Molecular Basis of Inheritance: Class 12 Biology Chapter 5 Hinglish mein

Ek saaf toothpick se apne gaal ke andar ki taraf halka sa scrape karo, aur tumhare paas kuch sau cells aa jaate hain. Har cell mein tumhare instructions ki poori copy hoti hai. Agar ek hi cell ke DNA ko end to end kheencha jaaye to woh lagbhag do metre lamba hoga, phir bhi woh sirf kuch micrometres chaude nucleus ke andar baitha hai, aur kamaal ki precision ke saath copy hota hai, padha jaata hai aur on-off hota hai.

Pichhle chapter mein tumne Mendel ke 'factors' ko parents se bachchon tak follow kiya, bina yeh pooche ki woh bane kis cheez se hain. Yahan hum us factor ko khol kar dekhte hain. Tum dekhoge ki DNA kaise banta aur pack hota hai, scientists ne kaise prove kiya ki heredity DNA carry karta hai, protein nahi, aur kyun RNA shayad dono ka ancestor hai. Phir hum information ko follow karte hain jab woh copy hoti hai (replication), RNA mein dobara likhi jaati hai (transcription) aur protein mein decode hoti hai (translation), aur dekhte hain ki bacterium kaise decide karta hai ki gene kab kaam kare. Chapter do bade applications ke saath khatam hota hai: poore human genome ko padhna, aur DNA fingerprinting. Hindi mein is chapter ka naam वंशागति का आणविक आधार hai.

5.1 The DNA

Jo molecule ek metre lamba ho sakta hai, use naapte kaise hain? Uske building blocks gin kar. DNA, jo deoxyribonucleotides ka polymer hai, single-stranded ho to nucleotides mein aur double-stranded ho to base pairs (bp) mein naapa jaata hai. Har species ka ek characteristic genome size hota hai, aur range bahut badi hai: chhote viruses mein kuch hazaar units, mammals mein billions (Table 5.1).

5.1.1 Structure of Polynucleotide Chain

Yeh building block tum Class XI mein dekh chuke ho. Har nucleotide mein ek phosphate group, ek five-carbon (pentose) sugar aur ek nitrogenous base hota hai. Is level par do details DNA aur RNA ko alag karti hain. DNA ki sugar deoxyribose hai aur RNA ki ribose; aur dono adenine (A), guanine (G) aur cytosine (C) use karte hain, lekin jahan RNA uracil (U) use karta hai wahan DNA thymine (T) use karta hai. Shape ke hisaab se bases do families mein aate hain: purines (A aur G) double ring par bane hote hain, pyrimidines (C, T aur U) single ring par.

Yeh parts ek fixed tareeke se judte hain (Fig. 5.1). Base sugar ke carbon 1′ se ek N-glycosidic bond ke through judta hai, aur nucleoside banta hai. Phir ek phosphate carbon 5′ wale OH se phosphoester bond ke through judta hai, aur nucleotide banta hai. Chain badhane ke liye ek nucleotide ka phosphate agle nucleotide ki sugar ke 3′ OH se bond banata hai; yeh hai 3′–5′ phosphodiester bond. Do nucleotides se dinucleotide banta hai; sainkdon ya laakhon se polynucleotide.

Kyunki har link ek 3′ carbon se agle 5′ carbon tak jaata hai, isliye chain ki ek direction hoti hai. Ek end par 5′ carbon par free phosphate hota hai (5′ end); doosra end 3′ carbon par free OH par khatam hota hai (3′ end). Sugars aur phosphates backbone banate hain, aur bases usse bahar nikle rehte hain. RNA do details mein alag hai: har ribose ke carbon 2′ par ek extra OH hota hai, aur jahan DNA mein thymine hai wahan uracil hota hai (thymine bas 5-methyluracil hai).

Model ko famous base pairing ne banaya. Kyunki A sirf T ke saamne aa sakta hai, aur G sirf C ke, isliye har strand mein uske partner ki poori information hoti hai: dono strands complementary hain. Isse turant samajh aaya ki DNA inherit kaise ho sakta hai. Strands ko alag karo, har ek par same rules se partner dobara banao, aur tumhe original jaise do identical molecules mil jaate hain (Section 5.4).

1. Do chains. Do polynucleotide chains ek hi axis ke around lipti hoti hain; sugar–phosphate backbones bahar aur bases andar.
2. Antiparallel. Chains opposite directions mein chalti hain: agar ek 5′→3′ chalti hai, to uska partner 3′→5′ chalta hai.
3. Specific pairs. A, T ke saath do hydrogen bonds se pair karta hai; G, C ke saath teen se. Purine ke saamne hamesha pyrimidine hota hai, jisse width lagbhag constant (karib 2 nm) rehti hai.
4. Right-handed twist. Ek full turn (pitch) 3.4 nm lamba hota hai aur usmein karib 10 base pairs hote hain, isliye paas-paas ke pairs lagbhag 0.34 nm door hote hain.
5. Stacking. Flat base pairs sikkon ke dher ki tarah ek ke upar ek rakhe hote hain. Stacking aur hydrogen bonds milkar helix ko mazboot rakhte hain.

Double helix ke kuch hi samay baad Crick ne summary di ki cell mein genetic information kaise flow karti hai: DNA RNA mein copy hota hai, aur RNA se protein banta hai. Unhone ise central dogma kaha (Fig. 5.3). Cell divide hone se pehle DNA khud ko bhi copy karta hai. Kuch RNA viruses, jaise HIV, is raaste ka kuch hissa ulta chalte hain: reverse transcriptase naam ka enzyme unke RNA se DNA banata hai, aur is step ko theek hi reverse transcription kehte hain.

5.1.2 Packaging of DNA Helix

Tumhare ek cell mein DNA kitna lamba hai? Ek diploid human nucleus mein lagbhag $6.6\times {10}^9$ bp hote hain. Ise 0.34 nm per pair se multiply karo to karib 2.2 m double helix milta hai, jo sirf kuch micrometres (${10}^{-6}$ m) chaude nucleus mein pack hai. Cells is problem ko tight aur orderly folding se solve karte hain.

Bacterium bina nucleus ke kaam chala leta hai. Uska chromosome cell ke ek hisse, nucleoid, tak simit rehta hai, jahan woh bade loops mein fold hota hai jinhe proteins jagah par pakde rakhte hain; un proteins par positive charges negatively charged DNA ko pakad lete hain.

