Discovering the Structure of DNA - Online Tutor, Practice Problems & Exam Prep

Rosalind Franklin's x-ray diffraction image, known as photo 51, was pivotal in revealing DNA's double helix structure. In 1953, James Watson and Francis Crick utilized this image to describe DNA as two antiparallel strands of nucleotides, with specific base pairing rules: adenine pairs with thymine, and cytosine pairs with guanine. Each nucleotide consists of a phosphate group, a five-carbon sugar, and a nitrogenous base. The strands are held together by hydrogen bonds, forming a sugar-phosphate backbone that supports the nitrogenous bases in the center.

Discovering the Structure of DNA

In this video, we're going to talk about some of the scientists that helped with discovering the structure of DNA. Way back in the early 1950s, a female scientist named Rosalind Franklin actually used a technique called X-ray diffraction on DNA. She captured an incredibly important photo that is well known as Photo 51. If we take a look at our image down below on the left-hand side, you can see an image of the scientist Rosalind Franklin and an image of Franklin's Photo 51, which shows an X-ray diffraction pattern of DNA. In this X-ray diffraction pattern, these bands create a kind of X-like formation. Through very complicated concepts and math, it turns out that this photo is actually evidence to show that DNA has a double helix structure.

However, it wasn't until 1953 that the scientists James Watson and Francis Crick were able to use Franklin's Photo 51, along with other information, to describe the structure of DNA as a double helix structure with two antiparallel strands of nucleotides. This is information that we had already covered in some of our previous lesson videos when we first introduced DNA. If you don't remember the information from those older videos on DNA, be sure to go back and check out those videos.

Now, Watson and Crick also came up with how these base pairing rules apply, and it's known as Watson and Crick base pairing. This basically describes how the nucleotides on opposite strands of DNA will pair with each other via hydrogen bonds, where all of the adenines (A's) would base pair with all of the thymines (T's) on opposite strands, and all of the cytosines (C's) would base pair and hydrogen bond with all of the guanines (G's) on opposite strands. So, A's base pair with T's and C's with G's. This is going to be really important information for you to keep in mind how the base pairing works.

If we take a look at our image down below on the right-hand side, we are showing you the images of James Watson and Francis Crick, who were able to use Rosalind Franklin's Photo 51, along with other information they knew, to help reveal the structure of DNA with two strands of nucleotides that are antiparallel with respect to each other. Recall from our previous lesson videos when we first introduced DNA that "antiparallel" is referring to the fact that one strand will go from 5 prime to 3 prime in one direction from left to right here, whereas the other strand would go from 5 prime to 3 prime in the opposite direction, and that's why they are called antiparallel. This image also shows how the Watson-Crick base pairing works, where all of the cytosines (C's) base pair with all the guanines (G's), and all of the adenines (A's) base pair with all the thymines (T's) on opposite strands. The blue backbone that you see here of the molecule represents a sugar phosphate backbone.

We'll get to talk a little bit more about the details of the DNA structure in our next lesson video. But for now, this here concludes our introduction to how the DNA structure was discovered, and we'll be able to get some practice as we move forward in our course. So I'll see you all in our next video.

The scientist/s that was/were given credit for first determining the structure of DNA is/are:

• A. Hershey and Chase.
• B. Watson and Crick.
• C. Chargaff.
• D. Griffith.
• E. Hershey and Crick.
• F. Watson and Chase.

The scientist/s that used x-ray diffraction to help reveal the structure of DNA is/are:

• A. Watson and Crick.
• B. Hershey and Chase.
• C. Avery and Macleod.
• D. Chargaff.
• E. Franklin.

Detailed DNA Structure

In this video, we're going to talk a little bit more about the detailed DNA structure. First, it's helpful to recall the information that we covered about DNA in our previous lesson videos where we first introduced DNA. If you don't know anything about DNA structure, then please be sure to go back and check out those older videos on DNA before continuing here. Now that being said, recall from those older videos that DNA actually consists of two strands of nucleotide monomers or these nucleotide building blocks that are repetitively linked together.

If you take a look at our image down below, notice that we're showing you three different representations of the DNA molecule. We've got one representation of the DNA molecule over here, another representation of DNA here in the middle, and a third representation of the DNA molecule over here on the right. Previously, in our lesson videos, it was shown that DNA forms a double helix where there are two strands, one strand there and another strand here, that are wrapped around each other and twisted upon each other to create a double helix, this twisting ladder-type formation. But if you were to take this DNA double helix and you were to untwist the DNA double helix so that it's a straight formation, it would look something like what you see here. Then if you were to zoom into this structure, you would get this image right here, a more detailed view of the DNA molecule. Again, what you would notice is that the DNA molecule consists of these nucleotides that are repetitively linked together. Here is one nucleotide; this is one nucleotide here; this is another nucleotide here; here is another nucleotide. These nucleotides are just repetitively linked together to create a DNA strand. We have two DNA strands here. We have one DNA strand right here, and then we have a second DNA strand over here.

