Cell Cycle and Cell Division: Class 11 Biology Chapter 10 Hinglish mein

Tumhari shuruaat ek single cell se hui thi: ek fertilised egg jo lagbhag ek millimetre ke dasven hisse jitna chauda tha. Aaj tumhare body mein lagbhag 37 trillion cells hain, aur har ek cell usi pehle cell se division ke through aaya hai. Agar har division sirf count ko double karta, to lagbhag 45 rounds kaafi hote, kyunki $2^{45}\approx 3.5\times {10}^{13}$. Asli development isse zyada uljha hua hai, cells alag-alag speed se divide hote hain aur kai cells tay samay par mar bhi jaate hain, lekin principle wahi rehta hai: ek organism un cells se banta hai jo baar-baar grow karte hain, apni copy banaate hain aur split hote hain.

Chapter 8 mein tumne jo cell theory padhi thi, woh kehti hai ki naye cells sirf pehle se maujood cells se hi bante hain. Yeh chapter poochta hai ki ek cell yeh kaise karta hai, bina apne genetic instructions ko khoye ya bigaade. Is sawaal ke jawab do tarah ke division hain: mitosis, jo aise daughter cells deta hai jo genetically parent jaise hote hain, aur meiosis, jo sexual reproduction ke cells banane ke liye chromosome number ko aadha kar deta hai.

10.1 Cell Cycle

Ek cell do working cells ban sake, usse pehle use teen kaam poore karne hote hain: grow karna (zyada cytoplasm, membranes aur organelles banana), apne DNA ki theek ek baar copy banana, aur sab kuch barabar baantna. Socho, agar cell replication poora hone se pehle hi split ho jaaye: to ek daughter ko toota hua genome milega. Agar woh do baar replicate kar le: to daughter ko extra copies mil jaayengi. Isliye yeh teen kaam ek timetable se chalte hain, aur yeh timetable khud cell ke apne genes tay karte hain.

Cell cycle mein ek asymmetry par dhyaan do. Poocho ki cell cytoplasm kab badhata hai, to jawab hai lagbhag hamesha; poocho ki woh DNA kab copy karta hai, to jawab hai sirf ek window mein, jise S kehte hain. Phir division ko har chromosome ki ek copy har daughter nucleus tak pahunchani hoti hai (Fig. 10.1).

10.1.1 Cell Cycle ke Phases

Cycle ki length mein bahut fark hota hai. Ek yeast cell lagbhag 90 minutes mein chakkar poora kar leta hai; culture dish mein grow ho rahe human cells ko lagbhag ek din lagta hai; aur ek hi body ke alag-alag tarah ke cells bilkul alag schedules follow karte hain.

Ek cultured human cell ki time-lapse film dekho to lagta hai ki woh din ke zyada hisse mein kuch khaas nahi karta, phir lagbhag ek ghante mein split ho jaata hai. Woh ek ghanta M phase (mitosis phase) hai. Ek M phase ke end se agle M phase ki shuruaat tak ka lamba hissa interphase hai. 24 mein se lagbhag 23 ghante wahin beette hain, isliye interphase cycle ka 95 % se zyada hissa leta hai.

M phase ke khud ke do parts hain. Pehle nucleus divide hota hai, aur alag hue daughter chromosomes do naye nuclei mein ikattha ho jaate hain: yeh karyokinesis hai (Greek karyon, yaani kernel ya nucleus, aur kinesis, yaani movement). Aam taur par uske baad cytoplasm do hisson mein kat jaata hai, jo cytokinesis hai (cyto-, yaani cell).

Isi dikhne wali susti ki wajah se interphase ko purana nickname resting phase mila. Yeh naam galat impression deta hai: interphase asal mein taiyaari ka samay hai, jab cell ek tay order mein grow karta hai aur apna DNA replicate karta hai. Iske teen sub-phases hain:

• G1 (gap 1): pehle grow karo. Naya bana daughter cell chhota hota hai. Full metabolic speed par kaam karte hue woh proteins, organelles aur cytoplasm banata hai, jabki uska DNA bilkul waisa hi rehta hai: abhi koi replication nahi.
• S (synthesis): genome copy karo. Har DNA molecule ek baar replicate hota hai, aur cell 2C se 4C ho jaata hai. Centromeres double nahi hote, isliye cell mein abhi bhi 2n chromosomes hote hain, aur ab har ek sister chromatids ka ek pair hai. Animal cell mein isi samay cytoplasm mein centriole bhi copy hota hai.
• G2 (gap 2): mitosis ki taiyaari. Growth chalti rehti hai, aur cell woh proteins banata hai jo division ke liye chahiye honge.