Eukaryotes mein packing ki basic unit nucleosome hai: karib 200 bp DNA jo aath histone proteins ke core, yaani histone octamer, ke around lipta hota hai (Fig. 5.4). Histones chhote proteins hain jinmein lysine aur arginine bharpoor hote hain, aise amino acids jinki side chains par positive charges hote hain, isliye negatively charged DNA unse chipak jaata hai. Chromatin, nucleus ka woh material jo stain leta hai, asal mein DNA par ek ke baad ek nucleosomes hi hai, aur electron micrographs mein yeh dhaage par beads jaisa dikhta hai.

Aur folding ke liye proteins ka ek aur group chahiye, non-histone chromosomal (NHC) proteins. Inki madad se beads ka dhaaga coil hokar mote chromatin fibres banata hai, aur metaphase mein yeh itna tightly fold hote hain ki har chromosome light microscope mein dikh jaata hai. Non-dividing nucleus mein bhi packing jagah-jagah alag hoti hai, jisse do tarah ka chromatin banta hai.

5.2 The Search for Genetic Material

Miescher ne 1869 mein nuclein dhoondha, Mendel ke apne laws announce karne ke sirf chaar saal baad, phir bhi dashakon tak kisi ne dono ko connect nahi kiya. 1920s tak Mendel, Walter Sutton, Thomas Hunt Morgan aur kai aur logon ke kaam ne genes ko nucleus ke andar chromosomes par rakh diya tha. Lekin chromosomes mein DNA aur protein dono hote hain. Instructions kaun carry karta hai? Kai biologists ne protein par daav lagaya: bees tarah ke amino acids ke saath woh chaar bases wale DNA se kahin zyada rich lagta tha.

Transforming principle

Pehla clue ek medical microbiologist se mila. 1928 mein Frederick Griffith Streptococcus pneumoniae study kar rahe the, ek bacterium jo pneumonia karta hai. Culture plate par yeh do forms mein ugta hai. S strain har cell ko ek chipchipe polysaccharide capsule mein lapet deta hai aur smooth, chamakdaar colonies banata hai; yeh mice ko maar deta hai. R strain mein capsule nahi hota, yeh rough colonies banata hai aur harmless hai.

Griffith ne mice mein chaar alag preparations inject kiye (Fig. 5.5). Jaisa expected tha, live S cells ne mice ko maar diya aur live R cells ne nahi. Heat se maare gaye S cells bhi harmless the. Lekin heat-killed S cells aur live R cells ka mixture mice ko maar gaya, aur unke blood se live S cells isolate kiye ja sake. Dead S cells ki kisi cheez ne R cells ko capsule banane wale S cells mein transform kar diya tha, aur unki offspring bhi S hi rahi. Unhone ise 'transforming principle' kaha. Yeh zaroor hereditary material tha, lekin iski chemistry pata nahi thi.

Biochemical characterisation of transforming principle

1940s tak zyadatar scientists ab bhi maante the ki genes proteins hain. 1933 se 1944 ke beech Oswald Avery ne Colin MacLeod aur Maclyn McCarty ke saath milkar Griffith ke principle ki chemical identity pakki karne ka kaam kiya. Unhone heat-killed S cells ke components alag kiye aur test kiya ki kaunsa component live R cells ko transform kar sakta hai. Sirf DNA fraction ne kaam kiya. Phir enzymes ne decisive test diya (Table 5.4).

Protein ya RNA ko destroy karne se koi fark nahi pada, lekin DNA ko destroy karte hi transformation ruk gaya, isliye principle DNA hi hona chahiye tha. Sab log convince nahi hue: kuch ko shak tha ki DNA mein chhupa protein ka thoda sa trace yeh kaam kar raha tha.

5.2.1 The Genetic Material is DNA

1952 mein Alfred Hershey aur Martha Chase ne ek bahut simple system se ek sharp sawaal poocha. Bacteriophage, yaani bacteria par attack karne wala virus, protein shell mein pack DNA se zyada kuch nahi hai. Jab yeh cell ko infect karta hai, to kuch cheez bacterium mein jaakar control le leti hai, aur cell naye phages banane lagta hai. Agar woh cheez protein hai, to protein genetic material hai; agar DNA hai, to DNA ka case pakka.

Unhone ek chemical difference ka fayda uthaya. DNA mein phosphorus hota hai lekin sulfur nahi, jabki coat protein mein sulfur hota hai lekin phosphorus nahi. Isliye radioactive phosphorus (${}^{32}$P) ke saath ugaye gaye phages ka DNA labelled tha, aur radioactive sulfur (${}^{35}$S) ke saath ugaye gaye phages ke coats labelled the. Har batch ne E. coli ko infect kiya. Phir ek kitchen blender ne khaali coats ko cells se jhatak kar alag kiya, aur centrifuge ne bhaari bacteria ko ghuma kar pellet bana diya, aur halke coats upar ke liquid mein reh gaye (Fig. 5.6).

${}^{32}$P phages se infect hue bacteria radioactive the; ${}^{35}$S phages se infect hue bacteria nahi the, aur unki radioactivity liquid mein rahi. Protein kabhi cells mein nahi gaya, jabki DNA gaya, aur phages ki agli generation usi se bani. Virus se bacterium tak jaane wala genetic material DNA hai, protein nahi.

5.2.2 Properties of Genetic Material (DNA versus RNA)

Hershey aur Chase ke baad protein race se bahar ho gaya. Phir bhi DNA akela answer nahi hai: kuch viruses mein, jaise tobacco mosaic virus, bacteriophage Qβ aur influenza virus, genes RNA ke bane hote hain. To phir zyadatar life ne DNA ko kyun chuna, jabki RNA messenger aur adapter ka kaam karta hai? Answer is baat mein hai ki kisi bhi genetic material ko kya-kya karna padta hai.

1. Replicate: apni accurate copies banana.
2. Stable rehna: organism ki poori life mein apni chemistry aur structure banaye rakhna.
3. Dheere change hona: kabhi-kabhi mutations hone dena, jo evolution ka raw material hain.
4. Express hona: khud ko aise traits ke roop mein dikhana jo Mendelian tareeke se inherit hote hain.

Replication. Base pairing ki wajah se dono nucleic acids apni copying khud direct kar sakte hain. Proteins mein aisa koi pairing rule nahi hota, isliye woh pehle hi test mein fail ho jaate hain.

