Watson and Crick (Double Helix) model of DNA

  • James Watson and Francis Crick has made a major contribution in 1953 to the development of the structure of DNA.
  • They combined the physical and chemical data and proposed DNA as double helical twisted molecules connected together by hydrogen bonds.
  • DNA is a backbone for all life and this model has helped to unlock the genetic code of all living organisms.
  • It has also made to believe that the genetic pieces of information are held inside the structure and makes a new copy of the entire molecule and transfers it in a series of a generation.

The Watson and Crick model of DNA, also known as the double helix model, is a scientific breakthrough that revolutionized our understanding of genetics. In 1953, James Watson and Francis Crick proposed that DNA has a double helix structure made up of two complementary strands, each consisting of a sugar-phosphate backbone and nitrogenous bases. The nitrogenous bases, adenine, thymine, guanine, and cytosine, form base pairs that hold the two strands together via hydrogen bonds. This model explained how DNA could replicate itself, transmit genetic information from generation to generation, and carry out the essential functions of life. Today, the Watson and Crick model remains a foundational concept in the field of molecular biology.

Four major features of Watson and Cricks DNA model:

(Source: https://biologyreader.com/watson-and-crick-model-of-dna.html )

  • DNA molecule comprises two individual strands of polynucleotide molecule which are linked together by a hydrogen bond. Each base of nucleotide pairs up with one another i.e, purine bases always pair up with pyrimidine bases.
  • Most double helix DNA is right-handed i.e, in a clockwise direction, and completes one turn at a distance of 34A°. It runs in a direction when you hold your right thumb pointed up which represents the axis and fingers curled around are the sugar-phosphate backbones that run outside the molecule while nitrogen bases remain inside. One of the exceptions of left-handed DNA is the Z-DNA.
  • The DNA strands are anti-parallel to each other i.e, if the starting point of one strand is 5’ then the starting point of another strand is 3’ and vice versa where the nucleotide links together with the sugar group of 3’ and 5’ end which is due to the polarity of phosphodiester linkage.
  • The outer exposed molecules of the nitrogen bases have also potential in hydrogen bonding with other molecules. This hydrogen bond provides easy access to DNA replication and expression.

Components of DNA:

DNA molecules are composed of a sequence of four nucleotide monomers namely Adenine (A), Cytosine (C), Thymine (T), and Guanine (G).

Each nucleotide monomers are made up of three molecular parts:

  1. Nitrogen bases
  2. Deoxyribose sugar
  3. A phosphate group

1. Nitrogen bases

  • Nitrogen-containing base functions in bonding nucleic acids and are also termed nucleobases due to their role in nucleic acid.
  • DNA is composed of two major nitrogen bases: Purine and Pyrimidine.
  • Two nucleotides of purine are Adenine (A) and Guanine (G) and pyrimidine are Cytosine (C) and Thymine (T).
  • Two bases of a nucleotide pair each other with a hydrogen bond and forms two strands of double helix structure.
  • Hydrogen atoms of amino groups act as a donor while carbonyl oxygen and ring nitrogen act as an acceptor.
  • Adenine always pairs up with Thymine by two hydrogen bonds (A=T) similarly, Guanine pairs up with Cytosine forming triple hydrogen bonds (GC).
  • Chargaff’s rule states that purine and pyrimidine bases should have a 1:1 stoichiometric ratio i.e, the amount of adenine must be equal to the amount of thymine and that similar to guanine and cytosine.
  • The helix contains ten nucleotides in each turn with an internucleotide distance of 3.4A° whereas full turn helix length is 34A° with 20A° diameter.
  • This helical turning forms two grooves: Major grooves and minor grooves.
  • The major groove appears deep and wide and is the specific binding site of proteins while the minor groove is the distance between two strands.
Nitrogen bases
Nitrogen bases

2. Deoxyribose sugar

  • Deoxyribose sugar is a five-carbon atom sugar molecules that lack one hydroxyl group on the 2’ carbon i.e, one oxygen molecule is absent thus its name deoxyribose sugar.
  • It is a cyclic molecule with five carbon atoms, among which four are carbon molecules and one oxygen molecules which are arranged in a cyclic manner.
  • Due to its flexible structure, it can be twisted into various conformations like in canonical B-DNA, the sugar configuration is C2’ endo.
  • The deoxyribose sugar molecule for all nucleobase is the same. When a nucleobase is attached to a deoxyribose sugar it becomes a nucleoside.
  • The 1’ carbon of the pentose sugar bonds with the nitrogenous base and the 5’ end carbon atom bonds to the phosphate group.
  • Then, the phosphate group bonds to the 5’ end of the nucleotide. All these bonding are followed by hydrogen carbon.
  • During replication, several enzymes are involved in the breakdown of hydrogen bonding which later forms a newly stranded DNA molecule.
  • These newly formed ribose molecules attach with nitrogenous bases and a phosphate group before being deoxygenated and later become deoxyribose and become an independent DNA molecule.
Deoxyribose sugar
Deoxyribose sugar

3. A phosphate group

  • A phosphate group is a backbone for every single strand of the DNA molecule.
  • The chemical structure of the phosphate group comprises a functional group with a phosphorous atom bonded with four oxygen atoms in which 3 are singly bonded and one with a double bond.
  • The phosphate group remains exterior of the DNA and are attached to the 5’ of each sugar atom and forms phosphodiester bond.
  • The phosphate group functions as an energy donor when DNA synthesizes.
  • Phosphate groups when attaches to the nucleoside form nucleotide molecules.
Phosphate group


  1. https://www.nature.com/scitable/topicpage/discovery-of-dna-structure-and-function-watson-397/
  2. https://aklectures.com/lecture/structure-and-function/watson-crick-model-of-dna
  3. https://www.khanacademy.org/science/high-school-biology/hs-molecular-genetics/hs-discovery-and-structure-of-dna/a/discovery-of-the-structure-of-dna
  4. https://www.biologydiscussion.com/dna/watson-and-cricks-model-of-double-helix-of-dna-biochemistry/65076
  5. https://biologydictionary.net/deoxyribose/

Leave a Reply

Your email address will not be published. Required fields are marked *