When a child has the same colour hair or the same-shaped nose as one of his or her parents, it is said that there is a family resemblance. Throughout history, humans have had many ideas about what type of substance might be passed from parent to child that accounts for family resemblances. Before the exact molecule responsible for heredity was identified, Mendel and other scientists determined that genes, carried on chromosomes (Figure 1), control phenotypic characteristics. Chromosomes are passed from parents to offspring through gametes. In this way, the diversity of individuals, and indeed the diversity of all life, is a reflection of the different genes organisms inherit.
Figure 1 A human chromosome
The Discovery of DNA as Genetic Material
Modern science has shown that deoxyribonucleic acid (DNA) is the molecule that stores and transmits genetic information from parents to offspring. Many scientists have contributed to the discovery of the hereditary molecule DNA.
The story began in 1869 when Friedrich Miescher, a Swiss scientist, investigated a compound found in the nucleus of the cells he studied; Miescher called this compound nuclein (now known as DNA). After Miescher's discovery, numerous scientists conducted experiments in an attempt to determine if nuclein was the material that stored and passed on genetic information. The chemical components of nuclein were discovered in the 1920s, but it took decades of research before it was proven that DNA is responsible for heredity.
nuclein: the original name given to DNA when it was discovered in the nucleus of cells by Friedrich Miescher in 1869
In the 1930s, Danish biologist Joachim Hammerling verified that genetic material was contained in the nucleus. Hammerling's experiments involved large, green, single-celled algae called Acetabularia that have three distinct regions: a foot, a stalk, and a cap. Using a microscope, he saw that the foot contained the nucleus. In his first experiment, he cut off the cap of several algal cells, and the algae were able to regenerate new caps. In the second experiment, he cut off the foot, but this time none of the algal cell regenerated (Figure 2). Therefore, he reasoned, the material that was directing new growth must be located in the nucleus, found in the foot. Hammerling could not verify what the genetic material was, only that it was found in the nucleus. This experiment was a vital step in the study of genetics. [Go To Nelson Science]
Figure 2 Since a new cap grew only from an existing foot that contained the nucleus, Hammerling concluded that the nucleus must contain the hereditary material.
Image: An illustration of two experiments that were performed by Hammerling to find out what contained the heredity material. Experiment 1 the cap has been removed from the stalk and the foot, leaving just the stalk and the foot, the cap was then able to regenerate. In experiment 2 the foot was removed leaving the cap and stalk and there was no foot regeneration.
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At this point, scientists hypothesized that DNA was the hereditary material. However, the hypothesis was not proven until 1952, when Alfred Hershey and Martha Chase conducted experiments using a bacterial virus. Bacterial viruses invade bacterial cells and then use the organelles found in the bacteria to produce more bacterial viruses. Hershey and Chase showed that the viruses needed only to inject their DNA into the bacteria to produce more bacterial viruses. They concluded that DNA directed the production of new viruses and that DNA was the hereditary material.
Although Hershey and Chase worked only with bacteria, they realized that the idea could be applied to all organisms. Their discovery confirmed the work done by Hammerling, who had concluded that hereditary material is located in the nucleus.
The Chemical Composition of DNA
In the 1920s, Phoebus Levene discovered that DNA had three main components: - a pentose sugar (a cyclic, 5-carbon sugar) - a phosphate group that has a negative charge - a nitrogenous base
Together, the three components are called a nucleotide. About 3 billion pairs of nucleotides make up the human genome. There are four possible bases for the nucleotides of DNA: adenine (A), guanine (G), thymine (T), and cytosine (C) (Figure 3).
nucleotide: the repeating unit in DNA; it comprises a deoxyribose sugar, a phosphate group, and one of the four nitrogenous bases
Figure 3 The four nitrogenous bases, A, C, T, and G, in DNA
In 1940, Erwin Chargaff discovered a key relationship among the nitrogenous bases in DNA: - The amount of adenine (A) is always equal to the amount of thymine (T). - The amount of guanine (G) is always equal to the amount of cytosine (C).
Figure 4 shows a nucleotide, comprising a phosphate group, a pentose sugar, and a nitrogenous base. Notice from the figure that DNA has a negative charge because of the phosphate ions in its chemical backbone.
Figure 4 The three main components of DNA are shown here (with thymine as the nitrogenous base).
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The Structure of the DNA Molecule
Scientists had now gathered a great deal of information about the chemical composition of DNA, but they were still missing a few pieces of the puzzle. To understand how DNA stores and transmits genetic information, scientists needed to establish the precise structure of the molecule.
In 1951, researcher Rosalind Franklin began to study DNA using X-ray crystallography. Working together with another scientist, Maurice Wilkins, Franklin determined that DNA molecules form a helix, or corkscrew, shape (Figure 5).
