Chapter 8 - Genetic Recombination PDF

Chapter 8 Genetic Recombination Have you ever played the lottery?  Why are we so diverse? 1. Mutation 2. Random Fertilization 3. Recombination Why is it important to be diverse? Mechanism of Genetic Recombination a. Requires 2 DNA molecules that similar but non- identical b. Hom...

Chapter 8 Genetic Recombination Have you ever played the lottery?  Why are we so diverse? 1. Mutation 2. Random Fertilization 3. Recombination Why is it important to be diverse? Mechanism of Genetic Recombination a. Requires 2 DNA molecules that similar but non- identical b. Homology allows DNA on different molecules to line up and recombine precisely c. Enzymatic cutting and pasting of both DNA backbones from each of 2 DNA molecules required for recombination So what kinds of DNA are similar but not identical? REMEMBER… Homologous chromosomes Simplified Model of Genetic Recombination Homologous 2 similar but nonidentical DNA Enzymatic ‘Cut and Paste’ 2 Recombined DNA Genetic Recombination in Bacteria a. Genetic recombination occurs in E. coli b. Bacterial conjugation brings DNA of two cells into close proximity c. Transformation and transduction provide additional sources of DNA for recombination Genetic Recombination in Bacteria Some bacteria genetically reshuffle – Genes transferred from one individual to another recombine with existing DNA How did they figure this out? – Genetically identical clones allow molecular genetics studies  Genetic Recombination in E. coli Prototrophs - Bacteria grow on minimal media because they make their own amino acids Auxotrophs – Bacteria with mutations does ____ grow on minimal medium – Three letter gene name: – + normal, – mutated allele – Ex. arg+ and arg¯ Complete medium - full complement of nutrients Minimal media – missing some nutrients  Replica Plating Replica plating – is a technique that is used to: 1. identify prototrophs versus auxotrophs 2. identify and count genetic recombinations in bacterial colonies  Replica Plating Which are prototrophs and which are auxotrophs? Prototrophs Prototrophs only and auxotrophs NO Auxotrophs  What is the experimental evidence for Genetic Recombination in Bacteria? Lederberg & Tatum But how does this happen?? 1. Bacterial Conjugation Bacterial recombination by conjugation – Bacteria are haploid – ________ connects two bacteria – Donor sends DNA via cytoplasmic bridge to recipient Recipient undergoes recombination – Plasmids: Circular, nonchromosomal transferable DNA – R plasmids confer resistance to antibiotics __________ Gene Transfer Conjugating E. coli Cells F Factor and Conjugation Donor cell must have ________(fertility) plasmid – F+ cells are donors with F factor – F- cells are recipients without F factor F factor has genes that encode for sex pilus – cytoplasmically connects F+ cell to F- cell – F- cell converts to F+ cell – ___ recombination occurs Transfer of Genetic Material During Conjugation Bacterial chromosome Rolling Circle Replication Yes F plasmid is transferred ___ chromosomal DNA transferred ___ DNA recombination  Hfr Cells and Recombination Hfr (high frequency) cells integrate F factor into bacterial chromosome through recombination – Hfr cells can conjugate with F- cells – Recipient becomes _________ Genetic recombination occurs by double crossing- over in recipient – New generations have __________  Transfer of Genetic Material During Conjugation Partial Diploid ___ partial chromosomal DNA F factor into transferred chromosomal DNA = Hfr cell ___ DNA recombination Mapping Genes by Conjugation Mated Hfr and F- cells that differed in number of alleles At regular intervals after conjugation commenced, remove cells and break apart mating pairs Cultured separated cells and analyzed for recombinants Mapping Genes by Conjugation Greater time to conjugate before separation, the greater number of donor genes into recipient  greater number of ________ Note order and time at which genes were transferred --> able to map and assign relative positions of several genes of E. coli chromosome  2. Transformation Transformation occurs when bacteria take up DNA from disintegrated bacteria – Linear fragments recombine by double crossovers – Transformation bacteria usually have DNA binding protein in wall Artificial transformation – Alters cell membrane for DNA penetration – Electroporation _______Gene Transfer  3. Transduction Transduction occurs when bacterial phages transfer DNA from one bacteria to another Virus incorporates DNA fragments from host cell – If DNA fragments are homologous  bacteria become partial diploid – Recombination by double crossovers ________Gene Transfer  3a. Generalized Transduction Phage Proteins Phage Attachment Phage Assembly Phage Enzymes Phage Release Phage DNA Replication Phage with _______DNA Recombination can now occur 3b. Specialized Transduction Prophage Fig. 10-8, p. 208 Virulent vs. Temperate Bacteriophage Virulent Bacteriophage – uses only lytic cycle of infection  ____ host bacteria Temperate Bacteriophage – uses