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Class 12th Biology NCERT Exemplar Solutions
1. Reproduction In Organisms
This chapter provides a foundational overview of reproduction, the biological process essential for the continuity of a species. It contrasts the two primary modes. Asexual reproduction is detailed as a method involving a single parent, resulting in offspring that are genetically identical clones. Examples include binary fission in Amoeba, budding in yeast, and vegetative propagation in plants through structures like runners and rhizomes. In contrast, sexual reproduction involves the formation and fusion of male and female gametes, typically from two parents, leading to offspring with genetic variation. The chapter introduces the key stages of sexual reproduction: pre-fertilisation, fertilisation (syngamy), and post-fertilisation events.
2. Sexual Reproduction In Flowering Plants
This chapter offers an in-depth exploration of sexual reproduction in angiosperms. It details the structure of the flower, focusing on the male reproductive part (androecium) and the female reproductive part (gynoecium). The processes of microsporogenesis (formation of pollen grains) and megasporogenesis (formation of the embryo sac) are explained. The chapter covers pollination, the transfer of pollen, discussing its types (self and cross-pollination) and various agents. A key focus is the unique phenomenon of double fertilisation in flowering plants, which involves two fusion events: syngamy (fusion of a male gamete with the egg cell) and triple fusion (fusion of the second male gamete with the central cell), leading to the formation of the embryo and the nutritive endosperm, respectively.
3. Human Reproduction
This chapter provides a comprehensive account of the human reproductive process. It begins with a detailed anatomical description of the male and female reproductive systems. The process of gamete formation, spermatogenesis in males and oogenesis in females, is meticulously explained. The chapter details the menstrual cycle and the hormonal control that regulates it. It then traces the sequence of events from insemination to fertilisation, the formation of a zygote, its development into a blastocyst, and subsequent implantation in the uterine wall. The chapter concludes with a discussion of pregnancy, embryonic development, parturition (childbirth), and lactation.
4. Reproductive Health
This chapter addresses the crucial topic of reproductive health, defining it as total well-being in all aspects of reproduction. It discusses major societal issues, including population explosion and the importance of family planning. A significant portion is dedicated to various contraceptive methods—natural, barrier, IUDs, oral pills, and surgical techniques (vasectomy and tubectomy). The chapter also raises awareness about Sexually Transmitted Infections (STIs) like HIV/AIDS and their prevention. Furthermore, it explores the problem of infertility and explains the different Assisted Reproductive Technologies (ART) such as In Vitro Fertilisation (IVF), ZIFT, and GIFT, which help couples conceive.
5. Principles Of Inheritance And Variation
This chapter lays the foundation of classical genetics, exploring heredity and variation. It is centered around Gregor Mendel's pioneering work and his three Laws of Inheritance: the Law of Dominance, the Law of Segregation, and the Law of Independent Assortment. The chapter defines fundamental genetic terms like gene, allele, genotype, and phenotype, and explains monohybrid and dihybrid crosses using the Punnett square. It also covers concepts beyond simple Mendelian genetics, such as incomplete dominance, co-dominance (e.g., ABO blood groups), and polygenic inheritance. The Chromosomal Theory of Inheritance, linkage, recombination, and mechanisms of sex determination are also discussed.
6. Molecular Basis Of Inheritance
This chapter delves into the molecular world of genetics, identifying DNA as the genetic material through landmark experiments (Griffith, Hershey-Chase). It details the double-helix structure of DNA and explains the "Central Dogma" of molecular biology. The chapter provides a meticulous description of three core processes: DNA replication (the semi-conservative mechanism), Transcription (the synthesis of RNA from a DNA template), and Translation (the synthesis of a polypeptide chain on a ribosome based on an mRNA sequence). It also deciphers the genetic code, explains the regulation of gene expression using the lac operon as an example, and discusses the goals and impact of the Human Genome Project and DNA fingerprinting.
7. Evolution
This chapter traces the history of life on Earth, exploring the grand process of evolution. It discusses theories on the origin of life, focusing on the chemical evolution hypothesis of Oparin and Haldane. A wide range of evidence for evolution is presented, including fossil records, comparative anatomy (homologous and analogous structures), and molecular similarities. The chapter centers on Charles Darwin’s theory of Natural Selection as the key mechanism driving evolution. It also explains modern evolutionary concepts like adaptive radiation, the Hardy-Weinberg principle (which describes a non-evolving population), genetic drift, and provides a brief account of human evolution.
8. Human Health And Diseases
This chapter focuses on the maintenance of human health and the various ailments that affect it. It details numerous infectious diseases caused by pathogens, including bacterial (typhoid), viral (common cold, AIDS), protozoan (malaria), and helminthic diseases. A major part of the chapter is dedicated to the human immune system, explaining the difference between innate and acquired immunity (humoral and cell-mediated). The principles of vaccination and immunization are explained. Non-infectious diseases like cancer are also discussed, along with the societal menace of drug and alcohol abuse and its detrimental effects on individual and public health.
