12th Zoology One Mark Questions and Book Back Answers

About Zoology

Medium ~20 h study

Class 12 Zoology picks up where Class 11 left off. You already know cells, tissues and organ systems. This year asks how animals, and especially humans, reproduce, inherit traits, fight disease and fit into their environment. The first chapters deal with reproduction: asexual and sexual modes, gametogenesis, fertilisation, pregnancy and reproductive health. Those ideas come back later when you study hormones, immunity and biotechnology.

The middle of the book works at the level of chromosomes and DNA. You learn Mendel's laws as they apply to humans, multiple alleles and blood groups, sex determination, and then the molecular side: replication, transcription, translation and the lac operon. The evolution chapter connects these ideas. A change in a gene, spread through a population by selection, drift or migration, is what produces new species over geological time.

The last chapters are applied. Immunology explains vaccines, allergy and AIDS. Microbes in human welfare and applications of biotechnology show how bacteria and yeast make curd, antibiotics, biogas and recombinant insulin. The ecology chapters cover population attributes, biodiversity and its conservation, and environmental issues such as pollution and waste management. Public exam questions test whether you can explain a process in the right sequence and use the textbook's terms, so learn the steps, not just the names.

What you'll learn

• Explain human gametogenesis, fertilisation and embryonic development in the correct sequence.
• Work out inheritance of multiple alleles, blood groups and sex-linked traits, and describe sex determination in different animals.
• Describe DNA replication, transcription, translation and regulation of the lac operon.
• Use anatomical, embryological and fossil evidence to support organic evolution and place major events on the geological time scale.
• Distinguish innate from acquired immunity and label the parts of an antibody molecule.
• Name the microbes used in fermentation, antibiotic production, sewage treatment and bioremediation, and state what each does.
• Suggest ways to conserve biodiversity and reduce pollution, with examples from the textbook.

Before you start

• Cell structure, mitosis, meiosis and chromosome structure from Class 11.
• The basics of animal classification and the kingdom system.
• Simple chemistry of biomolecules: proteins, nucleic acids, carbohydrates and the bonds that hold them together.

Zoology — the syllabus in depth

What Class 12 Zoology Covers

Reproduction and Development

This theme starts with reproduction in organisms: binary fission, budding, fragmentation and regeneration, followed by sexual reproduction. Human reproduction covers the male and female reproductive systems, spermatogenesis and oogenesis, the menstrual cycle and its hormones (FSH, LH, oestrogen, progesterone), fertilisation, implantation, placenta formation, parturition and lactation. Reproductive health covers contraception, sexually transmitted infections, infertility and assisted reproductive technologies such as IVF, ZIFT, GIFT, ICSI and surrogacy. Learn the order of events in each process, because MCQs often swap two steps.

Principles of Inheritance and Variation

Here you study multiple alleles using the ABO blood groups, the Rh factor and erythroblastosis foetalis, and sex determination in humans (XX-XY), birds (ZW-ZZ) and honeybees (haplodiploidy). The chapter covers sex-linked inheritance such as haemophilia and colour blindness, karyotyping, pedigree analysis, Mendelian disorders like phenylketonuria, albinism, thalassemia, sickle cell anaemia and Huntington's chorea, and chromosomal disorders such as Down, Turner's and Klinefelter's syndromes. It ends with eugenics, euphenics and euthenics.

Molecular Genetics

This chapter explains DNA as the genetic material, the double helix, semi-conservative replication (the Meselson and Stahl experiment), transcription and translation. You will study the features of the genetic code, the lac operon in E. coli, the Human Genome Project and DNA fingerprinting. These ideas are the base for the biotechnology chapter that follows, where the same enzymes and techniques are used to make recombinant proteins, transgenic animals and gene therapy.

Evolution

The evolution chapter covers theories on the origin of life, including the Miller and Urey experiment, and the geological time scale. It sets out the evidence for evolution from anatomy (homologous and analogous organs), embryology and palaeontology. You then study Lamarckism, Darwinism, mutation theory, the Hardy-Weinberg principle, natural selection, speciation and the origin and evolution of man. Know one clear example for each type of evidence and each type of selection.

Human Health, Immunology and Microbes

Human health and diseases covers common infections caused by bacteria, viruses, protozoans, fungi and helminths, along with cancer and substance abuse. Immunology deals with innate and acquired immunity, active and passive immunity, the structure of an antibody, vaccines, hypersensitivity, autoimmune diseases and AIDS. Microbes in human welfare shows microbes at work in curd and bread, antibiotics, biogas, sewage treatment, biocontrol and bioremediation. Applications of biotechnology includes recombinant human insulin, the hepatitis B vaccine, gene therapy and transgenic animals.

Ecology and Environmental Issues

Organisms and populations looks at how organisms respond to abiotic factors, population attributes such as density, natality and mortality, and exponential and logistic growth. Biodiversity and its conservation covers the levels of biodiversity, hotspots, causes of biodiversity loss, in situ and ex situ conservation and sacred groves. Environmental issues deals with air, water and noise pollution, biomagnification, eutrophication, solid waste management and ecological sanitation.

How to study Zoology

Start with the reproduction chapters. They are long but descriptive, and the hormones and cell types you learn there reappear in immunology and biotechnology. Give genetics and molecular genetics the most time, since they need understanding rather than memory: work out blood group crosses and pedigree charts on paper, and trace transcription and the lac operon step by step. Finish with evolution and the ecology chapters, which are easier to revise once the genetics is clear.

