DNA bases

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The DNA double helix consists of two strands of complementary polynucleotide chains which are made up of four nucleotides. The four nucleotide bases are:

Each base has the same basic structure, a deoxyribose five carbon sugar with a base attached to C1 and a phosphate group attached to C5. This structure gives the nucleotide, and subsequently the DNA strand polarity[2]. The end with the phosphate group is denoted as the 5' end. And the end with the free hydroxyl group on C3 is denoted as the 3' end.

The nucleotide bases are joined by a phosphodiester linkage[3]. This is a covalent bond that forms between C5 on one nucleotide and C3 of the neighbouring nucleotide[4]. This forms the sugar phosphate backbone of DNA.

Watson-Crick base pairing occurs between Adenine and Thymine, Cytosine and Guanine. The base pairs form hydrogen bonds. Adenine (A) and Guanine (G) are purines as they both have double-ring structure. Whereas, thymine (T) and cytosine (C) are called pyrimidines as they both have a single-ring structure. Due to the different structure of the bases, A and T form two hydrogen bonds whereas C and G form three hydrogen bonds. It is by complementary base pairing that the two strands of DNA are held together[5].

References

  1. Daniel.L.Hartl and Maryellen Ruvolo (2012) Eighth Edition, Genetics: analysis of genes and genomes,Jones and Bartlett Learning International Student Edition, Editor: Megan.R.Turner, USA Chapter 1 page 7
  2. Alberts et. al. (2008) Molecular Biology of the Cell, 5th Edition, United States, Garland Publishing Inc, pp 116
  3. Alberts et. al. (2008) Molecular Biology of the Cell, 5th Edition, United States, Garland Publishing Inc, pp 197-199
  4. Alberts et. al. (2008) Molecular Biology of the Cell, 5th Edition, United States, Garland Publishing Inc, pp 117
  5. Daniel.L.Hartl and Maryellen Ruvolo (2012) Eighth Edition, Genetics: analysis of genes and genomes, Jones and Bartlett Learning International Student Edition, Editor: Megan.R.Turner, USA Chapter 2 pager 42-47

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