Lesson Video: DNA | Nagwa Lesson Video: DNA | Nagwa

Lesson Video: DNA Biology

In this video, we will learn how to describe the structure of DNA and explain how DNA can be extracted from organic material.


Video Transcript

In this video, we will learn about DNA. We’ll discuss its structure and learn about nucleotides, the sugar–phosphate backbone, and the complementary bases that hold the two strands of DNA together. We’ll also learn about the orientation of these strands. And finally, we’ll go over how we can extract DNA from organic material, such as fruit.

DNA is an incredibly important molecule. It’s inside nearly every cell of our body and contains the information that makes us who we are. In fact, if you were to take all the DNA out of a single cell and stretch it out, it would be about two meters long. That’s probably a bit taller than you are, but maybe not so much. This incredible molecule contains the information that gives us all of our characteristics. For example, it controls our eye color, whether or not we have a lot of hair or just a little, and can make us tall or short. All of these characteristics are coded by DNA. And in this lesson, we’ll learn more about DNA structure, what it looks like, and what it’s made up of.

To visualize what a molecule of DNA looks like, first picture a ladder. It has two parallel side rails and rungs or steps at regular intervals. Now, imagine twisting the ladder. This is the shape of a DNA molecule. There are two strands of DNA twisted around one another. This shape is called a double helix. Now let’s zoom in and take a closer look at DNA’s chemical structure. So here we can see the two strands of DNA. This strand on the left is indicated here, while this strand on the right is indicated here. Each strand of DNA is a polymer that’s made up of many repeating subunits called nucleotides. You can see one of these nucleotides circled here.

In this diagram, there’s a total of six nucleotides, and each nucleotide is made up of three distinct parts. The first part is a phosphate group. The second part is a pentose sugar, which is a sugar molecule that contains five atoms of carbon. In DNA, this pentose sugar is called deoxyribose. In fact, this is why we call it DNA, or deoxyribonucleic acid. The “deoxyribo” part refers to the deoxyribose sugar in DNA, while nucleic acid refers to how DNA is a nucleic acid, which is a polymer of nucleotides.

The third part of a nucleotide is the nitrogenous base. There’s actually four different nitrogenous bases in DNA: guanine, or G for short, represented here in orange; cytosine, represented in blue; adenine, represented in green; and thymine, represented in pink. You can see these different nitrogenous bases represented in both diagrams. These nucleotides can differ from one another depending on what kind of nitrogenous base they have. So that’s the basic structure of a nucleotide.

You might be wondering how nucleotides combine with one another. They are joined by the phosphate group and the two adjacent carbons in the deoxyribose sugar. This is called a phosphodiester bond. These repeating phosphodiester bonds attach one nucleotide to the next and form the backbone of DNA. This backbone is called the sugar–phosphate backbone, and it’s represented in black in this diagram and also black in this diagram. So there’s two sugar–phosphate backbones in DNA because there’s two strands of DNA in the double helix. So how does one strand of DNA combine with the other strand to form this double helix structure?

This has to do with these nitrogenous bases and how they bond to one another. Every nitrogenous base on one strand of DNA bonds to a nitrogenous base on the opposite strand. And this is how these rungs in this twisted ladder shape are formed. When these nitrogenous bases bond, they do so in a special way. In DNA, adenine can only bond to thymine and cytosine can only bond to guanine. These are called the rules of complementary base pairing. This is accomplished through forming hydrogen bonds. And between cytosine and guanine, three hydrogen bonds can form, while in adenine and thymine, two hydrogen bonds form. These hydrogen bonds aren’t very strong on their own. But collectively, over a large molecule of DNA, this can be very strong and is what holds the two strands together.

Now let’s talk about directionality of DNA. You probably noticed that the deoxyribose sugar of one strand seems to be pointing in the opposite direction of the other. That’s because these two strands are going in opposite directions. The way we talk about direction in DNA has to do with how the carbons in the deoxyribose are numbered. This carbon here is called one prime. This is two prime, three prime, four prime, and five prime. This atom here is actually an oxygen atom and not our carbon.

When we talk about direction in DNA, we’re interested in the five prime and three prime positions. This is because this is where the phosphodiester bond is formed. And when DNA is synthesized and new phosphodiester bonds are formed, new nucleotides are always added in the five prime to three prime direction. So this strand is pointing in this direction because that’s the direction that this strand would grow during DNA synthesis. This is just a convention that we use in biology to describe DNA’s direction. So, when we read a sequence of DNA, we read it in the five prime to three prime direction.

Let’s label this strand as the five prime to three prime strand because it’s pointing in that direction. And if we look at the diagram on the left and follow this strand, we can see this strand labeled here. Now let’s label the carbons on the opposing strand. Five prime to three prime in this strand is pointing in the opposite direction of the first strand. So let’s label this strand as the three prime to five prime strand because that’s the direction it’s pointing in. And we can see this in the diagram on the left here. Because these two strands are going in opposite directions, this means that these strands are antiparallel, meaning they run parallel to each other but in opposite directions.

