DNA Replication, Repair and Recombination 3 (Lec 5) Flashcards
What is a holiday junction?
DNA intermediate containing four DNA strands from two different helices; structures are present only transeintly
What resolves a holiday junction?
strands of the helices are cleaved by endonuclease (RuvC)
What are the two outcomes of Holiday junction resolution?
crossing over, gene conversion
Meiotic recombination occurs between what types of chromosomes?
paired maternal and paternal
Meiotic recombination begins with what? What happens after that?
begins with a double strand break; then strand invasion and double holiday junction formation follow, resolution
True or False?
in homologous recombination, crossing over and gene conversion can occur in the same chromosome
true, allows for multiple opportunities for genetic reassortment
Divergence from the expected distribution of alleles during meiosis is due to what?
gene conversion
How does gene conversion occur?
DNA synthesis during homologous recombination, repair of mismatches in regions of heteroduplex DNA
HR must be regulated because recombination between repeated sequences could do what?
scramble the genome
Which repair pathway can interrupt HR and stop recombination events?
mismatch repair
Describe transposons
specialized segments of DNA that move from one position in the genome to another
What benefits can transposons offer?
antibiotic resistance in bacteria, produce genetic variation, no sequence homology required meaning they can insert anywhere
What two techniques do DNA only transposons utilize?
cut and past transposition or replicative transposition
How does cut and paste transposition work?
sequences on each end of element bind transposase, two transposase molecules come together forming a loop juxtaposing two ends of element, transposase introduces cuts at base of loop - removes element and forms central intermediate, central intermediate catalyzes direct attack on random site of target DNA, hole in the donor chromosome is repaired
Retroviruses, unlike tranposons, encode proteins that package their genetic info into virus particles that can do what?
infect other cells
Retroviruses use what type of enzyme to infect cell?
reverse transcriptase
What does integrate do in retroviruses?
creates 3’-OH on the DNA that attacks the host DNA
In regards to retroviruses, what synthesizes the new viral RNA?
host’s RNA polymerase
How do retrovirus-like transposons work?
entire transposon is transcribed by host, reverse transcriptase makes a double-stranded DNA copy of RNA molecule via hybrid DNA/RNA intermediate, double-stranded DNA integrates into site on chromosome using integral, encoded by element
Describe nonretroviral transposons
comprise a large portion of our genome, most are immobile but few retain ability to move, require endonuclease and reverse transcriptase to move
How do nonretroviral transposons work?
endonuclease and reverse transcriptase bind to L1 RNA, endonuclease nicks the target DNA at insertion point, release 3’-OH to serve as primer in reverse transcription step, single-stranded DNA copy of L1 directly linked to target DNA, insertion of double-stranded DNA copy of L1 at target site
What does conservative site specific recombination mediate?
mediates rearrangements of other types of mobile DNA elements
How does conservative site specific recombination work?
break and join two DNA helices on each molecule
How does conservative site recombination differ from transposition?
needs special sites on each DNA that serve as recognition sites for recombinase, forms transient high energy covalent bonds and use this energy to complete DNA rearrangement
What does the outcome of conservative site specific recombination depend on?
orientation of DNA sites
A. if sites are in same orientation, DNA sequence can be excised
B. if sites are inverted in orientation, DNA sequence is inverted instead of excised
Can site-specific recombination be used to turn genes on and off?
yes
