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BIOLOGY, TEXTBOOK FOR CLASS XII, , 2020-21

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ISBN 81-7450-639-X, First Edition, December 2006 Pausa 1928, , ALL RIGHTS RESERVED, , Reprinted, November 2007 Kartika 1929, January 2009 Pausa 1930, December 2009 Pausa 1931, January 2011 Magha 1932, January 2012 Magha 1933, November 2012 Kartika 1934, November 2013 Kartika 1935, December 2014 Pausa 1936, January 2015 Pausa 1937, January 2017 Pausa 1938, January 2018 Magha 1939, January 2019 Magha 1940, August 2019 Bhadrapada 1941, , q, , No part of this publication may be reproduced, stored in a retrieval system, or transmitted, in any form or by any means, electronic, mechanical,, photocopying, recording or otherwise without the prior permission of the, publisher., , q, , This book is sold subject to the condition that it shall not, by way of trade,, be lent, re-sold, hired out or otherwise disposed of without the publisher’s, consent, in any form of binding or cover other than that in which it is, published., , q, , The correct price of this publication is the price printed on this page, Any, revised price indicated by a rubber stamp or by a sticker or by any other, means is incorrect and should be unacceptable., , OFFICES OF THE PUBLICATION, DIVISION, NCERT, NCERT Campus, Sri Aurobindo Marg, New Delhi 110 016, , Phone : 011-26562708, , 108, 100 Feet Road, Hosdakere Halli Extension, Banashankari III Stage, Bengaluru 560 085, , PD 450T BS, © National Council of Educational, Research and Training, 2006, , Phone : 080-26725740, , Navjivan Trust Building, P.O.Navjivan, Ahmedabad 380 014, , Phone : 079-27541446, , CWC Campus, Opp. Dhankal Bus Stop, Panihati, Kolkata 700 114, , Phone : 033-25530454, , CWC Complex, Maligaon, Guwahati 781 021, , Phone : 0361-2674869, , Publication Team, , ` 195.00, , Printed on 80 GSM paper with NCERT, watermark, , Head, Publication, Division, , :, , M. Siraj Anwar, , Chief Editor, , :, , Shveta Uppal, , Chief Production, Officer, , :, , Arun Chitkara, , Chief Business, Manager, , :, , Bibash Kumar Das, , Assistant Editor, , :, , Shashi Chadha, , Production Assistant, , :, , Mukesh Gaur, , Published at the Publication Division by the, Secretary, National Council of Educational, Research and Training, Sri Aurobindo Marg,, New Delhi 110 016 and printed at Nova, Publications, C-51, Focal Point Extn.,, Jalandhar-144 004, , Cover and Layout, Shweta Rao, Illustrations, Lalit Maurya, , 2020-21

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FOREWORD, The National Curriculum Framework (NCF) 2005, recommends that children’s life at school, must be linked to their life outside the school. This principle marks a departure from the, legacy of bookish learning which continues to shape our system and causes a gap between, the school, home and community. The syllabi and textbooks developed on the basis of NCF, signify an attempt to implement this basic idea. They also attempt to discourage rote learning, and the maintenance of sharp boundaries between different subject areas. We hope these, measures will take us significantly further in the direction of a child-centred system of, education outlined in the National Policy on Education (1986)., The success of this effort depends on the steps that school principals and teachers will, take to encourage children to reflect on their own learning and to pursue imaginative activities, and questions. We must recognise that, given space, time and freedom, children generate, new knowledge by engaging with the information passed on to them by adults. Treating the, prescribed textbook as the sole basis of examination is one of the key reasons why other, resources and sites of learning are ignored. Inculcating creativity and initiative is possible, if we perceive and treat children as participants in learning, not as receivers of a fixed body, of knowledge., These aims imply considerable change in school routines and mode of functioning., Flexibility in the daily time-table is as necessary as rigour in implementing the annual, calendar so that the required number of teaching days are actually devoted to teaching. The, methods used for teaching and evaluation will also determine how effective this textbook, proves for making children’s life at school a happy experience, rather than a source of, stress or boredom. Syllabus designers have tried to address the problem of curricular burden, by restructuring and reorienting knowledge at different stages with greater consideration, for child psychology and the time available for teaching. The textbook attempts to enhance, this endeavour by giving higher priority and space to opportunities for contemplation and, wondering, discussion in small groups, and activities requiring hands-on experience., The National Council of Educational Research and Training (NCERT) appreciates the, hard work done by the textbook development committee responsible for this book. We, wish to thank the Chairperson of the advisory group in science and mathematics,, Professor J.V. Narlikar and the Chief Advisor for this book, Professor K. Muralidhar,, Department of Zoology, University of Delhi, Delhi for guiding the work of this committee., Several teachers contributed to the development of this textbook. We are grateful to their, principals for making this possible. We are indebted to the institutions and organisations, which have generously permitted us to draw upon their resources, material and personnel., We are especially grateful to the members of the National Monitoring Committee, appointed, , III, , 2020-21

