CBSE Class 12 Biology Question Paper 2024 (Set 1- 57/5/1) Available- Download Solution PDF with Answer Key

Solution: When a single gene can exhibit multiple phenotypic expressions, such a gene is called a pleiotropic gene. For example, the gene responsible for phenylketonuria in humans affects multiple traits, such as hair color, skin pigmentation, and mental ability.

Solution: A person with trisomy of 21^st chromosome will be afflicted with Down's syndrome. This condition is characterized by a small round head, furrowed tongue, partially open mouth, and characteristic palm crease.

Solution: The diagram represents Intracytoplasmic Sperm Injection (ICSI), where a single sperm is directly injected into the cytoplasm of the egg using a fine needle. This technique is used in cases of severe male infertility or failed fertilization in conventional IVF.

Solution: Interferons are signaling proteins secreted by virus-infected cells to protect nearby uninfected cells. They play a crucial role in the immune response by interfering with viral replication.

Solution: In the biological treatment of sewage, flocs are masses of bacteria held together by fungal filaments, forming mesh-like structures. These flocs help in the breakdown of organic matter during secondary treatment.

Solution: In agarose gel electrophoresis, DNA is separated based on size. Staining with ethidium bromide allows the DNA bands to be visualized under UV light. Ethidium bromide binds to DNA and fluoresces under UV light, making the bands visible.

Solution: Pseudocopulation occurs when a male insect is deceived by the patterns or structures of a flower that resemble a female insect. The male attempts to copulate with the flower, leading to its pollination. This is a pollination strategy employed by certain orchids.

Solution: The correct labellings in the figure are:
- A represents degenerating synergids as they are no longer needed after fertilization.
- B is the zygote formed after the fusion of the male and female gametes.
- C is the primary endosperm nucleus (PEN) formed by the fusion of a sperm with two polar nuclei.
- D represents degenerating antipodals as their function is complete.

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: In the lac operon:
- The ‘i’ gene codes for the repressor protein, which inhibits the operon when lactose is absent.
- The ‘z’ gene produces β-galactosidase, which breaks down lactose into glucose and galactose.
- The ‘a’ gene encodes transacetylase, which helps in lactose metabolism.
- The ‘y’ gene produces lac permease, which facilitates the entry of lactose into the cell.

Solution: When both parents are carriers for thalassaemia (heterozygous), the genetic cross shows:
- 25% chance of an unaffected child (homozygous dominant).
- 50% chance of a carrier child (heterozygous).
- 25% chance of an afflicted child (homozygous recessive).
Thus, there is a 25% chance that the child will be afflicted with thalassaemia.

Solution: The RNA transcript is complementary to the template strand and identical to the coding strand (except that uracil (U) replaces thymine (T) in RNA). Therefore, the RNA transcript of the sequence 5' – ATGAATG – 3' is 5' – AUGAAUG – 3'.

Solution: HIV (Human Immunodeficiency Virus) causes AIDS (Acquired Immunodeficiency Syndrome) by damaging the immune system. However, HIV is an RNA virus, not a DNA virus. Therefore, Assertion (A) is true, but Reason (R) is false.

Solution: A probe is a single-stranded DNA or RNA tagged with a radioactive or fluorescent molecule, used to identify complementary sequences by hybridization. The probe searches and hybridizes with its complementary DNA in a clone of cells, making both (A) and (R) correct, with (R) explaining (A).

Solution: In birds, the sex of the offspring is determined by females, not males, as females are heterogametic (ZW), while males are homogametic (ZZ). Therefore, Assertion (A) is false, but Reason (R) is true.

Solution: Communities with more species are more stable due to the complementary roles of species, reducing year-to-year variation in total biomass and ensuring ecological stability. Both (A) and (R) are true, and (R) correctly explains (A).

Solution: 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.

Solution:
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.

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):
[Diagram of DNA with EcoRI cut sites and sticky ends]
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):
- Growth curve ‘A’ represents exponential growth, which is possible when resources are unlimited, and there are no environmental constraints.
- Growth curve ‘B’ represents logistic growth, which occurs when resources are limited, and the population stabilizes after reaching the carrying capacity.

