In this explainer, we will learn how to summarize the evidence, based on scientific investigations, that shows that DNA is the genetic material of cells.
All living organisms have a set of characteristics that define them and give them their identity. We know today that the genetic information controlling these characteristics is passed on from parents to offspring. Every single cell in our body has DNA, or deoxyribonucleic acid, which is the molecule that carries this information. It is composed of two strands that coil around each other to form a double helix, as you can see in Figure 1. DNA is made up of genes, which are sequences of genetic information that control every function that a cell performs.
Definition: DNA (Deoxyribonucleic Acid)
DNA is the molecule that carries the genetic instructions for life. It is composed of two strands of nucleotides that coil around each other to form a double helix.
A gene is a section of DNA that contains the information needed to produce a functional unit, for example, a protein. It is the functional unit of heredity.
There was a time in the history of science, however, when this was not an accepted fact. At the beginning of the 20th century, scientists knew that genetic information was passed from generation to generation, but they did not know which biomolecule actually carried this information.
In 1869, a scientist, Friedrich Miescher, had discovered an acidic, phosphorus-containing substance in the nuclei of cells. He called this substance “nuclein,” and we now know this molecule to be DNA. However, the function of “nuclein” was unknown. In fact, during this time, several groups of scientists believed that genes were carried by proteins and not DNA! In this explainer, we will be learning about the revolutionary discoveries of scientists in the 20th century that proved the fact we know very well today: DNA is the genetic material.
In 1902 and 1903, Walter Sutton and Theodor Boveri conducted research separately but reached the same conclusions. Sutton studied inheritance in grasshoppers, while Boveri conducted similar research on sea urchins. They put forth the “chromosomal theory of inheritance.” Chromosomes are linear structures in the nuclei of living cells, as shown in Figure 2. Sutton and Boveri both proposed that these chromosomes played a role in heredity.
Key Term: Chromosomes
A chromosome is a long molecule of DNA and associated proteins that contains the genetic information of an organism in the form of genes.
They proposed that the nucleus of every living cell has a certain number of these chromosomes. These chromosomes contain genes, or pieces of genetic information, that are passed on from parents to determine the characteristics of the offspring. Every cell in the body of an organism contains the same number of chromosomes, and this number is usually maintained from one generation to the next. For example, human cells each typically have 46 chromosomes.
Most organisms, like animals and some plants, reproduce through sexual reproduction. Each parent contributes one sex cell, or gamete, and these two gametes fuse together to form a zygote in a process called fertilization. The zygote undergoes several stages of development, eventually becoming a fetus. According to the chromosomal theory of inheritance, the cells of this fetus should have the same number of chromosomes as each of its parents. But if the zygote was formed by the fusion of one cell from each parent, would the number of chromosomes in the fetus not be double the number that each of its parents had?
Definition: Gametes (Sex Cells)
Gametes are an organism’s reproductive cells that contain half the genetic material of a normal body cell.
Sutton answered this question by suggesting that the gametes, or sex cells, of a living organism have only half the number of chromosomes that the body cells, or somatic cells, have. This provided the logic behind the fact that every organism belonging to a species has the same number of chromosomes. When a male and female gamete fuse together, each with half the normal number of chromosomes, the resulting zygote would have the normal somatic number of chromosomes.
Definition: Somatic Cells (Body Cells)
Somatic cells are the cells that make up the body of an organism, excluding the sex cells.
Figure 3 shows the process of fertilization in humans. A female gamete, or an ovum, fuses with a male gamete, or a sperm, to form a zygote. You can see that the ovum and the sperm have 23 chromosomes each, which is half the number of chromosomes that you can see in the zygote. Cells that have half the normal number of chromosomes are called haploid, and this is represented as n. Cells with the normal somatic number of chromosomes, like the zygote cell in the figure, are called diploid, and this is represented as 2n.
