Madhya Pradesh Board Class 12 Biology — NCERT Solutions

Given: We need to identify the floral parts where male and female gametophytes develop.

Male Gametophyte Development:
The male gametophyte (pollen grain) develops inside the microsporangia (pollen sacs) present in the anther of the stamen.

• The sporogenous tissue within the microsporangium undergoes meiosis (microsporogenesis) to form microspore tetrads.
• Each microspore matures into a pollen grain (male gametophyte).

Female Gametophyte Development:
The female gametophyte (embryo sac) develops inside the ovule, which is present within the ovary of the pistil (gynoecium).

• The megaspore mother cell in the nucellus undergoes meiosis (megasporogenesis) to form megaspores.
• One functional megaspore develops into the embryo sac (female gametophyte).

Answer:

• Male gametophyte → Anther (microsporangia/pollen sacs)
• Female gametophyte → Ovule (inside the ovary)

Differences between Microsporogenesis and Megasporogenesis:

Type of Cell Division:
Both microsporogenesis and megasporogenesis involve meiosis (meiotic division / reductive division).

Structures formed at the end:

• Microsporogenesis → Microspore tetrad (which further matures into pollen grains — the male gametophyte)
• Megasporogenesis → Megaspore tetrad (one functional megaspore, which develops into the embryo sac — the female gametophyte)

(a) Humans reproduce sexually.

(b) Humans are viviparous (they give birth to young ones directly, without laying eggs).

(c) Fertilisation is internal in humans (it occurs inside the female reproductive tract).

(d) Male and female gametes are haploid (they contain half the chromosome number, $n = 23$).

(e) Zygote is diploid (formed by fusion of two haploid gametes, $2n = 46$).

(f) The process of release of ovum from a mature follicle is called ovulation.

(g) Ovulation is induced by a hormone called Luteinising Hormone (LH) (the LH surge triggers ovulation).

(h) The fusion of male and female gametes is called fertilisation.

(i) Fertilisation takes place in the ampulla of the fallopian tube (oviduct).

(j) Zygote divides to form blastocyst which is implanted in the uterus.

(k) The structure which provides vascular connection between foetus and uterus is called placenta.

Diagram of Male Reproductive System

Although a hand-drawn diagram is required in the board exam, the key structures to be labelled are described below:

Key structures and their positions:

1. Testes — paired oval organs located in the scrotal sac; site of spermatogenesis and androgen production.
2. Epididymis — coiled tube on the posterior surface of each testis; site of sperm maturation and storage.
3. Vas deferens (Ductus deferens) — muscular tube that carries sperms from epididymis upward into the pelvic cavity.
4. Seminal vesicles — paired glands that secrete fructose-rich fluid forming part of semen.
5. Prostate gland — single gland surrounding the urethra; secretes slightly alkaline fluid.
6. Bulbourethral glands (Cowper's glands) — paired glands that secrete lubricating fluid.
7. Urethra — common passage for urine and semen.
8. Penis — male external genitalia; contains erectile tissue.
9. Scrotum — pouch of skin housing the testes, maintains temperature ~2–3°C below body temperature.
10. Ejaculatory duct — formed by union of vas deferens and duct of seminal vesicle.

Note to student: Draw a neat diagram showing the above structures in correct anatomical positions and draw arrows with labels for each part. The urethra should be shown passing through the prostate and penis.

Given/Concept: Reproductive health refers to total well-being in all aspects of reproduction — physical, emotional, behavioural and social.

Significance of Reproductive Health in Society:

1. Healthy population: Ensures physically and mentally healthy individuals capable of reproduction, leading to a healthy society.
2. Reduced maternal and infant mortality: Proper reproductive health care reduces deaths during pregnancy, childbirth and infancy.
3. Prevention of STIs: Awareness and medical care help in early detection and cure of sexually transmitted infections, preventing their spread.
4. Population control: Knowledge of contraception and family planning helps in controlling population explosion.
5. Elimination of sex-related crimes: Education and awareness reduce sexual abuse, rapes and exploitation.
6. Emotional and social well-being: Counselling and awareness programmes help individuals deal with adolescent changes, relationships and social pressures in a healthy manner.
7. Assistance to infertile couples: Reproductive health programmes provide medical assistance to couples unable to conceive naturally.
8. Legal protection: Legalisation of MTP (Medical Termination of Pregnancy) protects women from unsafe abortions.