Stability. Yahan dono molecules sach mein alag hain. RNA ki har ribose ke carbon 2′ par ek reactive OH group hota hai, jo RNA ko chemically nazuk aur jaldi tootne wala banata hai; kuch RNAs ka reactions catalyse kar paana khud reactivity ki nishaani hai. DNA mein yeh group nahi hota aur uska ek doosra, complementary strand hota hai. Heat bhi, jo strands ko alag kar deti hai, use barbaad nahi karti, kyunki thanda hone par strands phir se pair ho jaate hain; isi wajah se Griffith ke heat-killed S cells ka DNA ab bhi transform kar saka. Uracil ki jagah thymine use karna DNA ko aur bhi stable banata hai.11Cytosine dheere-dheere ek amino group kho kar uracil ban jaata hai. Kyunki normal DNA mein uracil nahi hota, repair enzymes har milne wale U ko pehchaan kar theek kar sakte hain; U wale DNA mein woh damage aur normal bases mein fark nahi kar paate.

Mutation. Change bhi zaroori hai, lekin dheere. Kam stable hone ki wajah se RNA mein DNA se zyada tezi se mutations aate hain. Ise viruses ke chhote generation times ke saath jodo, to RNA viruses bahut tezi se evolve hote hain; influenza itni jaldi badalta hai ki uski vaccine lagbhag har saal update karni padti hai.

Expression. Yahan RNA aage hai. Ribosome RNA ko seedha translate karta hai, jabki DNA ki information ko pehle RNA mein copy hona padta hai, aur protein banane wala apparatus khud RNA se bana hai. Hisaab yeh hai: archive ke roop mein DNA jeetta hai aur courier ke roop mein RNA, isiliye cells apne genes DNA mein store karte hain aur kaam ke liye RNA copies bahar bhejte hain.

5.3 RNA World

Aaj DNA ko copy hone ke liye proteins (uske polymerases) chahiye, aur proteins ko banne ke liye DNA ke instructions chahiye. To pehle kaun aaya? Jo molecule dono kaam kar sake woh is circle ko tod dega, aur RNA aisa kar sakta hai: yeh information store karta hai, aur kuch RNAs, yaani ribozymes, enzymes ki tarah reactions catalyse karte hain. Dhyaan dene wali baat yeh hai ki cell ke kuch sabse purane core processes, jaise translation, splicing aur metabolism ke kuch hisse, ab bhi RNA par chalte hain; har ribosome mein peptide bond RNA hi banata hai. Isiliye ek early RNA world ka idea aaya, jismein RNA pehla genetic material tha.

Is double life ki ek keemat thi: jo molecule catalyse karne layak reactive hai, woh toot jaane layak bhi reactive hai. DNA, yaani ek chemically modified RNA jismein 2′-OH nahi aur thymine hai, kahin zyada stable nikla, aur uske doosre strand ne ek backup copy di jisse damage repair ho sake. Isliye storage DNA ke paas chala gaya aur zyadatar catalysis proteins ke paas, aur RNA beech mein reh gaya (Fig. 5.7). Yeh idea tum evolution wale chapter mein phir dekhoge.

5.4 Replication

Watson aur Crick ne copying ka tareeka apne hi model mein dekh liya. Apne 1953 ke paper ke end ke paas unhone lagbhag yunhi kaha ki pairing rule seedha molecule ko duplicate karne ka raasta dikhata hai. Dono strands ko unzip karo, har ek ko template banne do, aur pairing rules ke hisaab se naye nucleotides jodo. Phir har bane hue molecule mein ek purana (parental) strand aur ek naya strand hoga: semiconservative replication.

Yeh ek hypothesis tha, aur do aur possible the. Conservative copying mein parent helix poora ka poora bana rehta aur uske bagal mein ek bilkul naya helix banta. Dispersive copying mein har daughter ke dono strands mein purane aur naye pieces bikhre hote. Ek experiment ko inmein fark karna tha.

5.4.1 The Experimental Proof

1958 mein Matthew Meselson aur Franklin Stahl ne purane DNA ko naye se uske weight se alag pehchaanne ka tareeka dhoondha. Pehle unhone bacterium ka saara DNA heavy bana diya. E. coli ko kai generations tak aise medium par ugaya gaya jismein nitrogen ka akela source ${\phantom{X}}_{\phantom{}}^{\phantom{15}}{\phantom{X}}_{\phantom{2}}^{\phantom{2}15}\mathrm{N}\mathrm{H}{\phantom{X}}_4\mathrm{C}\mathrm{l}$ tha, isliye nitrogen-15, jo ek heavy lekin stable isotope hai (yeh radioactive nahi hai), har base mein pahunch gaya. Phir cells ko normal ${\phantom{X}}_{\phantom{}}^{\phantom{14}}{\phantom{X}}_{\phantom{2}}^{\phantom{2}14}\mathrm{N}\mathrm{H}{\phantom{X}}_4\mathrm{C}\mathrm{l}$ par shift kiya gaya, aur uske baad unka banaya har strand light tha. Cells ke divide hote rehne ke dauraan, lagbhag har 20 minute mein ek baar, samples liye gaye. Har sample ko bahut tez speed par caesium chloride (CsCl) density gradient mein spin kiya gaya; yeh ek salt solution hai jo tube ke neeche ki taraf zyada dense hota jaata hai, jahan har DNA molecule, jo ab bhi double-stranded hai, us level par baith jaata hai jo uski apni density se match karta hai (Fig. 5.8).

• Start: ek single heavy band.
• Ek generation (20 min) ke baad: hybrid density ka ek single band, heavy aur light ke beech mein, aur koi heavy band bilkul nahi.
• Do generations (40 min) ke baad: barabar size ke do bands, ek hybrid aur ek light.

Results dono rivals ko rule out karte hain. Conservative copying ek generation ke baad ek heavy band aur ek light band predict karta hai, lekin real sample mein sirf hybrid DNA dikha. Dispersive copying har generation par ek single band predict karta hai jo dheere-dheere light ki taraf khiskta hai, lekin 40 minute par hybrid aur light DNA ne do alag bands banaye. Sirf semiconservative copying har sample se fit hota hai. Teeno models khud Fig. 5.9 mein try karo.

Bacteria koi special case nahi the. Lagbhag usi samay J. Herbert Taylor aur unke saathiyon ne broad bean Vicia faba ke divide hote root-tip cells ko radioactive thymidine diya aur follow kiya ki label kahan gaya. Chromosomes bhi semiconservatively replicate hue, aur baad mein doosre plants aur human cells ke liye bhi yahi dikhaya gaya.

