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With the new advancements in gene editing, DNA has became a much talked about subject. Most people get the gist of what it is and its purpose, but how did we come to distill our understanding of life into a single strand of molecule?
This is the story of how it all came about... |
"DNA neither cares nor knows. DNA just is. And we dance to its music."
Richard Dawkins, 'River Out of Eden: A Darwinian View of Life' (1995)
The molecule now known as DNA was first identified in the 1860s by a Swiss chemist called Johann Friedrich Miescher. Johann set out to research the key components of white blood cells, part of our body’s immune system. The main source of these cells was pus-coated bandages collected from a nearby medical clinic.
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Johann carried out experiments using salt solutions to understand more about what makes up white blood cells. He noticed that, when he added acid to a solution of the cells, a substance separated from the solution. This substance then dissolved again when an alkali was added. When investigating this substance he realised that it had unexpected properties different to those of the other proteins he was familiar with. Johann called this mysterious substance ‘nuclein’, because he believed it had come from the cell nucleus. Unbeknown to him, Johann had discovered the molecular basis of all life – DNA. He then set about finding ways to extract it in its pure form.
He was convinced of the importance of nuclein and came very close to uncovering its elusive role, despite the simple tools and methods available to him. However, he lacked the skills to communicate and promote what he had found to the wider scientific community. Ever the perfectionist, he hesitated for long periods of time between experiments before he published his results in 1874. Before then he primarily discussed his findings in private letters to his friends. As a result, it was many decades before Johann Friedrich Miescher’s discovery was fully appreciated by the scientific community. For many years, scientists continued to believe that proteins were the molecules that held all of our genetic material. They believed that nuclein simply wasn’t complex enough to contain all of the information needed to make up a genome. Surely, one type of molecule could not account for all the variation seen within species? |
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The Four Building Blocks of DNA (1881)
Albrecht Kossel was a German biochemist who made great progress in understanding the basic building blocks of nuclein. In 1881 Albrecht identified nuclein as a nucleic acid and provided its present chemical name, deoxyribonucleic acid (DNA). He also isolated the five nucleotide bases that are the building blocks of DNA and RNA: adenine (A), cytosine (C), guanine (G), thymine (T) and uracil (U). This work was rewarded in 1910 when he received the Nobel Prize in Physiology or Medicine. The Chromosome Theory of Inheritance (1902) In the early 1900s, the work of Gregor Mendel was rediscovered and his ideas about inheritance began to be properly appreciated. As a result, a flood of research began to try and prove or disprove his theories of how physical characteristics are inherited from one generation to the next. |
In the middle of the nineteenth century, Walther Flemming, an anatomist from Germany, discovered a fibrous structure within the nucleus of cells. He named this structure ‘chromatin’, but what he had actually discovered is what we now know as chromosomes. By observing this chromatin, Walther correctly worked out how chromosomes separate during cell division, also known as mitosis.
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The chromosome theory of inheritance was developed primarily by Walter Sutton and Theodor Boveri. They first presented the idea that the genetic material passed down from parent to child is within the chromosomes. Their work helped explain the inheritance patterns that Gregor Mendel had observed over a century before.
Interestingly, Walter Sutton and Theodor Boveri were actually working independently during the late 1900s. Walter studied grasshopper chromosomes, while Theodor studied roundworm embryos. However, their work came together in a perfect union, along with the findings of a few other scientists, to form the chromosome theory of inheritance. Building on Walther Flemming’s findings with chromatin, German embryologist Theodor Boveri provided the first evidence that the chromosomes within egg and sperm cells are linked to inherited characteristics. From his studies of the roundworm embryo he also worked out that the number of chromosomes is lower in egg and sperm cells compared to other body cells. |
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American graduate, Walter Sutton, expanded on Theodor’s observation through his work with the grasshopper. He found it was possible to distinguish individual chromosomes undergoing meiosis in the testes of the grasshopper and, through this, he correctly identified the sex chromosome. In the closing statement of his 1902 paper he summed up the chromosomal theory of inheritance based around these principles: (1) chromosomes contain the genetic material; (2) they are passed along from parent to offspring; and (3) they are found in pairs in the nucleus of most cells (during meiosis these pairs separate to form daughter cells). During the formation of sperm and eggs cells in men and women, respectively, chromosomes separate. Each parent contributes one set of chromosomes to its offspring.
Avery-Macleod-McCarty Experiment (1944)
In 1944, Oswald Avery, Colin MacLeod and Maclyn McCarty helped demonstrate the role of DNA as the carrier of genetic information by working with the bacterium that causes pneumonia, Streptococcus pneumoniae.
However, their work was given a head start by a British bacteriologist called Frederick Griffith, who identified something called the ‘transforming principle’.