One way to split the year into twelve weeks:

1. Weeks 1-3: Reproduction in organisms, human reproduction and reproductive health.
2. Weeks 4-7: Principles of inheritance, molecular genetics, DNA replication and gene expression.
3. Weeks 8-10: Human health and diseases, immunology, microbes and biotechnology.
4. Weeks 11-12: Evolution, organisms and populations, biodiversity and environmental issues.

Practise drawing and labelling the human sperm, the ovum, the structure of an antibody and the lac operon until you can do each without looking. After each chapter, answer the book back questions, take the MCQ test for that chapter here, and then attempt a previous public exam paper under timed conditions.

Common mistakes to avoid

• Mixing up spermatogenesis and spermiogenesis. Spermatogenesis is the whole process by which spermatogonia become sperm. Spermiogenesis is only its last stage, when spermatids change into motile spermatozoa.
• Crediting FSH with ovulation. FSH stimulates the growth of the ovarian follicles. Ovulation happens because of the LH surge in the middle of the cycle.
• Swapping the template and coding strands. RNA polymerase reads the template strand, which runs 3' to 5'. The coding strand runs 5' to 3' and has the same sequence as the mRNA, except that it has thymine where the mRNA has uracil.
• Confusing active and passive immunity. In active immunity your own body makes antibodies after meeting an antigen, through infection or a vaccine. In passive immunity ready-made antibodies are given from outside, as in colostrum or antivenom.
• Treating biomagnification and eutrophication as the same thing. Biomagnification is the rise in concentration of a non-biodegradable toxin such as DDT at each trophic level. Eutrophication is nutrient enrichment of a water body that leads to algal blooms.
• Mixing up eugenics, euphenics and euthenics. Eugenics aims to improve the genes of a population, euphenics treats the symptoms of genetic disorders, and euthenics improves the environment in which people grow.

Frequently asked questions

What is the difference between leading and lagging strands in DNA replication?

DNA polymerase can add nucleotides only to a free 3' end, so new DNA always grows 5' to 3'. On the leading strand this direction points towards the replication fork, so synthesis is continuous. On the lagging strand it points away from the fork, so DNA is made in short pieces called Okazaki fragments, which DNA ligase later joins.

How does the human body prevent polyspermy during fertilisation?

When the first sperm enters the ovum, the cortical granules just below the egg's plasma membrane release their contents. This changes the zona pellucida and forms a fertilisation membrane that other sperms cannot cross. Only one sperm nucleus fuses with the egg, so the zygote stays diploid.

How does the lac operon regulate genes in E. coli?

The lac operon is a group of structural genes (lac z, lac y and lac a) that code for enzymes needed to use lactose, controlled by a promoter and an operator. When lactose is absent, the repressor made by the regulator gene (i) binds to the operator and stops transcription. When lactose is present, it acts as the inducer: it binds the repressor, the repressor leaves the operator, and RNA polymerase transcribes the genes.

How is natural selection different from artificial selection?

In natural selection the environment decides which individuals survive and reproduce, so helpful variations become common over generations. In artificial selection humans decide, choosing and breeding animals or plants with traits they want, such as higher milk yield in cattle. Darwin used artificial selection as evidence that selection can change a species.

What is the difference between active and passive immunity?

Active immunity develops when your own immune system meets an antigen, through infection or vaccination, and makes antibodies and memory cells. It takes time to develop but lasts long. Passive immunity comes from ready-made antibodies given to the body, such as maternal antibodies in colostrum or antivenom after a snake bite. It acts at once but is short-lived.

Why is the biogenetic law not accepted as a universal rule?

Haeckel's biogenetic law said that ontogeny recapitulates phylogeny, meaning an embryo passes through the adult forms of its ancestors. Later work showed that embryos resemble the embryos of their ancestors, not the adults. A human embryo has pharyngeal slits like a fish embryo, but it never looks like an adult fish.

How does biomagnification happen in an aquatic food chain?

Non-biodegradable chemicals such as DDT or mercury enter water and are taken up by plankton. The body cannot break them down or excrete them, so they build up in fat tissue. Each consumer eats many organisms from the level below, so the concentration rises at every step. Fish-eating birds at the top can carry enough DDT to lay thin-shelled eggs that break.

What is the difference between in situ and ex situ conservation?

In situ conservation protects species in their natural habitat, through national parks, wildlife sanctuaries, biosphere reserves and sacred groves. Ex situ conservation protects them outside their habitat, in zoos, botanical gardens, seed banks and by cryopreservation of gametes and embryos. Ex situ methods are used when a species is too threatened to survive in the wild.

What are helper T cells and why does HIV harm them?

Helper T cells (CD4 cells) direct the immune response. They help B cells make antibodies and activate cytotoxic T cells. HIV infects and destroys helper T cells, so their number falls steadily. Without them the immune system cannot respond properly, and the person develops opportunistic infections that a healthy body would control.

What does the Hardy-Weinberg principle tell us?

It states that allele and genotype frequencies in a large, randomly mating population stay constant from one generation to the next if no evolutionary force acts. With two alleles of frequency p and q, p + q = 1 and p2 + 2pq + q2 = 1. If the observed frequencies differ from these values, some force such as mutation, gene flow, genetic drift or natural selection is changing the population.