NCERT Class 12 Biology PDF

llim.tll'l II  CONTENTS FoREWORD iii PREFACE vii UNIT VI REPRODUCTION 1-66 Chapter 1 : Reproducti...

llim.tll'l II  CONTENTS FoREWORD iii PREFACE vii UNIT VI REPRODUCTION 1-66 Chapter 1 : Reproduction in Organisms 3 Chapter 2 : Sexual Reproduction in F1owering 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 BIOWGY IN HUMAN WELFARE 143-190 Chapter 8 : Human Health and Disease 145 Chapter 9 : Strategies for Enhancement in 165 Food Production Chapter 10 : Microbes in Human Welfare 179 UNIT IX BIOTECHNOWGY 191-216 Chapter 11 : Biotechnology : Principles and Processes 193 Chapter 12 : Biotechnology and its Applications 207 IX UNIT X EcOWGY 217-286 Chapter 13 Organisms and Populations 219 Chapter 14 : Ecosystem 241 Chapter 15 Biodiversity and Conservation 258 Chapter 16 Environmental Issues 270 X Chapter I Biology in essence is the story of life on earth. While individual Reproduction in Organisms organisms die without fail, species continue to live through millions of years unless threatened by na !ural or anthropogenic Chapter:2 extinction. Reproduction becomes a vital process without Sexu at Re produc lion in which species cannot survive for long. Each individual leaves flowering Plants its progeny by as exu at or sexua I me a ns. Sexua I mode of reproduction enables creation ofnewvarianls,so that survival Chapter:3 advantage is enhartced. This unit examines the gerteral Human Re produc lion principles unde rtying reproductive processes in living orga nisrns and then explains the details of this process in flowering plants Chapter4 and humans as easy to relate representative examples. A related Reproductive Health perspective on human reproductive health and how reproductive ill health can be avoided is also presented to complete our understanding of biology ofreproductiort. _I Bom in November 1SD4 in Japur (Raja; than) Panchanan M aheshwai ros e to become o ne of the most disting u ished b otanists not only of India but of the e ntire w orld. He move d to A llahab ad fo r higher educ a tion where he o b ldned his D.Sc. Du ring hi; colleg e d ays, he w a; i n; p~ed by Dr W. Dudgeon. a n A m erican m iss ion ary te a cher, to clevelop interest in Bota ny and especially morphology. His teacher once expressed that if his student p rogress es ahead of him , it wil give him a great satisfac tion. Thes e w ord> encouraged Pcnchanan to enquire wha t he could d o for his te a cher in re turn. He worked on embryological aspe cts and pop u larise d the u;e of e mbryolog ical chra cters in taxonomy. He esta blished the D epartment of Botany, Unile rsity of Delhi a s an impo rta nt cen tre of researc h n e mbryology and I ssue c ulh.xe. He also e mphasised the need for initia tion of w ork o n a rtificial culture of im mature embryos. These d ays. tissue culture ha; beco m e a landmark in science. His work o n tes t tube PANC HA AAN MJ~..H ES HWARI ferlili; alion a nd intra -ovaricn p ollina tion won w orldwide a cclaim. (1904-1966) He wa; honoure d w ith felowship of Royal Socie ty of l o ndon (FRS), India n Na tional Science Acade my a nd several o ther in;titutions of excellence. He enco urag ed gene ral e d ucation and macle a significant contrib ution to scho ol e d ucation by h i; leadership in bring ing out the v e ry f irs t textbooks of Biology for Higher Secon dary Schools pub lished by NCERT in 1964.  