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bonds non covalent hydrogen bonds between the bases are shown as dashed lines the wiggly lines stand for the connection to the pentose sugar and point in the direction of the minor groove hydrogen bonding is the chemical interaction that underlies the base pairing rules described above appropriate geometrical correspondence of hydrogen bond donors and acceptors allows only the right pairs to form stably dna with high gc content is more stable than dna with low gc content crucially however stacking interactions are primarily responsible for stabilising the double helical structure watson crick base pairing s contribution to global structural stability is minimal but its role in the specificity underlying complementarity is by contrast of maximal importance as this underlies the template dependent processes of the central dogma e g dna replication 11 the bigger nucleobases adenine and guanine are members of a class of double ringed chemical structures called purines the smaller nucleobases cytosine and thymine and uracil are members of a class of single ringed chemical structures called pyrimidines purines are complementary only with pyrimidines pyrimidine pyrimidine pairings are energetically unfavorable because the molecules are too far apart for hydrogen bonding to be established purine purine pairings are energetically unfavorable because the molecules are too close leading to overlap repulsion purine pyrimidine base pairing of at or gc or ua in rna results in proper duplex structure the only other purine pyrimidine pairings would be ac and gt and ug in rna these pairings are mismatches because the patterns of hydrogen donors and acceptors do not correspond the gu pairing with two hydrogen bonds does occur fairly often in rna see wobble base pair paired dna and rna molecules are comparatively stable at room temperature but the two nucleotide strands will separate above a melting point that is determined by the length of the molecules the extent of mispairing if any and the gc content higher gc content results in higher melting temperatures it is therefore unsurprising that the genomes of extremophile organisms such as thermus thermophilus are particularly gc rich on the converse regions of a genome that need to separate frequently for example the promoter regions for often transcribed genes are comparatively gc poor for example see tata box gc content and melting temperature must also be taken into account when designing primers for pcr reactions citation needed examples edit the following dna sequences illustrate pair double stranded patterns by convention the top strand is written from the 5 end to the 3 end thus the bottom strand complementary strand is written 3 to 5 a base paired dna sequence atcgattgagctctagcg tagctaactcgagatcgc the corresponding rna sequence in which uracil is substituted for thymine in the rna strand aucgauugagcucuagcg uagcuaacucgagaucgc non canonical base pairing edit main article non canonical base pairing wobble base pairs comparison of hoogsteen to watson crick base pairs 12 in addition to the canonical watson crick pairing a t u g c some conditions can also favour base pairing with alternative base orientation and number and geometry of hydrogen bonds these pairings are accompanied by alterations to the local backbone shape citation needed the most common of these is the wobble base pairing that occurs between trnas and mrnas at the third base position of many codons during transcription 13 and during the charging of trnas by some trna synthetases 14 they have also been observed in the secondary structures of some rna sequences 15 additionally hoogsteen base pairing typically written as a u t and g c can happen when a different face of a purine base is used for pairing this happens in some dna sequences e g ca and ta dinucleotides in dynamic equilibrium with standard watson crick pairing 12 they have also been observed in some protein dna complexes 16 there is also a kind of reverse hoogsteen base pair in trna where both a purine and a pyrimidine uses a different face 17 18 in addition to these alternative base pairings a wide range of base base hydrogen bonding is observed in rna secondary and tertiary structure 19 these bonds are often necessary for the precise complex shape of an rna as well as its binding to interaction partners 19 base pairs and mutation edit mismatch repair edit mismatched base pairs can be generated by errors of dna replication and as intermediates during homologous recombination the process of mismatch repair ordinarily must recognize and correctly repair a small number of base mispairs within a long sequence of normal dna base pairs to repair mismatches formed during dna replication several distinctive repair processes have evolved to distinguish between the template strand and the newly formed strand so that only the newly inserted incorrect nucleotide is removed in order to avoid generating a mutation 20 the proteins employed