5.4.2 The Machinery and the Enzymes

Copying kitni tez honi chahiye? E. coli apne $4.6\times {10}^6$ bp ko lagbhag 18 minute mein duplicate karta hai, ek point se shuru karke apne circular chromosome ke dono taraf kaam karte hue. Isliye dono copying sites mein se har ek ko har second lagbhag 2000 nucleotides jodne hote hain, aur lagbhag har baar sahi nucleotide, kyunki bina theek ki gayi galti mutation ban jaati hai. Jo enzyme yeh sambhalta hai woh hai DNA-dependent DNA polymerase: yeh DNA template padhta hai aur deoxynucleotides ko jod kar matching strand banata hai.

Fuel building blocks ke saath hi aata hai. Nucleotides deoxyribonucleoside triphosphates (dATP, dGTP, dCTP aur dTTP) ke roop mein aate hain. Jab ek badhte strand mein juda jaata hai to uske do bahari phosphates release hote hain; yeh ATP jaise hi high-energy bonds hain, aur inke tootne se reaction chalta hai. DNA copy karna mehenga hai, aur dNTPs iska kharcha khud uthate hain.

DNA polymerase ek strict rule maanta hai: yeh nucleotide ko sirf badhte strand ke free 3′ end par jod sakta hai, isliye har naya strand 5′→3′ badhta hai. Cell copying se pehle poora helix nahi kholta, kyunki usmein bahut zyada energy lagegi; yeh ek baar mein chhota stretch kholta hai, yaani ek Y-shaped replication fork jo molecule ke saath aage badhta hai (Fig. 5.10). Fork par khule dono templates opposite directions mein point karte hain, isliye rule har ek par alag tarah se kaam karta hai. Jo template chalne ki direction mein 3′→5′ hai, uske saath naya strand usi taraf badhta hai jidhar fork chalta hai aur continuously banta hai. Doosre template par use fork se door badhna padta hai, isliye woh discontinuously banta hai, chhote pieces mein jo naya template khulte hi shuru hote hain, aur baad mein DNA ligase in pieces ko jod kar ek strand bana deta hai.

Copying shuru kahan se hoti hai? Random jagah se nahi, sirf khaas sequences se, jinhe origins of replication kehte hain; E. coli mein theek ek hota hai. Iska ek practical result hai. Ek organism se kaat kar bacterium mein daale gaye gene ka apna koi origin nahi hota, isliye woh kabhi copy nahi hoga. Isliye genetic engineers use ek vector se jodte hain, jo apne saath origin laata hai (Chapter 9). Yeh bhi dhyaan do ki DNA polymerase strand ko lamba kar sakta hai lekin zero se shuru nahi kar sakta; ek chhota RNA primer use starting point deta hai.

Eukaryotic cells mein copying cell cycle ke ek hisse, S phase, tak simit hoti hai, aur har chromosome ke saath kai origins se shuru hoti hai. Phir cell ko copying ke har round ke liye theek ek baar divide hona padta hai; jo cell apna DNA replicate kare lekin division skip kare, usmein chromosomes ke extra sets reh jaate hain, is state ko polyploidy kehte hain. Eukaryotic origins kaise kaam karte hain, iski kai details abhi bhi samjhi ja rahi hain.

5.5 Transcription

Replication sab kuch copy karta hai; transcription selective hai. Kisi ek cell mein sirf kuch genes RNA mein copy hote hain, har ek utni baar jitni zaroorat ho, aur har gene ke do strands mein se sirf ek use hota hai. Transcription us strand ki information ko jaane-pehchaane pairing rules se RNA mein copy karta hai, bas RNA adenine ke saamne uracil (U) lagata hai. Kyunki yeh process itna selective hai, DNA mein aise signals hone chahiye jo batayein ki transcribe hone wala stretch kahan shuru aur kahan khatam hota hai, aur kaunsa strand padha jaana hai.

Maan lo cell kisi gene ke dono strands copy kar de. Use do RNAs milenge jinke sequences complementary honge, identical nahi. Woh RNAs ek doosre ko dhoondh kar pair ho jaayenge, aur double-stranded RNA banega jise ribosomes padh nahi sakte, yaani saara kaam bekaar. Agar woh alag bhi rahein, to do bilkul alag messages likhenge, aur DNA ka ek stretch do alag proteins specify karega, jo confusion ka nuskha hai. Ek hi strand padhne se dono problems se bacha ja sakta hai.

5.5.1 Transcription Unit

Transcription unit DNA ka ek stretch hai jiske teen parts hote hain: ek promoter, ek structural gene aur ek terminator.

Kaunsa strand padha jaata hai? RNA banane wala enzyme, DNA-dependent RNA polymerase, sirf 5′→3′ bana sakta hai, aur uska product jis DNA ko copy karta hai uske antiparallel hona chahiye. Isliye structural gene ke andar yeh 3′→5′ direction wala strand padhta hai, yaani template strand. Partner strand copy nahi hota, lekin uska sequence RNA se match karta hai (U ki jagah T ke saath), isliye jab bhi hum kisi gene ko quote karte hain to wahi likha jaata hai. Ise coding strand kehte hain. Naam se confuse mat hona: coding strand woh copy hai jo hum padhte hain, woh nahi jo enzyme padhta hai (Fig. 5.11).

RNA polymerase gene ko uske promoter par dock karke dhoondhta hai, jo upstream end par hota hai, yaani coding strand ke hisaab se 5′ end ki taraf. Promoter ka orientation padhne ki direction bhi fix karta hai, aur usi se yeh bhi ki kaunsa strand template banega: agar promoter gene ke doosre end par hota to opposite strand template ban jaata. Downstream end par, coding strand ke 3′ end ki taraf, terminator aam taur par batata hai ki transcription kahan rukta hai. Doosre regulatory sequences aur upstream ya downstream ho sakte hain; unhe tum Section 5.8 mein dekhoge.

5.5.2 Transcription Unit and the Gene

Ek gene kahan khatam hota hai aur agla kahan shuru? Classical definition, yaani inheritance ki functional unit, bases ke kisi stretch mein saaf-saaf fit nahi hoti. Us DNA ke baare mein socho jo tRNA ya rRNA mein transcribe hota hai: woh kabhi protein code nahi karta, phir bhi woh bhi gene hai. Protein-coding DNA ke liye biologists ek zyada sharp term use karte hain, cistron: woh stretch jo ek polypeptide code karta hai. Ek cistron wala transcription unit monocistronic hai; kai cistrons wala polycistronic.