Example 1: Identifying the Relationship between the Chromosomal Number of Gametes and Somatic Cells
Which of the following is correct about the somatic cells (body cells) of an organism and the gametes (sex cells) that the organism produces?
- The genetic material of a somatic cell is DNA, whereas the genetic material of a gamete is RNA.
- Gametes contain around of the DNA that a somatic cell contains.
- The genetic material of a somatic cell is RNA, whereas the genetic material of a gamete is DNA.
- Gametes contain half the genetic material of a somatic cell.
- Gametes contain double the genetic material of a somatic cell.
When organisms reproduce, genetic material is passed from the parents to the offspring. This genetic material determines the characteristics of the offspring.
Walter Sutton and Theodor Boveri, in 1902 and 1903, proposed the chromosomal theory of inheritance. According to this theory, genetic information is carried in structures called chromosomes, which are found in the nucleus of every cell in an organism. Every single body cell or somatic cell of an organism contains the exact same number of chromosomes, and this number is maintained from one generation to the next. For example, human beings have 46 chromosomes, fruit flies have 8 chromosomes, and pea plants have 14 chromosomes in every somatic cell!
When organisms reproduce sexually, one male and one female gamete, or sex cell, fuse together to form a zygote. This zygote goes through several different stages of development and eventually becomes the fetus.
According to the chromosomal theory of inheritance, the zygote should have the same number of chromosomes as each of its parents. Here is where things get confusing! If a zygote is formed by the fusion of two cells, the genetic material carried in each of these cells should combine. But would that not mean that the zygote has twice the amount of genetic material as the parents? This does not make sense!
Sutton explained how this actually works. The exception to the chromosomal number rule is sex cells or gametes: while the rest of the cells of the body all have a fixed number of chromosomes, gametes have only half this number! This explains how zygotes will always have the same number of chromosomes as the body cells of their parents. Gametes are called “haploid” because they have half the number of chromosomes as somatic cells, which are called “diploid.”
If we take a look through the options in the question, we can see that the option that matches this relationship between the genetic information in gametes and somatic cells is that gametes contain half the genetic material of a somatic cell.
Although the chromosomal theory correctly identified that chromosomes are responsible for the inheritance of characteristics, it did not explain exactly what type of molecule carries the genetic information.
Let’s trace the journey leading to the identification of this molecule through the experiments of different scientists, beginning in 1928, with the work done by Frederick Griffith.
In a series of experiments on the bacteria Streptococcus pneumoniae, which causes pneumonia, Frederick Griffith discovered something that he called the “transforming principle.” Let’s understand the experiments he performed and go through the observations he made.
There are several different types, or strains, of Streptococcus pneumoniae. For his experiment, Griffith selected two different strains. One strain of bacteria had smooth surfaces and is called the smooth strain or S strain, while the other had rough surfaces and is called the rough strain or R strain. Bacteria belonging to the S strain have smooth surfaces because they synthesize a protective coating made of polysaccharide that forms the outermost layer. You can see a representation of the difference between the R strain and the S strain in Figure 4.
A strain is a genetic variant or subtype of an organism.
Aside from their morphological differences, Griffith found that the S and R strains of bacteria have another important difference: the S strain is the “virulent” strain that is capable of infecting another organism, while the R strain is the “nonvirulent” strain that will not cause an infection. How did Griffith discover this?
The word virulent is used to describe a pathogen that is capable of causing a harmful infection.
The word nonvirulent is used to describe a pathogen that is not capable of causing a harmful infection.
When Griffith injected these bacteria into mice, he observed that the mice infected with the virulent S strain died from pneumonia, while the mice infected with the nonvirulent R strain did not, as represented in Figure 5. These observations formed the basis for his further experiments.
Griffith then isolated the S strain and, by using heat, killed the bacteria. When he injected these heat-killed S bacteria into mice, he observed that the mice survived without developing pneumonia.