X-ray crystallography: a technique in which a pure substance is subjected to X-rays; the pattern in which the X-rays bend and spread helps reveal the structure of the pure substance
Figure 5 (a) Rosalind Franklin used X-ray crystallography to reveal the helical structure of DNA. In this technique, a crystal of DNA is exposed to X-rays in order to produce (b) a diffraction pattern. The diffraction pattern can then be used in an effort to reconstruct the positions of the atoms in the molecules of DNA.
In 1953, James Watson and Francis Crick, two scientists at Cambridge University, met with Wilkins, who shared the information he and Franklin had discovered. Using what they knew about the chemical structure of DNA, Watson and Crick built a model of DNA. Watson and Crick's model showed the molecular structure of DNA to be a double helix (Figure 6). This model was based on a single X-ray diffraction image that Franklin had taken, as well as on other chemical information about DNA that was known at the time. Their model accounted for the following information, discovered and shared by other scientists:
1. DNA is made of a pentose sugar, a phosphate group, and one of four nitrogenous bases (Levene, 1920s).
2. The proportion of adenine (A) to thymine (T) is equal. The proportion of cytosine (C) to guanine (G) is equal (Chargaff, 1940).
3. DNA has the shape of a helix or corkscrew (Franklin and Wilkins, 1951).
Figure 6 James Watson and Francis Crick with their model of a DNA molecule, which integrated all of the chemical information that was known about DNA at the time.
Watson and Crick used this information to put together all the "puzzle pieces" provided by their peers when creating their molecular model of DNA. Franklin's X-ray images of DNA crystals were of the highest quality, and were instrumental in Watson and Crick's final determination of the structure of DNA (now widely accepted as being accurate).
A Model for DNA
A scientific model can be a representation of a system or a concept. Watson and Crick built on the work of the scientists before them to create their model of DNA. Using this information, they proposed that DNA is made up of two strands of repeating DNA nucleotides that run in opposite directions.
scientific model: a simplified representation of a concept; can be tangible or conceptual
The chemical backbone of a DNA strand is made up of alternating phosphate groups and pentose sugars. Attached to this backbone are nitrogenous bases: adenine,
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guanine, cytosine, and thymine (Figure 7). The bases of one strand are paired with the bases of the other strand in the following arrangement:
- Thymine (T) is always bonded to adenine (A). - Cytosine (C) is always bonded to guanine (G).
This type of pairing is known as complementary base pairing. Watson and Crick proposed a complementary base-pairing molecule in order to explain Chargaff's observations of equal proportions of bases.
complementary base pairing: pairing of the nitrogenous base of one strand of DNA with the nitrogenous base of another strand; adenine (A) pairs with thymine (T), and guanine (G) pairs with cytosine (C)
Figure 7 DNA is double stranded. The outer backbone consists of alternating pentose sugar and phosphate groups. The inner part of the molecule contains the complementary bases.
LEARNING TIP
DNA Twist
DNA exists as a right-handed double helix as well as a left-handed double helix. The two types of helix are mirror images of each other. Watson and Crick built a model of a right-handed helix.
Since the bases are complementary, there is no need to know the nucleotide base sequence for both strands. Once the nucleotide base sequence is determined for one strand, the sequence on the complementary strand can be determined easily. For example, the complementary base sequence for ATGGCCATC would be TACCGGTAG. Figure 8 shows more examples.
Figure 8 Examples of complementary base pairing. T always pairs with A and G always pairs with C.
6.1 Summary
- Deoxyribonucleic acid (DNA) stores and transmits genetic information from parent to offspring.
- DNA is made up of repeating nucleotides. Each nucleotide consists of a sugar, a phosphate group, and a nitrogenous base.
- DNA has four nitrogenous bases: thymine (T), cytosine (C), adenine (A), and guanine (G).
- Thymine (T) and adenine (A) are complementary bases. Guanine (G) and cytosine (C) are complementary bases.
- James Watson and Francis Crick determined the structure of DNA. DNA consists of two strands that run in opposite directions. Each strand is made up of alternating phosphate and sugar molecules with nitrogenous bases attached to the backbone.
6.1 Questions
Key
K/U: Knowledge and Understanding T/I: Thinking and Investigation C: Communication A: Application
1. Outline the contributions of the following scientists in the determination that DNA is the hereditary material: K/U (a) Joachim Hammerling (b) Alfred Hershey and Martha Chase
2. What does DNA stand for? What are the three main components of DNA? K/U
3. Identify and explain Chargaff's contribution to the determination of DNA structure. K/U A
4. What is the complementary strand of TTGACAGTAAAA? A
5. Describe Watson and Crick's model of DNA. How does it account for all the experimental evidence about DNA composition that was known at the time? K/U C
When a child has the same colour hair or the same-shaped nose as one of his or her parents, it is said that there is a family resemblance. Throughout history, humans have had many ideas about what type of substance might be passed from parent to child that accounts for family resemblances. Before the exact molecule responsible for heredity was identified, Mendel and other scientists determined that genes, carried on chromosomes (Figure 1), control phenotypic characteristics. Chromosomes are passed from parents to offspring through gametes. In this way, the diversity of individuals, and indeed the diversity of all life, is a reflection of the different genes organisms inherit.
