Nucleic Acid Structure and Function

DNA (deoxyribonucleic acid) and RNA (ribonucleic acid) are composed of acidic phosphate groups and are found within the nucleus of eukaryotes.

DNA Structure

DNA is made up of purines (double-ring structure), pyrimidines (single-ring structure), sugar, and phosphate. The figure below depicts the difference between the purines and pyrimidines.

Purine vs Pyrimidine

  • Purines: consist of Adenine and Guanine
  • Pyrimidines: consist of Cytosine, Uracil, Thymine

DNA backbone is made up sugar-phosphate. This portion does not vary unlike the purine/pyrimidine bases. We need to clear up some definitions before moving on:

  • Nucleotide: Consists of a pentose sugar (ribose or deoxyribose), nitrogenous base (purine or pyrimidine), and a phosphate group.
  • Nucleoside: Consists of a pentose sugar (ribose or deoxyribose), nitrogenous base (purine or pyrimidine). There is NO phosphate group.

Watson-Crick Model

The Watson-Crick model states a few key characteristics about DNA:

  • DNA is double-stranded, such that two strands of DNA at connected to each other by hydrogen bonds
  • The two strands of DNA are connected in an anti-parallel manner. This means that one strand goes from 5′ to 3′ and the other goes from 3′ to 5′. These will be discussed furthermore in the future sections.
  • DNA is formed in a helix.

The following figure depicts DNA according to the Watson-Crick model:

Watson-Crick Model

Nucleotides are vertically attached to each other via the phosphate and hydroxyl groups with a phosphodiester bond. The figure below clarifies this phenomenon:

Base Pairing

  • The bases, hereafter will be referred to as A (adenine), T (thymine), G (guanine), and C (cytosine).
  • Bases pair with each other (remember the double stranded characteristic of DNA). Specifically A pairs with T and G pairs with C.
  • The bonds between A and T consist of 2 hydrogen bonds.
  • The bonds between G and C consist of 3 hydrogen bonds. Therefore, DNA with higher G-C pairs are harder to break and denature due to greater bond strength.

The figure below depicts the base pairing between nucleotides:

Nucleotide Base Pairing


Hopefully this section was able to cover the basic knowledge you require to progress further into the genetics subject. Please try to understand and visualize how bases are paired together and DNA strands are created.

It is important to understand the anti-parallel, complementary strands of DNA and the 5′ to 3′ phenomenon. For example, the following two strands can join together due to the anti-parallel and complementary nature.



It is this variation in base pairing that allows transmission and storage of unique genetic information.


  • Purine vs Pyrimidine
  • Watson-Crick Model
  • DNA Vertical Linkage
  • Nucleotide Base Pairing
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