Conclusion: A reproductively healthy society is one where people have responsible sexual behaviour, access to safe reproductive health care, and freedom from reproductive diseases and dysfunctions.

Aspects of Reproductive Health Needing Special Attention:

1. Sex Education: Proper, scientifically accurate sex education must be imparted in schools to adolescents to make them aware of reproductive organs, puberty changes, safe sexual practices and contraception.

2. Awareness about STIs/STDs: Creating awareness about sexually transmitted infections like HIV/AIDS, gonorrhoea, syphilis, etc., their modes of transmission and prevention.

3. Population Control: Propagation of contraceptive methods and family planning to check the rapidly growing population.

4. Maternal and Child Health Care: Improving ante-natal and post-natal care to reduce maternal and infant mortality rates.

5. Prevention of Female Foeticide: Strict enforcement of the ban on amniocentesis for sex determination to prevent female foeticide and maintain a balanced sex ratio.

6. Assistance to Infertile Couples: Providing access to ART (Assisted Reproductive Technologies) like IVF, GIFT, ZIFT, etc.

7. Safe MTP: Ensuring medically safe and legal termination of unwanted pregnancies.

8. Adolescent Health: Counselling adolescents about physical and emotional changes during puberty, peer pressure, substance abuse and responsible behaviour.

9. Elimination of RTIs (Reproductive Tract Infections): Promoting hygienic practices to prevent reproductive tract infections, especially in women.

10. Awareness about Contraceptives: Educating rural and urban populations about various contraceptive options to enable informed choices.

Given: Mendel chose the garden pea (Pisum sativum) for his experiments on inheritance.

Advantages of selecting pea plant:

1. Availability of contrasting characters: Pea plants have several sharply contrasting (discontinuous) characters such as tall/dwarf, round/wrinkled seeds, yellow/green seeds, etc., making it easy to distinguish traits.

2. Short life span: Pea plants complete their life cycle in one season, allowing several generations to be studied in a short time.

3. Bisexual flowers: The flowers are bisexual (hermaphrodite), so self-pollination occurs naturally, making it easy to maintain pure lines.

4. Easy cross-pollination: Cross-pollination can be carried out easily by emasculation (removal of anthers) and artificial pollination.

5. Large number of offspring: Each plant produces a large number of seeds, providing statistically significant data.

6. Easy to grow: Pea plants are easy to cultivate and require little maintenance.

7. Availability of pure breeding varieties: Pure breeding (true breeding) varieties were readily available in the market.

Conclusion: These features made pea plants an ideal experimental organism for studying the principles of inheritance.

Given: A list of six compounds — Adenine, Cytidine, Thymine, Guanosine, Uracil, Cytosine.

Concept: A nitrogenous base is a nitrogen-containing ring compound (purine or pyrimidine) that is NOT attached to a sugar. A nucleoside = nitrogenous base + pentose sugar (no phosphate group).

Classification:

Nitrogenous Bases:

• Adenine (purine)
• Thymine (pyrimidine)
• Uracil (pyrimidine)
• Cytosine (pyrimidine)

Nucleosides:

• Cytidine (Cytosine + Ribose sugar)
• Guanosine (Guanine + Ribose sugar)

Summary Table:

Given / Concept: Darwinian natural selection states that in any population, individuals show heritable variations. Those with favourable variations survive and reproduce better (survival of the fittest), passing the trait to offspring.

Explanation:

Step 1 – Pre-existing variation: In a large bacterial population, random mutations continuously produce a few individuals that carry a gene conferring resistance to a particular antibiotic (e.g., penicillin). These resistant variants exist even before the antibiotic is introduced.

