The DNA double helix complementary base is paired with antiparallel strands. So DNA is one of the most important molecules in life. It has a particular structure that needs to be aware of .it’s unique structure that is not seen by many other molecules. So in this particular part, the DNA is arranged in shape. That is known as a double helix. So the structure of DNA was discovered by two significant scientists known as Watson andCrick.
This was in 1953. So on the left, there was dr. Watson and on the right, there was dr. Crick. The structure of DNA was discovered in 1953. But actually, their discoveries were based on some of the research or the pioneering research. Another doctor did that. He is known as Rosalind Franklin. So it was the combined efforts of these people. That led to our current understanding of how DNA is structured. So the main discovery that Watson and Crick. They found DNA. That is that it’s arranged in a particular shape. It is known as a double helix. It’s like a ladder. That’s been twisted around this long axis. So it’s composed of two very long polynucleotide chains. They twisted up into a shape that is known as a double helix. The polynucleotide is if we have individual nucleotides as monomers.
When we join them all up and chain, we end up with a string of nucleotides. That’s known as a polynucleotide. What we have in this case is we have two of these. We have another one facing it or adjacent to the other pony nucleotide. If we imagine these two comings together, We form this overall kind of ladder. Their shape is looking like a stepladder. If we believe down the middle, we get the two ends of this ladder. They twist it about itself like it might twist an elastic band. End up with this double helix shape. So got one strand here. So one strand is known as a polynucleotide. That is labeled.
There this one’s going along this course down the molecule. Then the other strand kind of follows it halfway behind. So this would be strand number two. So the DNA is made up of two strands. They are polynucleotide chains twisted to make a double helix shape. So that’s what the overall way is it’s known as the double helix. So the actual double helix is made of and what forms this particular shape — the sides of the DNA double helix. The outer edges or the sides of the ladder, two parts make them of each of the nucleotides.
That’s the pentose sugar, which is represented by this Pentagon. The phosphate groups, so these form together with the sides of the ladder. That is twisting around each other. Those sides are known as the sugar-phosphate backbone. Poly nucleotide number is one. Then we have polynucleotide number two sort of going in the opposite kind of twist to it. That obviously, the chain is a polynucleotide. So it’s made of lots of individual nucleotides. We have one of those nucleotides. Then we have the next one. The next one is on. So forth and remember in the nucleotide. The Pentagon which represents the pentose sugar, That’s attached to this green circle.
A phosphodiester bond connects these nucleotides. This continued sating alternation of pentose sugars and phosphate groups. Then the next pentose sugar in an annex phosphate group. So overall, the actual strands that are making the outside of this ladder. That is called l the sugar-phosphate backbone because it’s an alternation of sugar units. Phosphate that they’re attached to and then the sugar in the phosphate of the next nucleotide. So forth making that backbone of this polynucleotide. But it is going into the inside of the DNA molecule, and they form different interactions. But the important thing is that the sides of the double helix. They are made from these two groups, which are known as the sugar-phosphate backbone. So that’s the outside of the DNA molecule at the inside that the DNA nucleotide has three parts. The organic bases face the inside of the ladder.
They’re known as the rungs of the DNA molecule. So the two strands of DNA the two nucleotide Hydrogen bond is more important. .So it’s a type of force that attracts the two strands together. These hydrogen bonds occur between the bases of each of the chains. And these are what we call the rungs. These hydrogen bonds between the bases represent the runs of the ladder. Our sugar-phosphate backbone is there. We have one organic base. So for this nucleotide, we’ve got this natural base here. We’ve got that natural base.
There the base of one polypeptide chain has to interact with the base of this nucleotide, which is at the opposite point. If they tie chain so these interactions, they are the hydrogen bonds between them. So this is the base of the chain. This one is the base of chain two or the other chain. They have to connect to make the rung of this DNA molecule. These are the rungs that you can imagine. If you were climbing a ladder, these lines would represent the Hodge bonds between them. That’s what makes the rungs of the ladder. We’ve got a chain of nucleotides.
A chain of nucleotides interacting with each other via the bases with the backbone being made of the phosphate and the sugars. They’re parallel because they run in the same direction. But actually, the strands of the DNA run in opposite directions. It goes into this in a bit more — Pentagon with the phosphate kind of coming up in that direction on this side. Then the base is forming. The inside of the DNA and They have to run in the opposite direction. The Pentagon’s have their phosphates facing. It’s almost like they have been turned 180 degrees around. One strand must run in one direction. The other strand runs in the opposite direction.
So that’s why we call it anti-parallel. Because they’re parallel, but they’re running in the opposite direction. We mean by complementary base pairing. This is an essential feature of DNA. Important in how DNA works in developing our body and the body of all organisms. So two polynucleotide strands are held together by hydrogen bonds between the two bases. Representing the rungs and we’ve got these hydrogen bonds that form between the bases in the middle. But the way that the bases interact with each other. It is essential. It’s very, very specific. It works the same way through all of life.
DNA has adenine, guanine, cytosine, and thymine, and they only pair up in specific ways. Adenine always pairs with thymine. So they would come together. This rung of ladder guanine still reacts with cytosine. They would come together as a ring. If you ever had to add facing cytosine or
guanine or any other mismatch. They would never match up. So when we look at DNA. We have the rungs between the two backbones. It’s always going to be either a pair to a T or a C pair to a G. It’s still going to be in this specific type.