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by the Department of Secondary and Higher Education, Ministry of Human Resource, Development under the Chairmanship of Professor Mrinal Miri and Professor G.P. Deshpande,, for their valuable time and contribution., As an organisation committed to systemic reform and continuous improvement in the, quality of its products, NCERT welcomes comments and suggestions which will enable us to, undertake further revision and refinement., , Director, New Delhi, , National Council of Educational, Research and Training, , 20 November 2006, , IV, , 2020-21

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PREFACE, Biology is the study of life in its entirety. The growth of biology as a natural science during, the last 1000 years is interesting from many points of view. One feature of this growth is, changing emphasis. Initially it was description of life forms. Identification, nomenclature,, classification of all recorded living forms enjoyed the attention of scientists for a long time., Description of their habitats and (in the case of animals) their behaviour was included in, this study. In later years, the focus was physiology and internal morphology or anatomy., Darwinian ideas of evolution by natural selection changed the perception completely. Classical, descriptive and clueless biology found a theoretical framework in the evolutionary theory of, Darwin., In the nineteenth and twentieth centuries, Physics and Chemistry were applied to Biology, and the new science of Biochemistry soon became the dominant face of biology. On one hand, Biochemistry was integrating with Physiology, becoming almost synonymous with it. On the, other hand it gave rise to Structural Biology (structure of biomacromolecules), originally called, Molecular Biology. The work of Bernal, Pauling, Watson and Crick, Hodgkins, Perutz and Kendrew,, Delbruck, Luria, Monod, Beadle and Tatum, Lederberg, Brenner, Benzer, Nirenberg, Khorana,, Mclintock, Sanger, Cohen, Boyer, Kornbergs (father and son), Leder, Chambon and scores of, others brought in and established a modern version of Molecular Biology dealing with life, processes at molecular level., Physics and Chemistry dominated public perception of science for a long time., Daytoday life of man was influenced by developments in Physics, Chemistry and their respective, manufacturing industries. Slowly and steadily, Biology, not to be left behind, demonstrated, its utility for human welfare. Medical practice, especially diagnostics, green revolution and, the newly emerging biotechnology and its success stories made the presence of biology felt, by the common man. Patent laws brought biology into political domain and commercial value, of biology became obvious., For more than a century, classical and so-called reductionist biology fought artifical, battles. The fact is both are important. Ecology brought in synthesis of both approaches, and emphasised integrated understanding of biology. Form and process are both equally, important. Systems biology, using mathematical tools, is bringing about a modern synthesis, of both the aspects of Biology., The Class XI and XII textbooks in biology essentially were to reflect these threads of, biological thought. While the Class XI book dealt with morphology, taxonomy, molecular and, cellular aspects of physiology, the Class XII book deals with the physiological process of, reproduction in flowering plants and humans, the principles of inheritance, the nature of, genetic material and its function, the contributions of biology to human welfare, basic, principles of biotechnological processes and their applications and achievements. The, Class XII book also relates genes to evolution on one hand and presents ecological interactions,, behaviour of populations and ecosystems on the other. Most important, the guidelines under, NCF-2005 have been followed in letter and spirit. The total learning load has been reduced, , V, , 2020-21