in mismatch repair during dna replication and the clinical significance of defects in this process are described in the article dna mismatch repair the process of mispair correction during recombination is described in the article gene conversion base analogs and intercalators edit main article nucleic acid analogue chemical analogs of nucleotides can take the place of proper nucleotides and establish non canonical base pairing leading to errors mostly point mutations in dna replication and dna transcription this is due to their isosteric chemistry one common mutagenic base analog is 5 bromouracil which resembles thymine but can base pair to guanine in its enol form 21 other chemicals known as dna intercalators fit into the gap between adjacent bases on a single strand and induce frameshift mutations by masquerading as a base causing the dna replication machinery to skip or insert additional nucleotides at the intercalated site most intercalators are large polyaromatic compounds and are known or suspected carcinogens examples include ethidium bromide and acridine 22 as a unit of length edit schematic karyogram of a human the blue scale to the left of each nuclear chromosome pair as well as the mitochondrial genome at bottom left shows its length in terms of mega base pairs further information karyotype the following abbreviations are commonly used to describe the length of a d r na molecule bp base pair one bp corresponds to approximately 3 4 å 340 pm 23 of length along the strand and to roughly 618 or 643 daltons for dna and rna respectively kb kbp kilo base pair 1 000 bp mb mbp mega base pair 1 000 000 bp gb gbp giga base pair 1 000 000 000 bp for single stranded dna rna units of nucleotides are used abbreviated nt or knt mnt gnt as they are not paired to distinguish between units of computer storage and bases kbp mbp gbp etc may be used for base pairs the centimorgan is also often used to imply distance along a chromosome but the number of base pairs it corresponds to varies widely depending on the patterns of chromosomal crossover in the human genome the centimorgan is about 1 million base pairs 24 25 unnatural base pair ubp edit see also artificial gene synthesis expanded genetic code nucleic acid analogue and synthetic genomics an unnatural base pair ubp is a designed subunit or nucleobase of dna which is created in a laboratory and does not occur in nature dna sequences have been described which use newly created nucleobases to form a third base pair in addition to the two base pairs found in nature a t adenine thymine and g c guanine cytosine a few research groups have been searching for a third base pair for dna including teams led by steven a benner philippe marliere floyd e romesberg and ichiro hirao 26 some new base pairs based on alternative hydrogen bonding hydrophobic interactions and metal coordination have been reported 27 28 29 30 in 1989 steven benner then working at the swiss federal institute of technology in zurich and his team led with modified forms of cytosine and guanine into dna molecules in vitro 31 the nucleotides which encoded rna and proteins were successfully replicated in vitro since then benner s team has been trying to engineer cells that can make foreign bases from scratch obviating the need for a feedstock 32 in 2002 ichiro hirao s group in japan developed an unnatural base pair between 2 amino 8 2 thienyl purine s and pyridine 2 one y that functions in transcription and translation for the site specific incorporation of non standard amino acids into proteins 33 in 2006 they created 7 2 thienyl imidazo 4 5 b pyridine ds and pyrrole 2 carbaldehyde pa as a third base pair for replication and transcription 34 afterward ds and 4 3 6 aminohexanamido 1 propynyl 2 nitropyrrole px was discovered as a high fidelity pair in pcr amplification 35 28 in 2013 they applied the ds px pair to dna aptamer generation by in vitro selection selex and demonstrated the genetic alphabet expansion significantly augment dna aptamer affinities to target proteins 36 in 2012 a group of american scientists led by floyd romesberg a chemical biologist at the scripps research institute in san diego california published that his team designed an unnatural base pair ubp 29 the two new artificial nucleotides or unnatural base pair ubp were named d5sics and dnam more technically these artificial nucleotides bearing hydrophobic nucleobases feature two fused aromatic rings that form a d5sics dnam complex or base pair in dna 32 37 his team designed a variety of in vitro or test tube templates containing the unnatural base pair and they confirmed that it was efficiently replicated with high fidelity in virtually all sequence contexts using the modern standard in vitro techniques namely pcr amplification of dna and pcr based applications 29 their results show that for pcr and pcr based applications the d5sics dnam unnatural base pair is functionally equivalent to a natural base pair and when combined with the other two natural base