:

It allows breeders to combine desirable traits from two different plants, such as disease resistance and high yield.
It enhances genetic variation in crops, increasing the potential for better adaptability and productivity.
It helps in developing new crop varieties with improved quality and performance.


Process of Artificial Hybridisation:

The process involves emasculation, where the anthers of a flower are removed to prevent self-pollination.
The emasculated flower is then covered with a bag (bagging) to protect it from unwanted pollen.
Pollen from the desired parent plant is collected and dusted onto the stigma of the emasculated flower, ensuring cross-pollination. Quick Tip: Artificial hybridisation is widely used to develop high-yielding, disease-resistant, and stress-tolerant crop varieties.

(a) Definition of Transgenic Animals:

Transgenic animals are those that have had their DNA modified by the introduction of a foreign gene or genes, making them capable of expressing traits that are not naturally found in their species.

(b) Transgenic Animal with the Largest Population:

Mice are the transgenic animals with the largest number among all existing transgenic animals. They are extensively used in genetic and biomedical research.

(c) Reasons for Producing Transgenic Animals:

To study the function of specific genes and their role in development and diseases.
For producing pharmaceutical proteins and drugs (e.g., recombinant insulin).
To create animal models for human diseases, enabling research on treatments and therapies. Quick Tip: Transgenic animals like mice and sheep are instrumental in biomedical research, drug production, and understanding genetic functions.

(a) Pyramid of Biomass in the Sea:

The pyramid of biomass in the sea is typically inverted. This is because:

The biomass of phytoplankton (producers) is much smaller than the biomass of the fishes (primary and secondary consumers).
Phytoplankton have a high turnover rate due to rapid reproduction and consumption by herbivorous fishes, maintaining their population despite low biomass.



Inverted Pyramid of Biomass (Marine Ecosystem) \[ Fishes (Consumers) - Large Biomass at the Top
Phytoplankton (Producers) - Small Biomass at the Bottom \]



(b) Conditions for an Upright Pyramid:

The pyramid of biomass will always remain upright in terrestrial ecosystems where:

The biomass of producers is always higher than the biomass of consumers.
Energy transfer follows the 10% rule, with significant energy loss at each trophic level. Quick Tip: - In marine ecosystems, the pyramid of biomass is often inverted due to the high productivity and turnover of phytoplankton. - Terrestrial ecosystems generally have upright pyramids due to greater producer biomass.

(a) Reason for Replication within the Replication Fork:

DNA replication occurs within the replication fork because DNA unwinds progressively at the fork to expose the two strands for replication.
The entire length of DNA cannot replicate simultaneously as it would require the entire molecule to be unwound at once, which is not feasible due to its length and complexity.
The replication fork allows for localized unwinding and replication, ensuring accuracy and efficiency.


(b) Continuous and Discontinuous Replication:

DNA polymerase synthesizes DNA only in the 5' to 3' direction.
On the leading strand, replication is continuous because it proceeds in the same direction as the unwinding of the helix.
On the lagging strand, replication is discontinuous, as it proceeds in short segments (Okazaki fragments) opposite to the direction of unwinding.
These fragments are later joined by DNA ligase to form a continuous strand.


Schematic Representation:

Quick Tip: - Leading strand: Continuous synthesis (5' to 3'). - Lagging strand: Discontinuous synthesis via Okazaki fragments. - DNA replication is semi-discontinuous due to the opposite orientation of strands.

(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. Quick Tip: - Menstrual cycle phases: Menses, Proliferative, Secretory. - Implantation occurs in the secretory phase due to high progesterone levels. - Progesterone = Maintains endometrium; Estrogen = Builds endometrium.

(a)

(i) In humans, Plasmodium enters in the form of sporozoites, which are injected by the female \textit{Anopheles mosquito during a bite.
(ii) In the female \textit{Anopheles body, \textit{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 \textit{Anopheles mosquito, after sucking blood from an infected person:

The gametocytes of \textit{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 \textit{Anopheles is \textit{Aedes aegypti, which carries the pathogen for dengue fever. Quick Tip:Plasmodium stages: Sporozoites enter humans; gametocytes enter mosquitoes.
- Aedes aegypti: Carries pathogens for dengue and chikungunya. - Malaria symptoms appear after the incubation period (10–14 days).

Functions of labelled parts:
• Head: Contains the nucleus with genetic material and acrosome for penetrating the egg.
• Middle Piece: Contains mitochondria that provide energy for motility.
• Tail: Facilitates movement of the sperm towards the egg.
• Plasma Membrane: Encloses the sperm and maintains its structural integrity.

(ii) The probability of an ovum being fertilized by more than one sperm is impossible due
to a phenomenon called cortical reaction, where the egg membrane changes after the first
sperm enters, preventing entry of additional sperms.
Solution (b):
(i) Diagram showing stages of embryo development in a dicot plant:

(ii) Endosperm development precedes embryo development because the endosperm pro-
vides nutrients to the developing embryo. It supports early stages of growth until the plant
can photosynthesize independently.
Quick Tip
- Human sperm: Acrosome helps penetrate the egg; mitochondria in the middle piece
generate energy.
- Cortical reaction prevents polyspermy in humans.
- Dicot embryos develop after endosperm formation, ensuring nutrient availability.

(a) Dihybrid Cross:


Let:

Violet axial flowers = ( VVAA )
White terminal flowers = ( vvaa )

Mendel’s Law Derived: Mendel’s law of Independent Assortment states that the inheritance of one trait is independent of the inheritance of another.

% Solution for part (b)
(b) Transcription in Prokaryotes:

Transcription in prokaryotes involves the synthesis of mRNA from DNA using RNA polymerase. The process includes:

Initiation: RNA polymerase binds to the promoter region of DNA, aided by sigma factors.
Elongation: RNA polymerase synthesizes RNA in the 5’ to 3’ direction using the DNA template strand.
Termination: Transcription ends when RNA polymerase reaches a terminator sequence, releasing the mRNA.


Differences from Eukaryotic Transcription:

In eukaryotes, transcription occurs in the nucleus, whereas in prokaryotes, it occurs in the cytoplasm.
Eukaryotic transcription requires multiple RNA polymerases and transcription factors, whereas prokaryotes use a single RNA polymerase.
Eukaryotic mRNA undergoes capping, tailing, and splicing, which are absent in prokaryotic transcription. Quick Tip: - In a dihybrid cross, the \( F_2 \) phenotypic ratio is always \( 9:3:3:1 \). - Prokaryotic transcription is simpler, whereas eukaryotic transcription involves additional post-transcriptional modifications.