Figure 1 A human chromosome
The Discovery of DNA as Genetic Material
Modern science has shown that deoxyribonucleic acid (DNA) is the molecule that stores and transmits genetic information from parents to offspring. Many scientists have contributed to the discovery of the hereditary molecule DNA.
The story began in 1869 when Friedrich Miescher, a Swiss scientist, investigated a compound found in the nucleus of the cells he studied; Miescher called this compound nuclein (now known as DNA). After Miescher's discovery, numerous scientists conducted experiments in an attempt to determine if nuclein was the material that stored and passed on genetic information. The chemical components of nuclein were discovered in the 1920s, but it took decades of research before it was proven that DNA is responsible for heredity.
nuclein: the original name given to DNA when it was discovered in the nucleus of cells by Friedrich Miescher in 1869
In the 1930s, Danish biologist Joachim Hammerling verified that genetic material was contained in the nucleus. Hammerling's experiments involved large, green, single-celled algae called Acetabularia that have three distinct regions: a foot, a stalk, and a cap. Using a microscope, he saw that the foot contained the nucleus. In his first experiment, he cut off the cap of several algal cells, and the algae were able to regenerate new caps. In the second experiment, he cut off the foot, but this time none of the algal cell regenerated (Figure 2). Therefore, he reasoned, the material that was directing new growth must be located in the nucleus, found in the foot. Hammerling could not verify what the genetic material was, only that it was found in the nucleus. This experiment was a vital step in the study of genetics. [Go To Nelson Science]
Figure 2 Since a new cap grew only from an existing foot that contained the nucleus, Hammerling concluded that the nucleus must contain the hereditary material.
Image: An illustration of two experiments that were performed by Hammerling to find out what contained the heredity material. Experiment 1 the cap has been removed from the stalk and the foot, leaving just the stalk and the foot, the cap was then able to regenerate. In experiment 2 the foot was removed leaving the cap and stalk and there was no foot regeneration.
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At this point, scientists hypothesized that DNA was the hereditary material. However, the hypothesis was not proven until 1952, when Alfred Hershey and Martha Chase conducted experiments using a bacterial virus. Bacterial viruses invade bacterial cells and then use the organelles found in the bacteria to produce more bacterial viruses. Hershey and Chase showed that the viruses needed only to inject their DNA into the bacteria to produce more bacterial viruses. They concluded that DNA directed the production of new viruses and that DNA was the hereditary material.
Although Hershey and Chase worked only with bacteria, they realized that the idea could be applied to all organisms. Their discovery confirmed the work done by Hammerling, who had concluded that hereditary material is located in the nucleus.
The Chemical Composition of DNA
In the 1920s, Phoebus Levene discovered that DNA had three main components:
- a pentose sugar (a cyclic, 5-carbon sugar)
- a phosphate group that has a negative charge
- a nitrogenous base
Together, the three components are called a nucleotide. About 3 billion pairs of nucleotides make up the human genome. There are four possible bases for the nucleotides of DNA: adenine (A), guanine (G), thymine (T), and cytosine (C) (Figure 3).
nucleotide: the repeating unit in DNA; it comprises a deoxyribose sugar, a phosphate group, and one of the four nitrogenous bases
Figure 3 The four nitrogenous bases, A, C, T, and G, in DNA
In 1940, Erwin Chargaff discovered a key relationship among the nitrogenous bases in DNA:
- The amount of adenine (A) is always equal to the amount of thymine (T).
- The amount of guanine (G) is always equal to the amount of cytosine (C).
Figure 4 shows a nucleotide, comprising a phosphate group, a pentose sugar, and a nitrogenous base. Notice from the figure that DNA has a negative charge because of the phosphate ions in its chemical backbone.
Figure 4 The three main components of DNA are shown here (with thymine as the nitrogenous base).
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The Structure of the DNA Molecule
Scientists had now gathered a great deal of information about the chemical composition of DNA, but they were still missing a few pieces of the puzzle. To understand how DNA stores and transmits genetic information, scientists needed to establish the precise structure of the molecule.
In 1951, researcher Rosalind Franklin began to study DNA using X-ray crystallography. Working together with another scientist, Maurice Wilkins, Franklin determined that DNA molecules form a helix, or corkscrew, shape (Figure 5).
