مواد ڏانھن هلو

ڊي اين اي

ڳنڍڻن کي سنواريو
کليل ڄاڻ چيڪلي، وڪيپيڊيا مان

ٻٽي پيچدار ڌاڳي نما ڊي اين اي جي شڪل جو حصو

ڊي آڪسي رائبو نيوڪليئڪ ائسڊ (Deoxyribonucleic Acid)، جنھن کي مختصرن ڊي اين اي (DNA) سڏيو ويندو آهي، ٻن ڌاڳي نما ھڪ ٻئي سان ويڙھيل لڙھن جو ٺھيل ماليڪيول آهي، جيڪا پيچدار شڪل جو ڌاڳي نما نيوڪليائي تيزاب ھوندو آھي، جنھن منجھ سمورن ڄاتل جاندارن ۽ وائرس جي پيدائش، وڏي ٿيڻ، ڪم ڪرڻ جي طريقن ۽ واڌ بابت جينياتي ترتيب موجود ھوندي آھي. ڊي اين اي ۽ آر اين اي نيوڪليئڪ تيزاب آهن. پروٽين، لپڊس (Lipids) ۽ پيچيده ڪاربوهائيڊريٽ (Polysaccharides) سان گڏ، نيوڪليڪ ايسڊ ڊگھي ماليڪيول (macromolecules) جي چئن وڏن قسمن مان هڪ آهن، جنھن ۾ ھڪ مخصوص جينياتي ڪوڊ ھوندو آھي، جيڪو کاڌن جي ترڪيبن واري ڪتاب وانگر جسم جي اندر سمورن قسمن جي پروٽين جي ٺاھڻ لاء ھدايتون رکندو آھي.[1] ۽ اها وراثت جي خاصيتن کي والدين کان اولاد ۾ منتقل ڪرڻ جو سبب ٿئي ٿو.

ٻن ڊي اين اي گڇن کي پولي نيوڪليوٽائڊس جي نالي سان سڃاتو وڃي ٿو ڇاڪاڻ ته اهي آسان مونوميرڪ يونٽن مان ٺهيل آهن جن کي نيوڪليوٽائڊس سڏيو ويندو آهي.[2][3] هر نيوڪليوٽائڊ چئن نائٽروجن تي مشتمل نيوڪليو بيسس (سائيٽوسائن، گيانائن، ايڊينائن ۽ ٿايامن)، هڪ شوگر جنهن کي ڊي آڪسائيربوز سڏيو ويندو آهي ۽ هڪ فاسفيٽ گروپ تي مشتمل هوندو آهي. نيوڪليوٽائيڊس هڪ ٻئي سان هڪ زنجير ۾ ڪوولنٽ بانڊز (جنهن کي فاسفوڊيسٽر لنڪج طور سڃاتو وڃي ٿو) هڪ نيوڪليوٽائيڊ جي شگر ۽ ٻئي جي فاسفيٽ جي وچ ۾ جڙيل آهن، جنهن جي نتيجي ۾ هڪ متبادل شگر-فاسفيٽ ريبون پيدا ٿئي ٿي. ٻن الڳ پولي نيوڪليوٽيڊ اسٽريڊس جي نائيٽروجني بنيادون، بنيادي جوڙيندڙ ضابطن جي مطابق، هائڊروجن بانڊن سان گڏ ڊبل اسٽرينڊ ڊي اين اي ٺاهڻ لاءِ، هڪٻئي سان جڙيل آهن. مڪمل نائيٽروجني بنيادن کي ٻن گروپن؛ واحد-رنگڊ پيريمائڊائنز ۽ ڊبل-رنگڊ پيورين، ۾ ورهايو ويو آهي. ڊي اين اي ۾، پيريميڊن، ٿائمن ۽ سائٽوسن آهن ۽ پيورين، ايڊينن ۽ گوانن آهن.

ڊي اين اي ڊبل هيلڪس (قسم B-DNA) جي جوڙجڪ. ڍانچي ۾ ايٽم عنصرن جي رنگ سان ڪوڊ ٿيل آهن ۽ ٻن بنيادي جوڙن جي تفصيلي جوڙجڪ هيٺئين ساڄي پاسي ڏيکاريل آهن

ڊبل اسٽرينڊ ڊي اين اي جا ٻئي گڇا ساڳي حياتياتي معلومات کي محفوظ ڪن ٿا. اها معلومات نقل نقل ٿي آهي، جڏهن ٻه گڇا الڳ ٿين ٿا. ڊي اين اي جو وڏو حصو (انسانن لاء %98 کان وڌيڪ) ڪوڊ ٿيل نه آهي، مطلب ته اهي حصا پروٽين جي ترتيبن لاء نمونن جي طور تي ڪم نه ڪندا آهن. ڊي اين اي جا ٻه حصا هڪ ٻئي جي مخالف طرفن ۾ هلن ٿا ۽ اهڙيءَ طرح ضد متوازي آهن. هر کنڊ سان جڙيل چار قسمن مان هڪ نيوڪليوبيس (يا اساس) آهي. اهو انهن چئن نيوڪليوبيسن جو تسلسل آهي جيڪو پٺي جي بون سان گڏ جينياتي معلومات کي انڪوڊ ڪري ٿو. آر اين اي اسٽرينڊز ڊي اين اي اسٽرينڊس کي ٽيمپليٽ جي طور تي، ٽرانڪرپشن نالي هڪ پروسيس ۾ استعمال ڪندي ٺاهيا ويندا آهن، جتي ڊي اين اي اساسن کي انهن جي لاڳاپيل اساسن کان تبادلو ڪيو ويندو آهي، سواءِ ٿائيمن (T) جي صورت ۾، جنهن لاءِ آر اين اي يوراسل (U) کي متبادل بڻائي ٿو.[4] جينياتي ڪوڊ جي تحت، اهي آر اين اي اسٽريڊس پروٽين جي اندر امينو تيزاب جي تسلسل کي بيان ڪن ٿا، جنهن کي ترجمو سڏيو ويندو آهي.

