Lesson Video: DNA Biology

In this video, we will learn to describe the structure of DNA.

15:27

Video Transcript

In this video, we will learn about the structure of DNA such as what it’s made of, patterns made by its parts, and about its shape. By the time we’re finished, you’ll be able to explain why DNA is a polymer, what sugar–phosphate backbones, nucleotides, and complementary bases are, and describe the orientation of the two strands that form the double helix.

First, let’s find out what DNA is made of. The letters D-N-A stand for deoxyribonucleic acid. That tells us at least two things. First, DNA is a nucleic acid, but we’ll come back to that when we talk about the patterns of DNA. Second, DNA contains deoxyribose, a five-carbon sugar shown here in black. The sugars run along the very long sides of a DNA molecule and are bonded between phosphate groups. Each chain of alternating deoxyribose sugars and phosphate groups is called a sugar–phosphate backbone.

These backbones are long. I mean really long. If you extract the DNA from just one human nucleus and lined the molecules up from end to end, they would be about two meters long. Connecting the sugar–phosphate backbones are nitrogenous bases of four different types that occur together in pairs: adenine or A, thymine or T, cytosine or C, and guanine or G. These bases bond to the deoxyribose sugars on the sugar–phosphate backbones. And they also bond to their complementary base, which we’ll talk about in the next section, which is called: What patterns do DNA molecules form?

Let’s take a closer look at DNA to examine patterns in its construction. And here’s a pattern we’ve already noted. The chains of alternating deoxyribose sugars and phosphate groups that run along the length of a DNA molecule form the sugar–phosphate backbones. You can see that these sugar–phosphate backbones head in opposite directions. If we think of the deoxyribose sugars as sugar houses with lollypop chimneys, the rooftops along one backbone point towards one end of the DNA molecule, while the rooftops of the other backbone head the other way. Since the sugar–phosphate backbones parallel each other as they head their opposite directions, we call this orientation antiparallel.

Now I’ve added the base adenine or A to our diagram. The base is bonded to one of the deoxyribose sugars, which, in turn, is bonded to a phosphate group. When a molecule of DNA is put together in your cells, its building blocks are not the individual sugars, phosphate groups, and bases. Instead, the building blocks that assemble to form the DNA are groups of one of each of these, bonded together. Each of these building blocks is called a nucleotide, and they are the fundamental repeating unit in each of the nucleic acids, DNA and RNA. Remember, the NA in DNA stands for a nucleic acid.

So how many nucleotides does it take to make a molecule of human DNA? DNA molecules vary in length, but the average number of nucleotides in a molecule of human DNA is about 270 million, over a quarter billion. Polymers are made of many repeating units. “Poly-” means many. “Mer” means unit. So polymers are made of many repeating units. And DNA is a polymer made of many nucleotides.

Now I’ve added another base to the diagram, guanine or G. It’s about the same length as A. Here are the bases cytosine or C and thymine or T. Their lengths are shorter than either A or G. But the width of a DNA molecule stays essentially constant, so the pairings of nucleotides across the DNA must include both a long base, either A or G, and a short base, either C or T, to fill or complete the gap to the other sugar–phosphate backbone. This pattern of pairing a long and short base is called complementary base pairing, since together they complete the distance between the sugar–phosphate backbones.

But there’s one more important detail: adenine pairs with thymine, and cytosine pairs with guanine. A simple way to remember this is that apples grow on trees, and cars go in the garage. These are called the complementary base pairing rules. So as far as the patterns in a DNA molecule, we’ve now discussed the alternating sugars and phosphates and antiparallel orientation in the two backbones, the repeating nucleotides of the DNA polymer, and complementary base pairing.

The final pattern we’ll talk about here is strands. DNA is made of two strands that meet where the complementary bases bond with hydrogen bonds. The function of DNA is to store genetic information, and the mechanism of the storage is order all the sequence of nucleotides in a particular section of one strand. The order of nucleotides in the strand outlined in pink here is AGCT. Or is it, if we start at the other end, the order is TCGA? Knowing which direction to read is critically important. So remember how the sugar–phosphate backbones are antiparallel? Well, we can use this information to indicate the direction of each strand.

Let’s take another look at the nucleotide we had marked earlier. The chimney end of the sugar house bonds to the phosphate group. But as nucleotides bond together to form DNA, they also bond to the ground level of the sugar house beneath the chimney. These positions on the deoxyribose are called the five prime and three prime positions. And we can mark each end of the strands with this notation. As a matter of fact, we can add the same notation to where I’ve written the base sequence from the strand outlined in pink so we know which direction to read it. The convention for writing nucleotide sequences, though, starts at the five prime position and goes to the three prime position.