pairs used by all organisms a t and g c they provide a fully functional and expanded six letter genetic alphabet 37 in 2014 the same team from the scripps research institute reported that they synthesized a stretch of circular dna known as a plasmid containing natural t a and c g base pairs along with the best performing ubp romesberg s laboratory had designed and inserted it into cells of the common bacterium e coli that successfully replicated the unnatural base pairs through multiple generations 26 the transfection did not hamper the growth of the e coli cells and showed no sign of losing its unnatural base pairs to its natural dna repair mechanisms this is the first known example of a living organism passing along an expanded genetic code to subsequent generations 37 38 romesberg said he and his colleagues created 300 variants to refine the design of nucleotides that would be stable enough and would be replicated as easily as the natural ones when the cells divide this was in part achieved by the addition of a supportive algal gene that expresses a nucleotide triphosphate transporter which efficiently imports the triphosphates of both d5sicstp and dnamtp into e coli bacteria 37 then the natural bacterial replication pathways use them to accurately replicate a plasmid containing d5sics dnam other researchers were surprised that the bacteria replicated these human made dna subunits 39 the successful incorporation of a third base pair is a significant breakthrough toward the goal of greatly expanding the number of amino acids which can be encoded by dna from the existing 20 amino acids to a theoretically possible 172 thereby expanding the potential for living organisms to produce novel proteins 26 the artificial strings of dna do not encode for anything yet but scientists speculate they could be designed to manufacture new proteins which could have industrial or pharmaceutical uses 40 experts said the synthetic dna incorporating the unnatural base pair raises the possibility of life forms based on a different dna code 39 40 data sources for base pair strengths edit main article nucleic acid thermodynamics the following sources have information on the free energy thermodynamic measures of strength of base pairs vendeix et al 2009 table 1 obtained by molecular simulation of rna including canonical and modified bases free energy for 300 k 41 however simply knowing what the minimal energy hydrogen bonded state between two nucleobases is not enough the stability of a nucleic acid molecule also comes from base stacking the stengths of which can vary with modified bases with respect to the original version the optimal hydrogen bonded state for two bases can also turn out to require an unnatural amount of bending of the nucleic acid backbone all of these contribute to the effective strength of a base pair in the context of nucleic acid secondary structure which is why predicting such structures need nearest neighbor models that describe base pairs in terms of free energy at 37 c and enthalpy for rescaling to different temperatures of 42 helix fragments such as aa uu and gguc cugg the sequence on the left of the colon is in usual 5 to 3 direction but the one on the right is written in reversed 3 to 5 direction terminal mismatches non pairs at the end of helices e g ca ga a list of nearest neighbor models can be found at nucleic acid structure prediction thermodynamic models see also edit list of y dna single nucleotide polymorphisms non canonical base pairing chargaff s rules references edit spencer m 10 january 1959 the stereochemistry of deoxyribonucleic acid ii hydrogen bonded pairs of bases acta crystallographica 12 1 66 71 bibcode 1959accry 12 66s doi 10 1107 s0365110x59000160 issn 0365 110x zhurkin vb tolstorukov my xu f colasanti av olson wk 2005 sequence dependent variability of b dna dna conformation and transcription pp 18 34 doi 10 1007 0 387 29148 2_2 isbn 978 0 387 25579 8 moran la 2011 03 24 the total size of the human genome is very likely to be 3 200 mb sandwalk blogspot com retrieved 2012 07 16 the finished length of the human genome is 2 86 gb strategicgenomics com 2006 06 12 retrieved 2012 07 16 one copy of the human genome consists of approximately 3 billion base pairs of dna national human genome research institute 2024 08 24 international human genome sequencing consortium october 2004 finishing the euchromatic sequence of the human genome nature 431 7011 931 945 bibcode 2004natur 431 931h doi 10 1038 nature03001 pmid 15496913 cockburn af newkirk mj firtel ra december 1976 organization of the ribosomal rna genes of dictyostelium discoideum mapping of the nontranscribed spacer regions cell 9 4 pt 1 605 613 doi 10 1016 0092 8674 76 90043 x pmid 1034500 s2cid 31624366 nuwer r 18 july 2015 counting all the dna on earth the new york times new york issn 0362 4331 archived from the original on 2022 01 01 retrieved 2015 07 18 the biosphere diversity of life aspen global change 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