Zyadatar eukaryotic genes split bhi hote hain. Unka coding sequence pieces mein toota hota hai, jinhe exons kehte hain, aur unke beech introns (intervening sequences) naam ke stretches hote hain. Dono pehle RNA transcript mein copy hote hain, lekin finished, mature RNA mein sirf exons bachte hain; introns hata diye jaate hain. Isliye eukaryotic gene ek continuous coding stretch nahi hai, aur use 'DNA ka ek piece' kehkar define karna aur bhi mushkil ho jaata hai.

Kisi trait ko badalne ke liye mutation ka coding sequence ke andar hona zaroori nahi: gene ke promoter ya doosre control sequences ka damage bhi use utni hi pakki tarah se band kar sakta hai. Isiliye aise sequences ko kabhi-kabhi loosely regulatory genes kaha jaata hai, halaanki strictly woh na RNA banate hain na protein.

5.5.3 Types of RNA and the process of Transcription

Protein banane ke liye teen tarah ke RNA ka teamwork chahiye, aur teeno DNA se transcribe hote hain. mRNA (messenger RNA) khud message hai. tRNAs (transfer RNAs) adapters hain jo amino acids laate hain aur code padhte hain. rRNAs (ribosomal RNAs) ribosome ka core banate hain aur uski chemistry ka kuch hissa karte hain.

Bacterium apne saare RNA ke liye ek hi RNA polymerase se kaam chala leta hai. Iske ek molecule ko gene ke saath follow karo (Fig. 5.12). Pehle yeh promoter par baithta hai: initiation. Phir yeh template ke saath aage badhta hai, apne theek aage helix ko kholta jaata hai aur template se sahi pair karne wale ribonucleoside triphosphates jodta jaata hai, jabki RNA peeche nikalta rehta hai aur sirf uska sabse naya end enzyme pakde rehta hai: elongation. Terminator par pehle finished RNA aur phir enzyme DNA ko chhod dete hain: termination.

Enzyme yeh sab akele nahi karta. Core polymerase sirf RNA ko lamba karne mein achha hai. Promoter dhoondhne aur shuru karne ke liye yeh thodi der ek partner protein saath rakhta hai, initiation factor σ (sigma); terminator par rukne mein termination factor ρ (rho) madad karta hai. Har partner badal deta hai ki polymerase kya pehchaanta hai, aur apna kaam hote hi chala jaata hai.

Bacterial mRNA bante hi use ke liye ready hota hai, aur yeh ribosomes wale compartment mein hi banta hai. Isliye ribosomes mRNA ke 5′ end par chadh kar translate karna shuru kar dete hain jabki RNA polymerase abhi gene mein aage busy hota hai: bacteria mein transcription aur translation coupled hote hain.

Eukaryotes do complications jodte hain. Pehla hai kaam ka bantwara. Unke nuclei teen alag RNA polymerases use karte hain, har ek apne RNAs ke set ke liye zimmedaar, jabki mitochondria aur chloroplasts ke apne polymerases hote hain (Table 5.7).

Doosra yeh ki gene ki pehli copy, yaani primary transcript ya hnRNA (heterogeneous nuclear RNA), mein exons ke saath introns bhi hote hain aur use waise hi use nahi kiya ja sakta. Teen tarah ki processing use mRNA banati hai (Fig. 5.13). Capping 5′ end par ek modified nucleotide lagata hai, yaani ek methylated guanosine jo triphosphate link se juda hota hai. Tailing mein 3′ end par bina kisi template ke 200–300 adenylate (A) residues jode jaate hain. Splicing mein introns kaat diye jaate hain aur exons sahi order mein jode jaate hain. Phir finished mRNA translate hone ke liye nucleus se bahar jaata hai.

Cells apne hi messages ko kaatne-jodne ki zehmat kyun uthayenge? Ek view historical hai. Introns bahut purane lagte hain, aur splicing machinery small nuclear RNAs ke around bani hai jo kaatne aur jodne ka zyadatar kaam khud karte hain, jo shayad RNA world ki nishaani hai (Section 5.3). Biologists jitna dekhte hain, utne hi zyada kaam milte hain jo RNA cells mein karta hai.

5.6 Genetic Code

DNA ko DNA mein, ya DNA ko RNA mein copy karne ke liye kisi dictionary ki zaroorat nahi: har base bas apne partner se pair karta hai. Translation alag tarah ki problem hai. Chaar tarah ke nucleotides ke sequence ko bees tarah ke amino acids ka sequence banna hai, aur amino acid ki shape mein aisa kuch nahi jo kisi base mein fit ho. Biologists itna jaante the ki genes ke mutations se proteins mein badle hue amino acids aate the. Isliye cell ko kuch rules follow karne honge jo base sequence ko amino acid sequence se jodte hain: yahi hai genetic code.

Pehla clue arithmetic tha, aur woh ek physicist, George Gamow, se aaya. Agar ek base ek amino acid ke liye hota, to chaar bases sirf chaar amino acids bata paate. Bases ke pairs se $4^2=16$ combinations milte hain, phir bhi bees se kam. Triplets se $4^3=64$ milte hain, jo kaafi se zyada hain, kai bach bhi jaate hain. Isliye codon shayad teen bases lamba tha. Yeh sach mein aisa hai, yeh dikhane ke liye chemists, biochemists aur geneticists ko saath kaam karna pada.

Decisive tool tha Marshall Nirenberg ka cell-free system, yaani cell contents ka test-tube mix jo daale gaye kisi bhi RNA se protein banata hai. 1961 mein sirf U wale ek artificial RNA se sirf phenylalanine wali chain bani, isliye UUU ka matlab phenylalanine hona tha. Aise tests ke liye artificial RNAs Severo Ochoa ke enzyme, polynucleotide phosphorylase, se banaye ja sakte the, jo bina template ke nucleotides jodta hai, jabki Har Gobind Khorana ke chemical methods se bilkul repeat hone wale sequences (homopolymers aur copolymers) wale RNAs bane jinse ek ke baad ek codon decode hue. 1966 tak saare 64 codons pata chal gaye (Table 5.8).

Table amino acid ke hisaab se arrange ki gayi hai taaki tum protein se wapas RNA tak ja sako. Doosri direction mein, yaani codon se uske amino acid tak, padhna Fig. 5.14 mein sabse aasaan hai. Dono tarah se, code ke kuch features saaf dikhte hain.

