Cell Cycle and Cell Division | Neet Free Hinglish Notes

1. Stages of Cellular Division and Replication Sequence

Cell division sabhi living organisms ke liye ek bahut hi important process hai, jo growth, repair, aur reproduction ke liye zaroori hota hai. Jab ek cell divide hota hai, to uske saath-saath DNA replication (DNA ki copying) aur cell growth bhi hoti hai. Ye saari steps coordinated aur organized tarike se hoti hain taaki naye daughter cells ko sahi genetic material mile.

Puri process, jisme ek cell apna DNA copy karta hai, naye cell parts banata hai, aur phir do daughter cells me divide hota hai, ise cell cycle kehte hain.

Cell growth (cytoplasm ka increase) lagbhag hamesha hoti rehti hai,
Lekin DNA synthesis (DNA ki nayi copies banana) ek specific stage me hoti hai, jo S-phase ke naam se jaane jati hai.

Jab DNA successfully replicate ho jata hai, uske baad chromosomes ko carefully organize karke har naye cell me equal tarike se distribute kiya jata hai. Ye process ek complex mechanism ke through hota hai, jo genetic control me hota hai. Iska matlab hai ki ye process genes aur regulatory proteins ke signal ke according operate karta hai, taaki koi error na ho.

2. Sequential Stages in Cellular Life Cycle Progression

In human cells jo labs me grow kiye jaate hain, cell cycle usually lagbhag 24 hours me complete hota hai. Lekin yeh time organism aur cell ke type par depend karta hai. For example, yeast cells sirf 90 minutes me cycle complete kar lete hain.

Cell cycle ke do main phases hote hain: Interphase aur M Phase (jise Mitosis phase bhi kehte hain). M Phase me cell actually divide hota hai. Interphase woh time hai jo do divisions ke beech hota hai. Human cells me, cell division lagbhag 1 hour leta hai, aur Interphase total cycle time ka 95% se zyada hota hai.

M Phase ki shuruaat nuclear division se hoti hai, jise karyokinesis kehte hain, jahan chromosomes alag ho jaate hain. Yeh usually cytokinesis par end hoti hai, jo cytoplasm ka division hota hai.

Jabki Interphase ko kabhi-kabhi resting phase kaha jaata hai, asal me cell yahan kaafi busy hota hai. Yeh division ke liye prepare karta hai, jisme cell growth aur DNA replication ek organized tareeke se hoti hai.

Interphase ko teen phases me divide kiya gaya hai:

G1 Phase (Gap 1)
S Phase (Synthesis Phase)
G2 Phase (Gap 2)

G1 Phase mitosis ke baad aur DNA replication ke start hone se pehle hota hai. Is phase me cell active hota hai aur grow karta hai, lekin apna DNA copy nahi karta.

S Phase me DNA copy ya replicate hota hai. Cell me DNA ki matra double ho jaati hai. Agar original DNA amount 2C tha, toh replication ke baad 4C ho jaata hai. Lekin chromosomes ki number nahi badalti. Agar cell diploid (2n) tha S Phase se pehle, toh S Phase ke baad bhi 2n hi rehta hai.

Animal cells me, S Phase ke dauraan, DNA replication nucleus me hoti hai, aur centriole cytoplasm me apna copy banata hai.

G2 Phase me cell mitosis ke liye proteins banata hai, aur grow karta rehta hai.

Kuch adult animals ke cells (jaise heart cells) dobara divide nahi karte. Doosre cells sirf tabhi divide karte hain jab zarurat hoti hai, jaise injury ke baad ya jab cells mar jaate hain. Ye cells G1 Phase chhod kar ek non-dividing stage me chale jaate hain, jise G0 Phase ya quiescent stage kehte hain.

G0 Phase me cells divide nahi karte, lekin apni normal cell activities continue karte hain. Agar organism ko zarurat ho, toh yeh dobara divide karna shuru kar sakte hain.

Animals me, mitosis usually sirf diploid somatic cells me hoti hai. Lekin kuch exceptions bhi hain, jaise male honey bees, jahan haploid cells bhi mitosis se divide karte hain.

Plants me, dono haploid aur diploid cells mitosis se divide kar sakte hain. Yeh aapne alternation of generations me pada hoga, jahan kuch haploid plant stages bhi mitosis dikhate hain.

3. Mitotic Phase of Cellular Division

M Phase cell cycle ka sabse dramatic part hota hai. Is phase ke dauraan, cell apne almost sabhi internal parts ka major reorganization karta hai.

Kyuki chromosomes ki number parent cell aur nayi cells dono me same rehti hai, is tarah ki division ko equational division bhi kaha jaata hai.

