Chapter 10, Problem 17AQP

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Summary Introduction

To explain:

The aspects of DNA structure which provide stability to the molecule upto thousands of years.

Introduction:

The stability of DNA is dependent on various features and structural composition of DNA. The intermolecular interactions, double helical structure base stacking, and surrounding water play a major role in the stability of DNA.

Explanation of Solution

The DNA is a biomolecule and the reasons of its long term stability are:

1. The structural and chemical stability of DNA comes from the linkage between nucleotide bases and deoxyribose sugar, the linkage is termed as phosphodiester linkage. The phosphodiester bonds are not easy to break by hydrolysis so, these bonds are considered as strong attachments.

2. Base stacking is a major reason of DNA stability. Two polynucleotide strands are attached to one another in the form of stack because of the complementary nature of nucleotide bases.

3. Hydrogen bonds are found between two successive nucleotides, though hydrogen bonds are weak bonds, but large amount of bonds present on two strands of DNA make its chemical and structural condition stable.

4. The nitrogenous bases are complementary to each other so, nitrogenous bases present on two separate strands get bind strongly and form double helical structure of DNA to make it stable.

5. Lacking a hydroxyl group at 2^nd carbon also provide stability to DNA; hydroxyl group tends to react fast. The ribose sugar of DNA contains only one hydroxyl group at 5^th position which is engaged in the formation of phosphodiester bonds with phosphate group. The absence of spare hydroxyl groups provides immunity to DNA towards several chemical reactions.

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Summary Introduction

To explain:

The stability of RNA is less in comparison to DNA.

Introduction:

RNA is a polynucleotide and it is the genetic material of various viruses. RNA and DNA are different from each other in composition as well as in the structure. RNA is less stable than DNA due to these structural and compositional differences.

Explanation of Solution

The reasons of RNA being less stable than DNA are:

1. RNA is single stranded while DNA is a double stranded helical structure. The double helical structure of DNA is rich in hydrogen bonds, being single stranded, RNA does not possess large number of hydrogen bonding. Large number of hydrogen bonds need high amount of energy to break the bonds.

2. Presence of hydroxyl group on the 2^nd carbon of ribose sugar of RNA makes it less stable than DNA, as hydroxyl group is very reactive and can undergo hydrolysis easily.

3. In RNA, uracil is found in the place of thymine, thymine contains a methyl group which supports the repairing of damaged DNA. Uracil is formed by the deamination of cytosine. The rapid deamination of cytosine will decrease guanine and cytosine base pairing.

Conclusion

DNA is a stable molecule due to the presence of large number of hydrogen bonds, complementary nitrogenous bases present on two separate strands form hydrogen bonds and provide a double helical structure to DNA, absence of hydroxyl group in the sugar part of DNA prevents it from hydrolysis, base stacking also plays a major role in the DNA stability. RNA is less stable than DNA because of being single stranded and the presence of hydroxyl group on the 2^nd carbon of sugar makes it susceptible tohydrolysis. Presence of uracil in the place of thymine is a disadvantage of RNA.