i I I j CHAPTER 1 REPRODUCTION IN ORGANISMS 1.1 Asexual Reproduction 1.2 Sexual Each a nd every organis m can live only for a cettaln period Reproduction of Ume. The period from birth to the na tural death of a n organism represents Its life span. Life spans of a few organisms are given In Figure 1.1. Several otherorganisms a re d rawn for w hich you s hould find out their life spans and write in the spaces provided. Examine the life s pans of organisms represented In the Figure 1.1. Is n't It both Interesting and Intriguing to note that It may be as shmt as a few days or as long as a few thousand years? Between these two extremes a re the life s pans of mos t other living organis ms. You may note that life s pans of organis ms are not necessarily correla ted With their s izes; the s izes of crows and pru1·ots ru·e not very different yet their life spans show a Wide difference. Similarly, a mango tree has a much s horter life s pan as compared to a peepal tree. Wha tever be the life s pan, death of evety Individual organis m Is a certainty, I.e., no IndiVIdual Is Immortal, except single-celled organisms. Why do we say there is no natural death in single-celled organisms? Given this reality, have you ever wondered how vast number of plant a nd a nima l species have exis ted on eruth for several thousands ofyeru·:::? There mus t be some processes In llvtng organisms tha t ens ure this continuity. Yes, we ru·e talking about reproduction, something that we take for granted.  81010GY Dot! _ _ B,_. lly ( 1-2 we bJ Crow' ( 15 Y"""'l llcnle(_) Fis,-ure 1.1 Approximate life~ of some or@r'Sms KEfROOOC ~0 N IN ORGMJI~il~$ Reproduction is defined as a biological process in which an organism gives lise to young ones (offsplingJ similar to itself. The offspling grow, ~lure and in tum produce new offspling. Thus, there is a cycle o fbirth, growth and death. Reproduction enables the continuity of the species, generation after generation. You will study later in Chapter 5 (Principles of lnhelitance and Variation) how genetic valiation is created and inhelited duling reproduction. There is a large diversity in the biological world and each organism has evolved its own mechanism to multiply and produce offspring. The orgo..'lism's hobito.t, its internal physiology o.nd severo.! other factors are collectively responsible for how it reproduces. Based on whether there is participation of one organism or two in the process of reproduction, it is of two types. When offspring is produced by a single parent with or without the involvement of gamete formation, the reproduction is asexual. When two parents (opposite sex) participate in the reproductive process and also involve fusion of male and female gametes, it is called sexual reproduction. 1.1 Asi·:xuAL R EPROiHJCTJON In this method, a single individual (parent) is capable of producing offspring. As a result, the offspring that are produced are not only itleuLical Lo oue auoLlier uu L are a!:so exacL copie:s of Llieir pareu L. Are these offspr(ng l(kety to be genet(calty (c/enttcal or different? The term clone is used to describe such morphologically and genetically similar individuals. Parent cell (b) (a) Figure 1.2 Cell division in unicellular orgar.ism: (a) Budding in yeast; (b) Binary fission in Amoeba Let u~ see how widespread asexual reproduction is, among different groups of organisms. Asexual reproduction is common among single-celled organisms, and in plants and anima:s With relatively simple organisations. In Protists and Monerans, the organism or the parent cell divides into two to give lise to new individuals (Figurel.2). Thus,  BIOLOGY (a) (c) (d) Figure 1. 