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11/27/2014 Genetic recombination Genetic recombination Obtaining of new gene combinations by rearrangements in genetic material: Between two DNA molecules Between two different chromosomes Between two sets of chromosomes 1 11/27/2014 Biological importance of recombination Genetic variability Natural selection Biologic evolution Recombination and phenotype consequences Genetic polymorphism Normal phenotypic polymorphism Different reaction to environment (drugs, food, toxins...) Susceptibility to infections Predisposition to diseases (cancers, diabetes...) Abnormal phenotypic polymorphism Types of recombination Genomic – between ♀ and ♂ genomes 23 chrs ♀ + 23 chrs ♂ = 46 Chromosomal During fertilisation Inter-chromosomal – between nonhomologous chromosomes; Intra-chromosomal – between homologous chromosomes Gene – between two alleles by gene fusion During meiosis During meiosis 2 11/27/2014 ? What is fertilization? Fusion of two sexual cells of different origin (haploid gametes; 23 chrs) formation of zygote (diploid cell 46 chrs). What is meiosis? Formation of haploid cells called gametes (by reductional division a cell with 46 chrs makes haploid cells with 23 chrs) Passing of genetic material from parents to children Mitosis, differentiation, growing Mitosis, differentiation, growing Copil (2n) Adulţi (2n) Zigot (2n) Meiosis Sperm (n) Fertilization Egg (n) OVOGENESIS SPERMATOGENESIS Peculiarities of segregation of chromosomes in meiosis during ovogenesis and spermatogenesis 3 11/27/2014 Mitosis vs. Meiosis 46 SC chrs 46 SC chrs Replication Replication 46 BC chrs 46 BC chrs Mitosis 46 SC chrs Meiosis 46 SC chrs 23 BC chrs I 23 BC chrs II Somatic cells 23 SC chrs 23 SC chrs 23 SC chrs 23 SC chrs Sexual cells = gametes 2n=2c Mitosis Meiosis ! Bivalents at equator 2n=4c MI M ! BC Chrs to poles AI A TI T 2n=2c ! n=2c MII 2n=2c AII TII ! MC Chrs to poles ! n=c Meiosis I division – reductional (2n=4cn=2c) Prophase I Metaphase I Anaphase I Telophase I II Division – equational (n=2cn=c) Prophase II Metaphase II Anaphase II Telophase II 4 11/27/2014 Meiosis I – reductional Prophase I Condensation of chromosomes Formation of spindle !!! Conjugation of homologous chromosomes bivalents !!! Crossing-over intra-chromosomal recombination There are 5 phases: L, P, Z, D, D Prophase I Leptotene Chromatin condensation Zygotene !!! Conjugation of homologous chromosomes Pachytene Diplotene Diakinezis !!! Crossingover Pushing of chromosomes Chiasma termination Conjugation of homologous chromosomes ♀ ♂ Takes place during prophase I Synapses of homologous chromosomes Ensured by synaptonemal complex Importance – formation of bivalents 5 11/27/2014 Synaptonemal complex Scaffold Chromatin of paternal sister chromatids Recombination nodules Chromatin of maternal sister chromatids Crossing-over Takes place during prophase I Exchange of fragments between homologous chromosomes intrachromosomal recombination Determined by synaptonemal complex Importance – genetic variability Crossover gametes Non-crossover gametes 6 11/27/2014 Molecular mechanism of crossing-over 7 11/27/2014 8 11/27/2014 Metaphase I Bivalents at equator !!! Random orientation of chromosomes to poles Important or Anaphase I Disjunction of homologous chromosomes Migration of BC chromosomes to poles (n chromosomes to each poles) !!! Independent combination of nonhomologous chromosomes = interchromosomal recombination 2n combination or AbC aBc or or ABC abc Abc aBC ABc abC Interchromosomal recombination Takes place during anaphase I Between non-homologous maternal and paternal chromosomes Random separation of non-homologous chromosomes Importance genetic variability of gametes 9 11/27/2014 Telophase I Reorganization of two nuclei (n=2c) Citokinesis Two gametocytes (haploid, chromosomes BC) !!! Reduction of number of chromosomes n=2c 2n=4c n=2c Interkinesis Preparation to second division (meiosis II) Meiosis II – equational division Prophase II Formation of spindle Maturation of kinetocores and 10 11/27/2014 Metaphase II At equator SC chromosomes and Anaphase II Longitudinal cleavage of centromere Disjunction of chromatids Migration of SC chromosomes (n=c chrs) and Telophase II Reorganization of nuclear envelopes and formation of haploid nuclei (n=c) Cytokinesis 4 haploid gametes n=2c II div I div n=c n=c II div n=c 2n=4c n=2c n=c 11 11/27/2014 Biological importance of meiosis Maturing of gametes Genetic recombination Multiplication of gametes n=2c n=c II div I div n=c II div n=c 2n=4c n=2c Ovogenesis Ovogonia n=c Spermatogenesis Spermatogonia Replication Replication 1st spermatocyte 1st ovocyte Meiosis I (reductional) 2nd ovocyte Meiosis I (reductional) Polar body 2nd spermatocytes Meiosis II (equational) Meiosis II (equational) Egg Spermatides Spermatozoon Polar bodies Formation of sperms Fecundation) Zygote Spermatozoon 12