IMPORTANCE Crossing_Over_and_Recombination.pptx

THE IMPORTANCE OF CROSSING OVER AND RECOMBINATION

1. Learning Objectives • Define crossing over and genetic recombination • Explain when and where crossing over occurs during meiosis • Describe the molecular mechanism of crossing over • Analyze the importance of crossing over for genetic diversity • Predict outcomes of crossing over in genetic problems • Identify disorders from meiosis malfunctions

2. Key Vocabulary Crossing Over: Exchange of genetic material between homologous chromosomes Recombination: New combinations of alleles from crossing over Homologous Chromosomes: Same genes, possibly different alleles Sister Chromatids: Identical copies of a chromosome Synapsis: Pairing of homologous chromosomes Bivalent/Tetrad: Paired homologous chromosomes Chiasma: Visible point of crossing over Alleles: Different versions of the same gene

3. Meiosis Recap Purpose: Produces gametes (sperm & egg) Chromosome number reduced from diploid to haploid Two divisions: Meiosis I & II Crossing over occurs ONLY in Meiosis I Importance: Creates genetic diversity, essential for reproduction, enables adaptation

4. When Crossing Over Happens Occurs ONLY in Prophase I of Meiosis I Stages: 1. Leptotene: Chromosomes condense 2. Zygotene: Homologs pair (synapsis) 3. Pachytene: Crossing over occurs 4. Diplotene: Chiasmata visible 5. Diakinesis: Ready for metaphase I

5. Molecular Mechanism 1. Synapsis formation: Synaptonemal complex forms 2. DNA breaking: SPO11 makes double-strand breaks 3. Strand exchange: Holliday junction forms 4. Resolution: DNA repaired and segments swapped

6. Before & After Crossing Over Before: Chromosomes carry parental combinations After: Recombinant chromosomes with new allele combos No mutation, only rearrangement

7. Eye Color Example Without crossing over: BC, bs only With crossing over: BC, bs, Bs, bC Increases genetic diversity in offspring

8. Crossing Over Frequency Frequency varies by chromosome size & gene distance Measured in map units (centimorgans) RF formula: (Recombinants / Total) × 100% Max RF: 50%

9. Recombination Types Intrachromosomal: Within same chromosome Interchromosomal: Between chromosomes (independent assortment) Crossing over = process, Recombination = result

10. Parental vs Recombinant Types Parental: Same allele combos as parents Recombinant: New combos from crossing over RF shows linkage degree

11. Importance Creates diversity, drives evolution Medical applications: Disease mapping, genetic counseling Agriculture: Crop & livestock improvement Conservation: Maintain genetic variation

12. Disorders from Meiosis Errors Nondisjunction → Aneuploidy Examples: Down syndrome, Klinefelter, Turner Crossing over errors → deletions/duplications Prevention: Prenatal screening, genetic counseling

13. Practice Problem NnGg fly → NG, ng, Ng, nG Without crossing over: Only NG, ng RF = (Recombinants / Total) × 100%

14. Common Student Questions Why not in mitosis? → No diversity needed Can it go wrong? → Yes, disorders All chromosomes? → Usually yes Max RF 50%? → Independent assortment

15. Applications Medical: Genetic counseling, personalized medicine Research: Genome mapping, evolutionary studies Agriculture: Crop improvement

16. Quick Quiz 1. b 2. b 3. b 4. c 5. b

17. Summary Crossing over happens in Prophase I Creates genetic diversity Involves homologous chromosomes Errors can cause disorders Essential for evolution and adaptation