ڊي اين اي جي سادي ڊاياگرام

يوڪريوٽڪ گھرڙن جي اندر، ڊي اين اي کي ڊگھي جوڙجڪ، جنهن کي ڪروموزوم سڏيو ويندو آهي، ۾ منظم ڪيو ويندو آهي. عام جيو گھرڙي جي تقسيم کان اڳ، اهي ڪروموزوم ڊي اين اي جي نقل جي عمل ۾ نقل ڪيا ويا آهن ۽ اهڙي طرح هر ڌي جيو گھرڙي لاء ڪروموزوم جو مڪمل سيٽ مهيا ڪري ٿو. يوڪريوٽڪ جاندار (جانور، ٻوٽا، فنگس ۽ پروٽسٽ) پنهنجي ڊي اين اي جو گهڻو حصو گھرڙي جي مرڪز (Nucleus) جي اندر نيوڪليائي ڊي اين اي (Nucleic DNA) طور ۽ ڪجهه مائٽوڪونڊريا (mitochondria) ۾ مائٽوڪونڊريائي ڊي اين اي طور يا ڪلوروپلاسٽ ۾ ڪلوروپلاسٽ ڊي اين اي طور، ذخيرو ڪندا آهن.[5] ان جي ابتڙ، پروڪاريوٽس (بيڪٽيريا ۽ آرڪيا) پنهنجي ڊي اين اي کي صرف سائٽوپلازم ۾، گول ڪروموزوم جي طور محفوظ ڪن ٿا. يوڪريوٽڪ ڪروموزوم جي اندر، ڪرومئٽن پروٽين، جهڙوڪ هسٽونيس، ڊي اين اي کي ملائي رکن ٿا ۽ منظم ڪن ٿا. اهي ٺهيل جوڙجڪ ڊي اين اي ۽ ٻين پروٽينن جي وچ ۾ رابطي جي رهنمائي ڪن ٿا، ڪنٽرول ۾ مدد ڪن ٿا ته ڊي اين اي جا ڪهڙا حصا نقل ٿيل آهن.

خاصيتون

[سنواريو]

ڊي اين اي ماليڪيولن جو ٺھيل ھوندو آھي جن کي نيوڪليوٽائڊ چوندا آهن. ھر نيوڪليوٽائڊ کنڊ ۽ فاسفيٽ گروپن جو ٺھيل ھوندو آھي جن جا بنياد نائٽروجن ھوندا آھن. انھن بنيادن جا چار قسم مٿي بيان ٿيل آھن.[6] انھن چئن بنيادن جي ڊي اين اي جي ڏاڪڻ جي ڏاڪن واري ترتيب کي جينياتي ڪوڊ يا جينياتي مجموعو چوندا آهن.[6]

ڊي اين اي جي ڪيميائي جوڙجڪ؛ هائڊروجن بانڊ ڊاٽ ٿيل لائينن جي طور تي ڏيکاريا ويا آهن. ڊبل هيلڪس جي هر آخر ۾ هڪ اسٽرينڊ تي هڪ بي نقاب 5' فاسفيٽ ۽ ٻئي تي هڪ بي نقاب 3' هائيڊروڪسيل گروپ (—OH) آهي.

ڊي اين اي هڪ ڊگهو پوليمر آهي جيڪو بار بار (repeating) ٿيندڙ يونٽن مان ٺهيل آهي جنهن کي نيوڪليوٽائڊس سڏيو ويندو آهي. ڊي اين اي جي جوڙجڪ ان جي ڊيگهه سان متحرڪ آهي، تنگ لوپس ۽ ٻين شڪلن ۾ ڪوئل ڪرڻ جي قابل آهي. سڀني نسلن ۾ اهو ٻن هيليڪل زنجيرن تي مشتمل آهي، جيڪو هڪ ٻئي سان هائيڊروجن بانڊن سان ڳنڍيل آهن. ٻئي زنجيرون هڪ ئي محور جي چوڌاري ڪوئل ٿيل آهن، ۽ 34 Å اينجسٽرام (3.4 nanometer) جي ساڳي پچ آهي. زنجيرن جي جوڙي جو ريڊيس 10 Å (1.0 اين ايم) آهي. هڪ ٻئي مطالعي مطابق، جڏهن هڪ مختلف حل (solution) ۾ ماپيو ويندو آهي، ته ڊي اين اي زنجير 22-26 Å (2.2-2.6 اين ايم) ويڪر ماپي ويندي آهي، ۽ هڪ نيوڪليوٽائيڊ يونٽ 3.3 Å (0.33 اين ايم) ڊگهو ماپي ويندي آهي. گھڻن ڊي اين اي جي تيز کثافت 1.7 گرام/سينٽي ميٽر 3 آهي. * ڊي اين اي عام طور تي هڪ واحد اسٽرينڊ جي طور تي موجود نه آهي، پر ان جي بدران تارن جي هڪ جوڙي جي طور تي موجود آهي جيڪي مضبوطيءَ سان گڏ جڙيل آهن. اهي ٻه ڊگها تار هڪ ٻئي جي چوڌاري ڊبل هيلڪس جي شڪل ۾ گڏ ٿين ٿا. نيوڪليوٽائيڊ ۾ ماليڪيول جي ريڙهه جو هڪ حصو (جيڪو زنجير کي گڏ رکي ٿو) ۽ هڪ نيوڪليوبيس (جيڪو هيلڪس ۾ ٻئي ڊي اين اي اسٽرينڊ سان رابطو ڪري ٿو) شامل آهن. هڪ نيوڪليوبيس جيڪو کنڊ سان ڳنڍيل آهي ان کي نيوڪليوسائيڊ سڏيو ويندو آهي، ۽ هڪ بنياد جيڪو کنڊ ۽ هڪ يا وڌيڪ فاسفيٽ گروپن سان ڳنڍيل آهي ان کي نيوڪليوٽائيڊ سڏيو ويندو آهي.