However, when Griffith introduced a mixture of heat-killed S bacteria and live R bacteria into the mice, they developed pneumonia and died, as represented in Figure 6. Furthermore, when he examined the blood of the dead mice, he found living S bacteria! The live R bacteria had changed their appearance and virulence to become S bacteria, causing pneumonia and killing the mice.
Griffith’s experiment demonstrated a phenomenon that had never been recorded before: the transformation of one type of bacteria into another. He concluded that some factor or biomolecule from the heat-killed S bacteria had entered the living R bacteria, giving them the ability to synthesize a polysaccharide coating and become virulent. This factor therefore “transformed” the R bacteria into S bacteria. Griffith called this factor the “transforming principle,” concluding that some genetic material had been carried by this factor from the S bacteria to the R bacteria. Today, this process is called bacterial transformation and is used in several significant applications of genetic engineering.
Key Term: Bacterial Transformation
Bacterial transformation is the process by which bacterial cells take up DNA from their environment and “transform” according to this foreign DNA.
Example 2: Identifying the Results of Griffith’s Experiments
The diagram provided shows a basic outline of Griffith’s experiment on bacterial transformation. He determined there were two strains of the bacteria that cause pneumonia, a smooth (virulent) strain and a rough (nonvirulent) strain. He injected samples of mice with different forms of these strains, as outlined in the diagram.
Which of the following was not determined by Griffith’s experiment?
- The material being passed between strains of bacteria was DNA.
- The heat-killed smooth strain of bacteria would not kill a mouse injected with it.
- Genetic material could be passed from the cells of the heat-killed smooth strain to the cells of the rough strain.
- The cells of the rough strain of bacteria could be altered to become virulent.
In 1928, Frederick Griffith conducted experiments on the bacteria that cause pneumonia, Streptococcus pneumoniae. He grew two types of this bacteria: a smooth type with a polysaccharide coating and a rough one without a polysaccharide coating.
He then injected both the rough strain or R strain and the smooth strain or the S strain into mice, as shown in experiment 1 and experiment 2 of the diagram in the question. He found that the mice injected with rough strain survived, while those injected with the smooth strain developed pneumonia and died. This is because the bacteria belonging to the smooth strain are virulent, which means they are capable of causing harmful disease, while the bacteria belonging to the rough strain are nonvirulent.
In the next step of Griffith’s work, which you can see represented as experiment 3 in the diagram in the question, he treated the smooth strain with heat, killing the bacteria. He then injected mice with this heat-killed smooth strain and observed that these bacteria no longer caused pneumonia in the mice—the mice survived with no signs of the disease.
In the final stage, or experiment 4, Griffith decided to combine the heat-killed smooth bacteria with some live rough bacteria. He injected this mixture into mice. Although neither the rough bacteria nor the heat-killed smooth bacteria should have been able to cause pneumonia in the mice, they all died from pneumonia within a few days! What was more, Griffith examined the blood of these dead mice and found live smooth bacteria, even though he had not injected them with any!
The rough bacteria had been altered to become virulent and had been given the ability to synthesize a smooth polysaccharide coating. He concluded that some factor carrying genetic material must have been transferred from the heat-killed smooth bacteria into the live rough bacteria, enabling them to transform. He called this factor the “transforming principle” but did not conduct any experiments to figure out exactly what molecule this transforming principle was composed of.
In our question, we are being asked which of the statements was not one of Griffith’s conclusions. If we take a look at each of the statements given, we can see that most of them were, in fact, results of Griffith’s work. However, the one thing Griffith did not determine was the exact molecule that was passed from the smooth strain to the rough strain.
The only option that was not discovered by Griffith’s experiments was, therefore, that the material being passed between strains of bacteria was DNA.
While Griffith’s experiments in 1928 were groundbreaking, they did not define the exact biochemical nature of this transforming principle. Scientists still believed that proteins were responsible for carrying genetic information. This leads us to the work carried out by Oswald Avery, Colin MacLeod, and Maclyn McCarty, which was reported in 1944.
