GENETICS FINAL EXAMS| GENETICS TEST BANK|415 QUESTIONS WITH 100% CORRECT ANSWERS| LATEST UPDATE 2024|2025

GENETICS FINAL EXAMS| GENETICS TEST BANK|415 QUESTIONS WITH 100% CORRECT ANSWERS| LATEST UPDATE 2024|2025 - Crossover or DNA recombination occurs in eukaryotes between non-sister chromatids during prophase of meiosis Recombination also occurs between prokaryotes (via conjugation) and as a means of viral integration into a host cell The molecular mechanisms of crossover differ by species, but always in- volves the exchange of DNA between adjacent chromosomes These mechanisms are still the subject of much research and our view of the process changes frequently: Molecular basis of crossover Occurs in eukaryotes between non-sister chromatids during prophase of meiosis: Crossover or DNA recombination Non-sister: Crossover or DNA recombination occurs in eukaryotes between - chromatids during prophase of meiosis The traditional model for recombination between DNA strands occurred in three stages: Stage 1= initiation An endonuclease causes a single-stranded break in both adjacent chromo- somes Stage 2= strand-exchange Breakage of H-bonds causes strands to separate and a single-strand is exchanged between chromatids DNA ligase joins each strand to its recipient chromatid; the fusion point between chromatids is called a Holliday junction Stage 3= Holliday junction resolution The joined chromatids take on a cross-shaped form called a chi-form The chromatids can rotate about the junction point to create more than one orientation Another endonuclease nicks the strand to release the two molecules (orien- tation has an effect on outcome): The single-strand nick model Endonuclease: In the initiation stage (stage 1) of the single-strand nick model, a(n) causes a single-stranded break in both adjacent chromatids Joins each strand to its recipient chromatid: In the strand-exchange stage (stage 2) of the single-strand nick model, what is the role of DNA ligase? Fusion point between chromatids: Holliday junction Pairing of DNA strands from different origins: Heteroduplex Chi-form: In the Holliday junction resolution stage (stage 3) of the sin- gle-strand nick model, the joined chromatids take on a cross-shaped form called a - - Results of Holliday junction resolution are due to both rotation/orienta- tion and nick-site Nicking at the horizontal plane creates patch-recombinants Non-crossover recombinants= parental except for a small hete region Nicking at the vertical place results in splice recombinants Crossover recombinants= true recombinants that also include a small het- eroduplex region: Outcome of the single-strand nick model Patch: In the single-strand nick model, nicking at the horizontal plane creates recombinants Noncrossover: Patch recombinants that are created as a result of nicking at the horizontal plane are recombinants and are parental except for a small heteroduplex region Splice: In the single-strand nick model, nicking at the vertical plane creates recombinants Crossover: Splice recombinants that are created as a result of nicking at the vertical plane are recombinants and are true recombinants that also include a small heteroduplex region - The major difference between the older model and the newer model is the first stage New evidence suggests that recombination is initiated by a double-stranded break