DNA is a long polymer made from repeating units called nucleotides.[7][8] The structure of DNA is dynamic along its length, being capable of coiling into tight loops and other shapes.[9] In all species it is composed of two helical chains, bound to each other by hydrogen bonds. Both chains are coiled around the same axis, and have the same pitch of 34 اينگسٽرام (3.4 nm). The pair of chains have a radius of 10 Å (1.0 nm).[10] According to another study, when measured in a different solution, the DNA chain measured 22–26 Å (2.2–2.6 nm) wide, and one nucleotide unit measured 3.3 Å (0.33 nm) long.[11] The buoyant density of most DNA is 1.7g/cm3.[12]

DNA does not usually exist as a single strand, but instead as a pair of strands that are held tightly together.[10][13] These two long strands coil around each other, in the shape of a double helix. The nucleotide contains both a segment of the backbone of the molecule (which holds the chain together) and a nucleobase (which interacts with the other DNA strand in the helix). A nucleobase linked to a sugar is called a nucleoside, and a base linked to a sugar and to one or more phosphate groups is called a nucleotide. A biopolymer comprising multiple linked nucleotides (as in DNA) is called a polynucleotide.[14]

The backbone of the DNA strand is made from alternating phosphate and sugar groups.[15] The sugar in DNA is 2-deoxyribose, which is a pentose (five-carbon) sugar. The sugars are joined by phosphate groups that form phosphodiester bonds between the third and fifth carbon atoms of adjacent sugar rings. These are known as the 3′-end (three prime end), and 5′-end (five prime end) carbons, the prime symbol being used to distinguish these carbon atoms from those of the base to which the deoxyribose forms a glycosidic bond.[13]

Therefore, any DNA strand normally has one end at which there is a phosphate group attached to the 5′ carbon of a ribose (the 5′ phosphoryl) and another end at which there is a free hydroxyl group attached to the 3′ carbon of a ribose (the 3′ hydroxyl). The orientation of the 3′ and 5′ carbons along the sugar-phosphate backbone confers directionality (sometimes called polarity) to each DNA strand. In a nucleic acid double helix, the direction of the nucleotides in one strand is opposite to their direction in the other strand: the strands are antiparallel. The asymmetric ends of DNA strands are said to have a directionality of five prime end (5′ ), and three prime end (3′), with the 5′ end having a terminal phosphate group and the 3′ end a terminal hydroxyl group. One major difference between DNA and RNA is the sugar, with the 2-deoxyribose in DNA being replaced by the related pentose sugar ribose in RNA.[13]

A section of DNA. The bases lie horizontally between the two spiraling strands[16] (animated version).

The DNA double helix is stabilized primarily by two forces: hydrogen bonds between nucleotides and base-stacking interactions among aromatic nucleobases.[17] The four bases found in DNA are adenine (A), cytosine (C), guanine (G) and thymine (T). These four bases are attached to the sugar-phosphate to form the complete nucleotide, as shown for adenosine monophosphate. Adenine pairs with thymine and guanine pairs with cytosine, forming A-T and G-C base pairs.[18][19]

Nucleobase classification

[سنواريو]

The nucleobases are classified into two types: the purines, A and G, which are fused five- and six-membered heterocyclic compounds, and the pyrimidines, the six-membered rings C and T.[13] A fifth pyrimidine nucleobase, uracil (U), usually takes the place of thymine in RNA and differs from thymine by lacking a methyl group on its ring. In addition to RNA and DNA, many artificial nucleic acid analogues have been created to study the properties of nucleic acids, or for use in biotechnology.[20]

Non-canonical bases

[سنواريو]

Modified bases occur in DNA. The first of these recognized was 5-methylcytosine, which was found in the genome of Mycobacterium tuberculosis in 1925.[21] The reason for the presence of these noncanonical bases in bacterial viruses (bacteriophages) is to avoid the restriction enzymes present in bacteria. This enzyme system acts at least in part as a molecular immune system protecting bacteria from infection by viruses.[22] Modifications of the bases cytosine and adenine, the more common and modified DNA bases, play vital roles in the epigenetic control of gene expression in plants and animals.[23]

A number of noncanonical bases are known to occur in DNA.[24] Most of these are modifications of the canonical bases plus uracil.

  • Modified Adenine
    • N6-carbamoyl-methyladenine
    • N6-methyadenine
  • Modified Guanine
    • 7-Deazaguanine
    • 7-Methylguanine
  • Modified Cytosine
    • N4-Methylcytosine
    • 5-Carboxylcytosine
    • 5-Formylcytosine
    • 5-Glycosylhydroxymethylcytosine
    • 5-Hydroxycytosine
    • 5-Methylcytosine
  • Modified Thymidine
    • α-Glutamythymidine
    • α-Putrescinylthymine
  • Uracil and modifications
    • Base J
    • Uracil
    • 5-Dihydroxypentauracil
    • 5-Hydroxymethyldeoxyuracil
  • Others
    • Deoxyarchaeosine
    • 2,6-Diaminopurine (2-Aminoadenine)

Grooves

[سنواريو]
DNA major and minor grooves. The latter is a binding site for the Hoechst stain dye 33258.

Twin helical strands form the DNA backbone. Another double helix may be found tracing the spaces, or grooves, between the strands. These voids are adjacent to the base pairs and may provide a binding site. As the strands are not symmetrically located with respect to each other, the grooves are unequally sized. The major groove is 22 اينگسٽرام (2.2 nm) wide, while the minor groove is 12 Å (1.2 nm) in width.[25] Due to the larger width of the major groove, the edges of the bases are more accessible in the major groove than in the minor groove. As a result, proteins such as transcription factors that can bind to specific sequences in double-stranded DNA usually make contact with the sides of the bases exposed in the major groove.[26] This situation varies in unusual conformations of DNA within the cell (see below), but the major and minor grooves are always named to reflect the differences in width that would be seen if the DNA was twisted back into the ordinary B form.

