 Now, we talk about the chemical composition of chromosomes. Chemically it is found by different types of biochemistry experiments and by x-ray diffraction studies that chemically DNA consist of 3 major ingredients, a sugar, a phosphate group and a nitrogenous base, DNA is a molecule that is about 2 nanometers thick. This is running continuously means without a break throughout the chromosome, DNA is acidic in nature. The biochemical analysis shows that DNA in its nature is acidic. So, you know the DNA is present in huge quantities inside the nucleus and it is packed. If we talk about DNA in a single cell all of its chromosomes and we simply elongate them and straighten them, these will be about 6 feet long, you can guess 6 feet about more than the height of an average person and this is packed in a microscopic cell and even inside its nucleus, how this much long DNA molecule is packed in a microscopic nucleus, very small one which we cannot see with the naked eye, how is this possible? The answer is coiling and the super coiling. We again have to look on a diagram, look at the diagram which is in front of you right now. We know that chromosomes are present in pairs. If we look at the bottom right, we can see a homologous pair of chromosomes, the metaphase chromosomes which are present in the metaphase of the mitosis cell division. If we simply uncoil a chromosome and spread it in a linear fashion, then we will see that this is a highly complex molecule, founder just we can say it is coiled, coiled and coiled. If we open these coiles more, we can see in the geolocal or if you can see the flower like structures, these are called the solenoids. These are the lengths of DNA wound around or wrapped around the histone protein. If we even open them more, if we open the solenoids as well, then we can see the bead like structures. These beads, these individual beads are actually lengths of DNA wound around or wrapped around a set of histone molecule, we call it a nucleosome. One nucleosome is actually a set of histone molecules and two wraps of DNA molecule around it. This DNA which is wrapped around in two circles around the histone molecule is about 200 base pairs long. Then you can see that these beads are attached to each other. This part of DNA which is attaching them with each other is called the linker DNA. That is it is linking nucleosomes with each other. This linker DNA is roughly about 80 base pairs long. Then if we open this, if we still open the nucleosome and separate the DNA from the proteins or unwrap the DNA from the nucleosomes, then we can see the DNA is a very long helical structures, two strands going side by side together in a helical fashion. This part of DNA actually which is now without the proteins, just two DNA strands going side by side together makes the genes. It means that these are the, these are only the DNA molecules with no association protein. In this form, these are capable of making the RNA molecule and then the proteins. We have a closer look in this next diagram on the nucleosomes. You can see three nucleosomes here. If we start from the right, you can see one nucleosome, a central core of proteins. We can call it a protein core made up of histone molecules. Then two wraps of DNA, then the linker DNA, then two wraps again around the next histone set and then there is a third and so on. If we look this structure under the electron microscope because we know that electron microscope is a microscope which can show us extremely small structure which are not visible under the light microscope even. If we look at the chromatin material during non-dividing phase, we can see a bead like structure. These beads as we can see just, this bead like structure is actually the straightened DNA which have nucleosomes and the linker DNA, then nucleosome and the linker DNA and so on. So this how, this is how this much long DNA molecule which is extremely long longer than the height of the person is coiled or is packed inside a very, very small nucleus which is microscopic, which is not visible to us, visible to our eye. We can say that some scientist says that if we can straighten the whole DNA present in a human body and straighten it and make a thread, this thread will reach from the earth to the moon. This is that long, but you can see that how nature packed it into a very small microscopic nucleus and control the whole characteristics of the organisms using this genetic material. Now we talk about the chemical composition of DNA. DNA is a complex large molecule, it is a macromolecule. It consists of the subunits, subunits are called the nucleotides. Each nucleotide consists of three major molecules. One is a sugar that is ribose sugar, a five carbon sugar and that is a deoxyribose. The other one is a phosphate group and the third one is a nitrogenous base. What is DNA actually? The nucleotides they join together that is one nucleotide is joined to the next and next to the next and so on. These nucleotides makes the long chains of the DNA molecule itself and there are two chains running side by side and these two chains are bound together. They are different parts we will look next. They bind together to make a helical structure. They are wound around each other in a helix. We call it a double helix because it has two strands helically winding around each other. If you look at the diagram it shows you a deoxyribose sugar, a five carbon sugar. Its five corners are showing its five carbons then you can see a phosphate group on its left and a nitrogenous base which is a six carbon one on its right. This is the structure of a single nucleotide. But nucleotides even are of different natures. Nucleotides are categorized into two groups according to their differences in their structures. There are two called purines and there are two others called pyrimidines. We have a look on their structure. Purines are two. One is adenine and the other is guanine. The pyrimidines are thymine and cytosine. There is a slight difference in their structure. The purines as you look at in the diagram, the purines are double ring structure. They have two rings. Their structure is like this that there are two rings joined together. But in the pyrimidines as you can see in the diagram the cytosine and thymine they are single ring structures. According to these four bases we call them nitrogenous bases because you can see there are lot many nitrogen molecules are present inside these structures. According to these nitrogenous bases we categorize the nucleotides into four major categories according to one with which adenine is attached, adenosine, guanosine, cytosine and thymine base. So, bases are adenine, guanine, cytosine and thymine. Now we have a look on DNA molecule more closely how these nucleotides are arranged in front of each other. If you look at the diagram we can see that a thymine is pairing with an adenine base. In DNA actually thymine always pairs with an adenine and a guanine always pairs with a cytosine. So, we can find out the GC pairs that is guanine cytosine pairs and AT pairs the adenine and thymine pairs. Now we know that the DNA is the main genetic material it produces the RNA ribonucleic acid there is a ribonucleic acid is also similar in its single standard structure to the DNA and is always complementary to the DNA. But the difference is this that RNA do not have a thymine in place of thymine the RNA molecule have another base called uracil. So, if this is an RNA molecule it will have uracil in place of the thymine and the uracil will pair with the adenine rather than thymine and the RNA is single standard RNA do not have these two strands running together. The DNA molecule as you can see that there are two parallel chains these chains are attached with each other by hydrogen bonding. If we look at in this next diagram the guanine and cytosine bases are joined together joined to each other by hydrogen bonding and guanine and cytosine the GC pair is attached to each other at three places by three hydrogen bonds while an AT pair adenosine to thymine pair is attached to each other by two hydrogen bonds. So, hydrogen bonding is the force that keep these two winding strands of the DNA together.