DNA Double Helix - Online Tutor, Practice Problems & Exam Prep

The DNA double helix, discovered by Franklin, Watson, and Crick, consists of two antiparallel strands of nucleotides forming a twisted ladder structure. Each nucleotide comprises a phosphate group, a pentose sugar, and a nitrogenous base. The double helix has a width of 20 angstroms and typically contains 10 base pairs per turn, with a total length of 34 angstroms. The major groove (22 angstroms) and minor groove (12 angstroms) allow protein binding, crucial for regulating RNA transcription and DNA replication.

DNA Double Helix Concept 1

We can say that the researchers Franklin, Watson, and Crick are credited with helping describe the structure of DNA. Here, we describe DNA as being a double helix with 2 antiparallel strands of nucleotides. Now, when we say antiparallel, we mean that the 2 strands run parallel to each other but with opposite directions. And we're going to say that the double helix represents a twisted ladder where our phosphate sugar backbone is on the sides and the bases are on the inside. If we take a look here at this image, we have here in yellow, highlighted in yellow, a phosphate group, a pentose ring, and a nitrogenous base. Remember, these three components represent a nucleotide. Now we know that these three components are the main things that contribute to the structure of a nucleotide. Remember, the phosphate itself isn't directly attached to the pentose sugar ring. It's attached to the 5' carbon, and that 5' carbon is connected to carbon number 4 of the pentose ring. Here we're going to say that stringing a bunch of nucleotides together through phosphodiester bonds represents the primary structure of our nucleotides. So, we have a primary structure here.

We can say here that it runs from the 5' end down to the 3' end, but we said it runs antiparallel to the other one. So for the bases to be complementary to each other, this 3' end would sync up with the 5' end of this other primary structure. This 5' end would link up with the 3' end of this primary structure. We have in the middle the nitrogenous bases forming hydrogen bonds with each other, or H bonds. So here we have our primary structures running antiparallel to each other, connected through hydrogen bonds. We say here that our sugar phosphate backbones are these blue parts, and, again, our bases are on the interior. This is how we're able to transition from our antiparallel DNA strands to our DNA ladder. Well, remember, this DNA ladder really represents a twisted ladder. So the way we do this is we would rotate the top part to the left, and then this bottom part to the right. Twisting it that way, doing that gives us our DNA double helix. So we can see that it resembles the ladder that we had but it's twisted. Alright. So this is the structure that Franklin, Watson, and Crick helped us to discover.

Right? So just remember, when we're talking about DNA, it's basically adding together everything we've learned thus far. What is a nucleotide? What are phosphodiester linkages? What is a primary structure? How do our bases pair with one another in a complementary fashion by utilizing this antiparallel designation DNA has. Remember, this helps to create the ladder which really is a twisted ladder to give us our DNA double helix. So just remember these key facts when it comes to the DNA double helix.

DNA Double Helix Example 1

Here in this example, it says, predict the sequence of bases in the DNA structure that is complementary to the DNA structure or strand shown below. Remember, we're running anti-parallel. This top strand, this is the 5' end and this is the 3' end. Anti-parallel means the other strand runs parallel to it but in the opposite direction. So this 3' end will link up with this 5' end. This 5' end would be next to this 3' end. Now, we just have to remember our base pairings. Remember, this is DNA so A links up with T, and G links up with C. So here we have C, so this would have to be G. This is G, so this would have to be C. We have 3 A, so this would be T, T, and T. Here we have a C, so this would be a G again. T here would be A. C and C would be G and G. T would be with A, and A would be with T. Remember, we have hydrogen bonds in between them. G and C make 3 hydrogen bonds. A and T only make 2. And then these make 3 again. 3 hydrogen bonds. 2, 3, 3, 3, 2, and 2. So this is how we'd show the sequence of bases. It didn't ask for us to do hydrogen bonds. I decided to add those. But just remember, this is what we mean when we're talking about anti-parallel when it comes to our two DNA strands and how the bases form complementary base pairings with one another. A bonds with T, G bonds with C.

Which of the following statements is false?

• A. The nitrogenous bases extend inward within the double helix. 
• B. The two polynucleotide strands in the DNA double helix are identical to one another. 
• C. Hydrogen bonding occurs between the two polynucleotide strands within DNA.
• D. The complementary strand of DNA segment of 5’ GCATTCAT 3’ would be 3’ CGTAAGTA 5’. 
• E. Phosphate groups and sugars form the backbone of the DNA double helix.

DNA Double Helix Concept 2

Hey, everyone. So in this video, we take a more detailed look at the DNA double helix. Now here we're going to say the DNA double helix has a width of 20 Ångströms. Remember, an Ångström is a capital A with that little circle over it. And we're going to say typically, we're going to have 10 base pairs per turn, and within a full turn, a length of approximately 34 Ångströms. So, if we took a look here, we would say that the width of this DNA double helix is 20 Ångströms. So that's from this distance to this distance. We said that the full length of a turn, which is composed of 10 base pairs – 1, 2, 3, 4, 5, 6, 7, 8, 9, 10 – that's a full turn. Its length is 34 Ångströms.