Base pairing

[سنواريو]
Top, a GC base pair with three hydrogen bonds. Bottom, an AT base pair with two hydrogen bonds. Non-covalent hydrogen bonds between the pairs are shown as dashed lines.

In a DNA double helix, each type of nucleobase on one strand bonds with just one type of nucleobase on the other strand. This is called complementary base pairing. Purines form hydrogen bonds to pyrimidines, with adenine bonding only to thymine in two hydrogen bonds, and cytosine bonding only to guanine in three hydrogen bonds. This arrangement of two nucleotides binding together across the double helix (from six-carbon ring to six-carbon ring) is called a Watson-Crick base pair. DNA with high GC-content is more stable than DNA with low GC-content. A Hoogsteen base pair (hydrogen bonding the 6-carbon ring to the 5-carbon ring) is a rare variation of base-pairing.[27] As hydrogen bonds are not covalent, they can be broken and rejoined relatively easily. The two strands of DNA in a double helix can thus be pulled apart like a zipper, either by a mechanical force or high temperature.[28] As a result of this base pair complementarity, all the information in the double-stranded sequence of a DNA helix is duplicated on each strand, which is vital in DNA replication. This reversible and specific interaction between complementary base pairs is critical for all the functions of DNA in organisms.[8]

ssDNA vs. dsDNA

[سنواريو]

Most DNA molecules are actually two polymer strands, bound together in a helical fashion by noncovalent bonds; this double-stranded (dsDNA) structure is maintained largely by the intrastrand base stacking interactions, which are strongest for G,C stacks. The two strands can come apart—a process known as melting—to form two single-stranded DNA (ssDNA) molecules. Melting occurs at high temperatures, low salt and high pH (low pH also melts DNA, but since DNA is unstable due to acid depurination, low pH is rarely used).

The stability of the dsDNA form depends not only on the GC-content (% G,C basepairs) but also on sequence (since stacking is sequence specific) and also length (longer molecules are more stable). The stability can be measured in various ways; a common way is the melting temperature (also called Tm value), which is the temperature at which 50% of the double-strand molecules are converted to single-strand molecules; melting temperature is dependent on ionic strength and the concentration of DNA. As a result, it is both the percentage of GC base pairs and the overall length of a DNA double helix that determines the strength of the association between the two strands of DNA. Long DNA helices with a high GC-content have more strongly interacting strands, while short helices with high AT content have more weakly interacting strands.[29] In biology, parts of the DNA double helix that need to separate easily, such as the TATAAT Pribnow box in some promoters, tend to have a high AT content, making the strands easier to pull apart.[30]

In the laboratory, the strength of this interaction can be measured by finding the melting temperature Tm necessary to break half of the hydrogen bonds. When all the base pairs in a DNA double helix melt, the strands separate and exist in solution as two entirely independent molecules. These single-stranded DNA molecules have no single common shape, but some conformations are more stable than others.[31]

Amount

[سنواريو]
Schematic karyogram of a human. It shows 22 homologous chromosomes, both the female (XX) and male (XY) versions of the sex chromosome (bottom right), as well as the mitochondrial genome (to scale at bottom left). The blue scale to the left of each chromosome pair (and the mitochondrial genome) shows its length in terms of millions of DNA base pairs.

In humans, the total female diploid nuclear genome per cell extends for 6.37 Gigabase pairs (Gbp), is 208.23 cm long and weighs 6.51 picograms (pg).[32] Male values are 6.27 Gbp, 205.00 cm, 6.41 pg.[32] Each DNA polymer can contain hundreds of millions of nucleotides, such as in chromosome 1. Chromosome 1 is the largest human chromosome with approximately 220 million base pairs, and would be 85 mm long if straightened.[33]

In eukaryotes, in addition to nuclear DNA, there is also mitochondrial DNA (mtDNA) which encodes certain proteins used by the mitochondria. The mtDNA is usually relatively small in comparison to the nuclear DNA. For example, the human mitochondrial DNA forms closed circular molecules, each of which contains 16,569[34][35] DNA base pairs,[36] with each such molecule normally containing a full set of the mitochondrial genes. Each human mitochondrion contains, on average, approximately 5 such mtDNA molecules.[36] Each human cell contains approximately 100 mitochondria, giving a total number of mtDNA molecules per human cell of approximately 500.[36] However, the amount of mitochondria per cell also varies by cell type, and an egg cell can contain 100,000 mitochondria, corresponding to up to 1,500,000 copies of the mitochondrial genome (constituting up to 90% of the DNA of the cell).[37]

Sense and antisense

[سنواريو]

Script error: No such module "redirect hatnote".</ includeonly> A DNA sequence is called a "sense" sequence if it is the same as that of a messenger RNA copy that is translated into protein.[38] The sequence on the opposite strand is called the "antisense" sequence. Both sense and antisense sequences can exist on different parts of the same strand of DNA (i.e. both strands can contain both sense and antisense sequences). In both prokaryotes and eukaryotes, antisense RNA sequences are produced, but the functions of these RNAs are not entirely clear.[39] One proposal is that antisense RNAs are involved in regulating gene expression through RNA-RNA base pairing.[40]

A few DNA sequences in prokaryotes and eukaryotes, and more in plasmids and viruses, blur the distinction between sense and antisense strands by having overlapping genes.[41] In these cases, some DNA sequences do double duty, encoding one protein when read along one strand, and a second protein when read in the opposite direction along the other strand. In bacteria, this overlap may be involved in the regulation of gene transcription,[42] while in viruses, overlapping genes increase the amount of information that can be encoded within the small viral genome.[43]

