Learn about Watson and Crick's model of DNA. Discover who Watson and Crick are, study their experiment on DNA, the base pairing of DNA, and the DNA model structure.
Updated: 02/03/2022
Watson & Crick Model of DNA: History, Basis, & Experiment
Who Are Watson and Crick?
James Watson and Francis Crick were scientists during the middle part of the twentieth century. The work of Watson and Crick extends back several decades prior to their actual scientific discoveries as they utilized information and research of several other scientists to ultimately complete their own experiments. In the late 19th century, a German biochemist determined that nucleic acids were composed of long chains of sugar, phosphoric acid and nitrogenous bases. American Oswald Avery proposed that DNA was the molecule that carried genetic information. Watson and Crick became interested in the shape and form of DNA molecules after reading about the research of Linus Pauling. Pauling determined that proteins took on an alpha helix shape. DNA, or deoxyribonucleic acid, is the genetic material of chromosomes. After attending a lecture on DNA and considering Pauling's theories, Watson and Crick attempted to propose the idea that DNA was a double stranded molecule in which the two strands ran in opposite directions. After more than one failed model, they ultimately surmised a three dimensional model proposal for the shape of DNA molecules that has become the accepted structure of DNA.
Watson and Crick Experiment
Science is often collaborative, whether intentional or not. This collaborative nature is demonstrated through Watson and Crick's experiments, which were based on the ideas and research of several other scientists including:
- A German biochemist who proposed that the composition of DNA contained sugar, phosphoric acid, and bases containing nitrogen.
- Linus Pauling's idea that proteins took the form of an alpha helix, causing Watson and Crick to consider that DNA might also take on a specific three-dimensional shape. This idea caught the attention of James Watson particularly after he heard Rosalind Franklin speak about DNA.
- Erwin Chargaff, who studied the composition of DNA molecules and discovered that no matter how much of the nucleic acid adenine he found in a sample, there would be the same amount of thymine. The same held true for cytosine and guanine. The pattern never varied with any of the DNA he analyzed. This became known as Chargaff's Rule.
- Rosalind Franklin, a scientist who worked with X-Rays. She used X-Rays to examine the shape and structure of DNA under different conditions of humidity. What she ultimately determined was that DNA had a unique shape that laid the foundation of Watson and Crick's model. If not for another researcher showing the work of Rosalind Franklin to Watson and Crick without her knowledge, they may or may not have been able to correctly create the three dimensional model of DNA.
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History of DNA Models
Up until James Watson and Francis Crick's experiments, we had very little understanding of exactly what made living things look and appear as they do. Other models had been proposed, but Watson and Crick believed that these models were inferior for numerous reasons that we'll not get into here. Their own model became the basis for our concept of DNA and helped unlock the genetic code of all living things.
In 1953, Francis Crick and James Watson developed what would be henceforth known as the Watson and Crick Model of DNA, which supposes that DNA exists in a double-helical twisted ladder structure of phosphate chains with matching nucleotides. DNA, as we well know, stands for deoxyribonucleic acid and is the backbone or blueprints for all life as we understand it.
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Watson and Crick Base Pairing
DNA is a long chain polymer composed of many monomers. Monomers are singular units of the long chain known as a polymer. In DNA the monomers are called nucleic acids. A complete, individual molecule of genetic material contains a phosphate group, a five carbon sugar (deoxyribose) and a nitrogenous base. The four nitrogenous bases found in DNA include guanine, cytosine, adenine and thymine, or G, C, A and T. In a single strand segment of DNA, each monomer has a phosphate molecule that acts as its backbone, followed by the five-carbon deoxyribose. Attached the deoxyribose is the nucleic acid; adenine, thymine, cytosine or guanine. To make DNA double-stranded and form the double helix, a complementary strand is attached to the original strand via a bond called a hydrogen bond. The second strand is considered complementary because it runs in the opposite direction, as proposed by Watson and Crick. The nucleic acids that are hydrogen bonded follow Chargaff's Rule of adenine always being attached to thymine and cytosine bonding to guanine. Unless an error occurred in the replication of the DNA during the cell cycle, there is no exception to this rule. This matching of nitrogenous bases is called Watson and Crick base pairing.