Now, this full turn is made up of two types of grooves. We're going to say the helical shape of DNA creates two grooves that are present on the exterior of its two strands. We have first our major groove. This is the wider and steeper groove of the DNA double helix, and then we have our minor groove. This is going to be the less wide and more shallow groove. If we take a look here, look how deep this groove is. That is our major groove. And here, this one is not as deep, so this is our minor groove.

We're going to say, taking a look at both the major and minor grooves within a given turn adds up to this 34 Ångströms. But how much of it is composed of major groove and how much of it is composed of minor groove? Well, the major groove is about 22 Ångströms in terms of length, and then the remainder, which is about 12 Ångströms, belongs to the minor groove.

Now here we're going to say proteins can bind to these grooves in order to regulate RNA transcription and DNA replication. So when we're talking about proteins binding to DNA, if we're talking about medications that deal with affecting our DNA, they're typically binding to these grooves, the major and minor grooves, and affecting them, and then eliciting a different type of response within our genetic makeup. So just remember that when we're talking about a more detailed look at DNA, we have to take into consideration the width of the DNA double helix. We have to take a look at its length. And remember, in a given turn, which is made up of, on average, 10 base pairs, we're going to have an overall length composed of this major and minor groove.

DNA Double Helix Example 2

Everyone, in this example question, it says, "Calculate the approximate length of a DNA segment that is composed of 171 base pairs." Alright. So we're starting out with our given amount of 171. Here, we're just going to write bp for base pairs. And remember that the connection is that for every one turn, we have 10 base pairs. Here, base pairs will cancel out. And next, we're going to say that one turn is approximately 34 angstroms in terms of length. Here, our turns would cancel out. So, at the end, we're going to have our length. So this is going to come out to 171÷10×34, which comes out to approximately 581.4 angstroms. Alright. So here, if we look at our options, the one that's closest to that answer is option C. It'll be approximately 581 angstroms in length when we have 171 base pairs. Alright. So this is the approach we would take in order to solve this type of question.

Clinical trials include the use of DNA-binding drugs that facilitate the breaking of double-stranded DNA in their function. A hypothetical drug used in the treatment of leukemia requires interaction with the grooves of DNA. If the minor groove is A–T rich and the major groove is G–C rich, which of the following statements can be true?

• A. The drug would more readily bind to the minor groove of DNA. 
• B. The drug would more readily bind to the major groove of DNA. 
• C. The drug would equally bind to the minor and major grooves of DNA. 
• D. Not enough information is given. 

One turn of a DNA sequence is comprised of 30% guanine (G). Calculate the approximate number of hydrogens bonds likely to be found within this DNA sequence.  

• A. 15
• B. 26
• C. 38
• D. 24

One turn of a DNA sequence is comprised of 15% adenine (A). If the bonding energies of A–T and G–C are 24 kJ/mol and 18 kJ/mol respectively, calculate the approximate total bonding energy in this one turn. 

• A. 110 kJ/mol
• B. 432 kJ/mol
• C. 222 kJ/mol
• D. 198 kJ/mol

Here’s what students ask on this topic:

What is the structure of the DNA double helix?

The DNA double helix consists of two antiparallel strands of nucleotides forming a twisted ladder structure. Each nucleotide is composed of a phosphate group, a pentose sugar, and a nitrogenous base. The strands run in opposite directions, with the 5' end of one strand aligning with the 3' end of the other. The sugar-phosphate backbones form the sides of the ladder, while the nitrogenous bases pair in the center through hydrogen bonds. This structure was discovered by Franklin, Watson, and Crick.

What are the dimensions of the DNA double helix?

The DNA double helix has a width of 20 angstroms. Each full turn of the helix, which includes 10 base pairs, measures approximately 34 angstroms in length. The helical structure creates two grooves: the major groove, which is 22 angstroms wide, and the minor groove, which is 12 angstroms wide. These grooves are essential for protein binding, which regulates RNA transcription and DNA replication.

How do the major and minor grooves of the DNA double helix function?

The major and minor grooves of the DNA double helix are critical for protein binding. The major groove is wider and deeper, measuring about 22 angstroms, while the minor groove is narrower and shallower, measuring about 12 angstroms. Proteins that regulate RNA transcription and DNA replication often bind to these grooves, influencing genetic processes. The specific shape and size of these grooves allow for selective binding of regulatory proteins.

What is the significance of antiparallel strands in the DNA double helix?

The antiparallel nature of the DNA double helix means that the two strands run in opposite directions, with one strand oriented 5' to 3' and the other 3' to 5'. This arrangement is crucial for the complementary base pairing, where adenine pairs with thymine and cytosine pairs with guanine through hydrogen bonds. The antiparallel orientation also facilitates the replication process, as enzymes can read and synthesize the DNA strands in the correct direction.

Who discovered the structure of the DNA double helix?

The structure of the DNA double helix was discovered by Rosalind Franklin, James Watson, and Francis Crick. Franklin's X-ray diffraction images provided critical insights into the helical structure, while Watson and Crick built the first accurate model of DNA, describing it as a double helix with antiparallel strands and complementary base pairing.