Frederick studied two strains of the Streptococcus pneumoniae bacteria. One, called the S strain, had smooth walls and was fatal when injected into mice. The second strain, R, had rough walls and was not fatal when injected into mice. The S strain was smooth due to a coat made out of sugars that helped protect it from the mouse immune system. The rough R bacteria were rough because it did not have a sugar coat, and so was not protected from the mouse immune system.
Frederick carried out a series of experiments to investigate the strains further. First he killed S bacteria with heat and injected them into the mice; the mice survived. He then injected heat-killed S bacteria along with living R bacteria; the mice died. After studying the blood of these mice he was surprised to find living S bacteria in it, somehow the rough R bacteria had transformed into smooth S bacteria. He then came to the conclusion that there was a ‘transforming principle’ responsible for this.
In 1944, Oswald Avery, Colin MacLeod and Maclyn McCarty helped demonstrate the role of DNA as the carrier of genetic information by working with the bacterium that causes pneumonia, Streptococcus pneumoniae.
However, their work was given a head start by a British bacteriologist called Frederick Griffith, who identified something called the ‘transforming principle’.
Frederick studied two strains of the Streptococcus pneumoniae bacteria. One, called the S strain, had smooth walls and was fatal when injected into mice. The second strain, R, had rough walls and was not fatal when injected into mice. The S strain was smooth due to a coat made out of sugars that helped protect it from the mouse immune system. The rough R bacteria were rough because it did not have a sugar coat, and so was not protected from the mouse immune system.
Frederick carried out a series of experiments to investigate the strains further. First he killed S bacteria with heat and injected them into the mice; the mice survived. He then injected heat-killed S bacteria along with living R bacteria; the mice died. After studying the blood of these mice he was surprised to find living S bacteria in it, somehow the rough R bacteria had transformed into smooth S bacteria. He then came to the conclusion that there was a ‘transforming principle’ responsible for this.
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But what exactly was it? Was it the proteins in the bacteria, the sugar coat on the S bacteria, the immune system of the mouse or the nucleic acids RNA and DNA.
Enter Oswald Avery and his colleagues. Working in test tubes, they used detergent to break open the heat-killed S cells to separate out the different components: They then destroyed the components one by one to identify which component was the ‘transforming principle’. The team published their results in 1944. |
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First they combined the heat-killed S bacteria with an enzyme that broke down the smooth sugar coat. They then mixed the sugar-coatless S bacteria with the R bacteria and found that the R bacteria still transformed into S bacteria. So, the ‘transforming principle’ was not in the sugar coat.
Next they added protein-digesting enzymes to destroy all of the protein in the bacteria, and yet again when mixed with R bacteria, the R transformed into S. So the ‘transforming principle’ clearly wasn’t a protein either.
Next they isolated the nucleic acids, DNA and RNA, using alcohol.They then destroyed the RNA using the RNase enzyme, leaving just the DNA behind. They mixed it with the R bacteria, and transformation from R to S still occurred. So, it wasn’t RNA.
Finally, they destroyed the DNA in the solution using DNase, mixed it with the R bacteria, and no transformation occurred, the R bacteria remained rough. So, the ‘transforming principle’ must be DNA!
Next they added protein-digesting enzymes to destroy all of the protein in the bacteria, and yet again when mixed with R bacteria, the R transformed into S. So the ‘transforming principle’ clearly wasn’t a protein either.
Next they isolated the nucleic acids, DNA and RNA, using alcohol.They then destroyed the RNA using the RNase enzyme, leaving just the DNA behind. They mixed it with the R bacteria, and transformation from R to S still occurred. So, it wasn’t RNA.
Finally, they destroyed the DNA in the solution using DNase, mixed it with the R bacteria, and no transformation occurred, the R bacteria remained rough. So, the ‘transforming principle’ must be DNA!
The Hershey-Chase Experiments (1952)
In 1952, experiments by Alfred Hershey and Martha Chase further supported the findings from the work of Avery, Macleod and McCarty. Hershey and Chase used a virus called T2 to investigate if the genetic information was passed on via DNA or proteins. Although T2 is made up of only a shred of DNA and a scrap of protein, the virus can hijack bacterial cells to make more copies of itself. Scientists knew that the instructions for making new viruses must therefore have been carried in the DNA or protein, but they didn't know which.
When Hershey and Chase added a radioactive label to the DNA of the original T2 virus, they found that the viruses produced were also radioactive. However, when they repeated the experiment labelling the protein rather than the DNA of the original virus, they found that the viruses produced were not radioactive. Hershey and Chase concluded that DNA carried the instructions to make new viruses, which was passed on to subsequent generations.
