The code that carries the hereditary characteristics of an organism is called genetic code. The code formed by the combination of three adjacent nitrogen bases in the DNA molecule is called the genetic code. It is also called mRNA code. The genetic code is the biochemical basis of heredity. Scientist Francis Crick proved that the genetic code is a triplet or 3-letter code. Nirenberg was able to synthesize artificial mRNA using poly U (polyuridylic acid) in a cell-free or in vitro system. Indian scientist Hargobind Khorana synthetically synthesized the 5GUGUGUGUGU3 nucleotide chain and used it to create the valine-cysteine valine-cysteine polypeptide chain. Nirenberg and Hargobind Khorana won the Nobel Prize in 1969 for this discovery.

The unit of gene expression in a primitive cell is called an operon. Jacob and Mond (1961) proposed the operon model. According to them, an operon is formed by the combination of several genes and their interaction in primitive cells results in the expression of functional genes. They discovered the lactose operon. The operon consists of four segments.
1. Structural genes: It synthesizes enzymes.
2. Driver Gene: It controls the production of protein.
3. Regulatory Gene: A gene that controls the functions of an operon is called a regulatory gene. An increase in the transcription of a gene in a cell is called induction and a decrease is called repression.
4. Promoter or stimulator gene: RNA polymerase enzyme is attached to it.

In 1950, scientist Erwin Chargaff mentioned the principles about the structure of DNA through research called Chargaff’s rule. Scientist Chargaff found that DNA molecules have equal amounts of adenine and thymine. Again, the amounts of Guanine and Cytosine are also equal. This is called Chargaff principle. Half of the nitrogenous bases in the DNA molecule will be purines (A+G) and half will be pyrimidines (T+C). Hence A pairs with T and G with C.

Several nucleotides are linked together by phospho-diester bonds to form a long chain. It is called polynucleotide. In polynucleotides, the phosphate molecule is attached to the 5th carbon of the pentose sugar on one side and to the 3rd carbon of the pentose sugar on the other side. Nucleotides in polynucleotides are arranged in a 5→3 carbon orientation. Each single helix of a DNA molecule is a polynucleotide chain.

Two molecules of nucleotide together are called dinucleotide. The 5th carbon of the pentose sugar of one nucleotide and the 3rd carbon of the pentose sugar of the other nucleotide are joined by a phospho-diester bond to form a dinucleotide.

If the nitrogenous base forming the nucleotide is pyrimidine, it is called pyrimidine nucleotide. It is of three types. Thymidine nucleotide, cytidine nucleotide and uridine nucleotide. Pentose sugars and thymine combine to form thymidine nucleotides, pentose sugars and cytosine combine to form cytidine nucleotides, and pentose sugars and uracil combine to form uridine nucleotides.

If the nitrogenous base forming the nucleotide is purine, it is called purine nucleotide. It is of two types. Adenosine nucleotide and guanosine nucleotide. Pentose sugars and adenine combine to form adenosine nucleotides and pentose sugars and guanine combine to form guanosine nucleotides.

A molecule of phosphate and a molecule of nucleoside combine to form a nucleotide. That is, one molecule of pentose sugar, one molecule of phosphate and one molecule of nitrogenous base are called nucleotides. Nucleotide is the structural unit of nucleic acid. A nitrogenous base is attached to the 1st carbon of a pentose sugar and a phosphate is attached to the 3rd or 5th carbon to form a nucleotide. It is of two types. Purine nucleotides and pyrimidine nucleotides.
(i) Purine Nucleotide: If the nitrogenous base forming the nucleotide is purine, it is called purine nucleotide. It is of two types. Adenosine nucleotide and guanosine nucleotide. Pentose sugars and adenine combine to form adenosine nucleotides and pentose sugars and guanine combine to form guanosine nucleotides.
(ii) Pyrimidine Nucleotide: If the nitrogenous base forming the nucleotide is pyrimidine, it is called pyrimidine nucleotide. It is of three types. Thymidine nucleotide, cytidine nucleotide and uridine nucleotide. Pentose sugars and thymine combine to form thymidine nucleotides, pentose sugars and cytosine combine to form cytidine nucleotides, and pentose sugars and uracil combine to form uridine nucleotides.
Different types of nucleotides are AMP, GMP, CMP, UMP, TMP, dAMP, dGMP, dCMP, dUMP, dTMP etc.

A molecule of phosphate and a molecule of nucleoside combine to form a nucleotide. That is, one molecule of pentose sugar, one molecule of phosphate and one molecule of nitrogenous base are called nucleotides. Nucleotide is the structural unit of nucleic acid. A nitrogenous base is attached to the 1st carbon of a pentose sugar and a phosphate is attached to the 3rd or 5th carbon to form a nucleotide. It is of two types. Purine nucleotides and pyrimidine nucleotides.

When the nitrogenous base forming the nucleoside is pyrimidine, it is called a pyrimidine nucleoside. The 1st carbon of the pentose sugar is joined to the 1st nitrogen of the base by a glycosidic bond to form a pyrimidine nucleoside. It is of three types. Thymidine nucleoside, cytidine nucleoside and uridine nucleoside. Pentose sugars and thymine combine to form thymidine nucleosides, pentose sugars and cytosine combine to form cytidine nucleosides and pentose sugars and uracil combine to form uridine nucleosides.