Step 2 – Selection pressure: When an antibiotic is administered, it acts as a selecting agent (environmental pressure). The non-resistant (sensitive) bacteria are killed, while the rare resistant individuals survive.

Step 3 – Differential reproduction: The surviving resistant bacteria reproduce rapidly (binary fission), passing the resistance gene to all daughter cells. Over successive generations, the resistant type becomes the dominant form in the population.

Step 4 – Result: The entire bacterial population eventually becomes antibiotic-resistant. This is not the antibiotic causing the mutation; the mutation was already present. The antibiotic merely selected the pre-existing resistant variant.

Conclusion: Antibiotic resistance is a classic, real-time demonstration of Darwinian natural selection — variation exists in the population, the environment (antibiotic) selects the fittest variant, and that variant proliferates. This is why over-use of antibiotics accelerates the emergence of multi-drug-resistant (MDR) bacteria such as MRSA.

Given/Concept: Public health measures are community-level steps taken to prevent the spread of infectious diseases.

Answer:

The following public health measures can be suggested as safeguards against infectious diseases:

1. Maintenance of personal and public hygiene: Regular washing of hands, keeping surroundings clean, and proper disposal of garbage.

2. Safe drinking water: Proper treatment and decontamination (chlorination, boiling) of drinking water to prevent water-borne diseases like cholera and typhoid.

3. Proper disposal of waste: Sewage treatment and proper disposal of human excreta to prevent contamination of soil and water.

4. Control of vectors: Elimination of breeding sites of mosquitoes (e.g., not allowing stagnant water to collect), use of insecticides, mosquito nets, and larvicides to control vector-borne diseases like malaria and dengue.

5. Vaccination and immunisation: Mass immunisation programmes against diseases like polio, tuberculosis, hepatitis, etc., to build herd immunity in the population.

6. Isolation of infected individuals: Quarantining patients with highly contagious diseases to prevent spread.

7. Health education: Educating the public about modes of disease transmission and preventive measures.

8. Regular health check-ups and early diagnosis: Timely detection and treatment of diseases to prevent their spread.

9. Control of food adulteration: Ensuring food safety standards to prevent food-borne infections.

10. Use of sterilised needles and syringes: Especially in hospitals and clinics to prevent transmission of blood-borne pathogens.

Conclusion: A combination of personal hygiene, environmental sanitation, vector control, and immunisation forms the backbone of public health measures against infectious diseases.

Sample to carry: Curd (or a piece of bread with mould, or a sample of buttermilk).

Best choice — Curd:

Given: We need a sample that is readily available at home and contains microbes that can be demonstrated under a microscope.

Reason:

• Curd is prepared by the action of Lactic Acid Bacteria (LAB), specifically Lactobacillus species.
• These bacteria are present in very large numbers in curd (millions per mL).
• A thin smear of curd on a glass slide, when stained and observed under a microscope, clearly shows the rod-shaped or spherical bacterial cells.
• Curd is easily available at home, is safe to handle, and the bacteria in it are non-pathogenic.
• Other suitable samples: a piece of mouldy bread (shows fungal hyphae of Rhizopus), or a sample of fermenting dough (shows yeast cells).

Conclusion: Curd is the most convenient and safe sample to carry from home to demonstrate the presence of microbes under a microscope.

The following are 10 recombinant proteins used in medical practice:

1. Recombinant Human Insulin (Humulin) – Used in the treatment of diabetes mellitus (Type 1 and Type 2).
2. Recombinant Human Growth Hormone (Somatotropin) – Used to treat growth hormone deficiency and dwarfism.
3. Recombinant Erythropoietin (EPO) – Used to treat anaemia, especially in patients with chronic kidney disease or undergoing chemotherapy.
4. Recombinant Tissue Plasminogen Activator (tPA / Alteplase) – Used as a thrombolytic agent in the treatment of heart attacks and strokes.
5. Recombinant Factor VIII – Used in the treatment of Haemophilia A (clotting factor deficiency).
6. Recombinant Factor IX – Used in the treatment of Haemophilia B.
7. Recombinant Hepatitis B Vaccine (HBsAg) – Used for immunisation against Hepatitis B virus infection.
8. Recombinant Interferon-alpha (IFN-α) – Used in the treatment of viral infections (e.g., Hepatitis C) and certain cancers.
9. Recombinant Interleukin-2 (IL-2 / Aldesleukin) – Used in the treatment of metastatic renal cell carcinoma and melanoma.
10. Recombinant DNase (Dornase alfa) – Used in the treatment of cystic fibrosis to reduce mucus viscosity in the lungs.

