Rollin Hotchkiss and Julius Marmur studied transformation in the bacterium Streptococcus pneumoniae (R. D. Hotchkiss and J. Marmur. 1954. Proceedings of the National Academy of Sciences of the United States of America 40:55-60). They examined four mutations in this bacterium: penicillin resistance (P), streptomycin resistance (S), sulfanilamide resistance (F), and the ability to use mannitol (M). They extracted DNA from strains of bacteria with different combinations of different mutations and used this DNA to transform wild-type bacterial cells (P* St F* M*). The results from one of their transformation experiments are shown here. Recipient Percentage of all cells Donor Transformants DNA DNA MSF M* s* F* M* SF* 4.0 M* s* F 4.0 M S* F* 2.6 MSF* 0.41 M* SF 0.22 M S* F 0.0058 MSF 0.0071
Genetic Recombination
Recombination is crucial to this process because it allows genes to be reassorted into diverse combinations. Genetic recombination is the process of combining genetic components from two different origins into a single unit. In prokaryotes, genetic recombination takes place by the unilateral transfer of deoxyribonucleic acid. It includes transduction, transformation, and conjugation. The genetic exchange occurring between homologous deoxyribonucleic acid sequences (DNA) from two different sources is termed general recombination. For this to happen, an identical sequence of the two recombining molecules is required. The process of genetic exchange which occurs in eukaryotes during sexual reproduction such as meiosis is an example of this type of genetic recombination.
Microbial Genetics
Genes are the functional units of heredity. They transfer characteristic information from parents to the offspring.
Hotchkiss and Marmur noted that the percentage of cotransformation was higher than would be expected on a random basis. For example, the results show that 2.6% of the cells were transformed into M and 4% were transformed into S. If the M and S traits were inherited independently, the expected probability of cotransformation of M and S (M S) would be 0.026
× 0.04 = 0.001, or 0.1%. However, they observed 0.41% M S cotransformants, four times more than they expected. What accounts for the relatively high frequency of cotransformation of the traits they observed?
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