18 



CELL HEREDITY 



Penicillin-agar 



Penicillin-agar 



Penicillin-sensitive 

 strain 



100 Cells 

 plated 



100 Cells 

 plated 



Result: 100 colonies 



obtained in presence 



and absence of penicillin 



10^ Cells 



plated: 



result is 



confluent 



growth 



FIGURE 1.6. Selection of a drug-resistant strain. To select a drug-resistant strain, 

 a heavy suspension of drug-sensitive cells is placed on agar containing the drug. 

 Most cells die, and any colonies that appear are probably drug-resistant. They are 

 further tested by growth for several generations in liquid culture in the absence of 

 the drug, follov/ed by plating again on drug-containing agar. If the strain is now 

 resistant, a colony should form from every cell, both in the presence and in the 

 absence of the drug. If a heavy suspension (e.g., 10 cells) of drug-resistant cells is 

 plated on drug-containing agar, the growth will be confluent. The suspension must 

 be diluted so that only about 100 cells are plated; the resulting growth will be in 

 the form of discrete countable colonies. 



individual drug-resistant cells arising by transformation can be isolated 

 from a population of many million sensitive cells simply by plating the 

 mixture on drug-containing solid medium, low frequencies of transforma- 

 tion can be measured with precision. 



The results of a double transformation experiment are given in Table 

 1.3. When sensitive cells were treated with a DNA preparation from a 

 penicillin-resistant, streptomycin-resistant strain, three classes of trans- 

 formants were found: double transformants and two classes of singles. 