Watson and Crick Model of DNA
The Watson and Crick Model of DNA shows a double stranded helix, that resembles a twisted ladder. From this model, it was discovered that:
- DNA is double stranded
- DNA takes on the shape of a double helix, or twisted ladder
- The two strands are complementary to each other due to Chargaff's Rule
- The nitrogenous bases following Chargaff's Rule always pair up as follows: Adenine (A)- Thymine (T) and Cytosine (C)- Guanine (G)
- The two complementary strands held together where the nitrogenous bases connect via hydrogen bonding
- Each strand has a sturdy backbone of phosphate molecules
The Watson and Crick DNA structure is the currently accepted model of DNA.
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Lesson Summary
James Watson and Francis Crick are the men credited with discovering the currently accepted model of the structure of DNA, or deoxyribonucleic acid. They based their model on the research and findings of several other scientists, including Linus Pauling, Erwin Chargaff and Rosalind Franklin. Although the work of Rosalind Franklin has been historically overlooked, it was essential to the development of Watson and Crick's model of a double helix that resembles a twisted ladder.
Erwin Chargaff's research determined that the four nucleic acids appeared in samples of DNA so that adenine and thymine had the same percentage and cytosine and guanine were of equal percentages. This became known as Chargaff's Rule. Due to Chargaff's Rule, Watson and Crick could surmise that the DNA strands ran in opposite directions. The two opposite or complementary strands were attached with hydrogen bonds between complementary base pairs of adenine with thymine and cytosine with guanine. Watson and Crick also determined that the two strands of DNA had backbones of phosphate groups that kept the nucleic acids evenly spaced at the correct angles, thus completing the Watson and Crick Model of DNA.
Basis for the Model
Prior to Watson and Crick, it was understood that there was a blueprint for all life and that this blueprint consisted of nucleotides and phosphate strands. Throughout the late 1800s and into the early 1900s advances had been made in finding this blueprint, but nothing as concrete as what was to come. Watson and Crick were able to determine that the four nucleotides contained in DNA (cytosine, guanine, thymine, and adenine) would only pair up in certain ways: adenine to thymine, guanine to cytosine. They had borrowed this idea from Erwin Chargaff, and they also ran with a helical theory proposed by Rosalind Franklin, whose photographs of DNA gave them a basis from which to start. Unfortunately, Franklin was not asked permission to see the photos and was also not credited for her contribution in the discovery.
DNA's Structure
The model of DNA that they proposed look like none before, consisting of a double-helix structure very similar to an extension ladder that had been twisted into a curving sort of look. It was discovered that there were two phosphate chains, each consisting of nucleotides that would match up in an arrangement, giving us the double helix twisted ladder look. It had already been suggested earlier that DNA consisted of strands of nucleotides, but it was not understood how they were arranged. What Watson and Crick did was develop this idea that during cell division the strands were separated but would only align with the exact opposite strand containing the opposite nucleotides.
Watson and Crick were able to show that:
- DNA consisted of two strands of phosphates to which nucleotides were attached
- The nucleotides kept similar distances and angles from the phosphates to which they were attached
- One side of the strand went up while the other attached going down so that each nucleotide attached to another opposite of it.
Their model for DNA was generally accepted as the best model at the time, and ever since. However, they did not develop the entire concept on their own. They owed much to the scientists that came before them.
Lesson Summary
In 1953, scientists Francis Crick and James Watson developed what we call the Watson and Crick model of DNA, which supposes that DNA exists in a double-helical twisted ladder structure of phosphate chains with matching nucleotides. Their model has become how we view DNA, the blueprint of all life, ever since.
