The Helical Structure of DNA: Watson and Crick’s Opinion Essay

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Introduction

The Deoxyribose Nucleic Acid (DNA) is an important component of biological studies. In this paper, the author examines the structural information about the DNA. To achieve this, the author of this essay examines the opinions of key researchers and scholars in this field. The works of two scholars, Watson and Crick, are examined in this essay. Information about the structure of the DNA is drawn from two manuscripts written by the two aforementioned biologists. In addition, the author of this paper makes a comparison between the structure proposed by the two biologists and the information provided in recent textbooks. Finally, the author will provide a descriptive analysis of how other scholars support the DNA structure proposed by Watson and Crick.

The Structural Characteristics and Predictions Put Forth in the Initial Watson and Crick Paper Dated April 25, 1953

Watson and Crick (1953a) propose a structure which, in their opinion, best suits the description of the Deoxyribose Nucleic Acid. In proposing the structure of the DNA, the two break away from earlier descriptions made by other scholars in the field. According to Watson and Crick (1953a), the DNA structure is shaped in such a way that it comprises of two helical chains. The biologists point out that the aforementioned helical chains share one axis.

When proposing any theory or hypothesis in a given study, researchers are expected to make some assumptions. In the case of Watson and Crick (1953a), chemical assumptions are made to this effect. First, the two authors mention that each of the two chains is made up of phosphate disaster groups (Watson & Crick, 1953a). The two groups join the beta-D-deoxyribofuranose residues. Watson and Crick (1953a) further suggest that the helical chains find relation in dyad at right angles to the axis. The said relation does not include the bases of these strands.

When describing the helical structure, Watson and Crick (1953a) point out that the chains take a right handed helical arrangement. Interestingly, the chains run opposite to each other. With regards to the details of each chain, Watson and Crick (1953a) suggest that they consist of a base and a phosphate. The base is on the inner part of the helix, whereas the phosphates are located on the outer part of the same.

Based on their description of the DNA structure, the two biologists are able to make further predictions. One of their predictions is drawn from the fact that the DNA structure is an open one (Watson & Crick, 1953a). In addition, the two biologists point out that such a structure is usually characterized by high water content. Based on this, the scholars predicted that whenever water content is low, the bases would tilt. The tilting will make the structure more compact.

In addition to the compaction of DNA, the scholars make another prediction with regards to how the bonding occurs. Watson and Crick (1953a) point out that the chains are held together by the “purine and pyrimidine bases”. The scholars describe the axis around which the chains are oriented. They note that the planes of the bases are at right angle to the axis. According to the description made, the bases of respective chains are joined by a hydrogen bond (Watson & Crick, 1953a). For the said bonding to occur, it is expected that each of the bases should lie on the same plane. Another prediction based on the structure of the DNA described is that only specific pairs of bases can bond together (Watson & Crick, 1953a).

Other Structures Proposed and Argued Against in the First Manuscript. What is Significant about these Alternative Structures?

As earlier mentioned, the first manuscript makes reference to other structures of the DNA. Watson & Crick (1953a) make reference to the structure proposed by Pauling and Corey. In this structure, Pauling and Corey suggest that DNA has three intertwined chains. Contrary to the structure proposed by Watson & Crick (1953a), the structure described by Pauling and Corey has its bases on the outside.

A similar three-chain structure is advanced by Fraser (Watson & Crick, 1953a). The structure differs from the previous one in that the bases are on the inside and the phosphates on the outside. The other two structures described by the scholars are quite important in that they provide the foundation on which the 2-chain helical structure is based. There are several shortcomings associated with three-chained structure. They involve the distances between the Van der Waal forces. The shortcomings need to be addressed to ensure that the actual bonding is fully understood.

What Structural Information was added in the Second Paper? How Does this Further Support the Information Presented in the First Paper?

In the second paper, Watson and Crick (1953b) discuss additional information about the DNA structure. The intention here is to understand how Deoxyribose Nucleic Acids replicate information. To understand this, the biologists provide a detailed discussion about the phosphate-sugar backbone (Watson & Crick, 1953b). Their understanding is that in a given bonding pair, the two formations must correspond. They argue that, “Adenine can occur on either chain….When it does, its partner on the other chain must always be thymine” (Watson & Crick, 1953b, p. 965).

Watson & Crick (1953b) look at other features of the phosphate-sugar backbone (see figure 1). The two argue that the sequence ought to be regular. An irregular sequence is quiet problematic. In such cases, for example, it is difficult to understand how, “the amount of adenine is close to the amount of thymine” (Watson & Crick, 1953b, p. 965). The additional information provided by the researchers in the second paper supports the pairing discussed in the first paper.

Phosphate-sugar backbone structure.
Figure 1. Phosphate-sugar backbone structure. Source: Watson & Crick (1953b).

How is the Information Presented in these Two Papers Similar or Different from that Presented in the Class Textbook?

Information contained in the two papers is similar to information about DNA contained in most of the course textbooks on the subject. According to the Estrella Mountain Community College (2007), the DNA has a helical structure. Moreover, genetic information about organisms is best understood by the pairing discussed by Watson and Crick.

A Brief Description of the Molecular Data Provided in the Articles by Wilkins, Stokes, and Wilson and Franklin and Gosling that Support the Structure Proposed by Watson and Crick

Franklin and Gosling (1953) studied the molecular configuration of Sodium Thymonucleate. Their study examined helical structures where the DNA was described according to Watson and Crick (1953a). The crystallization of the said molecule has a phosphate orientation similar to that described by Watson and Crick (Franklin & Gosling, 1953). As such, their study refutes the findings made by Pauling and Corey. Wilkins, Stokes, and Wilson (1953) also examined the molecular structure of DNA. In particular, Wilkins et al. (1953) examined the diffractive properties of the molecule. According to their study, helices have similar diffractive behavior. The uniformity is similar to that in the DNA, which is proposed to have a helical structure. The separate studies affirm that DNA, indeed, has a helical structure.

References

Estrella Mountain Community College. (2007). Online biology book. Web.

Franklin, E. R., & Gosling, G. R. (1953). Molecular configuration in Sodium Thymonucleate. Nature, 171(4356), 740-741.

Watson, J. D., & Crick, C. H. (1953a). Molecular structure of nucleic acids: A structure for Deoxyribose Nucleic Acid. Nature, 171(4356), 737-738.

Watson, J. D., & Crick, C. H. (1953b). Genetic implications of the structure of Deoxyribonucleic Acid. Nature, 171(4361), 964-967.

Wilkins, M. H., Stokes, A. R., & Wilson, H. R. (1953). Molecular structure of Deoxypentose Nucleic Acids. Nature, 171(4356), 738-740.

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