1. Yeh triplet code hai. 64 codons mein se 61 amino acids ke liye hain aur 3 (UAA, UAG aur UGA) kisi ko code nahi karte; yeh stop codons hain jo chain ko khatam karte hain.
2. Yeh degenerate hai. Zyadatar amino acids ke ek se zyada codons hain; leucine, serine aur arginine ke chhe-chhe hain. Sirf methionine aur tryptophan ka ek hi codon hai.
3. Yeh unambiguous hai. Har codon ka matlab ek hi amino acid hai, aur kuch nahi.
4. Ismein punctuation nahi hota. Codons bina gaps ya commas ke ek ke baad ek aate hain, isliye message ek fixed starting point se teen-teen bases karke padha jaata hai.
5. Yeh lagbhag universal hai. UUU ka matlab bacteria, plants aur insaanon, sab mein phenylalanine hai; kuch exceptions mitochondria aur kuch protozoans mein milte hain.
6. AUG ke do kaam hain. Yeh methionine code karta hai aur start (initiator) codon bhi hai.

5.6.1 Mutations and Genetic Code

Mutations woh natural experiments hain jo dikhate hain ki code kaise kaam karta hai. Ek bada deletion ya rearrangement poore genes hata ya duplicate kar sakta hai, jisse function saaf taur par ghatta ya badhta hai, lekin isse codons ke baare mein kam pata chalta hai. Ek ya kuch bases ke changes kahin zyada batate hain.

Point mutation ek single base pair badalta hai. Classic example hai sickle-cell anaemia. Haemoglobin ki β-globin chain ke gene mein ek base change codon GAG ko GUG bana deta hai, isliye finished chain ka chhatha amino acid, glutamic acid (glutamate), valine se replace ho jaata hai. Yeh ek hi badlaav oxygen kam hone par haemoglobin molecules ko aapas mein chipka deta hai, aur red blood cells ko sickle shape mein mod deta hai.

Kyunki codons bina punctuation ke padhe jaate hain, bases insert ya delete karne ka khaas effect hota hai. Ek ya do bases jodne ya hataane se us point se aage reading frame khisak jaata hai, isliye uske baad ka har codon galat padha jaata hai: yeh frameshift insertion ya deletion hai. Teen bases (ya teen ke multiple) jodne ya hataane se poore codons jud ya hat jaate hain. Ek ya zyada amino acids badh ya ghat jaate hain, aur change ke baad ka frame sahi rehta hai (Table 5.9).

5.6.2 tRNA – the Adapter Molecule

Codon aur amino acid ke beech kuch to hona chahiye. Transfer RNA yahi hai: yeh ek end par codon padhta hai aur doosre end par matching amino acid le kar chalta hai. Francis Crick ne logic ke basis par aise adapter ki prediction ki thi, kyunki amino acids ki koi aisi shape nahi jo codon pehchaan sake. Dilchasp baat yeh hai ki tRNA pehle hi discover ho chuka tha, soluble RNA (sRNA) naam se, kisi ko yeh pata chalne se pehle ki iska kaam yahi hai.

Flat banane par tRNA clover leaf jaisa dikhta hai; iski asli folded shape ek compact, ulta L hai (Fig. 5.15). Do jagah important hain. Anticodon loop mein teen bases hote hain jo codon se antiparallel pair karte hain. Amino acid acceptor end (3′ end) woh jagah hai jahan amino acid juda hota hai. Har tRNA ek khaas amino acid se charge hota hai, ek special initiator tRNA har chain shuru karne wala methionine laata hai, aur stop codons ke liye koi tRNA nahi hota.

5.7 Translation

Translation amino acids ko peptide bonds se jodta hai, bilkul mRNA ke codons ke order mein. Peptide bond apne aap nahi banta, isliye cell pehle se payment karta hai.

Translation se pehle ek enzyme ATP use karke har amino acid ko activate karta hai aur use uske matching tRNA (cognate tRNA) par load karta hai. Is loading ko tRNA ki charging, ya zyada sahi kahein to aminoacylation, kehte hain. Charged tRNA apne amino acid ko high-energy link se pakde rehta hai, isliye jab do charged tRNAs saath-saath baithte hain, to unke amino acids ko jodna energy ke hisaab se aasaan hota hai, aur ek catalyst ise tez bana deta hai.

Yeh kaam ribosomes par hota hai. Har ribosome ek small subunit aur ek large subunit se banta hai, jo khaali hone par alag-alag tairte hain; translation tab shuru hota hai jab small subunit kisi mRNA se bind karta hai. Dono subunits mein milkar structural RNAs aur lagbhag 80 tarah ke proteins hote hain. Large subunit mein charged tRNAs ke liye do paas-paas ki sites hoti hain (aam taur par P site aur A site kehte hain), jo do amino acids ko ek doosre ke bagal mein rakhti hain. Unke beech ka bond RNA banata hai: bacteria mein catalyst 23S rRNA hai, jo ek ribozyme hai.

mRNA us hisse se lamba hota hai jo padha jaata hai. Har end par ek untranslated region (UTR) hota hai: start codon se pehle 5′ UTR aur stop codon ke baad 3′ UTR. Yeh mRNA ko efficiently translate hone mein madad karte hain. Beech ka stretch, AUG se stop codon tak, translational unit hai jo asal mein polypeptide code karta hai.

1. Initiation. Ribosome mRNA par start codon AUG par assemble hota hai, jise sirf initiator tRNA (methionine wala) pehchaan sakta hai.
2. Elongation. Charged tRNAs ek-ek karke aate hain; har ek tabhi bind karta hai jab uska anticodon free site wale codon se pair kare. Ek peptide bond badhti chain ko naye amino acid se jodta hai, aur ribosome mRNA par ek codon aage badhta hai. Cycle repeat hota hai.
3. Termination. Jab stop codon ribosome tak pahunchta hai, to koi tRNA fit nahi hota. Uski jagah ek release factor bind karta hai, finished polypeptide free ho jaata hai, aur ribosome alag ho jaata hai.

5.8 Regulation of Gene Expression

Gene tab 'express' hota hai jab uska product, aam taur par ek polypeptide, banta hai, aur cell raaste mein kai points par is output ko badha ya ghata sakta hai. Eukaryotes mein regulation chaar levels par ho sakta hai:

1. transcriptional level, jo decide karta hai ki primary transcript banega ya nahi;
2. processing level, jaise RNA kis tarah splice hoga;
3. mRNA ka nucleus se cytoplasm tak transport;
4. translational level, jo decide karta hai ki har mRNA se kitna protein banega.

Cells har gene ko har waqt nahi chalate; isse energy aur materials barbaad honge. Liver cell aur nerve cell mein bilkul same DNA hota hai lekin woh genes ke alag sets use karte hain, aur embryo ko adult banane ka matlab hai genes ke poore groups ko sahi order mein on aur off karna. Ek single bacterium bhi apne genes ko apne surroundings ke hisaab se tune karta hai. E. coli enzyme β-galactosidase banata hai, jo energy ke liye lactose ko glucose aur galactose mein todta hai, aur yeh zyadatar tabhi banta hai jab lactose sach mein maujood ho. Short mein, gene expression cell ki metabolic, physiological aur environmental conditions ke hisaab se respond karta hai.

