Lecture 5 - DNA Replication, Repair, and Recombination 3 Flashcards
What are the two types of homologous recombination?
Crossing over and gene conversion
What are the steps of meiotic recombination that results in crossing over?
Formation of a double strand break by Spo11 followed by recognition and processing of the ends by Mre11.
Strand invasion occurs using Rec-A forming a Holliday junction(s).
If two Holliday junctions form and are cut in opposite directions, crossing over occurs.
If one Holliday junction forms or the double Holliday junction is resolved by cutting in the same direction, no crossing over occurs.
What is a Holliday junction
Transient structure consisting of 4 DNA strands. It is bound by RuvA and RuvB and resolved by RuvC.
What is branch migration and how does it happen?
When the exchange point of the invading strand moves. Happens spontaneously or with the help of special helicases (requires ATP).
What is a heteroduplex?
A region of DNA where DNA is paired with DNA its homolog. It occurs a sites of gene conversion and crossing over.
What is gene conversion and how does it occur?
Conversion of a stretch of heteroduplex due to repair of mismatched base pairs. results in 3 sets of a parental allele and 1 of the other.
What are the types of transpositional recombination?
DNA-only transposons
Retroviral-like retrotransposons
Nonretroviral retrotransposons
What are the characteristics of DNA-only transposons?
- short inverted repeats at each end
- transposase
- moves as DNA
What are the characteristics of retroviral-like retrotransposons?
- long terminal repeats at each end
- reverse transcriptase and integrase
- moves as RNA
What are the characteristics of nonretroviral retrotransposons?
- poly A 3’ RNA
- reverse transcriptase and endonuclease
- moves as RNA
Describe the process of DNA-only transposition.
Transposase binds short inverted repeats and come together to form a loop of DNA (transposome). Transposase then cuts transposome out creating a central intermediate. Central intermediate catalyzes a staggered break in target DNA. DNA inserts itself and the gaps are sealed with DNA polymerase and ligase.
How can a DNA-only transposon be restored at the donor site? What are the alternatives?
If DNA was recently replicated the identical copy can be used as a template to restore the transposon.
Homologous recombination results in the transposon not being restored.
Nonhomologous end joining results in mutation at the break site.
Describe the process of retroviral-like transposition.
Transposon DNA is transcribed into RNA. RNA transcript contains a reverse transcriptase and integrase. Reverse transcriptase creates DNA using RNA creating DNA/RNA intermediate and then replaces RNA to make double stranded DNA. Integrase creates a staggered break in target DNA where transposon is inserted. Gaps are filled by DNA polymerase and sealed with ligase.
Describe the process of nonretroviral transposition.
Transposon DNA is transcribed to into RNA. RNA transcript contains reverse transcriptase and endonuclease. Endonuclease nicks target DNA. RNA is used as template for DNA synthesis at the nick by reverse transcriptase. Complimentary DNA is made and double stranded DNA is inserted into genome.
What is conservative site specific recombination?
Movement of mobile DNA elements that require specific sequences on either end of the element.
