 Proteins like DNA are polymeric compounds and they are made up of amino acids. So in a protein molecule, amino acids are the repeating units. Different amino acids join together and they make protein molecules. Proteins range in size from small to very large. For example, some proteins may contain up to thousands of amino acids. So proteins are very big molecules. All the proteins are made up of 20 different amino acids. And as all the proteins are made up of these 20 amino acids, so these 20 amino acids are called standard amino acids. Here you can see the structure of an amino acid. So in this amino acid, there is one amino group and there is one carboxylic group. And both these groups are attached to a carbon atom, which is called alpha carbon. So to this alpha carbon, one hydrogen is attached and to the other side, one R group is attached. So if you see the structure of an amino acid, you will find in all the amino acids all these things constant. I mean, all the amino acids have carboxylic group, a amino group and one hydrogen. So what is variable? This R group is variable. In 20 different amino acids, there are 20 different types of R groups are present and these 20 R groups make 20 different types of amino acids. Here you can see the structures of different amino acids. For example, this is glycine and in glycine there is one carboxylic group, one amino group, one hydrogen and the R group is hydrogen. So this is simplest amino acid in which R group is hydrogen. Here you can see alanine. Again there is one carboxylic group, one amino group and the R group is methyl. This is another simple amino acid. Then you can see serine. This is R group of serine. Then trunine. This is a little bigger than the other amino acids. Then there is cysteine. This is R group of cysteine. And in cysteine, you can see here sulfur is present. So cysteine is a sulfur containing amino acid. Then there is valine. Here the R group is this. Then leucine. R group is this. Isoleucine has R group like this. Then another amino acid is methionine. Methionine also contains sulfur. So cysteine and methionine. These two are sulfur containing amino acids. Then there is proline. Proline has its R group in the form of this ring. So proline is quite different from all the other types of amino acids. Then there are some other amino acids like phenylalanine. Phenylalanine also contains this phenyl group. Then tyrosine. Also similar one. Then tryptophan. And aspartic acid and glutamic acid. So these aspartic acid and glutamic acid are charged amino acids. And these are negatively charged amino acids. Then aspargin glutamine. Glutamine has quite big R group. Then histidine, lysine and arginine. All these three amino acids are also charged amino acids. But they are positively charged amino acids. So here you have seen 20 different types of amino acids. With 20 different types of R groups. In a protein molecule. Each amino acid residue is joined to its neighboring amino acid. By a specific type of covalent bond which is called peptide bond. So two amino acids are joined together through a covalent linkage. And this covalent linkage is a bit different from normal covalent linkages. So this covalent linkage is called peptide bond. Here you can see two amino acids. One is this. The other one is this. These two amino acids combine together. One water molecule is eliminated. And with the elimination of this water molecule covalent bond is established. Between this carbon, carbonyl carbon and this nitrogen. This bond is actually covalent bond. But this is called peptide bond. So in this way two amino acids join together through a peptide linkage. Similarly third, fourth, fifth and so on. Other amino acids can also join to make a long chain of amino acids. A amino acid can successfully join to form dipeptides. Tripe peptides if they contain three amino acids. Tetra peptides containing four amino acids. Oligopeptides containing few amino acids. And polypeptide containing many amino acids. So if a polypeptide is formed we cannot say it is a protein. Because polypeptide is a polypeptide chain. And after some organization, conformational organization. This polypeptide is transformed into a protein molecule. For example here you can see this is a polypeptide chain. Different amino acids are joined together in this polypeptide. This is simply a polypeptide chain. This is not a protein. In this polypeptide chain there undergoes some conformational organization. And it forms this helical structure. This is secondary structure of protein. And this simple chain is called primary structure of protein. So after secondary organization further organization takes place in the polypeptide chain. And it forms tertiary organizational level. And now we can call this polypeptide as a protein. And in some other proteins this tertiary level go beyond. And it make quaternary level. So these are different levels of organization in a protein molecule.