Yahan do alag counts chal rahe hain, isliye unhe alag rakho. C ka matlab hai chromosomes ke ek haploid set mein DNA ki amount, isliye G1 mein ek diploid nucleus mein 2C hota hai. n chromosomes ko ginta hai, aur ek chromosome ko uske centromere se gina jaata hai. Replication C ko double karta hai, lekin koi naya centromere nahi jodta, isliye chromosome count wahi rehta hai (Fig. 10.2).

Cycle se bahar: quiescent stage (G0)

Ek surgeon kisi insaan ke liver ka ek hissa nikaal sakta hai aur organ wapas grow ho jaata hai, jabki pehle uske cells lagbhag divide hi nahi ho rahe the. To woh kahan the? Cycle ke bahar park kiye hue. Kuch cells, jaise adult heart ke muscle cells, lagta hai ki phir kabhi divide nahi hote; kai doosre cells tab tak intezaar karte hain jab tak body ko unki zaroorat na ho, jaise injury ya death se khoye cells ko replace karne ke liye.

Yeh parking place quiescent stage, G0, hai, jismein cell G1 se cycle chhod kar jaata hai. Park hone ka matlab band ho jaana nahi hai: G0 wala liver cell proteins banata rehta hai, glycogen store karta hai aur blood saaf karta hai. Woh bas tab tak multiply karna rok deta hai jab tak organism ki zaroorat use wapas na bulaaye.

Kaun se cells mitosis se divide ho sakte hain? Plants mein haploid aur diploid dono cells, jaisa unki alternating generations dikhati hain. Animals zyada strict hain: mitosis aam taur par sirf diploid somatic (body) cells tak seemit rehta hai. Male honey bee yeh rule todta hai, kyunki woh ek unfertilised egg se banta hai aur isliye poora haploid cells se bana hota hai, jo sab mitosis se bante hain.

10.2 M Phase

Biologists karyokinesis ko chaar naam wali stages mein baantte hain, prophase, metaphase, anaphase aur telophase, lekin cell kabhi rukta nahi. Yeh stages ek lagaataar chalti film se chune gaye frames jaisi hain: ek stage agli mein ghul jaati hai, aur har line kahan kheenchi jaaye yeh kuch had tak suvidha ki baat hai (Fig. 10.3).

Us ek ghante mein cell ka lagbhag har part reorganise hota hai. Chromatin compact chromosomes mein pack hota hai, nuclear envelope aur kai organelles toot jaate hain, ek spindle banta hai, aur phir sab kuch do baar dobara banaya jaata hai, har daughter mein ek baar. Har daughter ke paas aakhir mein theek parent jitna chromosome number hota hai, isiliye mitosis ko equational division kehte hain.

10.2.1 Prophase

Socho ki ek lambe necklace ki do copies drawer mein aapas mein uljhi padi hain: S aur G2 ke baad har chromosome ka DNA aise hi pada hota hai. Dono copies maujood hain, lekin tum unhe alag se pehchaan nahi sakte. Prophase tab shuru hota hai jab cell is chromatin ko coil aur fold karna shuru karta hai, aur ise tightly pack karna hi ise suljhata bhi hai. Har chromosome ek chhoti, moti body ke roop mein saamne aata hai jiske do chromatids alag pehchaane ja sakte hain (Fig. 10.4).

Isi beech cytoplasm mein ek doosri taiyaari chalti hai. S mein bane do centrosomes cell ke opposite ends ki taraf khisakte hain, aur har ek ke chaaron taraf phailte microtubules ka ek starburst hota hai jise aster kehte hain. Unke beech spindle fibres grow karte hain, aur yeh poora assembly, do asters plus spindle, mitotic apparatus kehlata hai.

Prophase ke end tak microscope mein dekha gaya cell yeh dikhata hai:

• compact chromosomes, har ek centromere par jude do chromatids se bana;
• do centrosomes jo opposite poles ki taraf ja rahe hain ya pahunch chuke hain, har ek apne aster ke saath;
• koi Golgi complex, endoplasmic reticulum, nucleolus ya nuclear envelope nahi.

10.2.2 Metaphase

Agar tum chromosomes ki photo lena chahte ho, to metaphase hi sahi waqt hai. Is samay woh sabse zyada condensed hote hain, cytoplasm mein phaile hote hain, aur koi nuclear envelope beech mein nahi aata, kyunki uska poori tarah tootna hi is stage ki shuruaat mark karta hai. Unka shape, size aur number abhi study karna sabse aasaan hai.