Bacteria zyadatar regulation bilkul pehle step par karte hain: RNA polymerase gene ka transcription shuru karega ya nahi. Promoter ke paas DNA se bind karne wale proteins yeh decide karte hain. Repressor polymerase ko block karta hai (negative control); activator uski madad karta hai (positive control). Kai bacterial repressors operator par baith kar kaam karte hain, jo promoter ke bagal ka ek chhota DNA sequence hai, aur har operator ko uska apna repressor pehchaanta hai: lac repressor lac operator se bind karta hai, kisi aur se nahi.

5.8.1 The Lac operon

Ek hi pathway ke liye kaam karne wale bacterial genes aksar saath-saath line mein hote hain aur ek polycistronic mRNA mein ek saath transcribe hote hain, ek hi promoter aur control sequences ka ek hi set share karte hue. Aisi unit ko operon kehte hain; trp, ara, his aur val operons iske examples hain. Sabse famous hai lac operon, jo lactose sugar ko handle karta hai. Iska control do French scientists ne samjha, geneticist François Jacob aur biochemist Jacques Monod ne, aur yeh transcriptional regulation ka pehla system tha jo samjha gaya.

Operon mein ek regulatory gene aur teen structural genes hain (Table 5.11, Fig. 5.17). Regulatory gene ko i kehte hain, inhibitor ka short form (inducer nahi). Teeno structural-gene products lactose ke use mein kaam aate hain, aur zyadatar operons mein bhi isi tarah grouped genes ek hi pathway mein kaam karte hain.

Aas-paas lactose na ho to operon off rehta hai. i gene bina ruke (constitutively) express hota hai, isliye repressor hamesha maujood hota hai, aur woh operator par baith kar RNA polymerase ko block karta hai. Ab cells ko lactose do, aur glucose nahi, jo unka pasandida khaana hai. Thoda sa transcription hamesha repressor se bach nikalta hai, isliye har cell mein kuch permease molecules hote hain, jo kuch lactose andar aane dene ke liye kaafi hain; inke bina sugar kabhi andar nahi aa paati. Andar lactose (zyada sahi kahein to usse bana allolactose) repressor se bind karke uski shape badal deta hai, aur woh operator ko chhod deta hai. Ab RNA polymerase z, y aur a se guzar sakta hai, aur lactose use karne wale enzymes bante hain. Kyunki lactose enzyme ka khaana bhi hai aur switch bhi palat deta hai, ise inducer kehte hain.

Yeh ek smart bachat hai: substrate khud un enzymes ko bulata hai jo use use karte hain. Products, glucose aur galactose, operon ko on nahi kar sakte. Har combination, do toote switches samet, Fig. 5.18 mein explore karo.

Kyunki yeh switch ek block hata kar kaam karta hai, ise negative regulation kehte hain. Lac operon mein ek doosra, positive control bhi hai, ek activator jo glucose bharpoor hone par ise low rakhta hai, lekin kahani ka woh hissa is chapter ke bahar hai.

5.9 Human Genome Project

1990 mein biologists ne insaanon ka poora DNA sequence padhne ka kaam shuru kiya: Human Genome Project (HGP). Yeh goal seedha is chapter se nikalta hai. Agar kisi insaan ka genetic make-up bases ke order mein likha hai, to woh order jaanna, aur yeh ki woh logon mein kaise alag hai, bahut kuch samjha dena chahiye. Do advances ne is koshish ko realistic banaya: genetic engineering se DNA ka koi bhi piece isolate aur clone ho sakta tha, aur sequencing tez aur routine ho gayi thi.

Yeh har tarah se ek mega project tha. Genome mein lagbhag $3\times {10}^9$ bp hain. Shuruaati planners ka andaaza tha ki har base pair padhne mein lagbhag US$3 lagenge, jo lagbhag US$9 billion banta hai. Data bhi utna hi bhaari hai. Har base ke liye ek letter store karo, to genome ki ek copy lagbhag 3 gigabytes ki text file bhar deti hai; bina ruke paanch letters per second ki speed se zor se padho, to lagbhag bees saal lagenge. Itni information ko store, search aur analyse karne ke liye tez computers chahiye the, aur project ek naye field ke saath bada hua, bioinformatics, jo biological data handle karne ke liye computers use karta hai.

Goals of HGP

1. Human DNA ke saare genes dhoondhna, jinka tab andaaza lagbhag 20 000–25 000 tha.
2. Lagbhag 3 billion base pairs ka poora order pata karna.
3. Information ko aise databases mein rakhna jo researchers ke liye open hon.
4. Data analyse karne ke liye behtar tools banana.
5. Nayi technologies ko industry aur doosre sectors ke saath share karna.
6. Project se uthne wale ethical, legal aur social issues (ELSI) par soch-vichaar karna.

HGP US Department of Energy aur National Institutes of Health ke under 13 saal chala, UK ka Wellcome Trust jaldi hi ek major partner ke roop mein juda aur Japan, France, Germany, China aur doosre countries ne bhi contribute kiya. 2003 mein sequence complete declare hua. DNA logon mein kaise alag hai, yeh map karne se bimaariyon ko detect, treat aur aakhir mein prevent karne ka tareeka badal sakta hai. Doosri species ke genomes bhi padhe gaye, jaise bacteria, yeast, nematode Caenorhabditis elegans, fruit fly Drosophila, rice aur Arabidopsis; inhe jaanne se hum aise organisms ko medicine, kheti, fuel production aur polluted sites ki safaai mein use kar paate hain.

Methodologies

Project ne problem ko do taraf se attack kiya. Expressed sequence tags (ESTs) ne cells jo RNA sach mein banate hain usse shuru karke genes chune. Whole-genome route ne sab kuch padha, genes aur unke beech ke lambe stretches bhi, bina chune, aur baad mein pata kiya ki kaunsa part kya karta hai; is labelling ko sequence annotation kehte hain.