Better understanding ke liye, mitosis ko nuclear division yaani karyokinesis ke four stages me divide kiya gaya hai. Lekin asal me, cell division ek continuous process hai, aur stages ke beech ki boundaries itni sharp ya clearly separated nahi hoti.

Karyokinesis ke four stages hain:

Prophase
Metaphase
Anaphase
Telophase

3.1 Initial Stage of Chromosomal Condensation in Mitosis

Prophase karyokinesis ka first stage hota hai mitosis ke dauraan. Yeh S aur G₂ phases of interphase ke baad aata hai. Un earlier phases me new DNA molecules bante hain, lekin woh abhi bhi mixed aur tangled hote hain, clearly visible nahi hote.

Prophase me, chromosomal material condense hona start karta hai. Iska matlab hai ki chromatin apna untangle hona shuru karta hai aur tight, visible structures banata hai, jise chromosomes kehte hain.

Isi time, centrosome, jo S phase ke dauraan apna copy bana chuka hota hai, cell ke opposite poles ki taraf move karna shuru karta hai.

Prophase ke end tak kuch important events hote hain:

Chromosomal material condense hota hai aur compact mitotic chromosomes banata hai. Har chromosome do chromatids se milkar bana hota hai, jo ek point par judte hain jise centromere kehte hain.
Centrosomes cell ke opposite sides par move kar jaate hain. Har centrosome se tiny fibers nikalte hain jise microtubules ya asters kehte hain. Dono asters, saath me spindle fibers, milkar mitotic apparatus banate hain jo cell division me help karta hai.
Jab prophase end hota hai, aur cell ko microscope me dekha jaata hai, toh kuch cell parts disappear ho jaate hain. Inme shamil hain: Golgi complexes, endoplasmic reticulum, nucleolus, aur nuclear envelope.

3.2 Midway Alignment Stage of Chromosomes During Cell Division

Metaphase mitosis ka second stage hota hai. Yeh tab start hota hai jab nuclear envelope completely break down ho jaata hai. Is wajah se chromosomes ab freely cytoplasm me spread ho jaate hain.

Is time tak, chromosomes fully condensed ho chuke hote hain, aur microscope ke neeche clearly dekhe jaa sakte hain. Isi wajah se, metaphase ko best stage maana jaata hai chromosomes ke shape aur structure study karne ke liye.

Har metaphase chromosome me do sister chromatids hote hain, jo ek point par judte hain jise centromere kehte hain.

Centromere par chhoti disc-like structures hoti hain jise kinetochores kaha jaata hai. Ye kinetochores wahi jagah hoti hain jahan spindle fibers chromosomes se attach hote hain.

Spindle fibers chromosomes ko cell ke center ki taraf move karne me help karte hain. Metaphase ke dauraan, saare chromosomes equator par ek straight line me arrange ho jaate hain. Is arrangement ko metaphase plate kehte hain.

Har sister chromatid apne kinetochore ke through spindle fibers se opposite poles par attach hoti hai — ek left pole se aur doosri right pole se.

Metaphase ke main features:

Spindle fibers chromosomes ke kinetochores se attach hote hain.
Chromosomes spindle equator par line up hote hain aur metaphase plate par arranged hote hain, dono poles se attached rehte hain.

3.3 Separation Phase of Sister Chromatids Toward Opposite Poles

Anaphase mitosis ka third stage hota hai. Yeh tab start hota hai jab chromosomes, jo metaphase plate par lined up hote hain, ek saath split ho jaate hain.

Har chromosome ke do daughter chromatids ab daughter chromosomes kehlate hain. Yeh daughter chromosomes cell ke opposite poles ki taraf move karna shuru karte hain.

Jab chromosomes center se door move karte hain, toh unke centromeres aage hote hain aur poles ki taraf point karte hain, jabki chromosome ke arms peeche trail karte hain.

Anaphase ke main features:

Centromeres split ho jaate hain, aur chromatids separate ho jaate hain.
Chromatids (ab daughter chromosomes kehlate hain) cell ke opposite poles ki taraf move karte hain.

3.4 Final Stage of Nuclear Division

Telophase karyokinesis ka final stage hota hai mitosis me. Is stage ki shuruaat me, chromosomes jo cell ke opposite poles par move ho chuke hote hain, decondense hona shuru karte hain — matlab woh uncoil hote hain aur apni clear structure lose kar dete hain.

Ab individual chromosomes visible nahi rehte. Iske bajay, chromatin material (loosely packed DNA) cell ke har pole par gather hona shuru hota hai.