Now let’s try reading the sequence of these nitrogenous bases on these strands. So here we can see that the sequence is CAC. And let’s do it on the diagram on the left also. So we have CAC. And then the strand twists around to TGC. Now let’s write this sequence up here. We’ll also indicate that it’s in the five prime to three prime direction. Now, what do we do when we want to figure out the sequence of the opposing strand?

Remember that these nitrogenous bases can form complementary base pairs that follow certain rules. So, whenever there’s a C in the sequence, we’ll know that it base-pairs with G. So, if you go up here to this sequence, we can fill it in, and the opposite is true too. So, if there’s a G, we know that it will pair with C. And if we have an A, we know that it will pair with T, so we can fill that in here, and T will pair with A. We can also double-check our work right here. So we have GTGACG, which matches our sequence.

Sometimes when we talk about the sequence of nitrogenous bases, we call them base pairs because they pair together. So, in this sequence, we have one, two, three, four, five, six base pairs. In humans, most cells have over six billion base pairs worth of DNA in a single cell. That’s why it’s two meters in length when stretched out because there’s so much of it.

Many DNA sequences provide the instructions for making proteins, which give us our unique characteristics. We call these DNA sequences genes. If we represent a segment of our double-stranded DNA as a line like this, we can see genes throughout our DNA indicated here in blue. Some have a very long DNA sequence, and some are very short. These different genes can code for all of our different characteristics, like our eye color, our hair color, or for how tall we might be. We have over 20,000 genes in our DNA, and together they make us who we are. We have learned so much about the structure of DNA, but how can we extract it so we can see it with our own eyes?

We can do this with different organic materials, like fruits or vegetables. In this example, we’ll be using strawberries. We’ll need a few more supplies: dishwashing liquid, salt. We’ll need some beakers, but you can use a cup. We’ll need some filter paper, but you can use a coffee filter, and ethanol, or you can use rubbing alcohol. So, first, you take a couple of strawberries and smash them up really good in a beaker or a cup. Then, take another beaker and add 100 milliliters of water, then 10 milliliters of dishwashing liquid, and half a teaspoon of salt. Now, mix it all up, and this is your extraction buffer. Now, add the extraction buffer to your smashed strawberries. Then, stir it all up for one minute and, if you can, leave it in a warm water bath for 10 minutes. The mixture might turn a bit pink from the strawberries. The dishwashing liquid and salt help to break down the cells of the strawberries so our DNA is now dissolved in this solution.

Next, let’s filter this solution to get rid of the strawberry bits and seeds. Now that we have our strawberry DNA solution, we need to precipitate the DNA out from this solution. You can do this right here in this beaker, or you can do it in a test tube if you have one. So you can add 10 milliliters of this DNA solution to a test tube. Then, carefully add an equal volume of cold ethanol so it forms a layer on top. Then, wait about five minutes, and a stringy substance should form in this top layer. This is the DNA.

Now that we’ve covered DNA structure and how to extract it, let’s try out a practice question.

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

DNA is a nucleic acid that stores the genetic information needed for life. It’s responsible for all of our different characteristics, such as the color of our eyes or how tall we are. It’s made up of two strands of DNA that are twisted around each other to make a double-helix shape as shown here.

Let’s untwist this DNA helix shape so we can talk more about its components. The black line is the backbone of these DNA strands and is called the sugar–phosphate backbone. It’s made up of phosphate groups and deoxyribose sugars. And in between these two strands are these different-colored boxes. These are called nitrogenous bases, or just bases for short. There are four different types of nitrogenous bases: guanine, or G for short, represented in orange; cytosine, represented in blue; adenine, represented in green; and thymine, represented in pink. These individual bases often come in pairs as you may have noticed.

You can see all the guanines indicated here, which are always paired with a cytosine, indicated here, whereas adenine always pairs with thymine. This isn’t a coincidence. These specific bases pair with one another because they have an affinity for each other. This is due to hydrogen bonding between these bases. We call two bases that pair together complementary. And DNA bases can pair according to certain rules, where G always pairs with C and A always pairs with T. So, when we have a sequence of these bases, like the one that’s given in the question, all we have to do is match these up. So guanine will pair with cytosine, and cytosine will pair with guanine. Adenine will pair with thymine, and thymine will pair with adenine, which now gives us our complementary sequence of bases. Therefore, the correct answer is TACGAATT.

Now let’s go over the key points that we covered in this video. DNA is a polymer made up of repeating monomer subunits called nucleotides. In DNA, a nucleotide is made up of a deoxyribose sugar, a phosphate group, and a nitrogenous base. DNA is made up of two strands of DNA that form a double helix with a sugar–phosphate backbone. This shape is maintained by complementary base pairs, where A binds to T and G binds to C. DNA is read in the five prime to three prime direction. You can extract DNA from organic material, such as fruits and vegetables.

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