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Nucleic Acid Structure
©2001 Timothy Paustian, University of Wisconsin-Madison
Nucleics acids, Ribonucleic acid (RNA) and Deoxyribonucleic acid (DNA), serve as storage units for our hereditary information. DNA can be thought of as a large cookbook with recipes for making every protein in the cell. RNA helps the ribosome translate the information in DNA into protein.
The building blocks
Despite their importance in cellular function, nucleic acid structure is surprisingly simple. RNA and DNA are long polymers of only 4 nucleotides, adenine, guanine, cytosine and thymine (or uracil for RNA). Figure 1.
Figure 1. The structure of nucleotides. The sugar in this case is ribonucleic acid. The bases U, A, C and G are found in ribonucleic acid and the ribsome sugar has two hydroxyls on it. Deoxyribonucleic acid is made up of T, A, C and G and the ribose has only on hydroxyl in the 3' position.
The nucleotide structure can be broken down into 2 parts. The sugar-phosphate backbone and the base. All nucleotides share the sugar-phosphate backbone. Nucleotide polymers are formed by linking the monomer units together using an oxygen on the phosphate, and a hydroxyl group on the sugar.
A, T (or U), G and C are capable of being linked together to form a long chain. The 3'-hydroxyl group on the ribose unit, reacts with the 5'-phosphate group on it's neighbor to form a chain.
The base on each nucleotide is different, but they still show similarities. adenine (A) and guanine (G) are purines, notice the two ring structure, with the differences in the molecules coming in the groups attached to the ring. Likewise, cytosine (C), thymine (T) and uracil (U) are pyrimidines and share a similar structure, but differ in their side groups.
If two strands of nucleic acid are adjacent to one another, the bases along the polymer can interact with complementary bases in the other strand. Adenine is capable of forming hydrogen bonds with thymine and cytosine can base pair with guanine. Adenine forms two hydrogen bonds with thymine, cytosine forms 3 with guanine.
Cells contain two strands of DNA that are exact mirrors of each other. When correctly aligned, A can pair with T and G can pair with C. Because these strands are mirrors of each other, the amount of A is equal to the amount of T and the amount of C is equal to the amount of G in any double stranded DNA molecule. In solution, the two strands will usually find each other and form a double helix. This reaction is favorable because of the numerous hydrogen bonds that can be formed between the complementary bases. The DNA molecule can stretch for millions of base pairs and the DNA sizes of organisms can vary greatly. Note however that the size of the DNA genome is not always a measure of how advanced an organism is, unless you think newts (genome size 19,000,000 kb) are more advanced than humans (genome size 3,500,000 kb).
RNA is similar in structure to DNA, except that uracil (U) takes the place of thymine in the molecule and the ribose unit on each sugar contains a hydroxyl group. The RNA in most cells exist as a single strands, but if complementary base sequences are present in the RNA it can fold back upon itself and base pair. This secondary structure of RNA often results in loops and stems that greatly affect the function of the molecule. RNAs with extensive secondary structure play important physiological roles in translation, in transcription and in DNA replication. RNA serves 3 functions in Ribosomes. These functions center around translating the genetic information in DNA into protein.
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