Roles of RecA and DnaB Genes in Escherichia coli: Correct Functional Insights

In Escherichia coli, the recA and dnaB genes encode proteins essential for DNA repair, recombination, and replication. Understanding their functions and the consequences of mutations in these genes is crucial for molecular biology and genetics.

Functions of RecA and DnaB Proteins

• RecA Protein:

  • Central to homologous recombination and DNA repair.

  • Facilitates strand invasion and exchange during recombinational repair.

  • Plays a key role in the SOS response, an inducible DNA repair system activated upon DNA damage.

  • RecA forms nucleoprotein filaments on single-stranded DNA and promotes homologous pairing.

• DnaB Protein:

  • Functions as the primary DNA helicase during replication.

  • Unwinds the DNA double helix ahead of the replication fork, enabling replication machinery access.

  • Prevents reannealing of separated strands, maintaining the replication fork structure.

Analysis of the Statements

• A. Mutation in recA gene is lethal.

  • Incorrect. recA mutants are viable but show defects in homologous recombination, DNA repair, and fail to induce the SOS response properly. They are sensitive to DNA damaging agents but not lethal.

• B. E. coli with mutated dnaB gene does not survive.

  • Correct. dnaB encodes an essential helicase; mutations disrupt DNA unwinding and replication, leading to inviability.

• C. DnaB after uncoiling DNA double strands, prevents further reannealing at the separated strands.

  • Correct. DnaB helicase unwinds DNA and helps maintain single-stranded regions for replication.

• D. recA gene is involved in SOS response and helps DNA repair.

  • Correct. RecA is a master regulator of the SOS response and is essential for recombinational DNA repair.

Correct Combination of Statements

• (1) B and C

  • Both statements B and C are correct.

• (2) A and B

  • A is incorrect; B is correct.

• (3) B and D

  • Both B and D are correct.

• (4) A and C

  • A is incorrect; C is correct.

Among the options, B and D are both correct and represent accurate statements about dnaB and recA functions.

Final Answer

(3) B and D

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Conclusion

In E. coli, mutations in the dnaB gene are lethal due to its essential role in DNA unwinding during replication, while recA mutations impair DNA repair and the SOS response but are not lethal. DnaB helicase prevents reannealing of DNA strands after unwinding, and RecA facilitates homologous recombination and SOS induction. Therefore, the correct answer is:

(3) B and D