DNA as the Genetic Material


Links to other Lectures and Information about DNA


The Discovery of DNA as the Genetic Material

Mendel helped establish that heredity was controlled by "factors" and chromosomes were soon suspected of carrying the factors (genes). Discover scientific proof that DNA is the genetic material by following the story below.

The story of DNA's discovery is outlined by the folks at Access Excellence.

 

The Early Efforts

Miescher identified DNA in 1869, and in 1914 Feulgen perfected a specific DNA stain (Feulgen stain); however the connection between DNA and heredity was not made until many years later.

Visit the following site to learn how researchers purify DNA.

 

Transformation

In 1928 Griffith, experimented with virulence in Pneumococcus. He determined that nonvirulent strains (rough-strain) could be transformed (genetically changed) to virulent (smooth) strains if the remains of dead virulent bacteria were made available to the living nonvirulent bacteria. Griffith called the genetic information which could be passed from one bacteria to another the "transforming principle."

In 1944, Avery et. al. showed that the transforming material was pure DNA not protein, lipid or carbohydrate. How is it possible for the rough-strain pneumococcus bacteria to transform itself into a virulent form?

Visit Access Excellence for a brief historical summary of the seminal work of Frederick Griffith and Oswald Avery.

 

The Experiment of Hershey and Chase

Using a bacteriophage (virus) and the bacterium Escherichia coli, Hershey and Chase were able to show that only viral DNA entered the host; thus it was DNA that directed the production of new viral particles. This strongly suggested that DNA was the genetic material.

 

The structure of DNA

Four different deoxynucleotides, or nucleotides, the structural units of DNA, are assembled into long polymers of DNA strands, or nucleic acids. Prior to assembly, they are in the form of nucleotide triphosphates similar to ATP.

 

DNA Nucleotides

 

  1. Each nucleotide contains three parts: a phosphate group, the sugar deoxyribose, and one of four nitrogen bases.
  2. The four bases of DNA, their designations and their triphosphate form are adenine (dATP), guanine (dGTP), thymine (dTTP), and cytosine (dCTP).
  3. In 1950, Chargaff developed the principle of base-pairing. He determined the relative amounts of A, T, C, and G in a variety of cells, proving that A = T and C = G and that there is exactly as much purine (adenine and guanine) in the nucleus as there is pyrimidine (thymine and cytosine).
  4. Through the use of X-ray crystallography, Wilkins and Franklin determined that DNA was double stranded and could form a helix.

Interactive models of the nucleotides and DNA using the Chime plug-in can be experienced at the following site at the University of Texas.

 

The Watson and Crick Model of DNA

In 1953, having used critical information from the work of others (Rosalind Franklin and Linus Pauling) and by constructing models of their own, Watson and Crick determined the double helictical structure of DNA, including its phosphate-sugar backbone, specific (A-T, G-C) base-pairing of purines and pyrimidines, and the meaning of the intramolecular distances.

In the double helix, the two polymers run in opposite directions (5'-to-3' and 3'-to-5'). The deoxyribose sugar is the hub of this numbering system -- the 3' carbon contains a hydroxyl group and the 5' carbon has a phosphate. In forming the phosphate sugar backbone of DNA the free 3' -OH group of deoxyribose in the first nucleotide reacts with the first 5' phosphate in the second nucleotide.

Many millions of nucleotides may be present in a single DNA molecule.

Follow this path to more information about Watson and Crick's discovery.

 

DNA Replication

Replication is the preparation of DNA copies prior to reproduction of the cell or organism.

Because of specific base pairing, upon separation of the DNA double helix, each strand can reproduce (serving as a template for) the other. This process is called semiconservative replication.

You should be aware that other replication models were considered by scientists, but eventually rejected.

Curious about how DNA replicates, then try the DNA workshop (it requires the shockwave plugin.

 

The Chemistry of DNA synthesis

When incorporated into DNA, a pyrophosphate (HPO3--O~HPO3-) is released from each nucleotide triphosphate. Nucleotides are joined by their phosphates and sugars, which form the backbone of the polymer with the nitrogen bases projecting off the side.

Synthesis of DNA polymers proceeds from the 5' end to the 3' end (each new nucleotide is added to the 3' end by hydrolysis) In its finished form, DNA is a double helix.

 

Origins of Replication

Replication is carried out by replication complexes, which include the unwinding enzyme helicase and the nucleotide adding enzyme DNA polymerase. The structure of helicase has now been revealed.

The helix is unwound, the separated strands form replication forks, and new nucleotide triphosphates are added according to base pairing.

The Replication complex untwists the helix in only one direction. The addition of bases to the leading end of a polymer (5' to 3') occurs smoothly, one base at a time. Replication on the complimentary strand also occurs in the 5'-to-3' direction but in short segments (Okazaki fragments) since the DNA polymerase must constantly wait for the original helix to unwind creating a large enough gap to get started. The Okazaki fragments are united by a special ligase enzyme.

An animation of this process can help you understand what is going on.

Because each new polymer is base-paired to an old one, DNA replication is called semiconservative (half is conserved).

 

DNA Replication in Eukaryotes and Prokaryotes

Replication sites "bubble out" as they form, and bubbles lengthen as replication proceeds in both directions.

In eukaryotic replication, multiple replication forks work simultaneously, forming many bubbles. In prokaryotes, only two replication forks (one bubble) form along the circular chromosome, but replication in prokaryotes is much faster (much smaller genome).

 

DNA and Genetic Information

Garrod identified metabolic disorders such as alkaptonuria by the presence of abnormal metabolites such as alkaptones in the urine. He determined that such conditions were inherited and surmised that they involved abnormalities in the enzymes of metabolic pathways. He correctly associated the abnormal enzymes and such enzymes with abnormal genes.

F. Crick proposed that all biological information is encoded in DNA, transmitted by DNA replication, transcribed into RNA, and translated into protein. This role for DNA is called the Central dogma of molecular biology.

The American Scientist has an article entitled "The Invention of the Genetic Code", which outlines the intellectual struggle to uncover just how DNA and its four nucleotides can encode the instructions for the assembly of millions of different proteins.

For reasons which became obvious once the structure of DNA was confirmed, DNA can be easily repaired. This is possible because DNA is made of complimentary strands. If one strand has a problem the other strand can be used as a template for repair. For more information discover "What is DNA Repair".

 


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Modified Dec. 6, 2002