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Video Transcript
History of DNA Models
Up until James Watson and Francis Crick's experiments, we had very little understanding of exactly what made living things look and appear as they do. Other models had been proposed, but Watson and Crick believed that these models were inferior for numerous reasons that we'll not get into here. Their own model became the basis for our concept of DNA and helped unlock the genetic code of all living things.
In 1953, Francis Crick and James Watson developed what would be henceforth known as the Watson and Crick Model of DNA, which supposes that DNA exists in a double-helical twisted ladder structure of phosphate chains with matching nucleotides. DNA, as we well know, stands for deoxyribonucleic acid and is the backbone or blueprints for all life as we understand it.
Basis for the Model
Prior to Watson and Crick, it was understood that there was a blueprint for all life and that this blueprint consisted of nucleotides and phosphate strands. Throughout the late 1800s and into the early 1900s advances had been made in finding this blueprint, but nothing as concrete as what was to come. Watson and Crick were able to determine that the four nucleotides contained in DNA (cytosine, guanine, thymine, and adenine) would only pair up in certain ways: adenine to thymine, guanine to cytosine. They had borrowed this idea from Erwin Chargaff, and they also ran with a helical theory proposed by Rosalind Franklin, whose photographs of DNA gave them a basis from which to start. Unfortunately, Franklin was not asked permission to see the photos and was also not credited for her contribution in the discovery.
DNA's Structure
The model of DNA that they proposed look like none before, consisting of a double-helix structure very similar to an extension ladder that had been twisted into a curving sort of look. It was discovered that there were two phosphate chains, each consisting of nucleotides that would match up in an arrangement, giving us the double helix twisted ladder look. It had already been suggested earlier that DNA consisted of strands of nucleotides, but it was not understood how they were arranged. What Watson and Crick did was develop this idea that during cell division the strands were separated but would only align with the exact opposite strand containing the opposite nucleotides.
Watson and Crick were able to show that:
- DNA consisted of two strands of phosphates to which nucleotides were attached
- The nucleotides kept similar distances and angles from the phosphates to which they were attached
- One side of the strand went up while the other attached going down so that each nucleotide attached to another opposite of it.
Their model for DNA was generally accepted as the best model at the time, and ever since. However, they did not develop the entire concept on their own. They owed much to the scientists that came before them.
Lesson Summary
In 1953, scientists Francis Crick and James Watson developed what we call the Watson and Crick model of DNA, which supposes that DNA exists in a double-helical twisted ladder structure of phosphate chains with matching nucleotides. Their model has become how we view DNA, the blueprint of all life, ever since.
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Create your account
Frequently Asked Questions
How did Watson and Crick determine the structure of DNA?
The Watson and Crick DNA structure is a collaboration of the work of many scientists dating back several decades before their model became a reality. Basing their ideas and research on the ideas of Linus Pauling's three dimensional proteins, Chargaff's composition studies of DNA, and Rosalind Franklin's X-ray images, they were able to create the double helix structure of DNA.
What are Watson-Crick base pairs?
Watson and Crick base pairs are based on the findings of Erwin Chargaff. When studying the chemical composition of DNA, he found that no matter how much adenine was present (percentage) there was an equal percent of thymine. The same proved true for guanine and cytosine. This became known as Chargaff's Rule and how Watson and Crick determined the base pairs of adenine to thymine and cytosine to guanine in complementary strands of DNA.
What two things did Watson and Crick discover?
Watson and Crick determined that DNA was double-stranded and took the shape of a twisted ladder or double helix. They also proved that complementary bases paired up with each other in the double strand of DNA. Adenine always pairs with thymine and guanine always pairs with cytosine.
What is Watson and Crick model of DNA?
The Watson and Crick model of DNA is a double stranded helix. Phosphate molecules make up the back bone of each strand and the complementary bases bond along the "rungs" of the twisted ladder shape with hydrogen bonding. The two connected strands of DNA run in opposite directions, making them complementary to each other.
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