Next, let’s take a look at the overall shape of DNA. So far, our diagrams have all looked a little bit like ladders with the sugar–phosphate backbones substituting for handrails and the complementary base pairs taking the place of steps. The actual shape of DNA, though, forms more of a twisted ladder as the complementary bases of the nucleotides bond together. This shape is called a double helix. Next up, let’s work some example problems.

The Given diagram shows a section of DNA. What structures are represented by labels one, two, and three? (A) Double helix, (B) a base, (C) nucleotides, (D) sugar–phosphate backbone, (E) hydrogen bonds.

Key knowledge required to select the correct option here is an understanding of DNA structure. So let’s review what DNA is made of, a couple patterns in its structure, and its overall shape. DNA is a polymer made of many similar repeating units called nucleotides. Each nucleotide is made out of a deoxyribose sugar, a phosphate group, and one of four nitrogenous bases. The deoxyribose sugars and phosphate groups make up the two long sugar–phosphate backbones of a DNA molecule, but the diagram in the question blends these together into these thick spiraling blue lines.

The bases in the diagram are represented by the four different colored short lines, which are held together by hydrogen bonds. And once it’s all put together, it forms a shape like a twisted ladder called the double helix. And now we’re ready to identify the structures that are represented by the labels one, two, and three. Solution option (A), double helix, can’t represent any of the three labeled structures because it, instead, describes the overall shape of the molecule. Solution option (B), a base, is represented in the diagram by the colorful short lines indicated by label two.

Solution option (C), nucleotides, are not indicated by the number label since it’s difficult to indicate a nucleotide without showing the sugars and phosphates in the sugar–phosphate backbone. Option (D), the sugar–phosphate backbone, are the thick spiraling blue lines and are indicated by label one. Finally, option (E), hydrogen bonds; the hydrogen bonds hold the two strands of the DNA molecule together and are indicated by label three. Therefore, the solution option for label one is sugar–phosphate backbone. The solution for label two is a base. And the solution for label three is hydrogen bonds.

A section of DNA contains the order of bases ATGCTTA. What would the complementary sequence of bases be? Option (A) ATCCAATT, option (B) TACCAATT, option (C) TACGAATT, option (D) TACGGATT, and option (E) TTCGAATT.

Key knowledge essential for selecting the correct solution option here is an understanding of DNA structure, specifically, complementary base pairing. Let’s start by reviewing some key terms in our question, such as DNA. DNA is a nucleic acid that stores genetic information. Another important term in this question is bases. DNA has four types of bases, and their sequence is the genetic code.

Before we discuss the term complementary, let’s take a look at the diagram of a strand of DNA that shows the bases from the diagram: A for adenine, T for thymine, G for guanine, and C for cytosine. These bases are attached to the sugar–phosphate backbones, which are made out of phosphate groups and deoxyribose sugars. So what does the term complementary mean? I’m sure you’ve heard of another word that sounds the same, but it means saying nice things like you’ve got really great hair. That’s being very complimentary. This complementary is different. It means things that tend to complete each other.

So when the question asks, what would the complementary sequence of bases be, they want you to identify the sequence of bases that complete or complement the sequence given in the first part of the question. So let’s take a look at that sequence of bases in our diagram. You’ve probably noticed that A and G are longer than the other bases C and T. A complementary pair of bases needs to include one long and one short base to fill or complete the gap between the sugar–phosphate backbones. But there’s another important detail. A bonds with T, and C bonds with G. These are known as the complementary base pairing rules.

And the answer to our question is right before our eyes in the top strand of our DNA diagram: TACGAAT.

Here are the key points on what we’ve learned about DNA. DNA stands for deoxyribonucleic acid. DNA is a polymer and a nucleic acid because it’s made of many repeating nucleotides. Nucleotides are made of a deoxyribose sugar, a phosphate group, and one of the four bases. The four bases are adenine, thymine, guanine, and cytosine.

The sugars and phosphate groups alternate in long chains along the DNA molecule and are called the sugar–phosphate backbones. These backbones are antiparallel, parallel while running in opposite directions. DNA has two strands attached by hydrogen bonds between base pairs. The base pairing rules are A bonds to T and C bonds to G. And finally, the overall shape of a DNA molecule is a double helix.

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