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] Under standard milieu, when there is no mutational or selective pressure, and under random distribution of nucleotides, there is an equal frequency of the four DNA bases (Adenine, Guanine, Thymine, and Cytosine) on a single strand of DNA.〔 In most prokaryotes (e.g. ''E. coli'') and some archaea (e.g. ''Sulfolobus solfataricus''), there is an asymmetry between the nucleotide compositions of the leading strand and the lagging strand.〔 The leading strand is enriched in Guanine (G) and Thymine (T), whereas the lagging strand shows richness in Adenine (A) and Cytosine (C).〔 This phenomenon is referred to as GC and AT skew, and it is represented as follow:〔 GC Skew = (G - C)/(G + C) AT Skew = (A - T)/(A + T) Sometimes this can also be labeled as "G-C skew" to further emphasize the relative subtraction order. ==Asymmetric nucleotide composition== Erwin Chargaff's work in 1950 demonstrated that in DNA the bases guanine and cytosine were found in equal abundance, and the bases adenine and thymine were found in equal abundance, although there was no equality between the amount of one pair versus the other.〔 Chargaff’s finding is referred to as Chargaff's rule or parity rule 1.〔 Three years later Watson and Crick used this fact during their derivation the structure of DNA, their double helix model. A natural result of parity rule 1, at the state of equilibrium, in which there is no mutation and/or selection biases in any of the two DNA strands, is that when there is an equal substitution rate, the complementary nucleotides on each strand have equal amounts of a given base and its complement.〔 In other words, in each DNA strand the frequency of occurrence of T is equal to A and the frequency of occurrence of G is equal to C because the substitution rate is presumably equal. This phenomenon is referred to as parity rule 2. Hence, the second parity rule only exists, when there is no mutation or substitution. Any deviation from parity rule 2 will result in asymmetric base composition that discriminates the leading from the lagging strand. This asymmetry is referred to as GC or AT skew.〔 There is a richness of guanine over cytosine and thymine over adenine in the leading strand and vice versa for the lagging strand. The nucleotide composition skew spectra ranges from -1, which correlates with G = 0 or A = 0, to +1, which correlates to T= 0 or C = 0.〔 Therefore, positive GC skew represents richness of G over C and the negative GC skew represents richness of C over G. As a result one expects to see a positive GC skew and negative AT skew in the leading strand, and a negative GC skew and a positive AT skew in the lagging strand.〔 GC or AT skew changes sign at the boundaries of the two replichores, which corresponds to DNA replication origin or terminus.〔〔〔 Originally this asymmetric nucleotide composition was explained as different mechanism used in DNA replication between leading strand and lagging strand. The DNA replication is semi-conservative and an asymmetric process itself.〔 This asymmetry is due the formation of the replication fork and its division into nascent leading and lagging strands. The leading strand is synthesized continuously and in juxtapose to the leading strand; the lagging strand is replicated through short fragments of polynucleotide (Okazaki fragments) in a 5' to 3' direction.〔 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「GC skew」の詳細全文を読む スポンサード リンク
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