 In this mini lecture, I'm going to talk to you a little bit more about the DNA structure. So how was the structure actually determined? The structure was actually determined by two scientists, James Watson and Francis Crick. And you may just hear them from what they called Watson and Crick. But Watson and Crick were not the only scientists that actually came up with the whole idea of the structure. They obtained data from other scientists such as Rosalind Franklin. She worked along with Maurice Wilkins to come up with some data that was used. Also Edwin Shargoff contributed data to determining the structure. Now I'm not going to go into detail into exactly the experiments that they did, but now we'll just remember that they were instrumental in determining the structure of DNA. So if you were to examine DNA, the structure that they actually proposed was this spiral structure, double helical structure. So this part is represented as a linear structure for clarity. So this is a linear representation. And I'll just put Rep for short. Whereas this, this is a part that is actually how it looks double helical. It's a double helix and it looks kind of like a spiral staircase. So it's made up of two strands and from the data they were able to deduce that. And the strands are basically running opposite direction to each other. So this strand is running in that direction and the other strand is running in the opposite direction. And also what is important is that these strands are held together by what are called hydrogen bonds and they exist between certain molecules that we're going to be talking a little bit more about. So these are the molecules that actually hold the structure together in order for the two strands to maintain this double helical pattern. So if we were to look at DNA on a closer level, if we were actually to look at the molecules that are involved, the DNA strand is a, or the DNA molecule is actually a polymer. And a polymer consists of monomers that join to each other to make this polymeric structure. So the monomers that are found in DNA are actually called nucleotides. So this is an example of a monomer. This is a nucleotide. This is another monomer. So these monomers join together to give one strand of DNA and the similar thing happens for the other strand. And in the middle of this DNA structure the bases, or they're called the bases of the nucleotides, these bases, they actually associate with each other to form the hydrogen bond. So these are the hydrogen bonds that I'm talking about. So what's important to know is that the nucleotides that are formed, they differ from each other depending on the base content that we have. So nucleotides normally look like this. So this is an example of an adenine nucleotide where you have this part being a sugar. So this sugar actually has five carbons and it's called a deoxyribose sugar. Then you have a phosphate group over here. So this is a phosphate group. And then you have the base which consists of a lot of nitrogen atoms. So this is the base. In this case this nucleotide has the adenine base. So you have four bases that make up nucleotides of DNA. You can either have an adenine base, a thymine base, a cytosine, or a guanine base. So they each form a base can combine with the sugar and the phosphate to make an adenine nucleotide, a thymine nucleotide, a cytosine nucleotide, and a guanine nucleotide. So from now on I'll just refer to the nucleotides as A, T, C, and G. So just remember when you see those letters we are talking about the nucleotides. So another thing that they found out, or they deciphered Watson and Crick, were based on Edwin Shargar's data. And the result of his data they concluded that adenine, which is the base, remember they form hydrogen bonds with each other. The one base in one strand of DNA forms a hydrogen bond with the base in the opposite strand of DNA. So what we learned is that adenine pairs with thymine and guanine pairs with cytosine. So that basically the structure of DNA are held together because of these base pairs. So the paren of adenine and thymine are the paren of guanine and cytosine called base pair. Base pair. So they're basically forming base pairs with each other. So another thing that is important about the structure of DNA is that they strands that make up the DNA molecule are complementary to each other. So if you were given this strand for example, and they asked you to find the complementary strand to it that would make the result in DNA molecule, you have to know your rules. So you know that adenine pairs with thymine and cytosine pairs. Adenine pairs with thymine, two hydrogen bonds, and cytosine forms three hydrogen bonds with guanine. So based on our base paren rule we can determine what the other strand is. So the other strand would be C, G, A, C, G, T, C, A, and G. So therefore we can determine our complementary strand if we know our existing strand. So this actually is the complete DNA molecule. So because DNA consists of series of nucleotides that are held together and the strand of the other strand in the DNA molecule is complementary to the first strand, this makes it suitable for its function. So it can function in inheritance and the reason for this is that it can get duplicated and we're going to be talking about DNA replication. So you can duplicate it and as a result it goes through cell division and because of this we can make, if you go through the cell division of meiosis, you're going to form gametes that will form the offspring. So these two individuals produce their offspring because of DNA replication. Also DNA replication is important in gene expression where you go from DNA to form a molecule called RNA. We'll talk about that later too and ultimately form in protein. So in order for DNA to be copied, the information in DNA to be copied to RNA, you have to have the complementary base pairing occurring and then you have translation occurring from RNA to protein. We will talk more about that just bear in mind that base pairing all tends to happen a lot when we are transmitting information from DNA to protein. So you have learned a little bit about the structure of DNA, now we're going to move on to talking about how it gets copied.