Supercoiling

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DNA can be twisted like a rope in a process called DNA supercoiling. With DNA in its "relaxed" state, a strand usually circles the axis of the double helix once every 10.4 base pairs, but if the DNA is twisted the strands become more tightly or more loosely wound.[44] If the DNA is twisted in the direction of the helix, this is positive supercoiling, and the bases are held more tightly together. If they are twisted in the opposite direction, this is negative supercoiling, and the bases come apart more easily. In nature, most DNA has slight negative supercoiling that is introduced by enzymes called topoisomerases.[45] These enzymes are also needed to relieve the twisting stresses introduced into DNA strands during processes such as transcription and DNA replication.[46]

Alternative DNA structures

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From left to right, the structures of A, B and Z-DNA

DNA exists in many possible conformations that include A-DNA, B-DNA, and Z-DNA forms, although only B-DNA and Z-DNA have been directly observed in functional organisms.[15] The conformation that DNA adopts depends on the hydration level, DNA sequence, the amount and direction of supercoiling, chemical modifications of the bases, the type and concentration of metal ions, and the presence of polyamines in solution.[47]

The first published reports of A-DNA X-ray diffraction patterns—and also B-DNA—used analyses based on Patterson functions that provided only a limited amount of structural information for oriented fibers of DNA.[48][49] An alternative analysis was proposed by Wilkins et al. in 1953 for the in vivo B-DNA X-ray diffraction-scattering patterns of highly hydrated DNA fibers in terms of squares of Bessel functions.[50] In the same journal, James Watson and Francis Crick presented their molecular modeling analysis of the DNA X-ray diffraction patterns to suggest that the structure was a double helix.[10]

Although the B-DNA form is most common under the conditions found in cells,[51] it is not a well-defined conformation but a family of related DNA conformations[52] that occur at the high hydration levels present in cells. Their corresponding X-ray diffraction and scattering patterns are characteristic of molecular paracrystals with a significant degree of disorder.[53][54]

Compared to B-DNA, the A-DNA form is a wider right-handed spiral, with a shallow, wide minor groove and a narrower, deeper major groove. The A form occurs under non-physiological conditions in partly dehydrated samples of DNA, while in the cell it may be produced in hybrid pairings of DNA and RNA strands, and in enzyme-DNA complexes.[55][56] Segments of DNA where the bases have been chemically modified by methylation may undergo a larger change in conformation and adopt the Z form. Here, the strands turn about the helical axis in a left-handed spiral, the opposite of the more common B form.[57] These unusual structures can be recognized by specific Z-DNA binding proteins and may be involved in the regulation of transcription.[58]

Alternative DNA chemistry

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For many years, exobiologists have proposed the existence of a shadow biosphere, a postulated microbial biosphere of Earth that uses radically different biochemical and molecular processes than currently known life. One of the proposals was the existence of lifeforms that use arsenic instead of phosphorus in DNA. A report in 2010 of the possibility in the bacterium GFAJ-1 was announced,[59][60] though the research was disputed,[60][61] and evidence suggests the bacterium actively prevents the incorporation of arsenic into the DNA backbone and other biomolecules.[62]

Quadruplex structures

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DNA quadruplex formed by telomere repeats. The looped conformation of the DNA backbone is very different from the typical DNA helix. The green spheres in the center represent potassium ions.[63]

At the ends of the linear chromosomes are specialized regions of DNA called telomeres. The main function of these regions is to allow the cell to replicate chromosome ends using the enzyme telomerase, as the enzymes that normally replicate DNA cannot copy the extreme 3′ ends of chromosomes.[64] These specialized chromosome caps also help protect the DNA ends, and stop the DNA repair systems in the cell from treating them as damage to be corrected.[65] In human cells, telomeres are usually lengths of single-stranded DNA containing several thousand repeats of a simple TTAGGG sequence.[66]

These guanine-rich sequences may stabilize chromosome ends by forming structures of stacked sets of four-base units, rather than the usual base pairs found in other DNA molecules. Here, four guanine bases, known as a guanine tetrad, form a flat plate. These flat four-base units then stack on top of each other to form a stable G-quadruplex structure.[67] These structures are stabilized by hydrogen bonding between the edges of the bases and chelation of a metal ion in the centre of each four-base unit.[68] Other structures can also be formed, with the central set of four bases coming from either a single strand folded around the bases, or several different parallel strands, each contributing one base to the central structure.

In addition to these stacked structures, telomeres also form large loop structures called telomere loops, or T-loops. Here, the single-stranded DNA curls around in a long circle stabilized by telomere-binding proteins.[69] At the very end of the T-loop, the single-stranded telomere DNA is held onto a region of double-stranded DNA by the telomere strand disrupting the double-helical DNA and base pairing to one of the two strands. This triple-stranded structure is called a displacement loop or D-loop.[67]

Branched DNA

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Single branch Multiple branches
Branched DNA can form networks containing multiple branches.

In DNA, fraying occurs when non-complementary regions exist at the end of an otherwise complementary double-strand of DNA. However, branched DNA can occur if a third strand of DNA is introduced and contains adjoining regions able to hybridize with the frayed regions of the pre-existing double-strand. Although the simplest example of branched DNA involves only three strands of DNA, complexes involving additional strands and multiple branches are also possible.[70] Branched DNA can be used in nanotechnology to construct geometric shapes, see the section on uses in technology below.

Artificial bases

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اصل مضمون جي لاءِ ڏسو Nucleic acid analogue

Several artificial nucleobases have been synthesized, and successfully incorporated in the eight-base DNA analogue named Hachimoji DNA. Dubbed S, B, P, and Z, these artificial bases are capable of bonding with each other in a predictable way (S–B and P–Z), maintain the double helix structure of DNA, and be transcribed to RNA. Their existence could be seen as an indication that there is nothing special about the four natural nucleobases that evolved on Earth.[71][72] On the other hand, DNA is tightly related to RNA which does not only act as a transcript of DNA but also performs as molecular machines many tasks in cells. For this purpose it has to fold into a structure. It has been shown that to allow to create all possible structures at least four bases are required for the corresponding RNA,[73] while a higher number is also possible but this would be against the natural principle of least effort.

