 Some bigger proteins may not only consist of single polypeptide chain. They may contain two or more separate polypeptide chains and these polypeptide chains are also called as subunits. These subunits may be similar or identical or different. The special arrangement of these subunits is known as proteins quaternary structure. So in a protein molecule there may be more than one polypeptide chain which are associated with each other and apparently they make a single protein molecule. So the special arrangement of these subunits in that overall molecule is called the quaternary structure. A multi subunit protein is also referred to as a multimer because it contains many subunits. And if a multimer contains few subunits that is called an oligomer and a single subunit or a group of similar subunits is called a protomer. As you can see in this diagram, this is the diagram of hemoglobin molecule which is a protein and it contains you can see four subunits. One, two, three, four. So four subunits are present in this molecule. But look at this, the two subunits, this and this, they are identical and the other two subunits, this one and this one, they are identical. So a hemoglobin molecule contains four subunits in which two alpha subunits are similar and two beta subunits are similar. So in this way, you can say hemoglobin is a tetramer. It contains four subunits. But some scientists say that if two or more subunits are identical, then those identical subunits are regarded as single protomer. So in this way, you can say hemoglobin is a dimer. So this is at its quaternary level of organization. Identical subunits of multi-meric proteins are generally arranged in a symmetric pattern. They are not arranged roughly, rather they have very regular symmetric patterns. And mainly two symmetries are present in the proteins and these are rotational symmetry or helical symmetry. There are several forms of rotational symmetry and the simplest rotational symmetry is the cyclic symmetry. In the cyclic symmetry, there is a single axis of rotation. That means the oligomers or the protomers, they rotate around a single axis. Either this is single axis and rotation is like this or this may be the rotational axis and the protomers are arranged around this axis. Here you can see clearly in this picture, you can see two protomers. One is this and the other one is this and they have a single rotational axis. Around this rotational axis, they can rotate. So there is only single rotational axis. This is called cyclic symmetry. It may contain more than two protomers like one, two, three, four. Four may be present, but again the rotational axis is only one. So this type of symmetry is called cyclic symmetry. A somewhat more complicated rotational symmetry is dihedral symmetry. This is another type of rotational symmetry. And in the dihedral symmetry, a two-fold rotational axis is present. That means there is one axis, rotational axis and protomers are arranged like this. And in the same molecule, there may be another rotational axis and molecules are arranged like this. So this is called, here two rotational axes are present which are normally at right angle to each other and such symmetry is called dihedral symmetry. As you can see in this diagram, for example in this, you can see one, two, three, four. Four protomers are present, but they have more than one rotational axis. One rotational axis is this, the other one is this. So this is for these two protomers and this rotational axis may be for these two protomers. So there are more than one rotational axis. Normally two rotational axes are present. Similarly in this, you can see the number of protomers is increasing. This symmetry is called D4, dihedral symmetry D4 in which again the four protomers, but again the rotational axis, one is this, the other one is this. So for these protomers, there are at least two rotational axes present in the molecule. A more complex rotational symmetry include icosahedral symmetry. In this, an icosahedron is formed. Icosahedron is a regular 12-cornered polyhedron with 20 triangular faces. For example, in this icosahedron, you can see 12 corners are present. These are corners of an icosahedron. There are 12 such corners in this icosahedron and there are 20 such triangular faces. So like one is this, this, this, this. So such 20 faces are present. So all these are actually protomers which are arranged in this fashion and they make an icosahedron. Such type of symmetries are present in the bacteriophages as you have seen in the head of a bacteriophage. The other major type of symmetry besides rotational symmetry is helical symmetry. So in the helical symmetry, these subunits, protein subunits or protomers, they are helically arranged. For example, in this figure you can see this is a single protomer and these protomers are attached together and they form a single helix segment. This is one helix segment and in this many such helix segments are arranged. So this helix is actually exhibiting the helical symmetry and this protein is at its quaternary level of organization. The example of such symmetry is also present in the capsules of different viruses like tobacco mosaic virus.