Chapter 13 notes
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Date Created: 04/05/16
Chapter 13: Meiosis and Sexual Life cycles In chapter 13 we will focus on a few basic questions: • What is sexual reproduction and how does it differ for asexual reproduction? • How are chromosomes passes for parents to offspring in sexual reproduction? • How does sexual reproduction contribute to genetic variation? Genetics: the scientific study of heredity and hereditary variation Genes: Hereditary units containing coded information • Genes are the genetic links to our parents • Our genes program the specific traits that emerge as we develop from fertilized eggs to adults • The genetic program is written in the language of DNA (AGTC nucleotides) • Most genes program cells to synthesize specific enzymes and other proteins whose cumulative action produce an organism’s inherited traits Gametes: vehicles that transmit genes from one generation to the next Traits: Freckles Example of a karyotype: A pair of homologous duplicated chromosomes Most DNA is found in chromosomes within the nucleus • How many pair of homologous chromosomes do human have? 23 • What species is this individual? Human • What is the sex of this individual? Male • What are these 2 copies of each chromosome (chromosomes 1-22) and two sex chromosmes? One copy comes from the male parent, the other copy comes from to female parent • What are the sex chromosomes? XX for female, XY for male Description of Chromosomes: example of a 2n cell depicted right after chromosome replication (G2 phase) Note: Chromosomes are shown in condensed form. However they are not condensed at the stage What are homologous chromosomes? A pair of chromosomes of the same length, centromere position, and staining pattern • Homologous chromosomes contain genes controlling the same inherited characters at the same location on the chromosomes (loci) What are alleles? Different versions of the same gene (A and a) Asexual Reproduction • Offspring are usually genetically identical to the parent cell (exceptions are due to mutation) • Single individual passes copies of all its genes to its offspring without fusion of gametes • These are many types of asexual reproduction such as fission, budding, fragmentation, and parthenogenesis • Binary fission: the two paramecium are identical Sexual Reproduction • Two parents give rise to offspring that have unique combinations or genes inherited from both parents • Offspring vary genetically from siblings and from both parents • Reproduction involves the fusion of gametes • Gametes are generated via meiosis • Family resemblance in sexual reproduction • Which of the following best describes the daughter cells after Meiosis? 4 haploid daughter cells • Which of the following best describes the daughter cells after Meiosis I? 2 haploid daughter cells • Which of the following best describes the daughter cells after mitosis? 2 diploid daughter cells Meiosis I: Separation of Homologous Chromosomes Prophase I • Chromosomes condense and homologous chromosomes align gene by gene • Synapsis takes place: this involves crossing over between nonsister chromatids within the homologous pairs • Synapsis ends in mid-prophase and chromosomes move apart slightly • Chiasmata: points where crossing over has occurred • This stage also entails centrosome movement, spindle formation, and nuclear envelope breakdown (as in prophase and prometaphase of mitosis) • Homologous pairs move as a unit towards the metaphase plate Crossing over and synapsis in prophase I: • DNA breaks at precisely corresponding points • Cohesion proteins hold sister chromatids together • Synaptonemal forming • This complex attaches one homolog to the other • Is fully “synapsis” • Broken DNA ends are joined. However, crossing over connects the corresponding ends of non sister chromatids • Homologs move slightly apart from each other but remain attached because of sister chromatid adhesion Closer Look: • Rec8 cohesion is the protein that hold sister chromatids together • After crossing over, homologous chromosomes are attached at chiasmata because of rec8 cohesion • In late prophase I, kinetochore microtubules originating from each pole attach to the kinetochores Metaphase I • Pairs of homologous chromosomes are arranged on the metaphase plate with one chromosome in each pair facing each pole • Both chromatids of one homolog are attached to kinetochore microtubules extending from the opposite pole • Those of the other homolog are attached to kinetochore microtubules extending from the opposite pole Anaphase I: • Breakdown of Rec8 cohesion by the enzyme, separase, allows the arms of homologs to separate • SGO Protects rec8 at centromere • This protection enables the sister chromatids to move as a unit towards the same pole • SGO is degraded after anaphase I is completed Telophase I: • At the beginning of telophase I, each half of the cell has a complete haploid set of replicated chromosomes • Each chromosome is composed of two sister chromatids • Note: sister chromatids are not identical due to crossover that occurred in prophase I • Cytokinesis occurs simultaneously with telophase I, forming two daughter cells Meisos II: Separation of sister chromatids Prophase II • Spindle apparatus forms • Chromosomes are still composed for two sister chromatid assosicated at the centromere • Chromsomes move towards metaphase II plate Metaphase II • Chromosomes are aligned on metaophase plate as in • The kinetochores of the sister chrimatids are attached to microtubles extending from opposite poles Anaphase II: • Lack of SGO allows the enzyme, Separase, to degrade rec8 at centromere region of sister chromatids • Enables sister chromatids to separate • Chromatids move towards opposite poles as individual chromosomes Telophose II and Cytokenisis • Nuclei form, the chromosomes begin to decondensem and cytokensis occurs • One parent produces 2 genetically different daughter cells • Remember: SGO was degraded right after anaphase I Meisos 1: Separation of homologous chromosomes Meiosis II: Separation of sister chromatids How od we account for the genetica variation that exists between offspring of orgnaisms taht reproduce sexually? 1. Independent Assortment of Chromosomes 2. Crossing over 3. Random Fertilization Independent assortment of chromosomes during :generates genetic variability in gametes • Can qualify the number of possible gametes comdinations = 2n where n=number of chromosome pairs • Crossing over during : generates gentic variablility in gametes • Key Point: crossing over generates identical chromosomes that carry genes from both parental and maternal chromsomes Random Fertilization: generates genetic variability in offspring • Number of gamete combinations: Egg= 2n = 2^23 = 8.4 million possible gamete combinations Sperm = 2n = 2^23=8.4 million possible gamete combinations • Number of Zygote combinations: 2^23 x 2^23 = 70 trillion possilbe zygote combinations • On top of this the varitation brought about by crossover!! Evolutionary Significance of Genetic Variation Within Populations: • New combinaitons of genes arise among offspring in a sexually reproducing population • Populations evolve through of its variant members • On average, individuals best suited for a local enviroment leave the - thus transmitting their genes • Results in the accumulation of genetic variants favored • As enviroment changes the population may survive if, in each generation, at least some of its members can cope effectively with the new conditions • Different combinations of alleles may work better than those that previously prevailed • Mutations are the original soucre of different alleles, which are then mixed and matched during meiosis • The ability of people to genate offspring is one of the most commonly proposed explainaiton for why sexual reproduction has persisted
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