Notice that these two DNA strands are both made up of nucleotides, and they are connected to each other via hydrogen bonds that form between the nitrogenous bases. Here, we can label these dotted lines as hydrogen bonds that form between the two strands and that connect and keep the two strands held together.

Now, recall from our previous lesson videos that a single nucleotide consists of three components: a phosphate group, a five-carbon sugar, and a nitrogenous base, either adenine, guanine, thymine, or cytosine, abbreviated as A, G, T, or C. Also, again, recall that these two DNA strands are antiparallel with respect to each other, which means that they go in opposite directions in terms of their 5' and 3' ends. So, you can see that this strand over here on the left is going from 5' to 3', top to bottom. However, this other strand over here on the right is going from 5' to 3' in the opposite direction, from bottom to top. The DNA strands are going to be antiparallel. When you compare the 5' end to the 3' end, what you'll notice is that at the 5' end of each strand is a free phosphate group, and at the 3' end of each strand is a free hydroxyl group.

Here, comparing the same sides of the two strands, you'll notice that they are chemically different: one has a free phosphate group and one has a free hydroxyl group. That's why it's important to keep in mind the directionality of these DNA strands in terms of their 5' and 3' ends, and that will be very important as we move forward in our course and talk about DNA replication.

Notice here that the nitrogenous bases are towards the middle of the DNA molecule, and on the perimeter of the molecule is the sugar-phosphate backbone. They call it the sugar-phosphate backbone because it is a repetitive sequence of sugar-phosphate groups. DNA molecules have a sugar-phosphate backbone, and that's why we represent that sugar-phosphate backbone here, using blue lines. This concludes our brief introduction to some of the detailed DNA structure, and we'll be able to get some practice applying these concepts that we've learned as we move forward in our course. So, I'll see you all in our next video.

In the polymerization of DNA, a phosphodiester bond is formed between a phosphate group of the nucleotide being added and which of the following atoms or molecules of the last nucleotide in the DNA strand?

• A. The 5' phosphate group.
• B. C6.
• C. The 3' OH.
• D. A nitrogen from the nitrogen-containing base.

Within a double-stranded DNA molecule, adenine (A) forms hydrogen bonds with thymine (T), and cytosine (C) forms hydrogen bonds with guanine (G). What is the significance of the structural arrangement?

• A. It allows variable width of the DNA double helix.
• B. It permits complementary base pairing.
• C. It determines the tertiary structure of the DNA molecule.
• D. It determines the type of protein produced from the DNA.

Here’s what students ask on this topic:

Who was Rosalind Franklin and what was her contribution to the discovery of DNA structure?

Rosalind Franklin was a pioneering female scientist in the early 1950s who used x-ray diffraction techniques to study DNA. Her most significant contribution was capturing an image known as photo 51, which revealed the x-ray diffraction pattern of DNA. This image provided crucial evidence that DNA has a double helix structure. Although Franklin's work was pivotal, it was James Watson and Francis Crick who later used her photo 51, along with other data, to describe the DNA structure in detail in 1953. Franklin's work laid the foundation for understanding the molecular structure of DNA.

What is the significance of photo 51 in the discovery of DNA's structure?

Photo 51 is an x-ray diffraction image of DNA taken by Rosalind Franklin. This image was crucial because it provided clear evidence of DNA's double helix structure. The x-shaped pattern in the diffraction image indicated that DNA strands are helical. James Watson and Francis Crick used this image, along with other data, to build their model of DNA in 1953. The insights gained from photo 51 were instrumental in confirming that DNA consists of two antiparallel strands forming a double helix, which was a groundbreaking discovery in molecular biology.

What are the base pairing rules in DNA and who discovered them?

The base pairing rules in DNA, discovered by James Watson and Francis Crick, describe how nucleotides on opposite strands pair with each other via hydrogen bonds. According to these rules, adenine (A) pairs with thymine (T), and cytosine (C) pairs with guanine (G). These specific pairings are due to the hydrogen bonding capabilities of the nitrogenous bases: A forms two hydrogen bonds with T, and C forms three hydrogen bonds with G. These base pairing rules are essential for the accurate replication and transcription of genetic information.

What is the structure of a DNA nucleotide?

A DNA nucleotide consists of three components: a phosphate group, a five-carbon sugar (deoxyribose), and a nitrogenous base. The nitrogenous base can be adenine (A), guanine (G), thymine (T), or cytosine (C). These nucleotides are repetitively linked together to form a DNA strand. The phosphate group of one nucleotide bonds with the sugar of the next, creating a sugar-phosphate backbone. The nitrogenous bases extend from this backbone and pair with complementary bases on the opposite DNA strand, forming the double helix structure.

What does it mean that DNA strands are antiparallel?

DNA strands are described as antiparallel because they run in opposite directions. One strand runs from the 5' (five prime) end to the 3' (three prime) end, while the other strand runs from the 3' end to the 5' end. This orientation is crucial for the structure and function of DNA, particularly during replication and transcription. The 5' end of a DNA strand has a free phosphate group, while the 3' end has a free hydroxyl group. This antiparallel arrangement allows the nitrogenous bases to pair correctly and form the double helix structure.