9. Strategies For Enhancement In Food Production
This chapter explores the scientific methods used to meet the increasing global demand for food. It covers strategies in both animal husbandry and plant breeding. Animal husbandry sections discuss modern practices in dairy and poultry farm management, apiculture (beekeeping), and fisheries. The plant breeding section details conventional methods for developing high-yielding and disease-resistant crop varieties, a cornerstone of the Green Revolution. It also introduces modern techniques like plant tissue culture, which allows for rapid propagation (micropropagation) of plants, and Single Cell Protein (SCP) as an alternative food source.
10. Microbes In Human Welfare
This chapter highlights the immensely beneficial roles of microorganisms. It showcases their use in producing household items like curd and bread, and in the industrial-scale production of beverages and antibiotics (e.g., Penicillin). A key application discussed is their role in sewage treatment plants (STPs), where they are used to break down organic waste and purify water. The chapter also explains how microbes are harnessed for the production of biogas (mainly methane) and as biocontrol agents and biofertilisers, offering an eco-friendly alternative to chemical pesticides and fertilizers, thereby promoting sustainable agriculture.
11. Biotechnology: Principles And Processes
This chapter introduces the revolutionary field of biotechnology and its core principles, focusing on genetic engineering. It details the essential tools of recombinant DNA (rDNA) technology: restriction enzymes (which act as 'molecular scissors' to cut DNA), cloning vectors (like plasmids, used to carry foreign DNA), and ligase enzymes (to join DNA fragments). The chapter outlines the complete process of creating a recombinant DNA molecule, including the isolation of the desired gene, its amplification using the Polymerase Chain Reaction (PCR), its insertion into a vector, and its introduction into a host organism to achieve cloning or gene expression.
12. Biotechnology And Its Applications
This chapter demonstrates the practical impact of biotechnology across various sectors. In agriculture, it discusses the creation of genetically modified (GM) crops, such as pest-resistant Bt cotton and nutritionally enhanced Golden Rice. In medicine, applications include the mass production of therapeutic drugs like genetically engineered insulin, the development of techniques for gene therapy to correct genetic defects, and the use of molecular diagnostic tools like PCR and ELISA. The chapter also explores the creation of transgenic animals and addresses the critical ethical issues and biosafety regulations associated with this powerful technology.
13. Organisms And Populations
This is the first chapter in ecology, focusing on the interactions between individual organisms and their environment, and the dynamics of populations. It describes the various adaptations (morphological, physiological, behavioural) that organisms evolve to cope with abiotic factors. The chapter then delves into population ecology, defining attributes like birth rate, death rate, and age structure. It explains the two primary models of population growth: the J-shaped exponential growth curve and the more realistic S-shaped logistic growth curve, which incorporates the concept of carrying capacity. Finally, it details the different types of interspecific interactions, such as predation, competition, parasitism, and mutualism.
14. Ecosystem
This chapter examines the ecosystem as the primary functional unit of nature. It describes the structure of an ecosystem, comprising biotic and abiotic components. The chapter focuses on four key ecosystem functions. Productivity refers to the rate of biomass generation. Decomposition is the breakdown of dead organic matter. Energy flow is traced through different trophic levels via food chains and food webs, governed by the 10% law of energy transfer. Nutrient cycling describes the movement of essential elements through biogeochemical cycles, with a focus on the carbon and phosphorus cycles. The chapter also introduces the concept of ecological succession.
15. Biodiversity And Conservation
This chapter highlights the importance of biodiversity—the variety of life on Earth at the genetic, species, and ecosystem levels. It discusses the patterns of biodiversity and its crucial role in maintaining ecosystem health and providing essential services to humans. The chapter identifies the major causes of biodiversity loss, termed the "Evil Quartet": habitat loss, over-exploitation, alien species invasion, and co-extinctions. It then details the two primary strategies for biodiversity conservation: in-situ conservation (protecting species in their natural habitats, like National Parks and Biosphere Reserves) and ex-situ conservation (protecting them outside their habitats, like in zoos and seed banks).
16. Environmental Issues
This chapter addresses the major environmental challenges facing humanity due to pollution and unsustainable practices. It discusses air pollution and its control, water pollution and the importance of sewage treatment, and solid waste management. The chapter explains the mechanisms and consequences of major global environmental problems, including the greenhouse effect and global warming, and the depletion of the stratospheric ozone layer. It also covers issues like deforestation and soil degradation, emphasizing the urgent need for sustainable development and responsible environmental stewardship to protect our planet's health.
Sample Paper
This entry provides a comprehensive Sample Paper for Class 12 Biology, designed to simulate the final board examination. It is an invaluable tool for self-assessment, covering the entire syllabus from all 16 chapters. The paper includes a diverse range of questions that test conceptual clarity, application skills, and analytical thinking. By solving this paper, students can practice time management, familiarize themselves with the exam pattern, identify areas requiring further revision, and build the confidence necessary to perform well in their examinations.