Question

Adenine, cytosine, guanine are present in both DNA and RNA. Thymine and uracil are exclusive for DNA and RNA respectively. Explain why it is advantageous for DNA to contain thymine, not uracil. 0. Gout is a common, painful form of arthritis. It causes swollen, red, hot and stiff joints. Gout happens when uric acid builds up in your body. It is treated with allopurinol. What is the biochemical bas is of this treatment?

Answer 1

Answer of Question 1 : The basic differences between Thymine and Uracil is Thymine is the pyrimidine base of the DNA, whereas Uracil is the pyrimidine base of the RNA. The occurrence of thymine and uracil is a crucial difference as thymine is only found in DNA and uracil is only found in RNA. If we analyse both structures then we can notice that Methyl group is absent in uracil whereas present in thymine at the C-5 position. So, for Uracil to be used in DNA, there are certain reasons on that. One major problem with using uracil as a base is that cytosine can be deaminated, which converts it into uracil. This is no major problem when using thymine, as the cell can easily recognize that the uracil doesn't belong there and can repair it by substituting it by a cytosine again.

Now, if we would use Uracil in DNA it would not be so easy to decide how to repair that error. It would prevent the usage of this important repair pathway. So, uracil cannot be used in DNA. Such matter does not comparable with RNA. But in DNA this error occurs in every replication this is a really bad problem. Rather, the existence of Thymine in DNA instead of Uracil is apparently due to evolution process which made DNA more stable.

Thymine has greater resistance to photochemical mutation, making the genetic message more stable because excitation energy in DNA is mobile, and is eventually transferred to thymine residues, which are the sites of radiation damage. So, by this mechanism DNA can be more stable using Thymine instead of Uracil.

Answer of Question 2 :

Basically Gout is a disease which happens in a human body when the amount of Uric acid is too much high in that body. In most of the cases it happens because of enzyme xanthine oxidase. Xanthine oxidase converts oxypurines (hypoxanthine and xanthine) to uric acid. It is found in many organs such as the liver, stomach, heart, brain, kidneys, and blood plasma. Xanthine oxidase converts hypoxanthine to xanthine and then xanthine to uric acid. Uric acid is a product of broken down foods and cells that are excreted by the kidneys. A decreased level of xanthine oxidase, an increased amount of hypoxanthine and xanthine, or a decreased kidney function can all increase the level of uric acid in the blood. Too much uric acid in the blood builds up around joints and causes the pain and swelling associated with gout. An increase in uric acid is also associated with organ damage and failure. Now, what Allopurinol does is that it  reaches its peak in thirty to sixty minutes once ingested and has a relatively short half-life. As it inhibits xanthine oxidase from converting enzymes to uric acid, it also helps to stop tophi (uric acid crystals) from forming. Tophi are responsible for causing gouty arthritis. That is the main basis of this treatment of Gout with Allopurinol.