both lysogenic and lytic cycle of infection  may or may not kill host bacteria Prophage – bacteriophage integrated into host DNA  Genetic Recombination in Eukaryotes: Meiosis a. Meiosis occurs in different places in different organismal life cycles b. Meiosis changes both chromosome number and DNA sequence c. Meiosis produces four genetically different daughter cells d. Several mechanisms contribute genetic diversity  Sexual Reproduction Sexual reproduction produces offspring by union of male and female gametes (sperm and egg) – Meiosis produces gametes with half chromosome number – Evolutionary advantage: __________  Fertilization Fertilization fuses nuclei of egg and sperm  Zygote – Restores parental chromosome number  Animal Life Cycles Diploid phase dominates animal life cycles – Meiosis followed directly by gamete formation – Haploid phase is reduced and short, no mitosis In males, all four nuclei from meiosis form separate sperm cells In females, only ______ nucleus becomes an egg Homologous Chromosome Pairs Paternal chromosomes from male parent Maternal chromosomes from female parent – Homologous chromosome pairs – Alleles may be different within homologous pairs Meiosis separates homologous pairs – Before meiosis - diploid (2n) – After meiosis - haploid (n) Meiosis I Meiosis I: First meiotic division – Recombination exchanges segments between homologues – Produces two haploid cells with chromatids attached  Meiosis II Meiosis II: Second meiotic division – Sister chromatids separate into separate cells – Produces 4 recombined haploid cells 2 Meiotic Divisions Give 4 Haploid Nuclei Diploid Sister Chromatids Recombination Haploid Non-identical  Meiotic Cell Cycle Prophase I - Sister chromatids condense Recombination chromosomes – Synapsis Synapsis pairing of homologs – Tetrads  fully paired homologs – Recombination  mixes alleles across tetrads Meiotic Cell Cycle Prometaphase I – Nuclear envelope breaks down – Kinetochores attach to polar spindles Meiotic Cell Cycle Metaphase I and Anaphase I – Tetrads align on metaphase plate – Homologs segregate - move to poles (sister chromatids attached) – Nondisjunction creates abnormal chromosome numbers Random Alignment  Meiotic Divisions Telophase I and Interkinesis – NO change in chromosomes – Spindle disassembles Fig. 10-11, p. 214 Meiotic Cell Cycle (cont’d) Prophase II, Prometaphase II – Chromosomes condense, spindles form – Nuclear envelope breaks, kinetochores attach to microtubules Meiotic Cell Cycle (cont’d) Metaphase II – Chromosomes align on metaphase plate Random Alignment Meiotic Cell Cycle (cont’d) Anaphase II and Telophase II – Spindles separate chromatids – Spindles disassemble – New nuclear envelopes form ___ genetically different haploid cells  Nondisjunction Both members of pair of homologous chromosomes connect to spindles from same pole Following anaphase, one pole then receives both copies of pair, and other pole receives none Result is gametes that have 2 copies of a chromosome After fertilization, zygote has 3 copies of chromosome instead of 2 Ex. Trisomy 21  Sex Chromosomes in Meiosis Meiosis and sex chromosome inheritance: – Gametes produced by females may receive either X chromosome – Gametes produce by males may receive either X or Y chromosome  Meiosis and Mitosis Compared Mitosis and meiosis compared – Both: Similar cell divisions, meiosis divides twice – Mitosis: Two identical daughter cells (diploid) – Meiosis: Four genetically different cells (haploid) Premeiotic interphases similar to mitotic interphase – Chromosomes copied into sister chromatids  Comparison of Key Steps in Meiosis and Mitosis Random Alignment 2 identical diploid cells 4 non-identical haploid cells Genetic Variability Four main ways: 1. Genetic recombination 2. Random segregated at anaphase I 3. Alternative combo at anaphase II 4. Random fertilization  1. Genetic Recombination Recombination (_________) – Key genetic shuffle of prophase I Tetrads held together at synaptonemal complex – 2 of 4 chromatids exchange alleles – Chiasmata or crossovers are points of exchange Crossing-Over Where does this happen? Between non-sister chromatids Synaptonemal Complex  2. Random Segregation Random segregation – Key genetic shuffle of metaphase I Each chromosome of a homologous pair may randomly end up at either spindle pole – Any combo of maternal and paternal chromosomes  segregated to gametes – 2X number of possible combinations  Random Spindle Connections At Metaphase I Chromosomes line up randomly 3. Alternative combo at anaphase II At Metaphase II Attachment of spindle to kinetochore on sister chromatids is random Therefore alignment is random.  4. Random Fertilization Random chance of male and female gamete forming zygote Meiosis allows randomness necessary for Mendelian laws of inheritance Putting it into perspective 1. How is diversity of genetic material advantageous? 2. What are the mechanisms in prokarytes to achieve genetic diversity? 3. What are the mechanisms in eukarytoes to achieve genetic diversity?