In 1952, experiments by Alfred Hershey and Martha Chase further supported the findings from the work of Avery, Macleod and McCarty. Hershey and Chase used a virus called T2 to investigate if the genetic information was passed on via DNA or proteins. Although T2 is made up of only a shred of DNA and a scrap of protein, the virus can hijack bacterial cells to make more copies of itself. Scientists knew that the instructions for making new viruses must therefore have been carried in the DNA or protein, but they didn't know which.
When Hershey and Chase added a radioactive label to the DNA of the original T2 virus, they found that the viruses produced were also radioactive. However, when they repeated the experiment labelling the protein rather than the DNA of the original virus, they found that the viruses produced were not radioactive. Hershey and Chase concluded that DNA carried the instructions to make new viruses, which was passed on to subsequent generations.
The results of experiments by an Austrian scientist Erwin Chargaff helped pave the way to understanding the three-dimensional structure of the DNA molecule.
Erwin set out to investigate if there were any key differences between the DNA of different species. Following this he came to two conclusions, which became known as ‘Chargaff’s rules’:
Erwin Chargaff’s rules were a crucial step in understanding the structure of DNA. In 1952, he met with James Watson and Francis Crick in Cambridge and discussed his findings with them. Although not the most cordial of encounters, Chargaff’s rules did help James and Francis to explain the three-dimensional structure of the DNA.
Unravelling the Double Helix (1953)
The function of DNA depends to a large extent on its structure. The three-dimensional structure of DNA was first proposed by James Watson and Francis Crick in 1953. It is one of the most famous scientific discoveries of all time.
James and Francis used evidence shared by others, particularly Rosalind Franklin and Maurice Wilkins, to determine the shape of DNA. Rosalind worked with Maurice at King's College London. She beamed X-rays through crystals of the DNA molecule and then used photographic film to record where the scattered X-rays fell. The shadows on the film were then used to work out where the dense molecules lie in the DNA. This technique is called X-ray diffraction. The DNA crystals resulted in a cross shape on the X-ray film which is typical of a molecule with a helix shape. The resulting X-ray was named Photograph 51 and Maurice shared it with James and Francis.
In 1953 James Watson and Francis Crick published their theory that DNA must be shaped like a double helix. A double helix resembles a twisted ladder. Each 'upright' pole of the ladder is formed from a backbone of alternating sugar and phosphate groups. Each DNA base (adenine, cytosine, guanine, thymine) is attached to the backbone and these bases form the rungs. There are ten 'rungs' for each complete twist in the DNA helix.
James and Francis suggested that each 'rung' of the DNA helix was composed of a pair of bases, joined by hydrogen bonds. According to Erwin Chargaff’s rules, A would always form hydrogen bonds with T, and C with G.
The concept that DNA was made of a sequence of paired bases along a sugar-phosphate backbone allowed James Watson and Francis Crick to draw two important conclusions:
The two strands of DNA provide a simple mechanism for copying the molecule. If separated, each strand provides a template for creating the other strand. By separating the double helix in this way two identical 'daughter' molecules can be created.
The order, or sequence, of bases on each strand makes up the digital code that carries the instructions for life. If we can understand the code, we are closer to understanding how cells work.
Decrypting the Code of Life
Even following the huge breakthrough of Francis Crick and James Watson, one big question remained unanswered. How do you get from a strand of DNA to a protein?
Many scientists set to the challenge, but three in particular, Marshall Warren Nirenberg, Har Gobind Khorana and Robert William Holley, were the first to discover how the four bases of DNA could be translated into the 20 building blocks of proteins, also known as amino acids.
To do this, they constructed a very simple strand of RNA, composed of a strand of only one base repeated over and over; in this case it was the base uracil, or U. In the lab, this led to the production of a protein made up of just one type of amino acid, the amino acid phenylalanine. By this simple experiment, in 1961, they had cracked the first letter of the code, a strand of Us translates into a strand of phenylalanine. They could then continue the experiment, but using the other bases, to find out the other letters of the code.
Eventually they identified that the letters in DNA are read in blocks of three called a ‘codon’. Each codon specifies an amino acid which is added to the protein during synthesis.
In 1968, the three scientists were rewarded for their work with the Nobel Prize in Physiology or Medicine.
There is no doubt James Watson and Francis Crick played a fundamental role in defining the structure and function of DNA. However, it is important to remember that this discovery was dependant on many other scientists before them. Miescher, Hershey and Chase, Chargaff, Wilkins and Franklin, and all the others mentioned here all deserve to be acknowledged for their work in helping to unravel the fundamental role of DNA in biology. Their research has provided the foundation on which the science of genomics is built and enabled the great strides being made today in our understanding of genetics.