Har chromosome par ek tug-of-war socho. Ropes spindle fibres hain aur handles kinetochores hain, yaani centromere ki surface par chhote discs, har sister chromatid ka ek, aur dono ulti dishaon mein mukh kiye. Jab ek pole ke fibres ek handle pakad lete hain aur doosre pole ke fibres doosra handle, to kheenchaav tabhi balance hota hai jab chromosome dono poles ke theek beech mein ho. Isliye chromosomes spindle ke equator ke aar-paar ek hi plane par, agal-bagal baith jaate hain: yahi metaphase plate hai.

To agar har chromosome dono taraf se bandha hai aur poora set cell ke beech mein ek line mein baitha hai, to tum metaphase dekh rahe ho.

10.2.3 Anaphase

Anaphase shuru karne ka signal har chromosome tak ek saath pahunchta hai: saare centromeres ek saath split hote hain. Us pal se har purana chromatid apne aap mein ek chromosome gina jaata hai, yaani daughter chromosome, aur use us pole ki taraf kheencha jaata hai jiski taraf uska kinetochore mukh kiye hai. Kinetochore aage chalta hai aur arms bina zor lagaye peeche chalte hain, jisse har chalta chromosome V ya J shape mein mud jaata hai jiski nok pole ki taraf hoti hai.

Ek quick test: chromosomes ke do milte-julte groups jo door ja rahe hain, aur abhi koi nuclear envelope nahi, matlab anaphase.

10.2.4 Telophase

Telophase, karyokinesis ki aakhri stage, prophase ko ulta chalata hai. Har pole par chromosomes ikattha hote hain aur khul kar chromatin ban jaate hain, aur unki alag outlines kho jaati hain. Har group ke chaaron taraf nuclear envelope dobara banta hai, jisse do daughter nuclei milte hain, aur nucleolus, Golgi complex aur endoplasmic reticulum, jo prophase mein gayab ho gaye the, wapas aa jaate hain.

10.2.5 Cytokinesis

Karyokinesis ke baad ek hi cell ke andar do nuclei reh jaate hain. Division tabhi poora hota hai jab cytoplasm bhi cytokinesis se split ho jaaye. Animal aur plant cells yeh ulti dishaon mein karte hain (Fig. 10.5).

Animal cell waise hi divide hota hai jaise tum aate ki loi ko dhaage ke loop se kaatte ho: woh apne beech mein ek ring kasta hai. Yeh ring, jo flexible plasma membrane ke theek neeche actin aur myosin filaments se bani hoti hai, sikudti hai aur membrane ko andar ki taraf kheench kar ek cleavage furrow banati hai, jab tak cell do hisson mein na bhinch jaaye.

Plant cell ek aisi wall mein band hota hai jo mushkil se khinchti hai, isliye use dabakar do hisson mein nahi kiya ja sakta. Iske bajaay woh andar se apna partition banata hai. Daughter nuclei ke theek beech mein ek cell plate banti hai aur side mein phailti jaati hai jab tak uske kinare purani walls se jud na jaayein. Yeh plate aage chalkar middle lamella banti hai, yaani do naye padosi cells ki walls ke beech ki cementing layer.

Har daughter ko apne hisse ke mitochondria aur plastids bhi lekar jaane hote hain. Yeh organelles shuru se assemble nahi kiye ja sakte: yeh khud grow aur divide karte hain, isliye har naye cell ko kuch virasat mein milne chahiye.

Cytokinesis hamesha karyokinesis ke baad nahi aata. Jab nuclei divide hote rehte hain lekin cytoplasm nahi, to result hota hai kai nuclei wala cytoplasm ka ek single mass, yaani syncytium. Tumne iska swaad chakha hai: kachche nariyal ka paani liquid endosperm hai, ek aisa cytoplasm jismein kai free nuclei hain aur unke beech koi cell walls nahi.

10.3 Mitosis ka Mahatva

Mitosis, yaani equational division, zyaadatar diploid cells mein hota hai. Lekin yeh sirf unhi tak seemit nahi hai: kuch lower plants ke haploid cells, aur bees jaise social insects ke haploid cells bhi isi tarah divide hote hain. Yeh jahan bhi ho, iska defining result ek hi hai: parent jitne chromosome number aur same genetic information wale do daughter cells. Yahi ek property mitosis ko kai tarah se useful banati hai:

• Growth. Ek multicellular body mainly cells jod kar grow karti hai: baar-baar mitosis se zygote embryo banta hai aur phir adult.
• Nucleo-cytoplasmic ratio wapas laana. Jaise-jaise cell grow karta hai, uska cytoplasm badhta hai lekin nucleus utni tezi se nahi badhta, isliye ek nucleus ko lagaataar bade volume ko control karna padta hai. Divide hone se nucleus aur cytoplasm ke beech balance wapas aata hai (Fig. 10.6).
• Repair aur replacement. Skin ke sabse bahari cells, gut ki lining ke cells aur blood cells lagaataar ghis kar khatam hote rehte hain, aur mitosis naye cells deta hai; ghaav bhi isi tarah bharta hai.
• Plants ki zindagi bhar growth. Meristems mitosis se divide hote rehte hain: root aur shoot tips ke apical meristems plant ko lamba karte hain, aur lateral meristems (cambium) use mota karte hain, isliye plants poori zindagi grow karte rehte hain.

