S. M. Zaigham Zia 1
,7.1 DNA structure & replication
Uncovering DNA
1. Outline the Hershey and Chase experiment, including the results
In the mid twentieth century, scientists were still unsure as to whether DNA or protein was the genetic
material of the cell. It was known that some viruses consisted solely of DNA and a protein coat and
could transfer their genetic material to hosts.
In 1952, Alfred Hershey and Martha chase conducted a series of experiments to prove that DNA was
the genetic material. Viruses (T2 bacteriophage) were grown in one of the two isotopic mediums in
order to radioactively label a specific viral component. Viruses grown in radioactive sulfur (S-35) had
radiolabeled proteins (sulfur is present in proteins but not DNA). Viruses grown in radioactive
phosphorus (P-32) had radiolabeled DNA (phosphorus is present in DNA but not in proteins).
The viruses were then allowed to infect a bacterium (E. Coli) and then the virus and the bacteria were
separated via centrifugation. The larger bacteria formed a solid pellet while the smaller viruses
remained in the supernatant.
The bacterial pellet was found to be radioactive when infected by the P-32 viruses (DNA) but not the
S-35 viruses (protein). This demonstrated that DNA, not protein, was the genetic material because DNA
was transferred to the bacteria.
2. Outline how X-ray crystallography provided clues about the structure of DNA
Rosalind Franklin and Maurice Wilkins used a method of x-ray diffraction to investigate the structure of
DNA. DNA was purified and then fibres were stretched in a thin glass tube (to make most of the strands
parallel). The DNA was targeted by a x-ray beam, which was diffracted when it was contacted by an
S. M. Zaigham Zia 2
, atom. The scattering pattern of the x-ray was recorded on a film and used to elucidate details of
molecular structure.
From the scattering pattern produced by a DNA molecule, certain inferences could be made about its
structure.
- Composition: DNA is a double stranded molecule
- Orientation: Nitrogenous bases are closely packed together in the inside and phosphates form an
outer backbone
- Shape: the DNA molecule twists at regular intervals (every 34 Angstrom) to form helix (two strands =
double helix)
Franklin’s data was shared by Wilkins with James Watson (without Franklin’s permission) who, with the
help of Francis Crick, used the information to create a molecular model of the basic structure of DNA.
In 1962, Watson, Crick, and Wilkins (but not Franklin) were awarded the Nobel Prize for their
contributions to DNA structure identification
3. Explain how the structure of DNA gave insights into the mechanism of DNA replication
Franklin’s x-ray diffraction experiments
demonstrated that the DNA helix is both tightly
packed and regular in structure. Phosphates (and
sugars) form an outer backbone and nitrogenous
bases are packaged within the interior.
Chargaff had also demonstrated that DNA is
composed of an equal number of purines (A+G)
and pyrimidines (C+T). This indicates that these
nitrogenous bases are paired (purine +
pyrimidines) within the double helix. In order for
this pairing between purines and pyrimidines to
occur, the two strands must run in antiparallel
directions.
When Watson & Crick were developing their DNA model they discovered that an A-T bond was the
same length as a C-G bond. Adenine and thymine paired via two hydrogen bonds, whereas guanine
and cytosine paired via three hydrogen bonds. if the bases were always paired this way, then this
would describe the regular structure of the DNA helix.
Consequently, DNA structure suggests two mechanisms for DNA replication
- Replication occurs via complementary base pairing (adenine pairs with thymine and guanine
pairs with cytosine)
- Replication is bidirectional (proceeds in the opposite direction on the strands) due to the
antiparallel nature of the strands
S. M. Zaigham Zia 3
,7.1 DNA structure & replication
Uncovering DNA
1. Outline the Hershey and Chase experiment, including the results
In the mid twentieth century, scientists were still unsure as to whether DNA or protein was the genetic
material of the cell. It was known that some viruses consisted solely of DNA and a protein coat and
could transfer their genetic material to hosts.
In 1952, Alfred Hershey and Martha chase conducted a series of experiments to prove that DNA was
the genetic material. Viruses (T2 bacteriophage) were grown in one of the two isotopic mediums in
order to radioactively label a specific viral component. Viruses grown in radioactive sulfur (S-35) had
radiolabeled proteins (sulfur is present in proteins but not DNA). Viruses grown in radioactive
phosphorus (P-32) had radiolabeled DNA (phosphorus is present in DNA but not in proteins).
The viruses were then allowed to infect a bacterium (E. Coli) and then the virus and the bacteria were
separated via centrifugation. The larger bacteria formed a solid pellet while the smaller viruses
remained in the supernatant.
The bacterial pellet was found to be radioactive when infected by the P-32 viruses (DNA) but not the
S-35 viruses (protein). This demonstrated that DNA, not protein, was the genetic material because DNA
was transferred to the bacteria.
2. Outline how X-ray crystallography provided clues about the structure of DNA
Rosalind Franklin and Maurice Wilkins used a method of x-ray diffraction to investigate the structure of
DNA. DNA was purified and then fibres were stretched in a thin glass tube (to make most of the strands
parallel). The DNA was targeted by a x-ray beam, which was diffracted when it was contacted by an
S. M. Zaigham Zia 2
, atom. The scattering pattern of the x-ray was recorded on a film and used to elucidate details of
molecular structure.
From the scattering pattern produced by a DNA molecule, certain inferences could be made about its
structure.
- Composition: DNA is a double stranded molecule
- Orientation: Nitrogenous bases are closely packed together in the inside and phosphates form an
outer backbone
- Shape: the DNA molecule twists at regular intervals (every 34 Angstrom) to form helix (two strands =
double helix)
Franklin’s data was shared by Wilkins with James Watson (without Franklin’s permission) who, with the
help of Francis Crick, used the information to create a molecular model of the basic structure of DNA.
In 1962, Watson, Crick, and Wilkins (but not Franklin) were awarded the Nobel Prize for their
contributions to DNA structure identification
3. Explain how the structure of DNA gave insights into the mechanism of DNA replication
Franklin’s x-ray diffraction experiments
demonstrated that the DNA helix is both tightly
packed and regular in structure. Phosphates (and
sugars) form an outer backbone and nitrogenous
bases are packaged within the interior.
Chargaff had also demonstrated that DNA is
composed of an equal number of purines (A+G)
and pyrimidines (C+T). This indicates that these
nitrogenous bases are paired (purine +
pyrimidines) within the double helix. In order for
this pairing between purines and pyrimidines to
occur, the two strands must run in antiparallel
directions.
When Watson & Crick were developing their DNA model they discovered that an A-T bond was the
same length as a C-G bond. Adenine and thymine paired via two hydrogen bonds, whereas guanine
and cytosine paired via three hydrogen bonds. if the bases were always paired this way, then this
would describe the regular structure of the DNA helix.
Consequently, DNA structure suggests two mechanisms for DNA replication
- Replication occurs via complementary base pairing (adenine pairs with thymine and guanine
pairs with cytosine)
- Replication is bidirectional (proceeds in the opposite direction on the strands) due to the
antiparallel nature of the strands
S. M. Zaigham Zia 3
