Question

Over the next week, we will be discussing the structure of macromolecules, extremely large molecules the masses of which can exceed 100,000 daltons. One such macromolecule is deoxyribonucleic acid (DNA). There are few discoveries in the history of science that have had as profound an effect on our understanding of how life “works” than the elucidation of the structure of DNA by Watson and Crick. For this unit’s forum, I have assigned the original Nature paper by James Watson and Francis Crick on the Molecular Structure of Nucleic Acids. Please read the paper and answer the following questions.

https://www.nature.com/scitable/content/molecular-structure-of-nucleic-acids-a-structure-13997975

1.       What are the two most significant differences between the structure for DNA as proposed by Watson and Crick and that proposed by Pauling and Corey?

2.       What important structural function do the purine and pyrimidine bases play? Besides the constraints on hydrogen bonding, what other reason must a purine pair with a pyrimidine in the structure proposed by Watson and Crick? (Hint: Look at the crude diagram of DNA presented in the paper.)

3.       In the penultimate paragraph of their paper, the authors state that their proposed base pairing scheme suggests a possible copying mechanism for DNA. Propose a mechanism for DNA copying based on the information provided in the paper.  

4.       Pairing of bases involves hydrogen bonding (see paragraph 6). What do you think would be the consequence if instead of hydrogen bonding, the bases were held together by covalent bonds?

Answer 1

1. The signification difference of the proposed DNA model by Watson-Crick and Pauling-Corey is in the number of strands, present in DNA. In the model, given by Pauling and Corey described the DNA as a triple stranded molecules whereas in the model of Watson-Crick, DNA molecules have two strands.

2. Purine and pyrimidins stack one by one and construct the central portion of a DNA molecules. Purins(A, G) contain two rings in there structure where as pyrimidins(C, T) contain single ring in their structure. Hence, maintaining the uniform structure of DNA molecules a purine should bind with a pyrimidin and vice versa.

3. A in-vovo DNA coping mechanism is DNA replication. In this case copy of both strands are produced.

4. There are numerous H-bonds between the alternative bases of the complementary DNA strands. Hydrogen bonds contain little amount of energy(1-40 Kcal/mol) to break it. The bond energy(100 Kcal/mol) of covalent bonds are much higher than the hydrogen bonds. At the time of of DNA replication, the strands must be separated from each other. If the bonds are made by covalent bonds, huge amount of energy is required to break them in comparison to the H-bonds.