Note: All these proteins are produced by inserting the human gene encoding the protein into a suitable host organism (bacteria, yeast, or mammalian cells) using recombinant DNA technology, allowing large-scale production.

Given / Concept:
Viruses spread systemically through the vascular tissue of plants, but they do not infect the apical meristem (shoot tip) because the meristematic cells divide rapidly and viruses cannot keep pace with the fast-dividing cells. Also, the vascular tissue (phloem) is not yet differentiated in the meristem region, so the virus cannot reach there.

Answer:
The shoot apical meristem (meristematic tip / apical dome) is best suited for making virus-free plants.

Reason:

1. Meristematic cells are actively and rapidly dividing; viruses cannot infect them as fast as the cells divide.
2. The vascular system (through which viruses travel) is absent or not yet differentiated in the apical meristem.
3. When explants from the shoot apex are cultured in vitro (meristem culture), the regenerated plants are free from viral infection.

This technique is widely used to produce virus-free varieties of potato, sugarcane, banana, etc.

Given/Concept: A population is a group of individuals of the same species in a defined area. Populations have emergent properties that a single individual cannot possess.

Attributes that populations possess but not individuals:

1. Birth Rate (Natality): The number of births per capita in a population per unit time. An individual can give birth, but a 'birth rate' is a property of the whole population.

2. Death Rate (Mortality): The number of deaths per capita in a population per unit time. Similarly, an individual can die, but a 'death rate' belongs to the population.

3. Sex Ratio: The ratio of males to females in a population (e.g., 60% females : 40% males). A single individual has a sex but not a sex ratio.

4. Age Distribution (Age Pyramid): The proportion of individuals in different age groups (pre-reproductive, reproductive, post-reproductive). An individual has an age, but age distribution is a population attribute.

5. Population Density: The number of individuals (or biomass) per unit area or volume at a given time.

6. Population Growth Rate: The rate at which the population size changes over time (expressed as $r$ or $\lambda$).

Conclusion: Birth rate, death rate, sex ratio, age distribution, population density, and growth rate are all attributes of populations, not of individual organisms.

(a) Plants are called as producers (autotrophs) because they fix carbon dioxide through the process of photosynthesis, converting inorganic $CO_2$ into organic compounds.

(b) In an ecosystem dominated by trees, the pyramid of numbers is of inverted type. This is because a single large tree (one producer) supports a very large number of herbivorous insects (primary consumers), which in turn support fewer secondary consumers, making the base narrow and the middle wide — giving an inverted shape.

(c) In aquatic ecosystems, the limiting factor for productivity is light (and sometimes nutrients like nitrogen and phosphorus). Light penetration decreases rapidly with depth, restricting photosynthesis to the photic zone.

(d) Common detritivores in our ecosystem are earthworms, millipedes, woodlice (isopods), and certain insects (e.g., beetles). They fragment dead organic matter (detritus) into smaller particles.

(e) The major reservoir of carbon on earth is the ocean (hydrosphere) / sedimentary rocks (fossil fuels and limestone). Among these, the oceans hold the largest active reservoir of carbon, while sedimentary rocks hold the largest overall geological reservoir.

Given/Concept: Biodiversity refers to the variety of life on Earth at all its levels.

Answer:
The three important components of biodiversity are:

1. Genetic diversity – Diversity at the genetic level within a species (e.g., different varieties/races of the same species).
2. Species diversity – Diversity at the species level, i.e., the variety of species in a given region.
3. Ecological (Community/Ecosystem) diversity – Diversity at the ecosystem level, i.e., the variety of habitats, communities and ecological processes in a biosphere.