Chapter 9 Genetic Recombination.docx

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University of Toronto Scarborough
Biological Sciences
Effiette Sauer

Chapter 9 Genetic Recombination 9.1 Mechanism of Genetic Recombination  Genetic Recombination requires two things: DNA molecules that differ from one another and DNA molecules in close proximity and enzymes that cut exchange and paste DNA together o 1) two molecules of DNA that have similar sequences are moved close together o 2) the DNA then breaks the H bonds between the bases and unwinds the DNA o 3) enzymes nick the DNA backbones and exchanges the ends and reattaches them  Homologous: when regions or whole chromosomes sequence of DNA are similar but not identical 9.2b Bacterial Conjugation Brings DNA of Two Cells into Close Proximity  In bacteria genetic transfer is in one direction from donor to recipient  They do not fuse together, they conjugate o They use a sex pillus that forms a cytoplasmic bridge from one cells to the next  Conjugation is initiated by a bacteria that has a plasmid in addition to their chromosome  Certain plasmid called the fertility plasmid, F factor carries genes that permit a copy to passed on to daughter cells during the process of binary fission, but there is vertical inheritance  F factor can be copied and sent to recipient cell, called horizontal inheritance  During conjugation F plamid replicates using a special type of replication called the rolling circle o Similar to a roll of tape, or a spool o No chromosomal DNA has been passed on to recipient cell 9.2c Transformation and Transduction Provide Additional Sources of DNA for Recombination  Transformation: bacterial take up pieces of DNA that are released into the external environment by degrading cells  Transduction: DNA is transferred from donor to recipient cells inside the head of an infecting bacterial virus o Bacterial viruses called bacteriophages o Generalized Transduction: all donor genes are equally likely to be transferred, is associated with some virulent phages, which are likely to kill the host cell upon injection  i.e. lytic cycle o specialized transduction: uses lambda is a temperate phage, will enter the cell and lines up with small regions of homology and combine itself with the bacteria’s chromosomal DNA  i.e. lysogenic cycle  can go from lysogenic cycle to lytic cycle by a trigger 9.3 Genetic Recombination in Eukaryotes: Meiosis  Sexual Reproduction: the production of offspring through the combination of male and female gametes  Gametes: eggs and sperm,  Meiosis: specialised process of cell division that recombines DNA sequence and produced cells with half the number of somatic chromosomes  Somatic cells: body cells  Fertilization: when an egg and sperm fuse together forming a zygote 9.3a Meiosis Occurs in Different Places in Different Organismal Life Cycles  One basic set of chromosome is haploid, two sets of chromosome is diploid 9.3b Meiosis Changes Both Chromosomes Number and DNA Sequence  Meiosis occurs in specialized tissues and promote difference through recombination and halved chromosomes  Diploid cells constitute homologous pairs o Same genes arranged in the same order o One is the paternal chromosome, the other is the maternal chromosome o Although they are the same genes they can have different alleles 9.3c Meiosis Produces Four Genetically Different Daughter Cells  Sexual Reproduction: the production of offspring through the combination of male and female gametes  Premeotic interphase cells enter the first of the two meiotic division o Meiosis I: homologous pairs find their partners and pair lengthwise gene for gene, process called synapsis  Recombination occurs the first division occurs  After meiosis II: cells are now haploid  Interkinesis: brief interphase that separates the two meiotic division but no replication occurs here  Prophase I:replicated chromosomes fold and condense, each consisting of two sister chromatids o Pairing or Synapsis: two chromosomes line up side by side in a zipperlike way o Tetrads: fully paired homologous pairs consists of four chromatids o When paired they exchange segments of DNA, allele exchange  Prometaphase I: nuclear envelope breaks down, spindles enter around the former nuclear area o Two Chromosomes are anchored to the spindles by the kinetochores  Metaphase and Anaphase I: spindle microtubules line up along the metaphase plate o Two chromosomes of the homologous pair separate and move to opposite poles o Delivering haploid structures to each pole, they still consist of two sister chromatids joined by a centromere  Telophase I and Interkinesis: telophase I very little occurs o Interkinesis disassembles the one spindle fibre and assembles 2 new spindles  Prophase II, Prometaphase II and Metaphase II: o Prophase II: ch
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