Nucleic Acids: DNA and RNA

by Anthony Carpi, Ph.D.

Table of contents

Living organisms are complex systems. Hundreds of thousands of proteins exist inside each one of us to help carry out our daily functions (see our Fats and Proteins module for more information). These proteins are produced locally, assembled piece-by-piece to exact specifications. An enormous amount of information is required to manage this complex system correctly. This information, detailing the specific structure of the proteins inside of our bodies, is stored in a set of molecules called nucleic acids.

The nucleic acids are very large molecules that have two main parts. The backbone of a nucleic acid is made of alternating sugar and phosphate molecules bonded together in a long chain, represented below:

Sugar molecular diagram phosphate Sugar molecular diagram phosphate  
sugar phosphate sugar phosphate ...

Each of the sugar groups in the backbone is attached (via the bond shown in red) to a third type of molecule called a nucleotide base:

nucleotide
base
  nucleotide
base
   
|   |  
sugar phosphate sugar phosphate ...

Though only four different nucleotide bases can occur in a nucleic acid, each nucleic acid contains millions of bases bonded to it. The order in which these nucleotide bases appear in the nucleic acid is the coding for the information carried in the molecule. In other words, the nucleotide bases serve as a sort of genetic alphabet on which the structure of each protein in our bodies is encoded.


DNA

In most living organisms (except for viruses), genetic information is stored in the molecule deoxyribonucleic acid, or DNA. DNA is made and resides in the nucleus of living cells. DNA gets its name from the sugar molecule contained in its backbone(deoxyribose); however, it gets its significance from its unique structure. Four different nucleotide bases occur in DNA: adenine (A), cytosine (C), guanine (G), and thymine (T).

These nucleotides bind to the sugar backbone of the molecule as follows: 

A   T   G   C  
sugar phosphate sugar phosphate sugar phosphate sugar ...

The versatility of DNA comes from the fact that the molecule is actually double-stranded. The nucleotide bases of the DNA molecule form complementary pairs: The nucleotides hydrogen bond to another nucleotide base in a strand of DNA opposite to the original. This bonding is specific, and adenine always bonds to thymine (and vice versa) and guanine always bonds to cytosine (and vice versa). This bonding occurs across the molecule, leading to a double-stranded system as pictured below:

sugar phosphate sugar phosphate sugar phosphate sugar ...
T A C G
¦ ¦ ¦ ¦
A T G C
sugar phosphate sugar phosphate sugar phosphate sugar ...

In the early 1950s, four scientists, James Watson and Francis Crick at Cambridge University and Maurice Wilkins and Rosalind Franklin at King's College, determined the true structure of DNA from data and X-ray pictures of the molecule that Franklin had taken. In 1953, Watson and Crick published a paper in the scientific journal Nature describing this research. Watson, Crick, Wilkins and Franklin had shown that not only is the DNA molecule double-stranded, but the two strands wrap around each other forming a coil, or helix. The true structure of the DNA molecule is a double helix, as shown at right.

The double-stranded DNA molecule has the unique ability that it can make exact copies of itself, or self-replicate. When more DNA is required by an organism (such as during reproduction or cell growth) the hydrogen bonds between the nucleotide bases break and the two single strands of DNA separate. New complementary bases are brought in by the cell and paired up with each of the two separate strands, thus forming two new, identical, double-stranded DNA molecules. This concept is illustrated in the animation below.


RNA

Ribonucleic acid, or RNA, gets its name from the sugar group in the molecule's backbone - ribose. Several important similarities and differences exist between RNA and DNA. Like DNA, RNA has a sugar-phosphate backbone with nucleotide bases attached to it. Like DNA, RNA contains the bases adenine (A), cytosine (C), and guanine (G); however, RNA does not contain thymine, instead, RNA's fourth nucleotide is the base uracil (U). Unlike the double-stranded DNA molecule, RNA is a single-stranded molecule. RNA is the main genetic material used in the organisms called viruses, and RNA is also important in the production of proteins in other living organisms. RNA can move around the cells of living organisms and thus serves as a sort of genetic messenger, relaying the information stored in the cell's DNA out from the nucleus to other parts of the cell where it is used to help make proteins.

A   U   G   C  
sugar phosphate sugar phosphate sugar phosphate sugar ...
RNA



Anthony Carpi, Ph.D. “Nucleic Acids” Visionlearning Vol. BIO-1 (1), 2003.

I was shown Rosalind Franklin's X-ray photograph and "Whooo! That was a helix!", and a month later, we had the structure...

-James Watson,
1928-



“The capacity to blunder slightly is the real marvel of DNA. Without this special attribute, we would still be anaerobic bacteria and there would be no music.”

- Lewis Thomas