3 Asexual reproductive structures: (a) Zoospores of chlamydomonas; (b) Conidia of PeniciUiwn; (c) Buds 1n Hydra; (d) Gemmules 1n sponge in these organisms cell division is itself a mode of reproduction. Many single-celled organisms reproduce by binary fission, where a cell di\ides into two halves and each rapidly grows into an adult (e.g. , Amoeba, Paramecium). In yeast. the di\ision is unequal and small buds are produced that remain attached initially to the parent cell which, eventually gets separated and mature into new yeast organisms [cells). Members of the Kingdom Fungi and simple plants such as algae reproduce through special asexual reproductive structures (Figure 1.3). The most common of these structures are zoospores that usually are microscopic motile structures. Other common asexual reproductive structures are conidia (PenidUtum), buds (Hydra) and gemmules (sponge).  (a) Adventitious (b) Root (c) (e) (d) Fi&....., 1.4 ITe@ut oe9etatwe reproduction in p!arlts in Class Xl. INhat do gou tNnl null'tler a fruits anclele. Another plant. ~obilattl\us 1a.u\ft\lm\(l(neelakuranji). !lowers once in 12 )"ars. As many a you 1!0Uicl be lu>owing that this plant flowered ctunng September -October 2006. Its mass OC>Wenile pha:se is lollowed by rrorphoiclgicaland phySiOlogical changes priOr to acUve reproc:lucuve tehalllour. The reproc:luCU;e phase is also of variable duraUonindifferentotganis ms. Cng animals. lbr example birds, do they tay eggs aU II'U"ough the yea!? Or is 11 a seasonal phenorrenon? What ab out other animals like frogs and Uzarcls? You will noUce that, birds liVing in nature lay egg,. oo!y seasonolly. Howe,.,r. birds in captiVity (as in poultry !Mms) can be made to Ia)' eggs throughout the year. In this case,lay;ng eggs is not related to repro:tucUOn but is a con:u:rercial exploitation for human ~lfare. Tile iomales aplaeental mammals exhibit c)'clicalchane>s in the aCUViUesof ooanes and accessa)' ducts as...nas b::t naes ctunng the roprcduCU"' phase. In non-pnmale manmalslilte rows, sheep. rals.cleers.clogs.liger. etc.. such c)'CIIcalchan~sdunngreptodu clicn are called oestrv.s cycle Whore as in pnmates (tn:llllrphologicallydistiftc\ \Jij)es (heterogametesj. In such org.arusms tll.e awe g.arrete is called llie antherozo1d or spenn ond the femalegomete is called the egg or ovum (Figure!.& b. c). Sexuality In organisms: Sexual reproductiOn in orgonisms gener.ny involves llie fusiOn of g.arretes from. tloi.'O different indiVidu.als. But tll.is is not alw.ays true. From gour recotledlon of examples stu.dled Ln C!a.ss M. ""'gou ot&s ~·...... Na.mt~of~m Ch:romo:some numbe.r in """"";-te (2n) Chromo.some numbe.r in (,vnote (n) ~ Hv:m«n bf"~gJ 46 23 H"-'ll)y 12 Rot 21 Dd. E\oerU of secual reprodmtion may be ca~rtsed 1rl.o p fertllls:ation far~n:md post,fertilisation.,..,..,ts. FN-fert111s>.tlon """"ts ll'clude ga.m.et.ogemslS 3.nd g3Jl'l.ete trMlSfier while pos t-fe:rUhn.Uon events !nOI\Ida \ha form..tlon of zygote =d embryo~. Org.ar.dsms m~ be bise:xu..31 or WliSexw.l. Sexuality tn plants ts ,..,led. p>rt.louWly 1n ~sperms. due lo Ue produc\Jon of dlvorse types o£ Oowers. P13rtls are defined as monoee:1ous.-rd. dloectous. Fbwcs m».y be btsexu.al or uniseXual flov.ws. ~ ""' hapb>i 1n nob.Un. It 11 reb.tl.valy easy 1n bisexual organs:ms. In uni.Sie:Xw.l.an.tna1s it oocu:rs bf copo 1"k'-'\ 01 sunult.ane::.us release. in a~~spaa1a.,. aspeoW prooess oallod poii!NI.>on Enitns trmsfar d p:>llen~- ~ u.. pollen I§""" to \he~ 6f~""f fkIe. 1he process of de,.elopmen\ of embryo &om U.. zygoia !s called ort~bryoea.-s. In~ lhe zyb'>te starts d...alopfie< 1ts '·....',:::. :, fonnatlon Animals may be either oViparous or Vi.Vipuou.s. Ewlbryonal protAotlon.a.r:d CMe axe better 1n ViViparous or@J'U~. ~~·:.'; ;.~.: In f1owenng pl.ms. oJ\er ferllltso.t!on. ovo.ry de''Oiops 1nto frutl =d : ;· ovu.t. m».tu.re tnto seeds. hls1de the mature seed 1S tht progenitor of th1JWCl ganer3lion ~ e:rnbryo. · ,,,,,,,,,,,,.......................................................................................................:.....\,., EXI F