Solution (b): ‘K’ in the graph represents the carrying capacity, which is the maximum population size that the environment can sustain indefinitely, given the available resources.

Solution: Plants form symbiotic associations with Glomus species, a type of mycorrhiza. These fungi help the plants in:
• Efficient absorption of water and minerals, especially phosphorus.
• Enhancing resistance to pathogens and tolerance to salinity and drought.

Solution: 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%.

Solution: The completed table is as follows:

Solution: Biologists explain the higher species richness in tropical regions due to:
• Favourable Climate: Stable climatic conditions in tropical regions support year-round growth and reproduction.
• Long Evolutionary Time: Tropical regions have remained undisturbed for millions of years, allowing species to diversify.
• Higher Productivity: High temperature and sunlight lead to higher productivity, supporting a larger variety of organisms.

Solution:
(i) An ecological pyramid represents the trophic structure and function of an ecosystem, depicting energy, biomass, or numbers at each trophic level.

(ii)
• Upright Pyramid: In most ecosystems, energy pyramids are upright because energy decreases with each trophic level. Example: Grassland ecosystem.
• Inverted Pyramid: Some aquatic ecosystems have an inverted biomass pyramid. Example: Phytoplankton (low biomass) support a higher biomass of zooplankton.

Solution:
(a) Transgenic animals are animals whose genomes have been altered by introducing foreign DNA, enabling them to express desired traits.

(b) The transgenic animal with the largest number is mice.

(c) Transgenic animals are produced for:
• Studying Diseases: Used as models for studying human genetic diseases.
• Producing Medicines: Engineered to produce proteins or medicines like insulin and human growth hormone.
• Improving Livestock: Modified to enhance productivity and disease resistance.

Solution:
The number of chromosomes in:
• Zygote: 10 chromosomes (formed by fusion of haploid male and female gametes, each with 5 chromosomes).
• Endosperm: 30 chromosomes (formed by triple fusion, involving two polar nuclei (10 + 10) and one male gamete (10)).

Process:
• Zygote Formation: Syngamy, where a male gamete fuses with an egg cell to form a diploid zygote.
• Endosperm Formation: Double fertilization, where one male gamete fuses with two polar nuclei to form a triploid endosperm.

Solution:
(a) DNA replication occurs within a replication fork because DNA polymerase synthesizes new strands only in the 5' to 3' direction. This restricts replication to occur progressively as the fork unwinds.

(b)
• The leading strand is synthesized continuously in the 5' to 3' direction.
• The lagging strand is synthesized discontinuously in small fragments (Okazaki fragments), which are later joined by DNA ligase.
[Diagram of DNA replication fork showing leading and lagging strand synthesis]

Solution: Processing of hnRNA into mRNA involves:
• Capping: Addition of a 5' methyl guanosine cap for stability and ribosome attachment.
• Splicing: Removal of introns (non-coding regions) and joining of exons (coding regions).
• Tailing: Addition of a poly-A tail at the 3' end for stability and nuclear export.
- This processing occurs in the nucleus of the eukaryotic cell.

Solution: Three human activities causing species extinction:
• Deforestation: Destruction of natural habitats reduces biodiversity.
• Pollution: Air, water, and soil pollution harm ecosystems and species survival.
• Overexploitation: Overfishing, hunting, and poaching lead to the depletion of species populations.

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) 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):
(i) A schematic diagram of the cloning vector pBR 322 is shown below:
[Diagram of pBR322 vector with labeled BamHI site, ampicillin resistance gene, ori, and rop gene]

(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.

Solution (a):
(i) Diagram of a human sperm:
[Diagram of human sperm with labeled parts: Acrosome, Head, Nucleus, Centrioles, Mitochondria, Middle Piece, Tail, Plasma Membrane, Axial Filament, Terminal ring, Principal piece, Endpiece]

Functions of labeled 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 getting 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:
[Diagram showing stages of dicot embryo development: Zygote, Globular Embryo, Heart-Shaped Embryo, Mature Embryo with labeled Suspensor, Radicle, Plumule, and Cotyledon]

(ii) Endosperm development precedes embryo development because the endosperm provides nutrients to the developing embryo. It supports early stages of growth until the plant can photosynthesize independently.

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 industrialization, 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.