 Have you ever wondered what makes you unique? Why do you look like a combination of your parents? And where is all of this information found? Well the answer to these questions all boil down to DNA, and today we will explore what makes up this important substance. In eukaryotes, DNA is found in the nucleus, where it is stored and packaged into thread-like structures known as chromosomes, which we can see in the presented diagram. These are found in almost every one of your cells, and these chromosomes contain genes and are inherited from your parents, and this is why you look similar to both of them. Now let's discuss the structure of DNA, specifically we are going to cover the basic unit of it, known as nucleotides, and these are compounds consisting of three molecules, which are a deoxyribose sugar, a phosphate group, and a nitrogenous base. And we can see this structure here to the right. There are four types of nucleotides, each of which contain different nitrogenous bases, and these bases include guanine, cytosine, thymine, and adenine, or G-C-T-A, respectively. And on the right side we can see what these bases look like. Whilst there are four different nitrogenous bases, the sugar and phosphate group are all the same in every nucleotide. Now let's talk about the bonding between the nitrogenous base and the sugar. These two molecules are paired by an N-glycosidic bond. When the two compounds are bonded, they form a nucleoside. Now note, it is not a nucleotide, which contain phosphates. So you may be wondering how do all of these nucleotides bind together? The sugars from one nucleotide bond covalently with the phosphate of the following nucleotide in the chain. This is called a phosphodiester bond, and these bonds are the high strength ones that form the backbone of DNA. Looking at the diagram shown here, there is a nucleotide and these bond together with adjacent nucleotides, via the spots indicated in the red circles. Then note that this high covalent bond strength makes the backbone difficult to break apart. So this is what this looks like. As we can see, the covalent bonds form the backbone, which is made up of alternating sugar and phosphate molecules. The bonds within this backbone, including those inside the nucleotide, are called phosphodiester bonds. Now we are going to talk about how the nitrogenous bases from one strand undergo hydrogen bonding with the bases from another strand, which forms the overall double-stranded DNA. Now this bonding is complementary, meaning that guanine binds with cytosine, and thymine binds with adenine. The reason why this occurs is because thymine and adenine can bond together with two hydrogen bonds, whereas cytosine and guanine bond together with three. Now when these two strands bond together via complementary bonding, they form the double-stranded structure of DNA, which creates a double helical shape as seen here. Another point to cover is the fact that DNA is bound to proteins. These proteins are called histones, and they help in the process of condensing DNA so that it can fit inside the nucleus. So essentially, DNA is tightly coiled, and it wraps around histone proteins so that it can be nicely packaged as chromatins within the nucleus. Now during cell division, chromatins duplicate and condense even further to become chromosomes, otherwise they just exist as chromatin network. The image seen here is a karyotype, which is the set of 46 chromosomes found within most of your cells in the body. As you can see, there are 23 pairs of these chromosomes, and this is because you get 23 from the maternal side and 23 from the paternal side. Each pair of chromosomes is known as a homologous pair. Now note that 44 of these chromosomes are autosomal, and 2 are your sex chromosomes, which determine an individual's genetic sex. Here, this is a karyotype of a genetic male, as we have an X and a Y chromosome. In summary, today we learnt that DNA has two strands that are made of a backbone, which consists of alternating sugar and phosphate molecules, and nucleotide bases, which undergo complementary bonding. And remember that this occurs between adenine and thymine, or cytosine and guanine. We also learnt that the two strands form a double helix. Lastly, we explored that in eukaryotes, DNA is condensed, coiled, and stored in the cells nucleus as chromatin or chromosomes.