The Hershey and Chase Experiments

In 1952 (7 years after Avery's demonstration that genes were DNA), two geneticists:

  • A. D. Hershey and
  • Martha Chase

provided further proof. They worked with a DNA virus, called T2, which infects E. coli (and so is a bacteriophage).

The figure shows the essential elements of the infective cycle of DNA bacteriophages like T2.

  • The virions attach to the surface of their host cell (a).
  • The proteins of the capsid inject the DNA core into the cell

Base Pairing

The rules of base pairing (or nucleotide pairing) are:

  • A with T: the purine adenine (A) always pairs with the pyrimidine thymine (T)
  • C with G: the pyrimidine cytosine (C) always pairs with the purine guanine (G)

This is consistent with there not being enough space (20 Å) for two purines to fit within the helix and too much space for two pyrimidines to get close enough to each other to form hydrogen bonds between them.

But why not A with C and G with T?

The answer

Index to this page

DNA Replication

 

Before a cell can divide, it must duplicate all its DNA. In eukaryotes, this occurs during S phase of the cell cycle.

The Biochemical Reactions

  • DNA replication begins with the "unzipping" of the parent