Nucleic Acids
7.1 DNA Structure and Replication
Hershey and Chase Experiments:
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 into hosts
In 1952, Alfred Hershey and Martha Chase conducted a series of experiments to prove that DNA was the
genetic material
1. Viruses (T2 bacteriophage) were grown in one of two isotopic mediums in order to radioactively
label a specific viral component
2. Viruses grown in radioactive sulfur (35S) had radiolabeled proteins (sulfur is present in proteins
but not DNA)
3. Viruses grown in radioactive phosphorus (32P) had radiolabeled DNA (phosphorus is present in
DNA but not proteins)
The viruses were then allowed to infect a bacterium (E.coli) and then the virus and 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 32P–viruses (DNA) but not the
35S–viruses (protein)
○ This demonstrated that DNA, not protein, was the genetic material because DNA was
transferred to the bacteria
,Structure of DNA:
Rosalind Franklin and Maurice Wilkins used a method of X-ray diffraction to investigate the structure of
of DNA
○ DNA was purified and then the 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 contacted an atom
○ The scattering pattern of the X-ray was recorded on a film and used to elucidate the
details of the molecular structure
From the scattering pattern produced by a DNA molecules, certain inferences could be made about its
structure
1. Composition: DNA is a double stranded molecule
2. Orientation: Nitrogenous bases are closely packed together on the inside and phosphates form
an outer backbone
3. Shape: The DNA molecule twists at regular intervals to from a 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
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 + pyrimidine) 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 G–C bond
Adenine and thymine paired via two hydrogen bonds, whereas guanine and cytosine paired via three
hydrogen bonds
7.1 DNA Structure and Replication
Hershey and Chase Experiments:
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 into hosts
In 1952, Alfred Hershey and Martha Chase conducted a series of experiments to prove that DNA was the
genetic material
1. Viruses (T2 bacteriophage) were grown in one of two isotopic mediums in order to radioactively
label a specific viral component
2. Viruses grown in radioactive sulfur (35S) had radiolabeled proteins (sulfur is present in proteins
but not DNA)
3. Viruses grown in radioactive phosphorus (32P) had radiolabeled DNA (phosphorus is present in
DNA but not proteins)
The viruses were then allowed to infect a bacterium (E.coli) and then the virus and 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 32P–viruses (DNA) but not the
35S–viruses (protein)
○ This demonstrated that DNA, not protein, was the genetic material because DNA was
transferred to the bacteria
,Structure of DNA:
Rosalind Franklin and Maurice Wilkins used a method of X-ray diffraction to investigate the structure of
of DNA
○ DNA was purified and then the 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 contacted an atom
○ The scattering pattern of the X-ray was recorded on a film and used to elucidate the
details of the molecular structure
From the scattering pattern produced by a DNA molecules, certain inferences could be made about its
structure
1. Composition: DNA is a double stranded molecule
2. Orientation: Nitrogenous bases are closely packed together on the inside and phosphates form
an outer backbone
3. Shape: The DNA molecule twists at regular intervals to from a 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
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 + pyrimidine) 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 G–C bond
Adenine and thymine paired via two hydrogen bonds, whereas guanine and cytosine paired via three
hydrogen bonds
