adenine pair thymine guanine cytosine Thymine pairs with

Why does adenine always pair with thymine and guanine always pair with cytosine?

A-T

Base pairing between adenine and thymine can be found in DNA only. There are two hydrogen bonds holding the two nitrogenous bases together. One of the hydrogen bonds is formed between one of the Hydrogen atoms of the amino group at C-6 of adenine and the Oxygen atom of the keto group at C-4 of thymine. Another bond is found between Nitrogen atom at position 1 of adenine and Hydrogen atom linked to N-3. The hydrogen bonds between adenine and thymine are important for DNA to maintain a double helix structure. Since they are not very strong bonds, they can be broken at elevated temperature. In DNA replication and transcription, the initiation of these reactions often starts at A-T rich sites because the breakage of two hydrogen bonds between A and T requires less energy than G-C rich sites which have three hydrogen bonds between G and C.

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C-G

Cytosine and guanine pairing can be found in both DNA and DNA-RNA hybrid formed during replication and transcription. The two nitrogenous bases are held together by three hydrogen bonds. The first hydrogen bond is found between Oxygen atom of the keto group at C-2 of cytosine and one of the Hydrogen atom of the amino group at C-2 of guanine. The second hydrogen bond is formed between N-3 of cytosine and Hydrogen atom attached to N-1 of guanine. The interaction between Hydrogen atom of the amino group at C-4 of cytosine and Oxygen atom of keto group at C-6 of guanine is the third hydrogen bond. DNA with higher G-C content is more stable than DNA with A-T rich regions. Having one more hydrogen bond between G-C than A-T needs more energy to break the nitrogenous bases apart. Thus, the melting temperature is relatively higher when DNA has higher C-G content. C-G content is used to predict the annealing temperature of primer to DNA in polymerase chain reaction.

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From The School of Biomedical Sciences Wiki

In Watson and Crick's model of DNA, the double helix, the two strands of DNA are joined to one another by hydrogen bonds between complementary nitrogenous bases. These hydrogen bonds have a strength of 4-21 kJ mol-1[1].

In DNA adenine always pairs with thymine and cytosine always pairs with guanine. In RNA uracil replaces thymine, therefore in RNA adenine always pairs with uracil. Thymine and uracil or adenine have two hydrogen bonds between them, whereas guanine and cytosine have three. Consequently, DNA with a larger proportion of guanine and cytosine is more stable and it takes more energy to break the two strands of DNA apart.

1 Structure
2 Purines and Pyrimidines
3 Chargaff's Rules
4 Base Stacking
5 References

Structure

The base pairing in the DNA helix helps to determine its structure. Due to the different interactions between the bases, the dsDNA helix completes a full turn on its axis every ten bases. Each base allows the helix to turn thirty-six degrees [2].

Purines and Pyrimidines

Adenine and guanine are both purine bases, this means that they have a double-ringed structure. Cytosine, uracil (only present in RNA) and thymine are pyrimidines and have single ringed structures. These bases contain nitrogen in their ring compounds.[3] Purines only ever pair with pyrimidines and pyrimidines only ever pair with purines. This is one of the reasons why a transversional base pairing change can have such disastrous effects on the structure of a protein as hydrogen bonds will not occur between two purines or two pyrimidines [4]. Before Watson and Crick presented the structure of DNA, Erwin Chargaff in the 1950s discovered a chemical technique in which he could determine the molar concentration of any one of the bases in a source of DNA. From what Chargaff discovered he noticed some patterns in the molar concentrations of the bases, from his results he devised some rules [5].

Chargaff's Rules

The amount of adenine is the same as the amount of thymine. [A] = [T]
The amount of guanine is the same as that of cytosine. [G] = [C]
The number of purine bases in equal to the number of pyrimidine bases. [A] + [G] = [T] + [C]

Base Stacking

In the DNA double helix, as well as the bases being complementary base-paired they are also stacked on top of one another. These bases also have interactions (Van der Waals) happening between one another which also contribute towards the DNAs structure. Base stacking in this way creates a hydrophobic core on the DNA [6].

References

↑ Hartyl, D. Jones, E.. (2005). DNA Structure and DNA Manipulation. In: Weaver, S. et al. Genetics, Analysis of Genes and Genomes. 6th ed. Sudbury,: Jones and Bartlett Publishers. p46-52.
↑ Berg, J. Stryer, L. Tymoczko, J.. (2007). DNA, RNA, and the Flow of Genetic Information. In: Ahr, K. et al. Biochemistry. 6th ed. New York: W.H. Freeman and Company. p107-112.
↑ Alberts et.al. (2007) Molecular Biology of the Cell 5th ed pg. 116
↑ Hartyl, D. Jones, E.. (2005). DNA Structure and DNA Manipulation. In: Weaver, S. et al. Genetics, Analysis of Genes and Genomes. 6th ed. Sudbury,: Jones and Bartlett Publishers. p46-52.
↑ Hartyl, D. Jones, E.. (2005). DNA Structure and DNA Manipulation. In: Weaver, S. et al. Genetics, Analysis of Genes and Genomes. 6th ed. Sudbury,: Jones and Bartlett Publishers. p46-52.
↑ Hartyl, D. Jones, E.. (2005). DNA Structure and DNA Manipulation. In: Weaver, S. et al. Genetics, Analysis of Genes and Genomes. 6th ed. Sudbury,: Jones and Bartlett Publishers. p46-52.

Why does adenine always pair with thymine and guanine with cytosine?

A always pairs with T and G always pairs with C because these are the only combinations that allow for hydrogen bonding to occur, given the spatial constraints of the double helix, which requires there to be one purine and one pyrimidine in each base pair.