X-ray crystallography: a technique in which a pure substance is subjected to X-rays; the pattern in which the X-rays bend and spread helps reveal the structure of the pure substance
Figure 5 (a) Rosalind Franklin used X-ray crystallography to reveal the helical structure of DNA. In this technique, a crystal of DNA is exposed to X-rays in order to produce (b) a diffraction pattern. The diffraction pattern can then be used in an effort to reconstruct the positions of the atoms in the molecules of DNA.
In 1953, James Watson and Francis Crick, two scientists at Cambridge University, met with Wilkins, who shared the information he and Franklin had discovered. Using what they knew about the chemical structure of DNA, Watson and Crick built a model of DNA. Watson and Crick's model showed the molecular structure of DNA to be a double helix (Figure 6). This model was based on a single X-ray diffraction image that Franklin had taken, as well as on other chemical information about DNA that was known at the time. Their model accounted for the following information, discovered and shared by other scientists:
1. DNA is made of a pentose sugar, a phosphate group, and one of four nitrogenous bases (Levene, 1920s).
2. The proportion of adenine (A) to thymine (T) is equal. The proportion of cytosine (C) to guanine (G) is equal (Chargaff, 1940).
3. DNA has the shape of a helix or corkscrew (Franklin and Wilkins, 1951).
Figure 6 James Watson and Francis Crick with their model of a DNA molecule, which integrated all of the chemical information that was known about DNA at the time.
Watson and Crick used this information to put together all the "puzzle pieces" provided by their peers when creating their molecular model of DNA. Franklin's X-ray images of DNA crystals were of the highest quality, and were instrumental in Watson and Crick's final determination of the structure of DNA (now widely accepted as being accurate).
A Model for DNA
A scientific model can be a representation of a system or a concept. Watson and Crick built on the work of the scientists before them to create their model of DNA. Using this information, they proposed that DNA is made up of two strands of repeating DNA nucleotides that run in opposite directions.
scientific model: a simplified representation of a concept; can be tangible or conceptual
The chemical backbone of a DNA strand is made up of alternating phosphate groups and pentose sugars. Attached to this backbone are nitrogenous bases: adenine,
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guanine, cytosine, and thymine (Figure 7). The bases of one strand are paired with the bases of the other strand in the following arrangement:
- Thymine (T) is always bonded to adenine (A).
- Cytosine (C) is always bonded to guanine (G).
This type of pairing is known as complementary base pairing. Watson and Crick proposed a complementary base-pairing molecule in order to explain Chargaff's observations of equal proportions of bases.
complementary base pairing: pairing of the nitrogenous base of one strand of
DNA with the nitrogenous base of another strand; adenine (A) pairs with thymine (T), and guanine (G) pairs with cytosine (C)
Figure 7 DNA is double stranded. The outer backbone consists of alternating pentose sugar and phosphate groups. The inner part of the molecule contains the complementary bases.
LEARNING TIP
DNA Twist
DNA exists as a right-handed double helix as well as a left-handed double helix. The two types of helix are mirror images of each other. Watson and Crick built a model of a right-handed helix.
Since the bases are complementary, there is no need to know the nucleotide base sequence for both strands. Once the nucleotide base sequence is determined for one strand, the sequence on the complementary strand can be determined easily. For example, the complementary base sequence for ATGGCCATC would be TACCGGTAG. Figure 8 shows more examples.
Figure 8 Examples of complementary base pairing. T always pairs with A and G always pairs with C.
6.1 Summary
- Deoxyribonucleic acid (DNA) stores and transmits genetic information from parent to offspring.
- DNA is made up of repeating nucleotides. Each nucleotide consists of a sugar, a phosphate group, and a nitrogenous base.
- DNA has four nitrogenous bases: thymine (T), cytosine (C), adenine (A), and guanine (G).
- Thymine (T) and adenine (A) are complementary bases. Guanine (G) and cytosine (C) are complementary bases.
- James Watson and Francis Crick determined the structure of DNA. DNA consists of two strands that run in opposite directions. Each strand is made up of alternating phosphate and sugar molecules with nitrogenous bases attached to the backbone.
6.1 Questions
Key
K/U: Knowledge and Understanding
T/I: Thinking and Investigation
C: Communication
A: Application
1. Outline the contributions of the following scientists in the determination that DNA is the hereditary material: K/U
(a) Joachim Hammerling
(b) Alfred Hershey and Martha Chase
2. What does DNA stand for? What are the three main components of DNA? K/U
3. Identify and explain Chargaff's contribution to the determination of DNA structure. K/U A
4. What is the complementary strand of TTGACAGTAAAA? A
5. Describe Watson and Crick's model of DNA. How does it account for all the experimental evidence about DNA composition that was known at the time? K/U C