Strand exchange in homologous recombination - Jim Haber (Brandeis)

TL;DR

Rad51 protein facilitates DNA strand exchange during homologous recombination.

Transcript

let me just say a little bit more about what's really going on in this process of strand exchange the rad 51 or re a protein uh forms a filament on single stranded DNA and when it does so uh each monomer of Rec a can bind uh three nucleotides of the single stranded DNA and it will bind and form this uh this filament and the filament as it binds str... Read More

Key Insights

• Rad51 protein forms a filament on single-stranded DNA, binding three nucleotides per monomer, and stretches the DNA to 1.5 times its normal length.
• The filament causes double-stranded DNA to underwind and extend to 18.6 base pairs per turn, facilitating strand exchange.
• Strand exchange involves forming base pairs between a single strand and one of the double-stranded DNA strands, releasing the other strand.
• RecA or Rad51 filament can carry out strand exchange reactions by searching for homologous sequences in double-stranded DNA.
• The process results in a nicked circle and a displaced single-strand DNA, observable on a gel.
• Radioactive phosphate labeling helps visualize the progression of strand exchange, showing the formation of joint molecules and nicked circles.
• The appearance of single-stranded DNA post-reaction confirms the strand exchange process mediated by Rad51 and RecA proteins.
• Strand invasion, catalyzed by base pairing, is a crucial step in homologous recombination processes.

Questions & Answers

Q: What role does Rad51 play in strand exchange?

Rad51 forms a filament on single-stranded DNA, binding three nucleotides per monomer, and stretches the DNA to facilitate strand exchange. It causes double-stranded DNA to underwind and extend, making it easier for strand exchange to occur by exposing DNA bases for base pairing.

Q: How does the RecA or Rad51 filament facilitate homologous recombination?

The RecA or Rad51 filament facilitates homologous recombination by searching for homologous sequences in double-stranded DNA. It causes the exchange of base pairs, leading to the formation of joint molecules and nicked circles, which are key intermediates in the recombination process.

Q: What is the outcome of the strand exchange process?

The outcome of the strand exchange process includes the formation of a nicked circle and a displaced single-strand DNA. These products can be visualized using gel electrophoresis, confirming the occurrence of strand exchange and the role of Rad51 and RecA proteins in this process.

Q: How is radioactive phosphate used in studying strand exchange?

Radioactive phosphate is used to label the ends of linear double-stranded DNA molecules, allowing researchers to visualize the progression of strand exchange on a gel. This technique helps identify the formation of joint molecules and nicked circles as the reaction proceeds.

Q: What is the significance of the nicked circle in strand exchange?

The nicked circle is a significant intermediate in the strand exchange process, indicating the successful exchange of strands between single-stranded and double-stranded DNA. Its formation is a key step in homologous recombination, facilitated by Rad51 and RecA proteins.

Q: Why is strand invasion important in homologous recombination?

Strand invasion is crucial in homologous recombination because it allows the single-stranded DNA to form base pairs with one strand of the double-stranded DNA, displacing the other strand. This step is essential for the exchange of genetic material and repair of DNA.

Q: What experimental evidence supports the role of Rad51 in strand exchange?

Experimental evidence, such as gel electrophoresis and radioactive labeling, supports Rad51's role in strand exchange. These methods show the formation of joint molecules and nicked circles, confirming Rad51's ability to mediate strand exchange through base pairing.

Q: How does the unwinding of double-stranded DNA aid in strand exchange?

The unwinding of double-stranded DNA increases its length and exposes the bases, facilitating the strand exchange process. This structural change, induced by the Rad51 filament, allows easier access for the single-stranded DNA to form base pairs and displace one of the strands.

Summary & Key Takeaways

• The Rad51 protein forms a filament on single-stranded DNA, extending its length and facilitating strand exchange by binding nucleotides. This process involves forming base pairs between the single-stranded DNA and one of the strands from the double-stranded DNA, releasing the other strand.

• The RecA or Rad51 filament searches for homologous sequences in double-stranded DNA, leading to strand exchange reactions. This results in the formation of a nicked circle and a displaced single-strand DNA, which can be observed using gel electrophoresis techniques.

• Using radioactive phosphate labeling, the progression of strand exchange can be visualized, showing the formation of joint molecules and nicked circles. The appearance of single-stranded DNA after the reaction confirms the effectiveness of Rad51 and RecA in catalyzing strand invasion during homologous recombination.