2. During growth and division of E. coli, the daughter strand is recognized due to
a. Hemimethylation of newly synthesized DNA
b. Nicks in newly synthesized DNA
c. double stranded breaks in newly synthesized DNA
d. DNA damage in newly synthesized DNA
E. coli Daughter Strand Recognition During Growth and Division
Hemimethylation of newly synthesized DNA allows E. coli to distinguish the daughter strand from the parental strand during replication and cell division. This mechanism ensures accurate mismatch repair and prevents reinitiation of replication. The correct answer is option a.
Option Analysis
a. Hemimethylation of newly synthesized DNA
In E. coli, DNA adenine methyltransferase (Dam) methylates adenine in GATC sequences on the parental strand. After replication, the daughter strand remains unmethylated briefly, creating hemimethylated DNA that SeqA and MMR proteins recognize to target the new strand for repair or sequestration. This transient state signals the newly synthesized strand during growth and division.
b. Nicks in newly synthesized DNA
Nicks occur naturally during replication but serve as entry points for excision in MMR after MutH cleavage at hemimethylated GATC sites. They do not primarily enable strand recognition; methylation directs MutH to nick the unmethylated daughter strand.
c. Double stranded breaks in newly synthesized DNA
Double-strand breaks arise from damage or repair errors but trigger RecBCD recombination, not routine daughter strand identification. They disrupt replication forks without distinguishing strands for growth or division.
d. DNA damage in newly synthesized DNA
General damage activates SOS response or excision repair but lacks specificity for strand discrimination. Hemimethylation provides the targeted cue for new strand processing.
E. coli daughter strand recognition relies on hemimethylation to maintain genomic stability during rapid growth and division. This process distinguishes newly synthesized DNA for precise repair and replication control.
Mechanism Overview
Dam methylase fully methylates GATC sites on parental DNA, but replication produces hemimethylated daughter strands. SeqA binds these sites to sequester oriC, preventing premature reinitiation, while MutH in MMR nicks the unmethylated strand for error correction. This ensures one replication cycle per cell division.
Biological Significance
During E. coli growth, hemimethylation coordinates replication with division by delaying full methylation. It reduces mutation rates 100-fold via strand-specific repair and supports cell cycle fidelity. Disruptions, like in dam mutants, cause over-replication or hypermutability.
Exam Relevance
For CSIR NET Life Sciences, this topic tests DNA replication, repair, and bacterial cell cycle integration. Focus on GATC methylation, SeqA function, and contrast with eukaryotic PCNA-directed repair.
