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Name: Sari
Status: Other
Grade:  Other
Location: AZ
Country: United States
Date: April 2007


Question:
Hello, one question. Why is DNA our genetic material and not RNA. What characteristics of RNA strip it from preferential characteristics that DNA has? There is something about reacting with OH-, but I'm not entirely sure of the reason. Your help is appreciated.



Replies:
The main reason DNA is better for 'safe' storage of information is its stability. There are several different ways DNA resists change more than RNA; here are some:

First, as you noted, the deoxyribose sugar in DNA is less reactive than the ribose sugar. In general C-H bonds are less reactive than C-OH (hydroxyl). Also, RNA is not very stable in alkaline conditions, while DNA is.

More broadly speaking, the double-strand DNA (dsDNA) has relatively small 'grooves' where damaging enzymes can attach, which makes it harder for them to 'attack' the DNA. Double-stranded RNA (dsRNA) has much larger grooves, so it would more subject to being broken down.

Second, the connection between the strands of dsDNA is tighter than dsRNA -- it's easier to 'unzip' dsRNA than it is to unzip dsDNA.

Overall, it's easier (faster, requires less energy) to break down and reform RNA than DNA -- since we want our genetic material to be stable, we want the substance that's harder to break down.

As an interesting side note, it is well known that DNA can be damaged by UV, but RNA is actually more resistant to damage by UV. Also, the sequence of DNA and its physical conformation (the shape the strands are folded into) seems to play a role as well.

This might be a chicken-egg point, but it's important to note that the body actively destroys enzymes that cleave DNA (called nucleases) -- when it needs to cleave DNA, it makes its own specific enzymes. It's one of several ways DNA is protected against damage. The body can actually "identify" foreign DNA and destroy it, and not destroy its own DNA.

Unlike DNA, RNA strands are continually made, broken down, and reused. If you add up the chemical stability, the energy it takes to break or make DNA and RNA bonds, and the availability of enzymes to do this work, a compelling case for DNA over RNA can be made.

Hope this helps,
Burr


This is a very interesting question. RNA and DNA are chemical cousins, RNA (ribonucleic acid) has an additional 2' OH group on its ribose sugar (which is why DNA is called deoxyribonucleic acid) and uses uracil instead of thymine (which differ only by a CH3 group). Like DNA, RNA can form a double helix structure and can store information because like DNA it is constructed from four bases along a sugar phosphate backbone.

One of the main reasons RNA is less stable than DNA is that the 2' OH group of RNA can react with the molecule's backbone in flexible regions, causing the molecule to cleave. Since long strands of RNA are therefore chemically less stable, organisms which evolved to use DNA instead of RNA to protect their biological code probably had a selective advantage. This may explain why almost all of life uses DNA as its genetic code (an exception to this are certain viruses which use RNA).

An important discovery by Tom Cech of the University of Colorado showed that RNA can actually adopt complex structures and can act as an enzyme (a molecule that catalyzes a reaction). Because RNA can encode biological information, many scientists speculate that life may have originated from an RNA molecule which contained the information to copy itself and the enzymatic ability to do so. This is known as the "RNA World hypothesis."

Ethan Greenblatt
Ph.D. Candidate
Stanford Department of Chemistry



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