Telophase ke important events:

Chromosomes opposite spindle poles par gather ho jaate hain aur apni clear shape lose kar lete hain, isliye unki separate chromosomes ki identity disappear ho jaati hai.
Har chromosome cluster ke around ek new nuclear envelope form hoti hai, jo two daughter nuclei banati hai.
Nucleolus, Golgi complex, aur endoplasmic reticulum (ER) har new nucleus ke andar re-form ho jaate hain.

4. Division of Cytoplasm

Cytokinesis ek process hai jahan cell ka cytoplasm do daughter cells me divide hota hai. Yeh mitosis ke baad hota hai, jahan chromosomes already do nuclei me separate ho chuke hote hain. Jab cytokinesis complete hoti hai, toh entire cell division finish ho jaati hai.

Animal cells me, cytokinesis ki shuruaat plasma membrane me ek furrow (chhoti groove) banne se hoti hai. Yeh furrow dheere-dheere deeper hota hai, aur aakhir me cell ko do parts me split kar deta hai, jahan cytoplasm equally divide hota hai.

Plant cells me, ek rigid cell wall hoti hai, isliye process alag hota hai. Furrow banne ke bajay, new wall cell ke center me form hona start karti hai aur outward grow karke existing side walls se jud jaati hai.

Yeh new wall cell plate se shuru hoti hai, jo wall ka starting structure hota hai. Cell plate aage chalke middle lamella ban jaata hai, jo do new plant cells ko separate karta hai.

Cytoplasmic division ke dauraan, cell parts jaise mitochondria aur plastids bhi do new cells me equally share kiye jaate hain.

Kuch organisms me, cytokinesis karyokinesis ke baad nahi hoti, jiska result hota hai ek multinucleate cell, jise syncytium kehte hain. Iska ek example hai liquid endosperm in coconut.

5. Importance and Role of Cell Division by Mitosis

Mitosis, jise equational division bhi kehte hain, usually diploid cells me hoti hai. Lekin kuch lower plants aur certain social insects me, haploid cells bhi mitosis ke through divide kar sakte hain.

Is tarah ki cell division ek organism ki life me bahut important hai. Zyada cases me, mitosis diploid daughter cells produce karta hai jo genetically identical hote hain parent cell ke.

Multicellular organisms ka growth mainly mitotic division ke wajah se hota hai. Jab ek cell grow karta hai, toh nucleus aur cytoplasm ke beech ka balance disturb ho jaata hai. Is balance ko restore karne ke liye, cell divide karna padta hai.

Mitosis ka ek aur important role hai cell repair. For example, upper layer of skin, lining of gut, aur blood cells constantly mitosis ke through replace hote rehte hain.

Plants me, mitotic division meristematic tissues me hoti hai jaise apical meristem aur lateral cambium. Isse plants apni entire life me grow karte rehte hain..

6. Reductional Cell Division (Meiosis Process)

Meiosis ek special type of cell division hai jo sexual reproduction ke dauraan hoti hai. Is process me, do gametes (reproductive cells) ek saath aate hain, aur har gamete me haploid set of chromosomes hota hai.

Ye gametes special diploid cells se bante hain. Meiosis me, chromosomes ki number aadhi ho jaati hai, isliye daughter cells haploid hote hain. Isi wajah se, meiosis ko reductional division bhi kehte hain.

Meiosis haploid phase create karta hai un organisms ke life cycle me jo sexually reproduce karte hain. Baad me, fertilization ke dauraan, haploid cells combine karke diploid phase restore karte hain.

Meiosis gamete formation (gametogenesis) ke dauraan hoti hai plants aur animals me, aur isse haploid gametes bante hain.

Meiosis ke main features:

Meiosis me do rounds of nuclear aur cell division hote hain: meiosis I aur meiosis II, lekin DNA replication sirf ek baar hoti hai meiosis ke start hone se pehle.
Meiosis I start hota hai jab DNA already S phase me copy ho chuka hota hai, jisse identical sister chromatids bante hain.
Homologous chromosomes pair up hote hain, aur genetic exchange (jise recombination kehte hain) non-sister chromatids ke, beech hota hai.,
Meiosis II ke end me, chaar haploid cells produce hote hain.

Meiosis ke different phases do stages me divide hote hain:

Meiosis I:

Prophase I
Metaphase I
Anaphase I
Telophase I

Meiosis II:

Prophase II
Metaphase II
Anaphase II
Telophase II

7. First Division Phase of Meiosis (Reductional Division Stage)

Meiosis I

Prophase I meiosis I ka first stage hai. Yeh mitosis ke prophase se longer aur more complex hota hai. Isse chromosome behavior ke basis par five sub-stages me further divide kiya jaata hai:

1. Leptotene

Is stage me, chromosomes microscope ke neeche visible hona start hote hain. Yeh slowly shorter aur thicker ho jaate hain. Yeh condensation process leptotene ke dauraan continue hoti hai.