Erwin set out to investigate if there were any key differences between the DNA of different species. Following this he came to two conclusions, which became known as ‘Chargaff’s rules’:
- In DNA, regardless of which organism it comes from, the amount of adenine (A) is usually the same as the amount of thymine (T), and the amount of guanine (G) is usually the same as the amount of cytosine (C).
- The composition of DNA varies between different species such that the amount of each base is different. This diversity in the composition of DNA made it a much more credible candidate for the genetic material than protein.
Erwin Chargaff’s rules were a crucial step in understanding the structure of DNA. In 1952, he met with James Watson and Francis Crick in Cambridge and discussed his findings with them. Although not the most cordial of encounters, Chargaff’s rules did help James and Francis to explain the three-dimensional structure of the DNA.
Unravelling the Double Helix (1953)
The function of DNA depends to a large extent on its structure. The three-dimensional structure of DNA was first proposed by James Watson and Francis Crick in 1953. It is one of the most famous scientific discoveries of all time.
James and Francis used evidence shared by others, particularly Rosalind Franklin and Maurice Wilkins, to determine the shape of DNA. Rosalind worked with Maurice at King's College London. She beamed X-rays through crystals of the DNA molecule and then used photographic film to record where the scattered X-rays fell. The shadows on the film were then used to work out where the dense molecules lie in the DNA. This technique is called X-ray diffraction. The DNA crystals resulted in a cross shape on the X-ray film which is typical of a molecule with a helix shape. The resulting X-ray was named Photograph 51 and Maurice shared it with James and Francis.
In 1953 James Watson and Francis Crick published their theory that DNA must be shaped like a double helix. A double helix resembles a twisted ladder. Each 'upright' pole of the ladder is formed from a backbone of alternating sugar and phosphate groups. Each DNA base (adenine, cytosine, guanine, thymine) is attached to the backbone and these bases form the rungs. There are ten 'rungs' for each complete twist in the DNA helix.
James and Francis suggested that each 'rung' of the DNA helix was composed of a pair of bases, joined by hydrogen bonds. According to Erwin Chargaff’s rules, A would always form hydrogen bonds with T, and C with G.
The concept that DNA was made of a sequence of paired bases along a sugar-phosphate backbone allowed James Watson and Francis Crick to draw two important conclusions:
The two strands of DNA provide a simple mechanism for copying the molecule. If separated, each strand provides a template for creating the other strand. By separating the double helix in this way two identical 'daughter' molecules can be created.
The order, or sequence, of bases on each strand makes up the digital code that carries the instructions for life. If we can understand the code, we are closer to understanding how cells work.
Decrypting the Code of Life
Even following the huge breakthrough of Francis Crick and James Watson, one big question remained unanswered. How do you get from a strand of DNA to a protein?
Many scientists set to the challenge, but three in particular, Marshall Warren Nirenberg, Har Gobind Khorana and Robert William Holley, were the first to discover how the four bases of DNA could be translated into the 20 building blocks of proteins, also known as amino acids.
To do this, they constructed a very simple strand of RNA, composed of a strand of only one base repeated over and over; in this case it was the base uracil, or U. In the lab, this led to the production of a protein made up of just one type of amino acid, the amino acid phenylalanine. By this simple experiment, in 1961, they had cracked the first letter of the code, a strand of Us translates into a strand of phenylalanine. They could then continue the experiment, but using the other bases, to find out the other letters of the code.
Eventually they identified that the letters in DNA are read in blocks of three called a ‘codon’. Each codon specifies an amino acid which is added to the protein during synthesis.
In 1968, the three scientists were rewarded for their work with the Nobel Prize in Physiology or Medicine.
There is no doubt James Watson and Francis Crick played a fundamental role in defining the structure and function of DNA. However, it is important to remember that this discovery was dependant on many other scientists before them. Miescher, Hershey and Chase, Chargaff, Wilkins and Franklin, and all the others mentioned here all deserve to be acknowledged for their work in helping to unravel the fundamental role of DNA in biology. Their research has provided the foundation on which the science of genomics is built and enabled the great strides being made today in our understanding of genetics.
Ponder this
How did a single molecule become the determining factor of an organism, at what point in the evolutionary history of life on Earth did this happen?
Why is the DNA in all the cells of an individual organism required to be exactly the same?
Discuss
Discuss the train of thought throughout the history of genetic research. Where did the idea came from? How did it develop into a single coalescing idea? Were there competing ideas which tried to explain the mechanism of inheritance? Why and how were these competing ideas wrong?
Further readings
Johann Miescher, Albrecht Kossel, and Gregor Mendel, the founders of genetics.
Avery–MacLeod–McCarty experiment, which discovered an elusive 'transforming principle' in bacteria.
Hershey–Chase experiment, which identifies DNA as the material which carries the information of inheritance.
History of molecular biology, for a broader macrotopic which includes the history of DNA research.