Whole-genome route ek giant jigsaw tha jise scientists ne pehle khud kaat kar tukde kiya (Fig. 5.19). Sequencing machines sirf chhote pieces padhti hain, isliye kai cells ke DNA ko random tareeke se fragments mein toda gaya. Har fragment ko bacteria ya yeast mein clone karke badhaya gaya, jo BACs (bacterial artificial chromosomes) aur YACs (yeast artificial chromosomes) naam ke vectors mein le jaaye gaye. Automated sequencers ne clones ko Frederick Sanger ke banaye method se padha, wahi Sanger jinhone sabse pehle kisi protein ka amino acid sequence pata kiya tha. Kyunki pieces random kaate gaye the, woh overlap karte the, aur overlaps se pata chala ki kaunse pieces ek doosre ke bagal mein the. Laakhon overlaps haath se match karna impossible hai, isliye special computer programs ne assembly ki.

Assemble kiye gaye sequences annotate kiye gaye aur unke chromosomes ko assign kiye gaye. 24 alag human chromosomes (22 autosomes plus X aur Y) mein se sabse aakhir mein chromosome 1 finish hua, May 2006 mein. Ek aur kaam tha genome ke genetic aur physical maps banana. Inmein restriction endonucleases se kaatne wali sites mein variation (polymorphism) aur microsatellites naam ke chhote repeated sequences use hue; agla section dikhata hai ki aise repeat polymorphisms kaise use hote hain.

5.9.1 Salient Features of Human Genome

Do findings par ek baat. Repeats ko kabhi 'junk' kehkar chhod diya gaya tha, lekin unki arrangement is baat ka record hai ki chromosomes kaise organised hain, kaise move aur change hote hain, aur kaise evolve hue. Single nucleotide polymorphisms (SNPs, bolte hain 'snips') human genetics ke sabse kaam ke markers ban gaye hain: families mein kaunse SNP variants kisi disease ke saath chalte hain yeh check karke researchers zimmedaar gene tak pahunch sakte hain, aur populations mein SNP patterns hamare ancestors ke migrations trace karte hain.

5.9.2 Applications and Future Challenges

Sequence ek shuruaat thi, ant nahi. Letters jaanna unhe samajhne jaisa nahi hai, aur sequence ko biology mein badalne mein labs aur companies ke hazaaron researchers dashakon tak lage rahenge. Sabse bada badlaav sawaal poochne ke tareeke mein hai. Ek baar mein ek gene study karne ki jagah biologists ab ek saath hazaaron genes measure kar sakte hain: tumour mein bagal ke healthy tissue ke comparison mein kaunse genes on hain, kaunse variants kisi patient mein dawa ko achha ya bura kaam karwate hain, ya koi crop variety sookhe ko kaise jhelti hai. Ab kisi insaan ka genome padhne mein kuch din lagte hain aur pehle wale ke kharche ka bahut chhota hissa lagta hai, jisse genome tests hospitals tak pahunch rahe hain, India mein bhi.

5.10 DNA Fingerprinting

Forensic scientists aur courts ko aksar jaanna hota hai ki do DNA samples ek hi insaan ke hain ya nahi, ya do log related hain ya nahi. Kyunki hum mein se koi bhi do log apne sirf lagbhag 0.1 % bases mein alag hain, har case ke liye poore genomes compare karna bekaar hoga. DNA fingerprinting ek shortcut leta hai: yeh sirf kuch regions compare karta hai jo insaan se insaan mein bahut zyada alag hote hain.

Chune gaye regions repetitive DNA hain, jismein ek chhota sequence baar-baar repeat hota hai. Iska kuch hissa pehli baar centrifugation ki ek ajeeb baat ke roop mein dikha. Density gradient mein cell ka zyadatar DNA ek bada peak banata hai, lekin uske bagal mein chhote extra peaks dikhte hain, jo aise repeats se bane hain jinka base composition unhe thodi alag density deta hai: satellite DNA. Satellites kai types ke hote hain, is baat par depend karke ki woh A aur T ya G aur C mein kitne rich hain, repeating unit kitni lambi hai aur kitni baar repeat hoti hai; microsatellites aur minisatellites inmein se do hain. Yeh koi protein code nahi karte, phir bhi genome ka bada hissa banate hain, aur repeats ki sankhya logon mein bahut zyada alag hoti hai.

Kisi insaan ke har tissue mein, chahe blood ho, hair roots, skin, bone, saliva ya semen, repeats ka same pattern hota hai. Isse yeh forensic kaam mein pehchaan ka ek powerful tool ban jaata hai. Aur kyunki repeats parents se inherit hote hain, wahi test parentage ke jhagde bhi suljha sakta hai.

Kyunki yeh method DNA polymorphism par tika hai, hume is term ko saaf samajh lena chahiye.

Polymorphisms inherit hote hain, isliye woh germ cells mein mutations ke roop mein shuru hote hain; body (somatic) cell ka mutation insaan ke saath hi khatam ho jaata hai. Agar germ-line change apne carriers ko bachche paida karne se nahi rokta, to woh kai generations mein population mein phail sakta hai. Zyadatar polymorphisms non-coding DNA mein hote hain, jahan change shayad hi kabhi health ya fertility par asar daalta hai, isliye woh hatta nahi aur jama hota rehta hai. Yeh ek single badle base se lekar bade rearrangements tak ho sakte hain, aur yahi woh variation dete hain jis par evolution aur nayi species ka banna depend karta hai.

Yeh method Alec Jeffreys ne 1984–85 mein invent kiya. Unka probe ek bahut variable satellite sequence tha, variable number of tandem repeats (VNTR), aur original procedure ise radioactive banaye gaye probe ke saath Southern blot hybridisation se detect karta tha.

Isse personal pattern kyun milta hai (Fig. 5.20)? VNTR ek minisatellite hai: ek chhota sequence jo head to tail repeat hota hai. Log is baat mein alag hote hain ki har VNTR site par unke paas kitne repeats hain, aur ek hi insaan mein chromosome ki do copies bhi aksar alag hoti hain, isliye repeats ke bagal mein kaatne se lagbhag 0.1 se 20 kb tak ke fragments milte hain. Probe har ek ko light up karta hai, aur film par bands ki ek ladder dikhti hai jo har insaan ke liye unique hai. Sirf identical (monozygotic) twins, jo ek hi zygote se aate hain, ise share karte hain.

Aaj yeh test kahin zyada sensitive hai. Polymerase chain reaction (PCR, Chapter 9) DNA ke chune hue stretches ko laakhon guna badha deta hai, isliye ek single cell ka DNA bhi kaafi ho sakta hai, aur kai alag probes aur markers use hote hain. Crime aur paternity cases ke alawa DNA fingerprints genetic diversity naapne mein madad karte hain, jaise kisi endangered species ke andar ya crop varieties ke beech.