Acidity

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The phosphate groups of DNA give it similar acidic properties to phosphoric acid and it can be considered as a strong acid. It will be fully ionized at a normal cellular pH, releasing protons which leave behind negative charges on the phosphate groups. These negative charges protect DNA from breakdown by hydrolysis by repelling nucleophiles which could hydrolyze it.[74]

Macroscopic appearance

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Impure DNA extracted from an orange

Pure DNA extracted from cells forms white, stringy clumps.[75]

موروثي بنيادي ايڪا

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ڊي اين اي ھڪ ورو ڪڙدار ڏاڪڻ وانگر ٺھيل ھوندو آھي، جن جا پاسا ڄڻ تہ ٻن ھڪ ٻئي سان وڪوڙيل ڌاڳن وانگر ھوندا آھن، جن جي وچ ۾ ڪروڙين ڏاڪا ھوندا آھن. ھر ڏاڪي ۾ ٻہ بنيادي ايڪا (Base) ڳنڍيل ھوندا آھن، يعني ھر ڏاڪو ھڪ جوڙو ھوندو آھي جن جو ڏاڪي جي وچ تي سنگم ھائڊروجن بانڊ سڏبو آهي.[1]

موروثي بنيادي ايڪن جي ترتيب وارو جينياتي ڪوڊ ھر جيو گھرڙي جي مرڪز ۾ موجود ھوندو آھي جيڪو مورثي ھوندو آھي يعني والدين مان ٻارن ۾ منتقل ٿيندڙ آهي.

اھي بنيادي ايڪا ھيٺين چئن قسمن جا ھوندا آھن:[1]

تبديل ٿيل بنيادي ايڪا

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ڊي اين اي (DNA) ۾ تبديل ٿيل غيرموروثي بنيادي ايڪا پڻ هوندا آهن. انهن مان پهريون سڃاڻپ ٿيل 5-ميٿائل سائٽوسن هو, جيڪو 1925ع ۾ مائڪوبيڪٽريم ٽيوبرڪلوسس جي جينوم (Genome) ۾ مليو.[76] بيڪٽيريا وائرس (بيڪٽريوفيج) ۾ انهن غير موروثي ايڪن جي موجودگي جو سبب بيڪٽيريا ۾ موجود (limiting) اينزائمز کان بچڻ آهي.[77] هي اينزائم جا سسٽم جزوي طور تي هڪ ماليڪيولي مدافعتي نظام جي طور تي ڪم ڪرڻ ٿا, جيڪو بيڪٽيريا کي وائرس جي انفيڪشن کان بچائيندو آهي. وڌيڪ عام بنياد ايڪن سائيٽوسائن ۽ ايڊينائن جي تبديليون ۽ تبديل ٿيل ڊي اين اي ايڪا ٻوٽن ۽ جانورن ۾ جين جي اظهار جي ايپي جنيٽڪ ڪنٽرول ۾ اهم ڪردار ادا ڪري ٿو.[78]

  • ڊي اين اي ۾ ڪيترائي غيرموروثي بنيادي ايڪن جي سڃاڻپ ٿي آهي. [79] انهن مان گھڻا موروثي بنيادي ايڪن ۽ يوراسل جي تبديلين جي ڪارڻ آهن.
  • ترميم ٿيل ايڊينائن
    • N6-ڪارباموئل-ميٿائل ايڊينائن
    • N6-ميٿائيڊينائن
  • ترميم ٿيل گوانائن
    • 7-ڊيزا گوانائن
    • 7-ميٿائل گوانائن
  • ترميم ٿيل سائيٽوسائن
    • N4-ميٿائل سائيٽوسائن
    • 5-ڪاربوڪسيل سائيٽوسائن
    • 5-فارميل سائيٽوسائن
    • 5-گلائڪوسل هائيڊروڪسي ميٿائل سائيٽوسائن
    • 5-هائيڊروڪسي سائيٽوسائن
    • 5-ميٿائل سائيٽوسائن
  • ترميم ٿيل ٿائيمائيڊائن
    • α-گلوٽا ميٿيميڊائن
    • α-پوٽرس سينائل ٿائيمائن
  • يوراسل ۽ تبديليون
    • بيس J
    • يوراسل
    • 5-ڊائيهائيڊروڪسي پينٽا يوراسل
    • 5-هائيڊروڪسي ميٿائل ڊي آڪسي يوراسل
  • ٻيا
    • ڊي آڪسي آرڪيو سائن
    • 2,6-ڊي امينو پيورين (2-امينو ايڊينائن)

ڪيميائي تبديليون ۽ تبديل ٿيل ڊي اين اي پيڪئجنگ

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حياتياتي ڪم

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پروٽين سان باهمي عمل

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جينياتي بحالي

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ارتقا

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ٽيڪنالاجي ۾ استعمال

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تاريخ

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سال 1856ع کان 1865ع تائين بيان ڪيل مينڊل جي تجربن سان وراثتي ايڪن جو تصور ٺھڻ لڳو. 1869ع ۾ سوئيٽزرلينڊ جي فريڊرڪ مائيسر (Friedrich Miescher) جيو گھرڙن جي مرڪزن ۾ ھڪ اھڙو مادو دريافت ڪيو جنھن ۾ تمام گھڻي فاسفورس موجود ھئي، پر کيس معلوم نہ ھيو تہ اھو ڊي اين اي سڏجندو. اهو مادو جيو گھرڙي جي مرڪز (Nucleus) مان حاصل ڪيو ويو. ھن سائٽوپلازم مان نيوڪليس کي ڌار ڪري، انکي الڪلي جي ڳار سان صاف ڪري، ان کي تيزابي شڪل ۾ آندو، جنھن کي ھن نيوڪليئن (nuclein) جو نالو ڏنو، جيڪو بعد ۾ ڊي اين اي سڏجڻ شروع ٿيو.[80] فريڊرڪ مائيسر اھو تيزابي مادو (دريافت ڪيل ڊي اين اي) ھڪ سرجيڪل پٽي (bandage) تي لڳل پيپ (pus) مان حاصل ڪيو. ھنجو خيال ھو تہ ھي مادو جنسي زرخيزي ۽ وراثتي خاصيتن جي منتقلي جو ذريعو ٿي سگهي ٿو.

