Structure of DNA and RNA
Nucleotides and polynucleotide formation
DNA and RNA are long molecules built from repeating smaller units. Each small unit is a nucleotide, and a chain made by joining many nucleotides is a polynucleotide. Nucleotides are therefore the monomers of the polymers DNA and RNA.
Nucleotide structure
Each nucleotide has three components:
- A pentose sugar: a sugar containing five carbon atoms.
- A phosphate group.
- A nitrogen-containing organic base.
DNA nucleotides contain the sugar deoxyribose. Their base is one of adenine, cytosine, guanine or thymine. RNA nucleotides contain the sugar ribose. Their base is one of adenine, cytosine, guanine or uracil. Thus, RNA has uracil in place of the thymine found in DNA.
Formation of a polynucleotide chain
Nucleotides join by condensation reactions. In each reaction, a bond forms between the phosphate group of one nucleotide and the sugar of another, and a molecule of water is released. This bond is called a phosphodiester bond.
Repeated condensation reactions join more nucleotides into a polynucleotide chain. The linked sugars and phosphate groups form the chain’s sugar-phosphate backbone, with a base attached to each sugar.
DNA structure and genetic information
DNA holds genetic information in living cells. A DNA molecule consists of two long chains twisted around each other to form a double helix.
Each chain is a polynucleotide: a chain made from many smaller units called nucleotides. Each nucleotide contains a sugar, a phosphate group and one of four bases: adenine, thymine, cytosine or guanine. The sugar and phosphate groups form the outside of each chain, while the bases lie between the two chains.
The chains are held together by hydrogen bonds between their bases. The bases pair in a specific, complementary way: adenine pairs with thymine, and cytosine pairs with guanine. Knowing the sequence of bases on one chain therefore tells us the complementary sequence on the other.
How DNA stores information
The genetic information in DNA is carried by the sequence of its bases along a chain. Although DNA uses only four types of base, those bases can be arranged in many different orders. Different sequences can therefore carry different information.
DNA’s apparent chemical simplicity once led many scientists to doubt that it could carry the genetic code. Its information capacity, however, depends on the order of its bases, not on having a large variety of nucleotide components.
RNA structure and genetic information
RNA is a polynucleotide: a chain made from many nucleotide units joined together. An RNA molecule is usually single-stranded and is relatively short compared with a DNA molecule. The order of its nucleotides can carry genetic information.
One type of RNA transfers genetic information copied from DNA to a ribosome. At the ribosome, this information is used to make a protein. The RNA that carries the information is called messenger RNA.
Ribosomes themselves are made from RNA and proteins. The RNA that forms part of a ribosome is called ribosomal RNA. RNA therefore has two roles here: messenger RNA carries information to a ribosome, while ribosomal RNA forms part of the structure that receives and uses that information.