2. Zygotene

Yahan, homologous chromosomes (har parent se same type) pair up hona start karte hain. Is pairing process ko synapsis kehte hain.

Paired chromosomes ab bivalents ya tetrads kehlate hain. Electron microscope ke neeche, ek special structure synaptonemal complex dekha ja sakta hai jo homologous chromosomes ko saath pakadta hai.

3. Pachytene

Is stage me, bivalent ke saare four chromatids clearly visible ho jaate hain.

Chhoti structures jise recombination nodules kehte hain, appear hoti hain. Yeh wahi sites hain jahan crossing over hota hai — ek process jahan genetic material non-sister chromatids ke beech exchange hota hai homologous chromosomes me.

Crossing over ek enzyme recombinase ke through control hota hai, aur iska result genetic recombination hota hai, jo genetic diversity ko increase karta hai.

Pachytene ke end tak, crossing over complete ho jaata hai, aur chromosomes ab bhi crossover sites par linked rehte hain.

4. Diplotene

Ab, synaptonemal complex dissolve ho jaata hai, aur homologous chromosomes separate hona start karte hain, lekin woh crossover points jise chiasmata kehte hain, par abhi bhi connected rehte hain.

Kuch organisms me, especially female egg cells (oocytes) me, yeh stage months ya even years tak chal sakta hai.

5. Diakinesis

Yeh prophase I ka final stage hai. Is phase me:

Chiasmata chromosomes ke ends ki taraf move karte hain (jise terminalisation kehte hain).
Chromosomes fully condensed ho jaate hain.
Spindle banana start hota hai.
Nucleolus disappear ho jaata hai.
Nuclear membrane break down ho jaati hai.

Yeh prophase I ka end aur metaphase I ka beginning mark karta hai.

Metaphase I

Is stage me:

Bivalent chromosomes (paired homologous chromosomes) cell ke center (equator) par line up hote hain.
Spindle fibers opposite poles se homologous chromosomes ke kinetochores se attach hote hain.

Anaphase I

Yahan, homologous chromosomes separate hote hain aur cell ke opposite poles ki taraf move karte hain.
Lekin, sister chromatids apne centromeres par abhi bhi attached rehte hain.

Telophase I

Is stage me:

Nuclear membrane aur nucleolus dobara appear hote hain.
Cytokinesis (cytoplasm ka division) hota hai, jisse do daughter cells bante hain, har ek me chromosome number ka aadha hota hai.
Is stage ko dyad stage kehte hain.
Chromosomes thode loosen ho jaate hain, lekin interphase ki tarah fully unwind nahi hote.

Interkinesis

Yeh short rest phase hai meiosis I aur meiosis II ke beech.

Is phase me DNA replication nahi hoti.
Iske baad Prophase II aata hai, jo Prophase I ke comparison me kaafi simpler aur shorter hota hai.

8. Second Division Phase of Meiosis (Equational Division Stage)

Meiosis II meiotic division ka second phase hai, aur yeh Meiosis I ke baad cytokinesis complete hone ke thodi der me start hota hai. Pehli meiotic division ke unlike, Meiosis II mitosis ke kaafi similar hota hai, kyunki isme sister chromatids separate hote hain, homologous chromosomes nahi. Isme char main stages hote hain: Prophase II, Metaphase II, Anaphase II, aur Telophase II.

Prophase II me, nuclear envelope break down hona start karta hai aur chromosomes, jo thode decondensed ho chuke hote hain Meiosis I ke baad, dobara condense hoke compact ho jaate hain. Yeh stage generally brief hoti hai aur cytokinesis ke turant baad occur karti hai, aksar chromosomes fully elongate hone se pehle.

Metaphase II me, chromosomes apne aap ko equatorial plane (middle) me align karte hain har do haploid cells me jo Meiosis I me bane the. Har chromosome do sister chromatids se bana hota hai, jo centromere se connected hote hain. Spindle fibers, jo microtubules se bane hote hain, opposite poles se extend hote hain aur sister chromatids ke kinetochores se attach hote hain.

Anaphase II tab initiate hota hai jab centromeres simultaneously split hote hain har chromosome me. Is separation se individual sister chromatids (ab daughter chromosomes) opposite poles ki taraf pull hote hain. Spindle fibers shorten hote hain, is movement me help karte hain aur ensure karte hain ki har daughter cell complete chromosome set receive kare.