ڊي اين اي دريافت ڪندڙ فريڊرڪ مائيسر

سال 1895ع ۾ ايڊمنڊ ڪروموسومز (موروثي ڌاڳا) ماء ۽ پي طرفان يڪسان منتقل ھجڻ جي بنياد تي انهن جي وراثت ۾ اھميت جو خيال ظاھر ڪيو. ان کان پھرين والٽر فليمنگ مک مرڪز (nuclein) جي مورثي ڪروموسومز سان ڳانڍاپو محسوس ڪري چڪو ھيو ۽ ان لاء پھريون ڀيرو ڪرومئٽن (chromatin) جو لفظ استعمال ڪيو. پوء غدود ۽ خمير جي جيو گھرڙن تي تجربو ڪندي جرمني جو البرخت ڪوسل اھو دريافت ڪيو تہ نيوڪلين اصل ۾ ٻہ تيزابي مادا آھن جن ۾ ھڪڙو ڊي اين اي ۽ ٻيو آر اين اي آھي. جاندارن جي اندر اھي ڪيميائي مادا يعني ڊي اين اي جينياتي ترتيبن جا مڪمل مجموعا ٺاھيندا آھن، جنهن کي جينوم سڏيو ويندو آهي. انساني جينوم ۾ ٽي ارب بنيادي ايڪن (Base) جا جوڙا ڳنڍيل ھوندا آھن ۽ ان ۾ 23 ڪروموسومن جا جوڙا پڻ شامل آهن.[1] سڀني انسانن ۾ انھن بنيادي ايڪن جو 99 سيڪڙو بنيادن جي ترتيب ھڪجھڙي ھوندي آھي.[6]

ميڪلن ميڪارٽي (کاٻي) فرانسس ڪرڪ ۽ جيمز واٽسن، ايڪسري تفاوت جي ڊيٽا ۽ روزالنڊ فرئنڪلن ۽ رئمونڊ گوسلنگ جي بصيرت جي بنياد تي، ڊبل هيلڪس ماڊل جي گڏيل شروعات ڪندڙ، سان هٿ ملائندي.

سال 1953ع ۾ جيمز واٽسن، فرانسس ڪرڪ، مورس ولڪنس ۽ روزالنڊ فرينڪلن ڊي اين اي جي ٻٽي وڪوڙيل ڌاڳن واري پيچدار ڏاڪڻ واري شڪل (double helix) دريافت ڪئي.[6] واٽسن، ڪرڪ ۽ ولڪنس کي سال 1962ع ۾ ڊي اين اي جي ماليڪيولن واري شڪل دريافت ڪرڻ تي طب جو نوبل انعام مليو.[6]

حياتياتي ڪم

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ڊي اين اي (DNA) هڪ پوليمر آهي جيڪو ٻن پولي نيوڪليوٽائيڊ زنجيرن مان ٺهيل آهي جيڪي هڪ ٻئي جي چوڌاري ڊبل هيلڪس ٺاهيندا آهن. پوليمر سڀني سڃاتل جاندارن ۽ ڪيترن ئي وائرسن جي ترقي، ڪم، واڌ ۽ پيدائش لاءِ جينياتي هدايتون ڏين ٿا. ڊي اين اي ۽ آر اين اي نيوڪليئڪ تيزاب آهن. پروٽين، لپڊس ۽ پيچيده ڪاربوهائيڊريٽ (پوليسڪچرائڊس) سان گڏ، نيوڪليئڪ تيزاب ميڪرو ماليڪولن جي چئن وڏن قسمن مان هڪ آهن، جيڪي زندگي جي سڀني ڄاڻايل شڪلن لاء ضروري آهن.

ٻن ڊي اين اي گڇن کي پولي نيوڪليوٽائڊس جي نالي سان سڃاتو وڃي ٿو ڇاڪاڻ ته اهي آسان مونوميرڪ يونٽن مان ٺهيل آهن جن کي نيوڪليوٽائڊس سڏيو ويندو آهي.[81] [82] هر نيوڪليوٽائڊ چئن نائٽروجن تي مشتمل نيوڪليو بيسس (سائيٽوسائن، گيانائن، ايڊينائن ۽ ٿايامن)، هڪ شوگر جنهن کي ڊي آڪسائيربوز سڏيو ويندو آهي ۽ هڪ فاسفيٽ گروپ تي مشتمل هوندو آهي. نيوڪليوٽائيڊس هڪ ٻئي سان هڪ زنجير ۾ ڪوولنٽ بانڊز (جنهن کي فاسفوڊيسٽر لنڪج طور سڃاتو وڃي ٿو) هڪ نيوڪليوٽائيڊ جي شگر ۽ ٻئي جي فاسفيٽ جي وچ ۾ جڙيل آهن، جنهن جي نتيجي ۾ هڪ متبادل شگر-فاسفيٽ ريبون پيدا ٿئي ٿي. ٻن الڳ پولي نيوڪليوٽيڊ اسٽريڊس جي نائيٽروجني بنيادون، بنيادي جوڙيندڙ ضابطن جي مطابق، هائڊروجن بانڊن سان گڏ ڊبل اسٽرينڊ ڊي اين اي ٺاهڻ لاءِ، هڪٻئي سان جڙيل آهن. مڪمل نائيٽروجني بنيادن کي ٻن گروپن؛ واحد-رنگڊ پيريمائڊائنز ۽ ڊبل-رنگڊ پيورين، ۾ ورهايو ويو آهي. ڊي اين اي ۾، پيريميڊين ٿايامن ۽ سائٽوسن آهن ۽ پيورين، ايڊينائن ۽ گيانائن آهن.