10.4 Meiosis

Sexual reproduction do gametes ko jodta hai. Agar har gamete mein poora diploid set hota, to har generation mein chromosome number double ho jaata: tumhare parents mein 46, tum mein 92, tumhare bachchon mein 184. Nature ise ek special division se rokti hai jo gametes bante samay chromosome number ko aadha kar deta hai. Woh division meiosis hai. Yeh sirf specialised diploid cells mein hota hai aur haploid daughter cells banata hai.

Life cycle ko do switches wale ek loop ki tarah socho. Ek switch, meiosis, cells ko diploid se haploid kar deta hai; doosra, fertilisation, unhe wapas palat deta hai. Plants aur animals dono pehla switch gametes banate samay dabaate hain, yaani gametogenesis ke dauraan.11Flowering plants mein meiosis pehle haploid spores (microspores aur megaspores) banata hai, jo phir mitosis se gametes banate hain. Class 12, Chapter 1 mein ise detail mein padhoge.

Meiosis chaar numbers mein:

• 1 round DNA replication ka, meiosis shuru hone se pehle wale S phase mein, isliye har chromosome meiosis I mein do sister chromatids ke roop mein aata hai;
• 2 divisions ek ke baad ek, meiosis I aur meiosis II, unke beech koi replication nahi;
• 2 homologues har pairing mein: har chromosome ki maternal aur paternal copies pair banati hain aur apne non-sister chromatids ke beech segments exchange karti hain;
• 4 haploid cells meiosis II ke end mein.

Har division jaani-pehchaani chaar stages se guzarta hai, jin par I ya II ka tag hota hai: pehle division mein prophase I, metaphase I, anaphase I aur telophase I, aur doosre mein prophase II, metaphase II, anaphase II aur telophase II (Fig. 10.7).

10.4.1 Meiosis I

Prophase I lamba aur ghatnaon se bhara hota hai, mitosis ke prophase se kahin zyada. Chromosomes kya karte hain yeh dekh kar cytologists ise paanch sub-stages mein baantte hain: leptotene, zygotene, pachytene, diplotene aur diakinesis (Fig. 10.8). Greek roots inhe yaad rakhne mein madad karte hain: lepto- patla, zygo- juda hua (yoked), pachy- mota, diplo- double, aur dia-kinesis alag hona.

Leptotene. Nucleus mein patle threads bhar jaate hain: chromosomes itne condense ho chuke hote hain ki light microscope se dikh sakein, aur stage aage badhne ke saath aur compact hote jaate hain.

Zygotene. Har chromosome apne partner ko dhoondhta hai, yaani doosre parent se mila homologue, aur dono agal-bagal zip ki tarah jud jaate hain. Is zipping ko synapsis kehte hain, aur khud zip, jo sirf electron microscope se dikhta hai, proteins ki ek seedhi jaisi structure hai jise synaptonemal complex kehte hain. Ek zipped pair ek unit gina jaata hai, yaani bivalent; ise tetrad bhi kehte hain, haalaanki iske chaar chromatids pachytene mein hi aasaani se dikhte hain.

Pachytene. Yeh lambi aur busy stage hai; iske comparison mein leptotene aur zygotene chhoti hain. Bivalent ab saaf taur par apne chaar chromatids dikhata hai, har homologue ke do. Jagah-jagah pair par chhote dense recombination nodules baithe hote hain, aur har ek ek crossing over ko mark karta hai: ek maternal aur ek paternal chromatid (non-sister chromatids) ek hi point par tootte hain aur ek doosre ke tukdon se jud jaate hain. Yeh cut-and-rejoin ek enzyme, recombinase, karta hai. Kyunki exchange hue tukdon mein genes ke alag versions hote hain, crossing over dono homologues ke genetic material ko recombine karta hai. Pachytene ke end tak exchanges poore ho jaate hain, aur jahan-jahan exchange hua tha wahan homologues bandhe reh jaate hain.