Finally, Telophase II me, separated chromosomes poles tak pahunch jaate hain, aur har group ke around new nuclear membranes form hote hain, jisse har do cells me do nuclei dobara bante hain. Iske baad cytokinesis hoti hai, jo cytoplasm divide kar deti hai aur total four haploid daughter cells bante hain, har ek me original diploid parent cell ke half chromosome number hota hai. Yeh four cells genetically distinct hote hain, kyunki Meiosis I ke dauraan genetic recombination events hue hote hain.

9. Biological Importance of Meiosis in Life Processes

Meiosis ka ek crucial role hai species ke constant chromosome number ko generations ke across maintain karne me, especially sexually reproducing organisms me. Chahe meiosis me chromosome number aadha ho jaata hai, yeh ensure karta hai ki jab gametes fertilization me fuse hote hain, toh offspring me original diploid chromosome number restore ho jaata hai. Isse genetic stability of a species time ke saath preserve hoti hai.

Meiosis ka doosra major significance hai genetic variation ka introduction. Prophase I me crossing over aur independent assortment of chromosomes ke processes ke dauraan, naye gene combinations bante hain. Yeh variations population me genetic diversity ko contribute karte hain, jo species ke adaptability aur evolution ke liye essential hai. Generations ke through, yeh diversity populations ko environmental changes aur selection pressures ke saath better respond karne me madad karti hai, is tarah evolution ka process drive hota hai.

10. Over View Of This Chapter

Cell theory ke according, sabhi cells pre-existing cells se cell division ke process ke through originate hote hain. Sexually reproducing organisms me, life single-celled zygote se shuru hoti hai, aur yeh cell divide aur grow karta hai complete organism banane ke liye. Cell division maturity ke baad bhi stop nahi hoti, balki organism ki life ke dauran continue rehti hai.

Ek cell ke life ke complete sequence of events from one division to the next ko cell cycle kehte hain. Yeh do main phases me divided hai: Interphase, jahan cell division ke liye prepare karta hai, aur M phase (mitosis), jo actual division phase hai.

Interphase me three stages hoti hain – G1 phase, S phase, aur G2 phase. G1 me, cell grow karta hai aur normal metabolic activities perform karta hai, saath hi organelle duplication start hoti hai. S phase me, DNA replication hoti hai, jisse chromosomes duplicate hote hain. G2 me, further cytoplasmic growth aur mitosis ke liye preparation hoti hai.

Mitotic phase ko Prophase, Metaphase, Anaphase, aur Telophase me subdivide kiya jaata hai. Prophase me, chromosomes condense, centrioles opposite poles ki taraf move karte hain, nuclear envelope aur nucleolus disappear hote hain, aur spindle fibers form hona start hote hain. Metaphase me, chromosomes equatorial plate par align hote hain. Anaphase me, centromeres split hote hain aur sister chromatids opposite poles ki taraf move karte hain. Telophase me, chromatids poles tak pahunchte hain, decondense hote hain, aur new nuclear membranes aur nucleoli form hote hain. Iske baad cytokinesis hoti hai, jahan cytoplasm divide hota hai aur do genetically identical daughter cells bante hain. Mitosis ko equational division kehte hain kyunki chromosome number unchanged rehta hai.

Contrast me, meiosis diploid cells me hota hai jo gametes banate hain, aur ise reductional division kehte hain kyunki yeh chromosome number aadha kar deta hai. Yeh sexual reproduction ke through chromosome number across generations maintain karne ke liye important hai. Meiosis do successive divisions me hota hai: Meiosis I aur Meiosis II.

Meiosis I me, homologous chromosomes bivalents banane ke liye pair up hote hain aur crossing over undergo karte hain. Prophase I ek long aur complex stage hai, jo leptotene, zygotene, pachytene, diplotene, aur diakinesis me subdivide hoti hai. Iske baad Metaphase I aata hai, jahan bivalents equator par align hote hain, aur phir Anaphase I, jahan homologous chromosomes, har ek me do chromatids, opposite poles ki taraf move karte hain, aur chromosome number aadha ho jaata hai. Telophase I me nuclear membrane aur nucleolus reform hote hain, uske baad interkinesis hoti hai.

Meiosis II mitosis ke similar hai aur sister chromatids ke separation ko involve karta hai Anaphase II me. Process Telophase II aur cytokinesis ke saath end hota hai, jisse four haploid daughter cells bante hain, har ek me unique genetic composition hota hai.

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Cell Cycle and Cell Division | Neet Free Hinglish Notes | Class 11 Biology