Building on the results established by Griffith’s experiments in 1928, Avery and his colleagues performed further experiments on the virulent S strain of Streptococcus pneumoniae. They knew that the possible carriers of genetic material were either proteins, RNA, or DNA. They isolated the S strain and used heat to kill the bacteria. They then divided the bacteria into three separate samples.
They added protease enzymes to the first sample, RNase to the second, and DNase to the third, as shown in the figure below. From the names of these three enzymes, we should be able to understand their functions: proteases digest proteins, RNase breaks down RNA, and DNase breaks down DNA. They then mixed each of these samples with live R bacteria, as shown in Figure 7.
A protease is an enzyme that breaks down proteins into peptides and amino acids.
Definition: RNase (Ribonuclease)
RNase is an enzyme that breaks down RNA into nucleotides or polynucleotides.
Definition: DNase (Deoxyribonuclease)
DNase is an enzyme that breaks down DNA into nucleotides or polynucleotides.
They observed that the protease-treated and RNase-treated mixtures both showed signs of bacterial transformation. This suggested that these enzymes did not prevent the R bacteria in the mixture from being transformed into virulent S bacteria. However, in the DNase-treated mixture, bacterial transformation was not observed!
Let’s think about why this happened. When the mixtures were treated with protein-digesting or RNA-digesting enzymes, their DNA remained intact and was able to transform the R bacteria into S bacteria. But when the DNA in these mixtures was broken down with DNase, the genetic material could not be passed from the heat-killed S bacteria to the live R bacteria, and therefore transformation could not take place! Avery and his team therefore concluded that the transforming principle described by Griffith had to be DNA.
Example 3: Understanding the Results of Avery’s Experiments
The diagram provided shows a modified version of the experiment conducted by Avery and his colleagues. The virulent S cells were killed by high temperatures and divided into three samples. A different enzyme was added to each sample, and then the solution was mixed with live, but nonvirulent, R cells.
Assume that mice injected with R cells that have successfully undergone bacterial transformation will die. In which of these three experiments will the mice die?
- 1 and 2
- 1 only
- 1 and 3
- 2 and 3
- 3 only
In 1944, Oswald Avery, Colin MacLeod, and Maclyn McCarty conducted experiments on Streptococcus pneumoniae, building on the work done by Frederick Griffith in 1928. Griffith had used two different strains of this bacteria. The rough bacteria or R bacteria are nonvirulent, meaning that they do not cause pneumonia. However, the smooth bacteria or S bacteria are virulent, and when mice are injected with this type of bacteria, they develop pneumonia and die.
Griffith had also demonstrated that when smooth bacteria are heat-killed, they lose their virulence. However, when heat-killed S bacteria are mixed with live R bacteria, this causes the R bacteria to “transform” into S bacteria, which makes them virulent. When this mixture is injected into mice, therefore, the mice develop pneumonia and die.
Avery, MacLeod, and McCarty took Griffith’s experiment a little further. They isolated S bacteria, killed them using heat, and divided them into three different samples. They added protease enzymes to the first sample, RNase to the second, and DNase to the third, as shown in the figure. From the names of these three enzymes, we should be able to understand their functions: proteases break down proteins, RNase breaks down RNA, and DNase breaks down DNA. When these molecules are broken down, they can no longer perform their functions. They then mixed each of these samples with live R bacteria.
We know that genetic material is carried by DNA. The “transforming principle” that Griffith described is actually referring to DNA, and it is this molecule that is transferred from the S bacteria to the R bacteria, making them virulent. If the DNA in the heat-killed S bacteria is degraded by enzymes, this transformation will not happen, and the R bacteria will remain nonvirulent!
Any other enzyme, like protease or RNase, will degrade other molecules but leave DNA untouched. The DNA can still pass from the heat-killed S bacteria to the R bacteria, making them virulent and killing the mice they are injected into.
