12. Genetic recombination Flashcards
Define genetic recombination
Genetic recombination - a process in which genetic material is exchanged between 2 or more DNA molecules or chromosomes
Define homologous recombination
Homologous recombination - crossing-over - a recombination process in meiosis which involves exchange of genetic material between homologous chromosomes
Define homologous sequence
Homologous sequence - a segment of genetic material that shares common ancestry or evolutionary origin with another segment, perform similar / related function
Define nuclease
Nuclease - an enzyme that cleaves / degrades nucleic acids
Define recombinase
Recombinase - an enzyme that breaks and rejoins DNA strands in exchange of genetic material in recombination in homologous chromosomes / plasmids
Define resolvase
Resolvase - an enzyme that cleaves and resolves Holliday junctions formed in homologous / site-specific recombination
Define Holliday junctions
Holliday junctions - four stranded DNA/RNA structure formed in genetic recombination processes like homologous recombination
Define non-homologous end joining
Non-homologous end joining (NHEJ) - DNA repair pathway used to repair ds breaks (DSB) in DNA - NO homologous template used (as used in HR)
May result in small insertions/deletions - mutations
Define transposition
Transposition - a process in which DNA sequences - transposons (“jumping genes”) - move within + between genomes
Define site-specific recombination
Site specific recombination - a precise + highly regulated process in which DNA strands are exchanged at specific recognition sites
Define genetic marker
Genetic marker - specific, identifiable DNA sequence that serves as a reference point for locating genes or other functional elements
Define chromosome dimer
Chromosome dimer - structure formed when 2 chromosomes / DNA molecules become fused / stuck together
Define phase variation
Phase variation - a phenomenon where certain genes / sequences undergo reversible / periodic / random changes in their expression / sequence
Ex: phase variation in bacterial antigen expression
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Lecture quiz question 5
What are the possible mechanisms for genetic recombination? Which of them don’t affect chromosome integrity?
Mechanisms for genetic recombination:
- homologous recombination (HR)
- non-homologous end joining (NHEJ)
- transposition
- site-specific recombination
- independent assortment of chromosomes -> doesn’t affect chromosome integrity - no direct effect on genome stability (the only)
Explain the process of homologous recombination
HR:
1) DNA ds break
2) RecBCD (nuclease) digest ends
3) RecA (recombinase) loaded onto 3’ - broken DNA invades other sequence
4) Synthesis of complimentary strands
5) RuvABC (resolvase) cleaves HJ: horizontally / vertically at 2 sites
6) Crossover only occurs if cut differently at each HJ site (1 change of direction)
When does a cell carry out HR?
HR occurs when:
- ds DNA break
- ds DNA fragment introduced
- ss DNA break that can invade other ds DNA
Explain the process of non-homologous end joining
NHEJ:
1) ds DNA break
2) Ku added
3) break end processing (+/-nt, +Pi by diff enzymes) without homologous template - new fragments added
4) Ligation of the ends by Lig4
What is site specific recombination? What are possible target site states?
Site-specific recombination: exchange between pairs of defined sequences - target sites - residing on the same DNA / different DNAs
Target site orientation:
- head-to-head (H-H)
- head-to-tail (H-T)
What are the possible outcomes of head-to-head and head-to-tail outcomes?
H-H: inversion
H-T: deletion / insertion
What are the examples of site specific recombination?
E. coli chromosome dimer resolution: circular molecule undergoes crossover:
- if one recombination -> one large circular
- if two recombinations -> two smaller circular chromosomes
Site specific recombination bacteriophage lembda example
Site specific recombination flagellar phase variation example in Salmonella
Site specific recombination in bacteriophage Mu tail fibre example
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