Welcome back students, we were seeing the topic molecular basis of inheritance. Now let's see about the flexibility of DNA. Now, the vast majority of DNA molecules present in the aquis protoplasm of the living cells almost certainly exist in what's an concrete model that is the double helic structure. That form of DNA is mainly called as B form of DNA and B form of DNA represent 92% of the relative humidity. In fact, intercellular B form of DNA appears to have an average of 10.4 nucleotides per turn rather than 10. So B form of DNA has more nucleotide per turn. Now mainly there are three forms of DNA, B DNA, A DNA and Z DNA. Now in high concentration of salts or in the dehydrated state, 75% humidity, we can say the DNA exists in the A form. So when there is 75% humidity or there is high concentration of salt, DNA exists in A form which has 11 nucleotides per turn. And some, there are some DNA which exist in the unique left handed form, left handed double helix form which are called as Z DNA. And A and B are mainly the right handed double helical form are in right handed double helical form. Now if you consider A form of DNA, it has 11 nucleotides per turn and the pitch is ranging about 24.6 Armstrong. B form of DNA has 10 nucleotides while Z form of DNA has 12 nucleotides per turn. Now as you can see in the figure, the B form of DNA, same you can consider of A form also, it is right handed DNA. That means it is starting from 3 prime end and ending to 5 prime end. It is in this sequence and it is right handed form. It is like this and it will be the complementary of 3 prime end would be 5 prime end and it is like this. So this is right handed double helical form of DNA while in Z DNA it is left handed. It is starting from 5 prime end, complementary to which will be the 3 prime end. So this 5 prime end would end to 2 3 prime end and this 3 prime end would end to 2 5 prime end. So this is left handed form of DNA. Students in this session we have seen about the flexibility of DNA and the main 3 forms of DNA. In further session we will have a more detailed look.