The Hershey Chase Experiment
The phage, which infects
, consists of a head, sheath, tail, and base plate made of different proteins.
DNA is packaged within the head of the virus. When T2 comes in contact with
, the phage attaches to the
bacterium by its tail. Next, the phage injects genetic material into the cell. The genetic material directs bacterial
enzymes to produce viral offspring. When the life cycle is complete, 100 to 200 progeny phages have been
assembled inside each bacterium. The bacterium breaks open, or lyses, and the phages are released. Since T2
consists of only DNA and protein, Hershey and Chase reasoned that the genetic material must be one of the two
components. So they designed an experiment to determine which it is. For their experiment, the scientists prepared
T2 phages that either had radioactive DNA or radioactive proteins.
DNA and RNA Structure
DNA and RNA are nucleic acids, polymers made of subunits called nucleotides. One difference between
DNA and RNA is the type of sugar their nucleotides contain. DNA contains the sugar deoxyribose, while RNA
contains the sugar ribose. Ribose has one more oxygen atom than deoxyribose. -the carbon atoms in a nucleotide's
sugars are numbered beginning with the carbon atom bonded to the nitrogenous base. DNA and RNA are each
composed of four different nucleotides, which differ in their nitrogenous bases. Three of the four bases are the same
in DNA and RNA— adenine, guanine, and cytosine. The fourth base in DNA is thymine. In RNA it is uracil. The
nitrogenous bases guanine and adenine each have two linked rings of atoms. They are called purines. Cytosine,
thymine, and uracil each have a single ring, and these three bases are called pyrimidines.
DNA the Double Helix
DNA consists of two complementary strands of nucleotides twisted together to form a double helix. In this
space-filling model, each atom is represented by a small sphere. The yellow and red phosphate groups make the
two sugar-phosphate backbones easy to spot. They're on the outside of the double helix. Each crossbar is a
hydrogen-bonded pair of bases, one from each DNA strand. An adenine is always paired with a thymine, and a
guanine with a cytosine. The double helix can also be represented by a ball-and-stick model. A ribbon model is
often used, with ribbons for the sugar-phosphate backbones.
DNA Replication: An Overview
In principle, copying DNA-- a process called DNA replication-- is very simple. The two
complementary DNA strands separate, and because each nucleotide can only pair with its complement--