What Makes You...You?
This young woman has naturally red hair. Why is her hair red instead of some other color? And, in general, what gives her the specific traits she has? There is a molecule in human beings and most other living things that is largely responsible for their traits. The molecule is large and has a spiral structure in eukaryotes. What molecule is it? With these hints, you probably know that the molecule is DNA.
Today, it is commonly known that DNA is the genetic material that is passed from parents to offspring and determines our traits. For a long time, scientists knew such molecules existed, that is, they were aware that genetic information is contained within biochemical molecules. However, they didn’t know which molecules play this role. In fact, for many decades, scientists thought that proteins were the molecules that contain genetic information.
Discovery that DNA is the Genetic Material
Determining that DNA is the genetic material was an important milestone in biology. It took many scientists undertaking creative experiments over several decades to show with certainty that DNA is the molecule that determines the traits of organisms. This research began in the early part of the 20th century.
Griffith's Experiments with Mice
The first important discovery was made in the 1920s. An American scientist named Frederick Griffith was studying mice and two different strains of a bacterium, called R (rough) strain and S (smooth) strain. He injected the two bacterial strains into mice. The S strain was virulent and killed the mice, whereas the R strain was not virulent and did not kill the mice. You can see these details in the diagram below. Griffith also injected mice with S-strain bacteria that had been killed by heat. As expected, the dead bacteria did not harm the mice. However, when the dead S-strain bacteria were mixed with live R-strain bacteria and injected, the mice died.
Griffith’s Experimental Results. Griffith showed that a substance could be transferred to harmless bacteria and make them deadly.
Based on his observations, Griffith deduced that something in the dead S-strain was transferred to the previously harmless R-strain, making the R-strain deadly. What was this "something?" What type of substance could change the characteristics of the organism that received it?
Avery and His Colleagues Make a Major Contribution
In the early 1940s, a team of scientists led by Oswald Avery tried to answer the question raised by Griffith’s research results. First, they inactivated various substances in the S-strain bacteria. Then they killed the S-strain bacteria and mixed the remains with live R-strain bacteria. (Keep in mind that the R-strain bacteria normally did not harm the mice.) When they inactivated proteins, the R-strain was deadly to the injected mice. This ruled out proteins as the genetic material. Why? Even without the S-strain proteins, the R-strain was changed, or transformed, into the deadly strain. However, when the researchers inactivated DNA in the S-strain, the R-strain remained harmless. This led to the conclusion that DNA — and not protein — is the substance that controls the characteristics of organisms. In other words, DNA is the genetic material.
Hershey and Chase Confirm the Results
The conclusion that DNA is the genetic material was not widely accepted until it was confirmed by additional research. In the 1950s, Alfred Hershey and Martha Chase did experiments with viruses and bacteria. Viruses are not cells. Instead, they are basically DNA (or RNA) inside a protein coat. To reproduce, a virus must insert its own genetic material into a cell (such as a bacterium). Then it uses the cell’s machinery to make more viruses. The researchers used different radioactive elements to label the DNA and proteins in DNA viruses. This allowed them to identify which molecule the viruses inserted into bacterial cells. DNA was the molecule they identified. This confirmed that DNA is the genetic material.
Chargaff Focuses on DNA Bases
Erwin Chargaff (1905-2002), an Austrian-American biochemist from Columbia University, analyzed the base composition of the DNA of various species. This led him to propose two main rules that have been appropriately named Chargaff's rules.
Chargaff determined that in DNA, the amount of one base, a purine, always approximately equals the amount of a particular second base, a pyrimidine. Specifically, that in any double-stranded DNA the number of guanine units equals approximately the the number of cytosine units and the number of adenine units equals approximately the number of thymine units.
Human DNA is 30.9% A and 29.4% T, 19.9% G and 19.8% C. The rule constitutes the basis of base pairs in the DNA double helix: A always pairs with T, and G always pairs with C. He also demonstrated that the number of purines (A+G) always approximates the number of pyrimidines (T+C), an obvious consequence of the base-pairing nature of the DNA double helix.
In 1947 Chargaff showed that the composition of DNA, in terms of the relative amounts of the A, C, G and T bases, varied from one species to another. This molecular diversity added evidence that DNA could be the genetic material.
Chemical structure of the four nitrogenous bases in DNA. Notice how the purines (A and G) are composed of two ring structures, whereas the pyrimidines (T and C) are composed of one ring structure. The DNA of all species has the same four nitrogen bases.The figure also illustrates that Adenine pairs with Thymine and Guanine pairs with cytosine.
