What is the difference between DNA and RNA?

DNA and RNA Introduction

Introduction:

The nucleic acids are informational molecules because their primary structure contains a code or set of directions by which they can duplicate themselves and guide the synthesis of proteins. The synthesis of proteins - most of which are enzymes - ultimately governs the metabolic activities of the cell. In 1953, Watson, an American biologist, and Crick, an English biologist, proposed the double helix structure for DNA. This development set the stage for a new and continuing era of chemical and biological investigation. The two main events in the life of a cell - dividing to make exact copies of themselves, and manufacturing proteins - both rely on blueprints coded in our genes.

There are two types of nucleic acids which are polymers found in all living cells. Deoxyribonucleic Acid (DNA) is found mainly in the nucleus of the cell, while Ribonucleic Acid (RNA) is found mainly in the cytoplasm of the cell although it is usually synthesized in the nucleus. DNA contains the genetic codes to make RNA and the RNA in turn then contains the codes for the primary sequence of amino acids to make proteins.

Nucleic Acid Parts List:

The best way to understand the structures of DNA and RNA is to identify and examine individual parts of the structures first. The complete hydrolysis of nucleic acids yields three major classes of compounds: pentose sugars, phosphates, and heterocyclic amines (or bases).

Phosphate: A major requirement of all living things is a suitable source of phosphorus. One of the major uses for phosphorus is as the phosphate ion which is incorporated into DNA and RNA.

Pentose Sugars:

There are two types of pentose sugars found in nucleic acids. This difference is reflected in their names--deoxyribonucleic acid indicates the presence of deoxyribose; while ribonucleic acid indicates the presence of ribose.

In the graphic on the left, the structures of both ribose and deoxyribose are shown. Note the red -OH on one and the red -H on the other are the only differences. The alpha and beta designations are interchangeable and are not a significant difference between the two.

Heterocyclic Amines:

Heterocyclic amines are sometimes called nitrogen bases or simply bases. The heterocyclic amines are derived from two root structures: purines or pyrimidines. The purine root has both a six and a five member ring; the pyrimidine has a single six member ring.

There are two major purines, adenine (A) and guanine (G), and three major pyrimidines, cytosine (C), uracil (U), and thymine (T). The structures are shown in the graphic on the left. As you can see, these structures are called "bases" because the amine groups as part of the ring or as a side chain have a basic property in water.

A major difference between DNA and RNA is that DNA contains thymine, but not uracil, while RNA contains uracil but not thymine. The other three heterocyclic amines, adenine, guanine, and cytosine are found in both DNA and RNA. For convenience, you may remember, the list of heterocyclic amines in DNA by the words: The Amazing Gene Code (TAGC).

http://www.elmhurst.edu/~chm/vchembook/580DNA.html

DNA Base Pairing Principle

The Base Pairing Principle is: Complementary base pairs are: adenine and thymine (A - T )
guanine and cytosine (G - C)

The base pairing is called complementary because there are specific geometry requirements in the formation of hydrogen bonds between the heterocylic amines. Heterocyclic amine base pairing is an application of the hydrogen bonding principle. In the structures for the complementary base pairs given in the graphic on the left, notice that the thymine - adenine pair interacts through two hydrogen bonds represented as (T=A) and that the cytosine-guanine pair interacts through three hydrogen bonds represented as (C=G).

Although other base pairing-hydrogen bonding combinations may be possible, they are not utilized because the bond distances do not correspond to those given by the base pairs already cited. The diameter of the helix is 20 Angstroms.

http://www.elmhurst.edu/%7Echm/vchembook/582dnadoublehelix.html

Solving the DNA Puzzle

In the late 1940's, the members of the scientific community were aware that DNA was most likely the molecule of life, even though many were skeptical since it was so "simple." They also knew that DNA included different amounts of the four bases adenine, thymine, guanine and cytosine (usually abbreviated A, T, G and C), but nobody had the slightest idea of what the molecule might look like.

In order to solve the elusive structure of DNA, a couple of distinct pieces of information needed to be put together. One was that the phosphate backbone was on the outside with bases on the inside; another that the molecule was a double helix. It was also important to figure out that the two strands run in opposite directions and that the molecule had a specific base pairing.

As in the solving of other complex problems, the work of many people was needed to establish the full picture.

The original DNA model by Watson and Crick. Photo: Cold Spring Harbor Laboratory Archives

http://nobelprize.org/educational_games/medicine/dna_double_helix/readmore.html