In this article we will discuss about the Watson and Crick’s Model of Double Helix of DNA.
Watson and Crick studied in advance the manuscript of Pauling and Corey in 1953.
They concluded that:
(i) Polynucleotide chain of DNA has a regular helix,
(ii) The helix has a diameter of about 20 Å, and
(iii) The helix makes a full turn at every 34 Å along its length and contains a stack of ten nucleotides per turn because the inter-nucleotide distance is 3.4 Å.
They considered the density of DNA molecule and concluded that the density of a cylinder having 20 Å diameter and 34 Å length is too low if it contained a single stack of ten nucleotides and the density will be too high if it contains three or more stacks often nucleotides. Therefore, the helix must contain two polynucleotide chains or two stacks of ten nucleotides per turn.
They also took into account that if the DNA is to contain heredity information which is inscribed as specific sequence of four bases along the polynucleotide chain, then the molecular structure of DNA must include any arbitrary sequences along its polynucleotide chain.
The dimension of purine ring is more than the dimension of pyrimidine ring; therefore, the two chain helix may contain a constant diameter provided a complementary relationship existed between the two nucleotide stacks, one harbouring purine and the other pyrimidine bases.
To provide thermodynamic stability they thought over the formation of hydrogen bonds between amino (-NH2) or hydroxyl (-OH) hydrogen’s and ketone oxygen or amino-nitrogen of the two bases. On the basis of these considerations they got success in constructing a double helix model for the DNA molecule.
Watson and Crick’s Model for DNA:
J.D.Watson and F.H.C. Crick (1953) combined the physical and chemical data, and proposed a double helix model for DNA molecule. This model is widely accepted. According to this model, the DNA molecule consists of two strands which are connected together by hydrogen bonds and helically twisted.
Each step on one strand consists of a nucleotide of purine base which alternates with that of pyrimidine base. Thus, a strand of DNA molecule is a polymer of four nucleotides i.e. A, G, T, C. The two strands join together to form a double helix. Bases of two nucleotides form hydrogen bonds i.e. A combines with T by two hydrogen bonds (A = T) and G combines with C by three hydrogen bonds (G = C) (Fig. 5.5).
However, the sequence of bonding is such that for every ATGC on one strand there would be TACG on the other strand. Therefore, the two chains are complementary to each other i.e. sequences of nucleotides on one chain are the photocopy of sequences of nucleotides on the other chain.
The two strands of double helix ran in antiparallel direction i.e. they have opposite polarity. In Fig. 5.6A the left hand strand has 5′ → 3′ polarity, whereas the right hand has 3′ → 5′ polarity as compared to the first one. The polarity is due to the direction of phosphodiester linkage.
The hydrogen bonds between the two strands are such that they maintain a distance of 20 A. The double helix coils in right hand direction i.e. clockwise direction and completes a turn at every 34 A distance (Fig. 5.6 B). The turning of double helix results in the appearance of a deep and wide groove called major groove. The major groove is the site of bonding of specific protein.
The distance between two strands forms a minor groove. One turn of double helix at every 34Å distance includes 10 nucleotides i.e. each nucleotide is situated at a distance of 3.4Å. Sugar-phosphate (nucleoside) makes the backbone of double helix of DNA molecule (Fig. 5.6B).
The DNA model also suggested a copying mechanism of the genetic material. DNA replication is the fundamental and unique event underlying growth and reproduction in all living organisms ranging from the smallest viruses to the most complex of all creatures including man. DNA replicates by semiconservative mechanism which was experimentally proved by Mathew, Meselson and Frank W. Stahl in 1958.
If changes occur in sequence or composition of base pairs of DNA, mutation takes place. Though the presence of adenine, guanine, thymine and cytosine is universal phenomenon, yet unusual bases in DNA molecule are also found.
In some bacteriophages 5-hydroxy-methyl-cytosine (HMC) replaces cytosine of the DNA molecule when methylation of adenine, guanine and cytosine occurs. This results in changes in these bases.