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considerably and themes like environmental issues, adolescent problems and reproductive, health have been dealt with in some detail. Studied together, the class XI and class XII, textbooks in Biology would enable the student to —, (i) become familiar with the diversity of biological material., (ii) appreciate and believe in the Darwinian evolutionary process exhibited by the living, world., (iii) understand the dynamic state of constituents of living bodies, i.e., metabolic, basis of all physiological processes in plants, animals and microbes., (iv) realise the structure and function of genetic material in directing the inherited, phenotype pattern as well as a mediator of evolutionary process., (v) appreciate the profound contributions of biology to human welfare., (vi) reflect on the physico-chemical basis of living processes and at the same time, realise the limitation of reductionism in understanding behaviour of organisms., (vii) experience the humbling effect of this realisation that all living organisms are, related to each other by virtue of shared genetic material., (viii) realise that biology is the story of the struggle of living organisms for existence, and survival., One may notice a perceptible change in the writing style. Most of the chapters are written, in an easy dialogue style engaging the student constantly while some chapters are in the, form of critical comments on the subject matter. A number of questions have been provided, at the end of each chapter though answers to some may not be found in the text. Students, have to read supplementary material, upon advise from the teacher, to answer such questions., I am thankful to Professor Krishna Kumar, Director NCERT; Professor G. Ravindra, Joint, Director, NCERT and Professor Hukum Singh, Head, DESM, NCERT for constant support. I, must place on record my deep appreciation for Dr B.K. Tripathi, Reader, DESM, NCERT for, his relentless efforts as coordinator in bringing out the Biology textbook for both the, Class XI and XII. All the members of the development team, the experts and reviewers, and, the school teachers have contributed enormously in the preparation of this book. I thank, them all. I am indeed highly thankful to the members of monitoring committee constituted, by Ministry of Human Resource Development for their valuable observation that helped in, the improvement of the book at the final stage. The book is prepared keeping in mind the, guidelines of the NCF-2005 especially the emphasis on reducing the learning load. We hope that, the book would meet the expectations of all the stakeholders. All suggestions for further, improvement are always welcome., K. MURALIDHAR, Chief Advisor, Biology Textbook for Class XII, , Department of Zoology, University of Delhi, , VI, , 2020-21

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CONTENTS, FOREWORD, , iii, , PREFACE, , v, , UNIT VI, REPRODUCTION, , 1-66, , Chapter 1, , : Reproduction in Organisms, , 3, , Chapter 2, , : Sexual Reproduction in Flowering Plants, , 19, , Chapter 3, , : Human Reproduction, , 42, , Chapter 4, , : Reproductive Health, , 57, , UNIT VII, GENETICS, , AND, , EVOLUTION, , 67-142, , Chapter 5, , : Principles of Inheritance and Variation, , 69, , Chapter 6, , : Molecular Basis of Inheritance, , 95, , Chapter 7, , : Evolution, , 126, , UNIT VIII, BIOLOGY, , IN, , HUMAN WELFARE, , 143-190, , Chapter 8, , : Human Health and Disease, , 145, , Chapter 9, , : Strategies for Enhancement in, Food Production, , 165, , Chapter 10, , : Microbes in Human Welfare, , 179, , 2020-21

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UNIT IX, BIOTECHNOLOGY, , 191-216, , Chapter 11 : Biotechnology : Principles and Processes, , 193, , Chapter 12 : Biotechnology and its Applications, , 207, , UNIT X, ECOLOGY, , 217-286, , Chapter 13 : Organisms and Populations, , 219, , Chapter 14 : Ecosystem, , 241, , Chapter 15 : Biodiversity and Conservation, , 258, , Chapter 16 : Environmental Issues, , 270, , X, , 2020-21