RNA structure is similar to DNA structure. There are some differences, however. For one thing, the sugar found in RNA nucleotides is ribose rather than deoxyribose. (Check the structure in the nucleotide diagram.) Like DNA, RNA nucleotides contain the bases adenine, cytosine, and guanine, but in RNA thymine is replaced by uracil (uracil base pairs with adenine). RNA occasionally occurs as a double-stranded molecule, but mostly, RNA is single stranded (not base-paired to another strand).

There are three major classes of RNA: messenger RNA (mRNA), ribosomal RNA (rRNA), and transfer RNA (tRNA). Each of these types of RNA plays a different role in the cell, as we will see elsewhere. All types of RNA are produced as complementary copies of one strand of DNA, by the process of transcription.

Denaturation

Because the two strands of double-stranded nucleic acids are held together by hydrogen bonds, which are individually weak, if enough energy is added to a double-stranded molecule, it will separate into two individual strands. In other words, if you heat up a DNA molecule to a high enough temperature, it will separate into two strands. This process is known as denaturation or melting. If the temperature is then lowered again, then complementary strands will renature, or reform a double-stranded molecule. This property of nucleic acids forms the basis of some powerful analytical techniques, as will be seen in the module on molecular techniques.

Nucleic Acids: Summary of Key Points

• DNA is the genetic material of all organisms except for a few viruses. This was initially demonstrated in bacteria, where DNA was shown to be able to transform one type of bacterial cell into another.
• DNA is made of subunits called nucleotides. A nucleotide consists of a 5-carbon sugar (2-deoxyribose) linked to a phosphate group and a nitrogenous base.
• There are four nitrogenous bases used in DNA: adenine, thymine, guanine and cytosine. Adenine and guanine are purines; cytosine and thymine are pyrimidines.
• Nucleotides are joined together in a chain. A DNA molecule consists of two such chains, joined to each other by hydrogen bonding between the nitrogenous bases on the nucleotides of each chain. Adenine bonds to thymine, and guanine bonds to cytosine.
• RNA is similar to DNA, except that the bases include uracil instead of thymine, and the sugar is ribose instead of deoxyribose.
• Adding energy (heat) to DNA can overcome the hydrogen bonds, denaturing the double-stranded DNA into two single strands. Cooling the molecules can reverse the process.