The three experiments depicted in the diagram in the question show three different enzymes being added to the heat-killed S bacteria. From what we have learned, we know that unless the bacteria are treated with DNase, the DNA or the “transforming principle” will cause virulence in the R bacteria and kill the mice.
The mice will therefore die in experiments 1 and 2.
These experiments were another huge leap in the direction of establishing DNA as the genetic material, but many groups of scientists still remained unconvinced. It was not until Alfred Hershey and Martha Chase conducted their experiments in 1952 that the scientific community began to accept that DNA was, in fact, the transforming principle that carried genetic information.
In their experiments, Hershey and Chase used bacteriophages, which are viruses that infect bacterial cells. Let’s learn a little bit about bacteriophages before we look at Hershey and Chase’s experiments.
A bacteriophage is a type of virus that infects bacteria and replicates within it.
A bacteriophage is a virus that infects bacteria and replicates within bacterial cells. Each bacteriophage is made of a protein capsid that encloses genetic material, as shown in Figure 8. When a bacteriophage attacks a bacterium, it attaches to the surface of the bacterium and injects its genetic material into the bacterial cell. This causes the bacterium to start producing new viruses. Eventually, the bacterial cell bursts open, releasing the new viruses, which can then go on to infect more bacteria.
Hershey and Chase used bacteriophages to identify the molecule that carries genetic material. From the work of other scientists, they knew that DNA contains phosphorus, while proteins do not. They also knew that proteins contain sulfur, and DNA does not. They applied this knowledge to their experiments, as we will see.
Bacteriophages are produced in labs by growing bacteria that are infected with these bacteriophages, so that the bacteriophages can replicate. Hershey and Chase produced one set of bacteriophages using a medium that contained radioactive phosphorus. This meant that these bacteriophages would contain radioactive DNA. In the same way, they produced another set of bacteriophages using a medium containing radioactive sulfur, so that these bacteriophages would contain radioactive proteins. They then allowed these sets of bacteriophages to infect two different bacterial cultures, as you can see in Figure 9.
As we now know, when bacteriophages infect bacteria, they inject their genetic material into the bacterial cell. Once this had happened with the two bacterial cultures, Hershey and Chase agitated the two mixtures using a blender, which detached the viral capsids from the surfaces of the bacterial cells. They then separated the viral capsids from the bacterial cells by spinning the mixtures in a centrifuge, which separates the components of a mixture based on their weight.
They found that the bacteria that had been infected by bacteriophages with radioactive protein did not exhibit any signs of radioactivity. However, the bacteria that had been infected by bacteriophages with radioactive DNA were found to be radioactive, as represented in Figure 9. This observation proved, without a doubt, that DNA was the genetic material that had been inserted into the bacteria.
Each of the experiments we have learned about in this explainer was an important milestone in the journey of determining the biochemical nature of the genetic material. The knowledge that DNA is the genetic material has proven to be priceless to modern science and health care. For example, today, patients with genetic diseases like cystic fibrosis, hemophilia, and some types of cancer can be treated by targeting the specific stretch of defective DNA that causes the disease. Without the foundations laid by these groups of scientists in the 20th century, therefore, medicine would not be where it is today.
Let’s quickly go over everything we have learned in this explainer.
- Although we know today that DNA is the molecule that carries genetic information, at the beginning of the 20th century, this was not an accepted fact.
- In 1902 and 1903, Walter Sutton and Theodor Boveri proposed the chromosomal theory of inheritance, stating that all cells carry genetic material in the form of chromosomes.
- In 1928, Frederick Griffith discovered that a “transforming principle” that could transform nonvirulent bacteria into virulent bacteria existed in Streptococcus pneumoniae.
- In 1944, Oswald Avery, Colin MacLeod, and Maclyn McCarty used proteases, RNase, and DNase enzymes to determine that the “transforming principle” was DNA.
- In 1952, Alfred Hershey and Martha Chase used radioactive phosphorus and sulfur to conclusively prove that it is DNA, not proteins, that carries the genetic material.