DNA as Hereditary Material

Abstract

The relative simplicity of the structure of deoxyribonucleic acid (DNA) and the complexity of inheritance led to the assumption that DNA was not the hereditary material; it was thought that inheritance was controlled by proteins. In only 9 years, from 1944 to 1953, progress was rapidly made in the knowledge gained about the nature of hereditary material, and DNA (not proteins) was shown to be the hereditary material, elucidating the structure of this molecule. The knowledge gained during those 9 years allowed J. Watson and F. Crick to integrate the existing knowledge of the primary structure of DNA with new insights from the secondary structure of the molecule to formulate their model, which basically mentions that DNA is a helical molecule consisting of two antiparallel strands. Each strand consists of linear arrays of four types of nucleotides (each consisting of a deoxyribose molecule, one of the four nitrogenous bases adenine, thymine, cytosine, or guanine, and a phosphate group) linked together by covalent bonds on the same strand. The strands are joined together by specific bonds between adenine-containing nucleotides with thymine-containing nucleotides as well as between cytosine-containing nucleotides with guanine-containing nucleotides by hydrogen bonds. The structure of the DNA molecule allows it to contain large amounts of information characterized by high fidelity in its transmission to the next generation.

Knowledge Integration Questions

• 1. Describe two conclusive tests that determined that DNA was the hereditary material.

  Experiment by Avery and coworkers: knowing that there is a species of bacteria that has two morphologically distinguishable variants, and knowing that bacteria take up nutrients from the environment in which they are found and that some of them can transform a variant in another, the question was which of the macromolecules possessed by one variant of bacteria (called S bacteria) is responsible for the transformation of R bacteria into S bacteria.

  They isolated polysaccharides, lipids, proteins, RNA, and DNA from dead bacteria of the S strain and placed them in contact with live bacteria of strain R. Only R bacteria that were in contact with the DNA of the S bacteria were transformed into S bacteria, which demonstrated that DNA was responsible for bacterial transformation.

  Hershey and Chase experiment: viruses are made up of proteins and nucleic acids. The Bacteriophages are viruses that attack bacteria and use them to reproduce. In this case the question was which of the two constituents of bacteriophages (DNA or proteins) was the most important for virus replication. To determine this, viruses were used with radioactively labeled phosphorus infecting Escherichia coli, and on the other hand viruses with sulfur radioactively labeled also infecting Escherichia coli. Subsequently, the bacteria were separated from the viruses, and radioactivity was measured in both fractions. In the experiment in which work was performed with viruses carrying radioactive sulfur, the radioactivity was detected in the virus debris but not in the bacteria, while in the experiment on the viruses carrying radioactive phosphorus, the radioactivity was detected only in the bacteria. Knowing that sulfur is a constituent of proteins, and phosphorous is a constituent of DNA, it was concluded that viruses insert DNA into bacteria to direct their reproduction.

• 2. Relate the characteristics of DNA to its primary and secondary structure.

  The DNA molecule is made up of numerous units (called nucleotides), which are composed of a molecule of phosphoric acid, a molecule of pentose sugar called deoxyribose, and one of the nitrogenous bases purines (adenine or guanine) or pyrimidines (thymine or cytosine). The secondary structure of DNA was determined by Watson and Crick, and the model has the following features:

  • (a) DNA is a double helix made up of two antiparallel strands. These strands are constituted in its external part by the phosphate group and the deoxyribose, and in its internal part by nitrogenous bases.

  • (b) The bands are joined together by hydrogen bonds that form between the nitrogenous bases, with the specific binding of purine bases with pyrimidine bases (adenine with thymine by two hydrogen bonds, and cytosine with guanine by three hydrogen bonds).

  The DNA molecule is characterized by the following:

  • (a) Faithful transmission of the information it contains: this is a consequence of the specificity that occurs at the adenine-thymine and cytosine-guanine junctions.

  • (b) Capacity to store a large amount of information: this is due to the fact that DNA is a linear arrangement of a multitude of nucleotides.

  • (c) Invariable in the different stages of development of the individual: it is also a consequence of the specificity in base bonding.