Discovery of the Double Helix
After DNA was shown to be the genetic material, scientists wanted to learn more about it, including its structure. James Watson and Francis Crick are usually given credit for discovering that DNA has a double-helix shape like a spiral staircase, as shown in the illustrations below. In fact, Watson and Crick's discovery of the double helix depended heavily on the prior work of Rosalind Franklin and other scientists, who had used X rays to learn more about DNA’s structure. Unfortunately, Franklin and these other scientists have not usually been given credit for their important contributions to the discovery of the double helix.
Double helix structure of DNA resembles a twisted staircase. In this image A. Adenine B. Thymine C. Guanine D. Cytosine 1. Sugar, Phosphate, Backbone 2. Base pair 3. Nitrogeous base. this image also shows the application of 2 rules of Chargaff.
The double-helix shape of DNA, together with Chargaff’s rules, led to a better understanding of DNA. As a nucleic acid, DNA is made from nucleotide monomers. Long chains of nucleotides form polynucleotides, and the DNA double helix consists of two polynucleotide chains. Each nucleotide consists of a sugar (deoxyribose), a phosphate group, and one of the four bases (adenine, cytosine, guanine, or thymine). The sugar and phosphate molecules in adjacent nucleotides bond together and form the "backbone" of each polynucleotide chain.
Scientists concluded that bonds between the bases hold together the two polynucleotide chains of DNA. Moreover, adenine always bonds with thymine, and cytosine always bonds with guanine. That's why these pairs of bases are called complementary base pairs. If you look at the nitrogen bases in the figure below, you will see why the bases bond together only in these pairings. Adenine and guanine have a two-ring structure, whereas cytosine and thymine have just one ring. If adenine were to bond with guanine as well as thymine, for example, the distance between the two DNA chains would be variable. However, when a one-ring molecule (such as thymine) always bonds with a two-ring molecule (such as adenine), the distance between the two chains remains constant. This maintains the uniform shape of the DNA double helix. The bonded base pairs (A-T and G-C) stick into the middle of the double helix, forming, in essence, the steps of the spiral staircase.
Knowledge of DNA’s structure helped scientists understand how DNA replicates. DNA replication is the process in which DNA is copied. It occurs during the synthesis (S) phase of the eukaryotic cell cycle. DNA must be copied so that, after cell division occurs, each daughter cell will have a complete set of chromosomes.
DNA replication begins when an enzyme breaks the bonds between complementary bases in the molecule, as shown in the figure below. This exposes the bases inside the molecule so they can be “read” by another enzyme and used to build two new DNA strands with complementary bases. The two daughter molecules that result each contain one strand from the parent molecule and one new strand that is complementary to it. As a result, the two daughter molecules are both identical to the parent molecule.
DNA Replication. DNA replication is a semi-conservative process. Half of the parent DNA molecule is conserved in each of the two daughter DNA molecules.Green DNA strand represents a brand new DNA strand.
Helicase and Polymerase
DNA replication begins as an enzyme, DNA helicase, breaks the hydrogen bonds holding the two strands together and forms a replication fork. The resulting structure has two branching strands of DNA backbone with exposed bases. These exposed bases allow the DNA to be “read” by another enzyme, DNA polymerase, which then builds the complementary DNA strand. As DNA helicase continues to open the double helix, the replication fork grows.
The two new strands of DNA are “built” in opposite directions, through either a leading strand or a lagging strand. The leading strand is the DNA strand that DNA polymerase constructs in the 5' → 3' direction. This strand of DNA is made in a continuous manner, moving as the replication fork grows. The lagging strand is the DNA strand at the opposite side of the replication fork from the leading strand. It goes in the opposite direction, from 3' to 5'. DNA polymerase cannot build a strand in the 3' → 5' direction. Thus, this "lagging” strand is synthesized in short segments known as Okazaki fragments. On the lagging strand, an enzyme known as primase builds a short RNA primer. DNA polymerase is then able to use the free 3'-OH group on the RNA primer to make DNA in the 5' → 3' direction. The RNA fragments are then degraded and new DNA nucleotides are added to fill the gaps where the RNA was present. Another enzyme, DNA ligase, is then able to attach (ligate) the DNA nucleotides together, completing the synthesis of the lagging strand (Figure below).
DNA replication. The two DNA strands are opened by helicase. The strands are held open by a single strand of binding proteins, preventing premature reannealing. Topoisomerase solves the problem caused by tension generated by winding/unwinding of DNA. This enzyme wraps around DNA and makes a cut permitting the helix to spin and relax. Once DNA is relaxed, topoisomerase reconnects broken strands. DNA primase synthesizes a short RNA primer which initiates the Okazaki fragment. Okazaki fragments are attached by DNA ligase.
