Phage Particles

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FIGURE 17.08 Only the DNA of Bacteriophage T4 Enters the Cell

In a classic experiment by Hershey and Chase, the viral DNA was found to enter the bacterial cell but the protein coat did not. To trace the DNA and protein, bacteriophage T4 was first grown with radioactive precursors (32P-dATP and 35S-meth ionine, respectively). The radioactively labeled T4 were isolated and purified. Phage labeled with 32P (blue DNA) were added to new host E. coli, and the E. coli were separated from remaining virus particles (left panel). After viral replication, the progeny viruses were isolated. The 32P labeled DNA was found in these viral particles, implying that the original virus DNA entered the host E. coli, and was eventually packaged in new particles. The same experiment was performed with 35S-methionine labeled T4 (green coats), but the final viral particles did not contain any 35S-methionine (right panel). This suggested that none of the original protein coat entered the host E. coli.

FIGURE 17.09 Variety of Virus Genomes

Viral genomes come in all shapes and sizes. They may be comprised of DNA or RNA. They may be circular or linear, and they may be double-stranded or single-stranded.

which carry their genes as RNA in the virus particle, must first make a DNA copy of themselves to insert into the host chromosome (see below).

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