1. Structure of DNA

• DNA is a store of genetic information which is important for synthesis of polypeptides in a cell, cell differentiation and cell division.

• All cells of an individual have the same DNA.

1.1 Structure of DNA

• The basic unit of the DNA molecule is the nucleotide.

• The nucleotide consists of a 5-carbon sugar known as deoxyribose, a phosphate group and a nitrogenous base.

• There are four types of nitrogenous base in a DNA, namely adenine, thymine, guanine and cytosine.

Above: Components of a nucleotide

• A deoxyribose sugar, phosphate group and a nitrogenous base are joined together to form a nucleotide. As there four types of nitrogenous bases, there are four types of nucleotides.

Above: Four types of nucleotides in a DNA molecule.

• Many nucleotides join together to form a polynucleotide. The bond between one nucleotide and the next nucleotide is known as the phosphodiester bond. The backbone consists of consecutive sugar and phosphate groups and it is known as the sugar-phosphate backbone.

Above: Five nucleotides join together to form a polynucleotide. The deoxyribose sugar of the top nucleotide joins to the phosphate group of the next nucleotide by forming a phosphodiester bond (represented by red lines). The backbone which consists of consecutive deoxyribose and phosphate groups is known as the sugar-phosphate backbone.

• DNA consists of two strands of polynucleotides that run in opposite directions (ie. anti-parallel).

• The two strands of polynucleotides are held in place by hydrogen bonds between complementary base pairs which is covered in Section 2.

Above: DNA consists of two strands of polynucleotides that are anti-parallel (ie they run in opposite direction). The two strands of DNA are held by hydrogen bonds between complementary base pairs.

• The two strands of polynucleotides of each DNA molecule coil around each other to form a double helix (See Section 3).

Interesting Fact:

The structure of DNA was discovered by Watson and Crick in 1953. Both of them received the Nobel Prize in recognition of their effort.