Given a loss of function mutant for dam the gene encoding the Dam methylase in E

Question

Given a loss of function mutant for dam the gene encoding the Dam methylase in E. coli, predict the phenotype one would observe with respect to spontaneous mutagenesis. spontaneous mutagenesis would decrease, as MutH would have no way to distinguish which strand is parental versus newly synthesized spontaneous mutagenesis would increase, as MutH would distinguish which strand is parental spontaneous mutagenesis would decrease, as MutH would distinguish which strand is parental spontaneous mutagenesis would increase, as MutH would have no way to distinguish which strand is parental versus newly synthesized

Explanation / Answer

Choice 4 is correct.

Reason: During bacterial replication, many replication errors might arise. Since the DNA repair machinery requires to identify the nascent and parental DNA strands, there should be some distinguishable features between the two. This is accomplished by methylation of the nascent DNA strand. The enzyme Dam methylase is encoded by a gene called dam. This enzyme is involved in methylation of the newly synthesized DNA strand in the daughter cells causing immediate hemi-methylation of the nascent DNA. When this DNA is hemi-methylated, it shows a special feature of being identified by the DNA repair machinery. In this repair machinery, the combination of various repair units such as mutL and mutH identify the nascent strand and bring about its repair.

However, if a cell as loss-of-function mutation for dam gene, it will not synthesize any DNA methylase enzyem and thus the repair machinery will not be able to identify the nascent and parental strand of DNA. Thus, no rectification of mutation/damage will occur in the absence of idenfication owing to un-methylation due to lack of Dam methylase. Slowly, this will accumulate spontaneous mutations in these cells.

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