Meiosis overview Living organisms are distinguished by their ability to reproduce their own kind Genetics is the scientific study of heredity and variation Heredity is the transmission of traits from one generation to the next Variation is demonstrated by the differences in appearance that offspring show from parents and siblings Genes are the units of heredity and are made up of segments of DNA Genes are passed to the next generation through reproductive cells called gametes sperm and eggs Copyright 2008 Pearson Education Inc publishing as Pearson Benjamin Cummings Comparison of Asexual and Sexual Reproduction Each gene has a specific location called a locus on a certain chromosome Most DNA is packaged into chromosomes One set of chromosomes is inherited from each parent In asexual reproduction one parent produces genetically identical offspring by mitosis A clone is a group of genetically identical individuals from the same parent In sexual reproduction two parents give rise to offspring that have unique combinations of genes inherited from the two parents Copyright 2008 Pearson Education Inc publishing as Pearson Benjamin Cummings Sets of Chromosomes in Human Cells Human somatic cells any cell other than a gamete have 23 pairs of chromosomes A karyotype is an ordered display of the pairs of chromosomes from a cell The two chromosomes in each pair are called homologous chromosomes or homologs Each pair of homologous chromosomes includes one chromosome from each parent Chromosomes in a homologous pair are the same length and carry genes controlling the same inherited characters Copyright 2008 Pearson Education Inc publishing as Pearson Benjamin Cummings APPLICATION Human males have one X and one Y chromosome The sex chromosomes are called X and Y The 22 pairs of chromosomes that do not determine sex are called autosomes TECHNIQUE 5 m Pair of homologous replicated chromosomes Centromere Sister chromatids Metaphase chromosome Human females have a homologous pair of X chromosomes XX The 46 chromosomes in a human somatic cell are two sets of 23 one from the mother and one from the father A diploid cell 2n has two sets of chromosomes For humans the diploid number is 46 2n 46 A gamete sperm or egg contains a single set of chromosomes and is haploid n Gametes are the only types of human cells produced by meiosis rather than mitosis For humans the haploid number is 23 n 23 In an unfertilized egg ovum the sex chromosome is X and in a sperm cell the sex chromosome may be either X or Y Copyright 2008 Pearson Education Inc publishing as Pearson Benjamin Cummings Behavior of Chromosome Sets in the Human Life Cycle Fertilization is the union of gametes the sperm and the egg The fertilized egg is called a zygote and has one set of chromosomes from each parent The zygote produces somatic cells by mitosis and develops into an adult At sexual maturity the ovaries and testes produce haploid gametes Gametes are the only types of human cells produced by meiosis rather than mitosis Meiosis results in one set of chromosomes in each gamete Copyright 2008 Pearson Education Inc publishing as Pearson Benjamin Cummings Fig 13 5 Key Haploid gametes n 23 Haploid n Egg n Diploid 2n Fertilization and meiosis alternate in sexual life cycles to maintain chromosome number Sperm n FERTILIZATION MEIOSIS Ovary Testis Diploid zygote 2n 46 Mitosis and development Multicellular diploid adults 2n 46 The alternation of meiosis and fertilization is common to all organisms that reproduce sexually In animals meiosis produces gametes which undergo no further cell division before fertilization Gametes are the only haploid cells in animals Gametes fuse to form a diploid zygote that divides by mitosis to develop into a multicellular organism Like mitosis meiosis is preceded by the replication of chromosomes Each replicated chromosome consists of two identical sister chromatids Copyright 2008 Pearson Education Inc publishing as Pearson Benjamin Cummings 1st Replication making copy of DNA chromosomes before Meiosis Key 2n 6 Maternal set of chromosomes n 3 Paternal set of chromosomes n 3 Two sister chromatids of one replicated chromosome Two nonsister chromatids in a homologous pair Centromere Pair of homologous chromosomes one from each set Meiosis reduces the number of chromosome sets from diploid to haploid Meiosis takes place in two sets of cell divisions called meiosis I and meiosis II The two cell divisions result in four daughter cells rather than the two daughter cells in mitosis Each daughter cell has only half as many chromosomes as the parent cell In the first cell division meiosis I homologous chromosomes separate In the second cell division meiosis II sister chromatids separate Copyright 2008 Pearson Education Inc publishing as Pearson Benjamin Cummings Fig 13 7 3 Interphase Homologous pair of chromosomes in diploid parent cell Two Stages of Meiosis Chromosomes replicate Homologous pair of replicated chromosomes 1 Meiosis I results in Two haploid daughter cells with replicated chromosomes Sister chromatids Diploid cell with replicated chromosomes Meiosis I 1 Homologous chromosomes separate Haploid cells with replicated chromosomes Meiosis II 2 Sister chromatids separate Haploid cells with unreplicated chromosomes 2 Meiosis II results in Four haploid daughter cells with unreplicated chromosomes Meiosis I Meiosis I is preceded by Interphase in which chromosomes are replicated to form sister chromatids The sister chromatids are genetically identical and joined at the centromere Division in meiosis I occurs in four phases 1 Prophase I 2 Metaphase I 3 Anaphase I 4 Telophase I and cytokinesis Copyright 2008 Pearson Education Inc publishing as Pearson Benjamin Cummings Fig 13 8a Prophase I Metaphase I Centrosome with centriole pair Sister chromatids Chiasmata Spindle Centromere with kinetochore Sister chromatids remain attached Metaphase plate Homologous chromosomes separate Homologous chromosomes Fragments of nuclear envelope Telophase I and Cytokinesis Anaphase I Microtubule attached to kinetochore Cleavage furrow 1 Prophase I crossing over happens Prophase I typically occupies more than 90 of the time required for meiosis This is when chromosomes begin to condense In synapsis homologous chromosomes loosely pair up aligned gene by gene In crossing over nonsister chromatids exchange DNA segments Each pair of chromosomes forms a tetrad a group of four chromatids Each tetrad usually has one or more chiasmata X shaped