A method of synthesizing a target nucleic acid preferentially relative to a nucleic acid with one or more nucleotides that differ from the target nucleic acid, the method comprising: a. exposing a cooperative nucleic acid molecule to a solution suspected of comprising a target nucleic acid and also potentially comprising a nucleic acid with one or more nucleotides that differ from the target nucleic acid (differing nucleic acid), wherein the cooperative nucleic acid molecule comprises, from 3' to 5': i. a first nucleic acid sequence, wherein the first nucleic acid sequence is complementary to a first region of a target nucleic acid, and further wherein a penultimate nucleotide of the 3' end of the first nucleic acid sequence is complementary to the target nucleic acid, but is not complementary to the differing nucleic acid, and yet further wherein the first nucleic acid is extendable on the 3' end; ii. a second nucleic acid sequence, wherein the second nucleic acid sequence is complementary to a second region of the target nucleic acid, wherein the complementarity between the second nucleic acid sequence and the target nucleic acid starts within one nucleotide or less of the first region of the target nucleic acid, such that it hybridizes to the target nucleic acid downstream from the 3' end of the first nucleic acid sequence, or overlaps on the 5' end with the 3' end of the first nucleic acid sequence; iii. a linker connecting said first and second nucleic acid sequences in a manner that allows both the said first and second nucleic acid sequences to hybridize to the target nucleic acid at the same time; b. providing conditions appropriate for nucleic acid synthesis wherein the polymerase extends from the 3' end of the first nucleic acid sequence through the second nucleic acid sequence, thereby synthesizing the target nucleic acid preferentially to the differing nucleic acid if it is present in the solution. a. exposing a cooperative nucleic acid molecule to a solution suspected of comprising a target nucleic acid and also potentially comprising a nucleic acid with one or more nucleotides that differ from the target nucleic acid (differing nucleic acid), wherein the cooperative nucleic acid molecule comprises, from 3' to 5': i. a first nucleic acid sequence, wherein the first nucleic acid sequence is complementary to a first region of a target nucleic acid, and further wherein a penultimate nucleotide of the 3' end of the first nucleic acid sequence is complementary to the target nucleic acid, but is not complementary to the differing nucleic acid, and yet further wherein the first nucleic acid is extendable on the 3' end; ii. a second nucleic acid sequence, wherein the second nucleic acid sequence is complementary to a second region of the target nucleic acid, wherein the complementarity between the second nucleic acid sequence and the target nucleic acid starts within one nucleotide or less of the first region of the target nucleic acid, such that it hybridizes to the target nucleic acid downstream from the 3' end of the first nucleic acid sequence, or overlaps on the 5' end with the 3' end of the first nucleic acid sequence; iii. a linker connecting said first and second nucleic acid sequences in a manner that allows both the said first and second nucleic acid sequences to hybridize to the target nucleic acid at the same time; i. a first nucleic acid sequence, wherein the first nucleic acid sequence is complementary to a first region of a target nucleic acid, and further wherein a penultimate nucleotide of the 3' end of the first nucleic acid sequence is complementary to the target nucleic acid, but is not complementary to the differing nucleic acid, and yet further wherein the first nucleic acid is extendable on the 3' end; ii. a second nucleic acid sequence, wherein the second nucleic acid sequence is complementary to a second region of the target nucleic acid, wherein the complementarity between the second nucleic acid sequence and the target nucleic acid starts within one nucleotide or less of the first region of the target nucleic acid, such that it hybridizes to the target nucleic acid downstream from the 3' end of the first nucleic acid sequence, or overlaps on the 5' end with the 3' end of the first nucleic acid sequence; iii. a linker connecting said first and second nucleic acid sequences in a manner that allows both the said first and second nucleic acid sequences to hybridize to the target nucleic acid at the same time; b. providing conditions appropriate for nucleic acid synthesis wherein the polymerase extends from the 3' end of the first nucleic acid sequence through the second nucleic acid sequence, thereby synthesizing the target nucleic acid preferentially to the differing nucleic acid if it is present in the solution.