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

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 two antiparallel strands of nucleotides. When we say antiparallel, we mean that the two strands run parallel to each other but with opposite directions. 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. The phosphate itself isn't directly attached to the pentose sugar ring. It's attached to the 5 prime carbon, and that 5 prime 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 prime end down to the 3 prime end, but we said it runs antiparallel to the other one. So, for the bases to be complementary to each other, this 3 prime end would sync up with the 5 prime end of this other primary structure. This 5 prime end would link up with the 3 prime end of this primary structure. We have in the middle the nitrogenous bases forming hydrogen bonds with each other. 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 prime end and this is the 3 prime end. Anti parallel means the other strand runs parallel to it but in the opposite direction. So this 3 prime end will link up with this 5 prime end. This 5 prime end would be next to this 3 prime 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 3a, 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. Remember, these make 3 hydrogen bonds. A and t only make 2. And then these make 3 again. 3 100 demands. 2, 3332, 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 antiparallel when it comes to our 2 DNA strands and how the bases, they 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.