Diplotene. Protein zip khul jaata hai aur dono homologues alag hone lagte hain. Woh poori tarah alag nahi ho paate, kyunki jahan bhi crossing over hua tha wahan woh abhi bhi bandhe hain; har bandhan ek X ki tarah dikhta hai, yaani chiasma (plural chiasmata). Diplotene hamesha chhota nahi hota: kuch vertebrates ke egg banane wale cells (oocytes) mein yeh mahinon ya saalon tak chalta hai. Human oocyte ladki ke janm se pehle hi diplotene tak pahunch jaata hai aur kai saal baad us egg ke ovulate hone se thoda pehle tak wahin rehta hai.

Diakinesis. Ab chiasmata arms ke saath-saath chromosome ends ki taraf khisakte hain, is movement ko terminalisation kehte hain, aur chromosomes maximum compaction tak pahunchte hain. Cell division ke liye raasta saaf karta hai: nucleolus gayab ho jaata hai, nuclear envelope toot jaata hai aur homologues ko alag kheenchne ke liye ek meiotic spindle banta hai. Isliye diakinesis seedhe metaphase I mein badal jaata hai.

Metaphase I. Bivalents equatorial plate par line mein lagte hain. Dono poles se aane wale spindle microtubules dono homologous chromosomes ke kinetochores se judte hain, har pole se ek homologue (Fig. 10.9). Mitosis se compare karo, jahan ek chromosome ke do sister chromatids opposite poles se bandhe hote hain.

Anaphase I. Ab poore homologues alag kheenche jaate hain, har pair ka ek-ek har pole ki taraf. Koi centromere split nahi hota, isliye jaane wala har chromosome apne dono sister chromatids saath rakhta hai, aur har pole par chromosome count aadha ho jaata hai. Yahi step meiosis ko reduction division banata hai.

Telophase I. Do naye nuclei bante hain, har ek apne nucleolus ke saath, aur cell aam taur par divide hokar do cells wala dyad deta hai. Chromosomes thode dheele padte hain, lekin interphase nucleus ki tulna mein kahin zyada compact rehte hain.

Dono divisions ke beech interkinesis aata hai, jo aam taur par chhota hota hai. Interphase ke ulat, isme koi S phase nahi hota, isliye DNA dobara copy nahi hota, aur yahi meiosis II ko DNA content ek baar aur aadha karne deta hai. Uske baad aane wala prophase II, prophase I ki tulna mein chhota aur simple hota hai.

10.4.2 Meiosis II

Meiosis II dyad ke dono cells ka mitosis jaisa division hai (Fig. 10.10):

• Prophase II. Beech mein lagbhag koi pause nahi hota: prophase II pehle cytokinesis ke turant baad shuru ho jaata hai, aksar jab chromosomes abhi bhi kuch had tak condensed hote hain. Woh phir se poori tarah compact hote hain, aur is stage ke end tak nuclear membrane gayab ho jaati hai.
• Metaphase II. Chromosomes ek naye spindle ke equator par ikattha hote hain, aur har sister chromatid ke kinetochore ko us pole ke microtubules pakad lete hain jiski taraf woh mukh kiye hai.
• Anaphase II. Saare centromeres ek saath split hote hain, aur sister chromatids alag ho jaate hain. Har ek ko apne pole ki taraf kheencha jaata hai jaise-jaise uske kinetochore se jude microtubules chhote hote hain.
• Telophase II. Chromosomes ke chaar groups ke chaaron taraf nuclear envelopes bante hain, aur cytokinesis do cells ko chaar haploid cells ke ek tetrad mein badal deta hai.

10.5 Meiosis ka Mahatva

Ek paheli hai. Jo division chromosome number ko aadha karta hai, wahi kisi species ka chromosome number kabhi na badalne ki wajah kaise ho sakta hai? Fertilisation ko jodo aur paheli sulajh jaati hai: egg se n plus sperm se n har zygote mein 2n wapas le aata hai. Isliye meiosis hi woh cheez hai jo sexually reproduce karne wali species ki generations mein chromosome number constant rakhti hai; iske bina number har baar double ho jaata.

Meiosis genes ko shuffle bhi karta hai, isliye population ki har generation seedhi copying ki tulna mein zyada varied hoti hai. Pachytene mein crossing over maternal aur paternal genes ke naye mixtures wale chromatids banata hai. Metaphase I mein har bivalent baaki bivalents se independently line mein lagta hai, isliye gamete ko maternal aur paternal homologues ka koi bhi mixture mil sakta hai (Fig. 10.12). Phir random fertilisation do aise gametes ko milata hai. Variation evolution ka raw material hai; Class 12, Chapter 4 iski genetics ko explore karta hai.