Key Concepts Meiosis is a type of nuclear division It results in cells that have half as many chromosomes as the parent cell In animals it leads to the formation of eggs and sperm Each cell produced by meiosis receives a different combination of chromosomes Because genes are located on chromosomes each cell produced by meiosis receives a different complement of genes Meiosis leads to offspring that are genetically distinct from each other and from their parents 2011 Pearson Education Inc Key Concepts The leading hypothesis to explain meiosis is that genetically variable offspring are more likely to thrive in environments where parasites and disease are common If mistakes occur during meiosis the resulting egg and sperm cells may contain the wrong number of chromosomes It is rare for offspring with an incorrect number of chromosomes to develop normally 2011 Pearson Education Inc Introduction During sexual reproduction a sperm and an egg unite to form a new individual This process is called fertilization Meiosis is nuclear division that precedes the formation of gametes egg and sperm and results in a halving of chromosome number 2011 Pearson Education Inc Chromosomes Come in Distinct Types Each organism has a characteristic number of chromosomes The karyotype is the number and types of chromosomes present in an organism Sex chromosomes determine the sex of the individual all other chromosomes are autosomes Humans have 46 chromosomes in every cell except their gametes 1 pair of sex chromosomes 22 pairs of autosomes 2011 Pearson Education Inc 2011 Pearson Education Inc Homologous Chromosomes Have the Same Genes Chromosomes of the same type are called homologous chromosomes or homologs Chromosomes carry genes A gene is a section of DNA that influences one or more hereditary traits in an individual Different versions of a specific gene are called alleles Homologs carry the same genes in the same locations but each one may contain different alleles 2011 Pearson Education Inc 2011 Pearson Education Inc The Concept of Ploidy The haploid number n indicates the number of distinct types of chromosomes present A cell s ploidy n 2n 3n etc indicates the number of each type of chromosome present 2011 Pearson Education Inc Ploidy Varies among Organisms Organisms whose cells contain just one of each type of chromosome are called haploid n Those whose cells contain two versions of each type of chromosome are termed diploid 2n Diploid cells have one paternal chromosome and one maternal chromosome Organisms with three or more versions of each type of chromosome are called polyploid 3n 4n etc 2011 Pearson Education Inc 2011 Pearson Education Inc An Overview of Meiosis Meiosis reduces chromosome number by half In diploid organisms the products of meiosis are haploid Just before meiosis begins each chromosome in the diploid 2n parent cell is replicated When replication is complete each chromosome consists of two identical sister chromatids attached at the centromere 2011 Pearson Education Inc 2011 Pearson Education Inc Meiosis Is Two Cell Divisions Meiosis consists of two cell divisions called meiosis I and meiosis II The two divisions occur consecutively but differ sharply 2011 Pearson Education Inc An Overview of Meiosis I During meiosis I the diploid 2n parent cell produces two haploid n daughter cells The homologs in each chromosome pair separate and go to different daughter cells Although the daughter cells are haploid n each chromosome still consists of two identical sister chromatids 2011 Pearson Education Inc An Overview of Meiosis II During meiosis II the sister chromatids of each chromosome separate and go to different daughter cells The four haploid daughter cells produced by meiosis II also have one of each type of chromosome but now the chromosomes are unreplicated 2011 Pearson Education Inc 2011 Pearson Education Inc Meiosis Is a Reduction Division The outcome of meiosis is a reduction in chromosome number For this reason meiosis is known as a reduction division In most plants and animals the original cell is diploid and the four daughter cells are haploid In animals these daughter cells become gametes via a process called gametogenesis 2011 Pearson Education Inc Fertilization Results in a Diploid Zygote When two haploid gametes fuse during fertilization a full complement of chromosomes is restored The cell that results from fertilization is diploid and is called a zygote In this way each diploid individual receives a haploid chromosome set from both its mother and its father Homologs are therefore referred to as being either maternal chromosomes from the mother or paternal chromosomes from the father 2011 Pearson Education Inc 2011 Pearson Education Inc The Life Cycle of a Sexual Organism An animal s life cycle summarizes life from fertilization through offspring production Meiosis in an adult produces haploid gametes that combine during fertilization to form a diploid zygote which develops through mitosis into an adult of the next generation 2011 Pearson Education Inc 2011 Pearson Education Inc The Phases of Meiosis I Meiosis I is a continuous process with five distinct phases These phases are as follows 1 Early prophase I 2 Late prophase I 3 Metaphase I 4 Anaphase I 5 Telophase I 2011 Pearson Education Inc 2011 Pearson Education Inc 2011 Pearson Education Inc 2011 Pearson Education Inc 2011 Pearson Education Inc 2011 Pearson Education Inc 2011 Pearson Education Inc 2011 Pearson Education Inc 2011 Pearson Education Inc The Phases of Meiosis I Early Prophase I The homolog pairs come together in a pairing process called synapsis The structure that results from synapsis is called a tetrad consisting of two homologs The chromatids of the homologs are called non sister chromatids Late Prophase I These non sister chromatids begin to separate Exchange or crossing over between homologous non sister chromatids occurs where chiasmata are formed during this stage Metaphase I The tetrads line up at the metaphase plate 2011 Pearson Education Inc The Phases of Meiosis I Anaphase I The paired homologs separate and begin to migrate to opposite ends of the cell Telophase I The homologs finish migrating to the poles of the cell Then the cell divides in the process of cytokinesis 2011 Pearson Education Inc The Result of Meiosis I The end result of meiosis I is that one chromosome of each homologous pair is distributed to a different daughter cell A reduction division has occurred The daughter cells of