Lecture 5: DNA Replication, Repair and Recombination 3 Flashcards
- Understand the mechanisms and possible consequences of gene conversion - learn the structure and transposition mechanisms of the 3 classes of transposons - discuss conservative site-specific recombination
_____ and _____ can occur in the same chromosome
crossing over; gene conversion
What does meiotic recombination occur with?
Paired maternal and paternal homologous chromosomes
What does homologous recombination mean?
to generate DNA molecules of novel sequence
What is the holiday junction?
DNA intermediate containing four DNA strands form two different helices
What are the two outcomes of resolution in homologous recombination?
- crossing over (rare)
- gene conversion (90% of Holliday junctions resolve this way)
What happens if both strands in each Holliday junction are cut in the same way?
They will separate with minimal exchange of sequences
What happens if strands in the Holliday junction are cut in opposite directions?
portions of each chromosome upstream and downstream are swapped, yielding crossing over
Once strand invasion occurs, the points of exchange can _______
move through branch migration
What happens in branch migration?
unpaired regions of one single strand displaces a paired region on the other
What is heteroduplex DNA?
region where a strand from the maternal homolog is base paired with a strong from the paternal homolog
What is gene conversion?
divergence from the expected distribution of alleles during meiosis
How does gene conversion occur?
- DNA synthesis during homologous recombination
- Repair of mismatches in regions of heteroduplex DNA
What are the three types of transpositional recombination?
- DNA only transposons
- Retroviral like retrotransposons
- nonretroviral like retrotransposons
Structure, enzyme and mode of movement for DNA only transposons
Structure: short inverted repeats at each end
enzyme: transposes
movement: moves as DNA, either by cut and paste or replicative pathways
Structure, enzyme and mode of movement for nonretroviral like retrotransposons
structure: poly A at 3’ end of RNA transcript, 5’ end is ofter truncated
enzyme: reverse transcriptase and endonuclease
moves: via an RNA intermediate that is often synthesized from a neighboring promoter
Structure, enzyme and mode of movement for retroviral like retrotransposons
structure: directly repeated long terminal repeats at each end
enzyme: reverse transcriptase and integrase
moves: via an RNA intermediate who’s production is driven by a promoter in the LTR
When two transposase molecules come together forming a loop juxtaposing two ends of element it forms a _____
transpososome
What does conservative site specific recombination do?
mediates rearrangements of other types of mobile DNA elements
How does conservative site specific recombination differ from transposition?
- need special sites on each DNA that serve as recognition sites for recombinase
- form transient high energy covalent bonds and use this energy to complete DNA arrangement
What does the outcome of conservative site specific recombination depend on?
orientation of the insertion site
If site are in the same orientation, then DNA sequence can ______
be integrated or excised
If sites are inverted in orientation, then DNA sequence is ______
inverted instead of excised
