Notes: DNA (DeoxyriboNucleic Acid)

Illustration showing how DNA comprises chromosomes in the nucleus of a cell

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DeoxyriboNucleic Acid

a complex molecule (polymer) found in all living things.

Animated image of DNA

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Historical Background on discovery of DNA

In 1928 scientist Frederick Griffith conducted experiments with two related strains of bacteria, one of which has a smooth coat and caused pneumonia in mice and the other with a rough coat that did not cause pneumonia. His procedure was as follows: (see illustration below)


Image detailing Griffith experiment on mice

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Griffith proposed an explanation: Transformation;  some unknown chemical component of the dead cells was used to transform the harmless rough cells into harmful smooth ones; 

Griffith's Experiment Animation

Narrated animation of Griffith's experiment


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1944  Avery, McCloud and McCarty first purified and then tested all the chemical components of the smooth bacteria with the rough ones to determine what component caused the transformation;  the only component that transformed the bacteria was DNA, but little was known about the composition or functions of DNA.

1947  Chargaff performed chemical tests on DNA and determined that there was always the same amount of Adenine as Thymine and there was always the same amount of Cytosine as Guanine.  Conclusion:  Adenine and Thymine were paired only with each other, while cytosine and Guanine were paired only with each other as well.  As a result of his findings, he proposed Chargaff's Rules, known more commonly as 'base pairing rules.' 

1952  Hershey & Chase conducted experiments to determine conclusively whether the transforming chemical components were genetic material, DNA or some type of proteins. 

Hershey and Chase used a virus, called a bacteriophage, that infects bacteria to get DNA into cells 

Since the protein coat of viruses contains sulfur, the viruses were treated with radioactive sulfur that would bind to the protein, but not DNA, and be easily detected with special equipment 

Since DNA has phosphate molecules, but not sulfur, the viruses were also treated with radioactive phosphate that would bind to the DNA but not the protein. 

After mixing the treated viruses with untreated bacteria, special techniques were used to track the location of the radioactive isotopes of sulfur and phosphate. 

Radioactive sulfur was found on the surface of bacterial cells and radioactive phosphate was found inside the bacterial cells.

Hershey and Chase concluded that it was only the genetic material, DNA, that entered cells and caused transformations. 

For a better understanding of this critical experiment, access the link below. Select the Narrated option.  After viewing, go back and try the quiz!

Hershey-Chase Experiment Animated

Screenshot of and link to narrated description of Hershey-Chase experiment  

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In 1953 James Watson, Francis Crick and Maurice Wilkins examined an amazing photograph made by Rosalind Franklin using a technique that she developed called X-ray crystallography. 

Rosalind Franklin Photo 51 that led to the discovery of the structure of DNA

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This is Franklin's photo that led to the discovery of the structure of DNA.

With what they had discovered about the composition of DNA and Franklin's photographs, Watson & Crick developed and proposed the first model for the structure of DNA. The PBS program The Secret of Photo 51 reveals more about this intriguing story.


Watson and Crick with their model of DNA

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Watson & Crick with their model


Watch the short Discovery Education video clip Double Helix: Discovery of the DNA Structure to get a better understanding of this amazing discovery.

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Their model for the DNA molecule consisted of two nucleotide chains that wrap around each other to form a double spiral. This shape is called a double helix. (The picture at the top of this page represents a double helix.) 

The primary function of DNA is to:

The monomers that make up DNA and RNA are called nucleotides. A structural formula for a sample nucleotide is shown below.

Structural formula for sample nucleotide

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Each nucleotide is composed of a sugar, a nitrogen base and a phosphate group:

A Sugar: Deoxyribose - a 5-carbon sugar in DNA Ribose - a 5-carbon sugar in RNA. This sugar has more oxygen than the sugar in DNA.

A nitrogen base:

Purines - have a double ring of carbon and nitrogen atoms.

Images of purine molecules

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 Pyrimidines - have a single ring of carbon and nitrogen atoms.

Images of pryimidine molecules

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A phosphate group: - PO4

The phosphate group is attached to the sugar, and the sugar is attached to the nitrogen base 

Two nucleotides pair to form one of the "rungs" of the ladder in a DNA double helix. 

The sugar and phosphate groups form the "backbone" or outer support of the DNA ladder-like double helix.


Base-pairing Rules

These rules describe the behavior of the bases.


  1. Cytosine always bonds with guanine by forming three hydrogen bonds. (C - G)
  2. Adenine always bonds with thymine by forming two hydrogen bonds. (A - T) In the RNA nucleotide, thymine is replaced by Uracil.
  3. A pair of bases that always bond together is known as a complementary base pair.



Replication: the process of duplicating the DNA molecule.

Watch the following Teacher's Domain video animation and explanation for the process of replication. It's a fascinating narrated animation from Interactive NOVA: The Secret of Life. It shows the double helix structure of DNA and how the molecule replicates. The segment illustrates how the DNA molecule chemically "unzips" to form templates for the new strands. The presentation also explains how complementary nucleotides bind together like the rungs of a ladder.

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 After watching the animation, continue with the following notes for a more detailed discussion of the process.  

The replication or duplication of DNA depends on one main idea, namely that the nitrogen bases of the nucleotides are complementary to each other on opposite sides of the molecule as shown below. 

This pairing is due to the shapes of the nucleotide bases, which allow weak hydrogen bonds to form between the complementary bases, holding the two halves of the DNA molecule together.

Image of DNA unwinding

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The double helix unwinds, unzips, and new nucleotides are added.

Replication occurs as follows:

  1. The parent DNA molecule unwinds and unzips (an enzyme called Helicase does this) exposing the two halves of the DNA molecule
  2. Each half of the parent DNA molecule serves as a template (model) for the complementary bases to be brought into their appropriate positions.
  3. Enzymes, (DNA Polymerase), attach free nucleotides to the open complementary positions.
  4. Additional enzymes, (Ligase), connect sugars to phosphates along the sides of the newly created ladder-like helix.
  5. Another enzyme checks for errors in base pair matching and corrects the majority of mistakes, which could result in mutations. A mutation is any change in the DNA.


Image of DNA replication

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An idea about DNA size:


An average cell nucleus is about 6 micrometers in diameter. The total length of the DNA in the human genome is 1.8 meters.


There are several levels of coiling and supercoiling in DNA with strands wrapped around special protein molecules called histones.