ڊبل اسٽرينڊ ڊي اين اي جا ٻئي گڇا ساڳي حياتياتي معلومات کي محفوظ ڪن ٿيون. اها معلومات نقل نقل ٿي آهي، جڏهن ٻه گڇا الڳ ٿين ٿا. ڊي اين اي جو وڏو حصو (انسانن لاء %98 کان وڌيڪ) ڪوڊ ٿيل نه آهي، مطلب ته اهي حصا پروٽين جي ترتيبن لاء نمونن جي طور تي ڪم نه ڪندا آهن. ڊي اين اي جا ٻه حصا هڪ ٻئي جي مخالف طرفن ۾ هلن ٿا ۽ اهڙيءَ طرح ضد متوازي آهن. هر کنڊ سان جڙيل چار قسمن مان هڪ نيوڪليوبيس (يا اساس) آهي. اهو انهن چئن نيوڪليوبيسن جو تسلسل آهي جيڪو پٺي جي بون سان گڏ جينياتي معلومات کي انڪوڊ ڪري ٿو. آر اين اي اسٽرينڊز ڊي اين اي اسٽرينڊس کي ٽيمپليٽ جي طور تي، ٽرانڪرپشن نالي هڪ پروسيس ۾ استعمال ڪندي ٺاهيا ويندا آهن، جتي ڊي اين اي اساسن کي انهن جي لاڳاپيل اساسن کان تبادلو ڪيو ويندو آهي، سواءِ ٿائيمن (T) جي صورت ۾، جنهن لاءِ آر اين اي يوراسل (U) کي متبادل بڻائي ٿو.[83] جينياتي ڪوڊ جي تحت، اهي آر اين اي اسٽريڊس پروٽين جي اندر امينو تيزاب جي تسلسل کي بيان ڪن ٿا، جنهن کي ترجمو سڏيو ويندو آهي.

يوڪريوٽڪ گھرڙن جي اندر، ڊي اين اي کي ڊگھي جوڙجڪ، جنهن کي ڪروموزوم سڏيو ويندو آهي، ۾ منظم ڪيو ويندو آهي. عام جيو گھرڙي جي تقسيم کان اڳ، اهي ڪروموزوم ڊي اين اي جي نقل جي عمل ۾ نقل ڪيا ويا آهن ۽ اهڙي طرح هر ڌي جيو گھرڙي لاء ڪروموزوم جو مڪمل سيٽ مهيا ڪري ٿو. يوڪريوٽڪ جاندار (جانور، ٻوٽا، فنگس ۽ پروٽسٽ) پنهنجي ڊي اين اي جو گهڻو حصو جيو گھرڙي جي نيوڪليس جي اندر نيوڪليائي ڊي اين اي طور ۽ ڪجهه مائٽوڪونڊريا ۾ مائيٽوڪونڊريائي ڊي اين اي طور يا ڪلوروپلاسٽ ۾ ڪلوروپلاسٽ ڊي اين اي طور، ذخيرو ڪندا آهن.[84] ان جي ابتڙ، پروڪاريوٽس (بيڪٽيريا ۽ آرڪيا) پنهنجي ڊي اين اي کي صرف سائٽوپلازم ۾، گول ڪروموزوم جي طور محفوظ ڪن ٿا. يوڪريوٽڪ ڪروموزوم جي اندر، ڪرومئٽن پروٽين، جهڙوڪ هسٽونيس، ڊي اين اي کي ملائي رکن ٿا ۽ منظم ڪن ٿا. اهي ٺهيل جوڙجڪ ڊي اين اي ۽ ٻين پروٽينن جي وچ ۾ رابطي جي رهنمائي ڪن ٿا، ڪنٽرول ۾ مدد ڪن ٿا ته ڊي اين اي جا ڪهڙا حصا نقل ٿيل آهن.

پڻ ڏسو

[سنواريو]

ٻاهريان ڳنڍڻا

[سنواريو]

حوالا

[سنواريو]
  1. 1 2 3 4 -https://www.yourgenome.org/facts/what-is-dna
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  3. ↑ "DNA", Basic Biology, اصل کان 5 January 2017 تي آرڪائيو ٿيل۔
  4. ↑ "Uracil", Genome.gov (ٻولي ۾ انگريزي), حاصل ڪيل 21 November 2019۔
  5. ↑ iGenetics. New York: Benjamin Cummings. 2001. ISBN 0-8053-4553-1. https://archive.org/details/igenetics0000russ_v6o1.
  6. 1 2 3 4 5 livescience.com-
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  80. ↑ Miescher, Friedrich (1871) "Ueber die chemische Zusammensetzung der Eiterzellen" (On the chemical composition of pus cells), Medicinisch-chemische Untersuchungen, 4 : 441–460. From p. 456: "Ich habe mich daher später mit meinen Versuchen an die ganzen Kerne gehalten, die Trennung der Körper, die ich einstweilen ohne weiteres Präjudiz als lösliches und unlösliches Nuclein bezeichnen will, einem günstigeren Material überlassend.سانچو:-" ("Therefore, in my experiments I subsequently limited myself to the whole nucleus, leaving to a more favorable material the separation of the substances, that for the present, without further prejudice, I will designate as soluble and insoluble nuclear material ('Nuclein').")
  81. ↑ Molecular Biology of the Cell (6th ed.). Garland. 2014. p. Chapter 4: DNA, Chromosomes and Genomes. ISBN 978-0-8153-4432-2. http://www.garlandscience.com/product/isbn/9780815344322.
  82. ↑ "DNA", Basic Biology, اصل کان 5 January 2017 تي آرڪائيو ٿيل۔
  83. ↑ "Uracil", Genome.gov (ٻولي ۾ انگريزي), حاصل ڪيل 21 November 2019۔
  84. ↑ iGenetics. New York: Benjamin Cummings. 2001. ISBN 0-8053-4553-1. https://archive.org/details/igenetics0000russ_v6o1.
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