What is RNA?
RNA structure differs from DNA structure in three specific ways. Both are nucleic acids and made out of nucleotides; however, RNA is single stranded while DNA is double stranded. RNA nucleotides, like those from DNA, have three parts: a 5-carbon sugar, a phosphate group and a base. RNA contains the 5-carbon sugar ribose, whereas in DNA, the sugar is deoxyribose. The difference between ribose and deoxyribose is the lack of a hydroxyl group attached to the pentose ring in the 2' position of deoxyribose.
Though both RNA and DNA contain the nitrogenous bases adenine, guanine and cytosine, RNA contains the nitrogenous base uracil instead of thymine. Uracil pairs with adenine in RNA, just as thymine pairs with adenine in DNA. Uracil and thymine have very similar structures; uracil is an unmethylated form of thymine.
The nucleotide sequence of RNA, which is complementary to the DNA sequence, allows RNA to encode genetic information. RNA though carries the genetic information of just one gene. Hence, compared to DNA, RNA molecules are relatively small.
A comparison of RNA and DNA is shown in Table below and the figure after that. .
moves to cytoplasm
stays in nucleus
3 types: mRNA, tRNA, rRNA
generally 1 type
Comparison of a single-stranded RNA and a double-stranded DNA with their corresponding nucleobases
- Determining that DNA is the genetic material was an important milestone in biology. The first important discovery was made in the 1920s, when Griffith showed that something in virulent bacteria could be transferred to nonvirulent bacteria and make them virulent as well.
- In the early 1940s, Avery and colleagues showed that the "something" Griffith found in his research was DNA and not protein. This result was confirmed by Hershey and Chase, who demonstrated that viruses insert DNA into bacterial cells so the cells will make copies of the viruses.
- In the mid-1950s, Chargaff showed that, within the DNA of any given species, the concentration of adenine is always the same as the concentration of thymine, and the concentration of guanine is always the same as the concentration of cytosine. These observations came to be known as Chargaff's rules.
- Around the same time, James Watson and Francis Crick, building on the prior X-ray research of Rosalind Franklin and others, discovered the double-helix structure of the DNA molecule. Along with Chargaff's rules, this led to a better understanding of DNA's structure and function.
- Knowledge of DNA's structure helped scientists understand how DNA replicates, which must occur before cell division occurs so each daughter cell will have a complete set of chromosomes. DNA replication is semi-conservative because each daughter molecule contains one strand from the parent molecule and one new strand that is complementary to it.
- DNA replication is the semi-conservative process by which a cell’s entire DNA is copied, or replicated.
- During DNA replication, the two new strands of DNA are “built” in opposite directions, starting at replication forks.
- RNA is a single-stranded nucleic acid. RNA contains the nitrogenous base uracil.
- Outline the discoveries that led to the determination that DNA, and not protein, is the biochemical molecule that contains genetic information.
- State Chargaff's rules. Explain how the rules are related to the structure of the DNA molecule.
- Explain how the structure of a DNA molecule is like a spiral staircase. Which parts of the staircase represents the various parts of the molecule?
- Describe the process of DNA replication.
- When does DNA replication occur, and why is the process said to be semi-conservative?
Why do you think dead S strain bacteria injected into mice does not harm the mice, but kills them when mixed with living (and normally harmless) R strain bacteria?
In Griffith’s experiment, do you think the heat treatment that killed the bacteria also inactivated the bacterial DNA? Why or why not?
Give one example of a specific piece of evidence that helped rule out proteins as the genetic material.
True or False. Two-ring bases always bind to each other.
True or False. DNA replication involves the breaking of one of the polynucleotide chains into individual nucleotides.
True or False. In DNA, each nucleotide has a sugar.
What would the complementary strand of this stretch of DNA bases be?
Which scientists detected labeled DNA that was transferred from one organism to another?
A. Hershey and Chase
________ break the bonds between complimentary bases and add new complementary nucleotides to the parental strands during DNA replication.
D. RNA molecules
Describe the differences between DNA and RNA.
- How is DNA replicated?
- What are the roles of the following enzymes?
- DNA polymerase
- DNA helicase
- DNA ligase
- Why is DNA replication called a "semi-conservative" process?
Rosalind Franklin was a British scientist who helped discover the structure of DNA. To learn more, check this out:
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