Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Inheritance of Genes Genes are the units of heredity Genes are segments of DNA Each gene has a specific locus on a certain chromosome One set of chromosomes is inherited from each parent Reproductive cells called gametes (sperm and eggs) unite, passing genes to the next generation

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Comparison of Asexual and Sexual Reproduction In asexual reproduction, one parent produces genetically identical offspring by mitosis In sexual reproduction, two gametes (egg and sperm) give rise to offspring that have unique combinations of genes inherited from the two parents What about hermaphrodites? Sexual reproduction or not? Video: Hydra Budding Video: Hydra Budding

LE 13-2 Parent 0.5 mm Bud

LE 13-5 Key Haploid (n) Diploid (2n) Haploid gametes (n = 23) Ovum (n) Sperm cell (n) Testis Ovary Mitosis and development Multicellular diploid adults (2n = 46) FERTILIZATIONMEIOSIS Diploid zygote (2n = 46)

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Sets of Chromosomes in Human Cells Each human somatic cell (any cell other than a gamete) has 46 chromosomes arranged in pairs 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 homologues Both chromosomes in a pair carry genes controlling the same inherited characteristics

LE µm Pair of homologous chromosomes Sister chromatids Centromere

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings The sex chromosomes are called X and Y Human females have a homologous pair of X chromosomes (XX) Human males have one X and one Y chromosome The 22 pairs of chromosomes that do not determine sex are called autosomes

LE 13-4 Key Maternal set of chromosomes (n = 3) 2n = 6 Paternal set of chromosomes (n = 3) Two sister chromatids of one replicated chromosomes Two nonsister chromatids in a homologous pair Pair of homologous chromosomes (one from each set) Centromere

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Onion Root Tip

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Meiosis: the REDUCTION division Like mitosis, meiosis is preceded by the replication of chromosomes in ? Meiosis takes place in two sets of cell divisions, called I and II The two cell divisions result in four daughter cells, cf the two daughter cells produced in mitosis Each daughter cell has only half as many chromosomes (n) as the parent cell (2n)

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings The Key Stages of Meiosis In the first cell division (meiosis I), homologous chromosomes separate Meiosis I results in two haploid daughter cells with replicated chromosomes In the second cell division (meiosis II), sister chromatids separate Meiosis II results in four haploid daughter cells with unreplicated chromosomes

LE 13-7 Homologous pair of chromosomes in diploid parent cell Interphase Homologous pair of replicated chromosomes Chromosomes replicate Meiosis I Diploid cell with replicated chromosomes Sister chromatids Meiosis II Homologous chromosomes separate Sister chromatids separate Haploid cells with replicated chromosomes Haploid cells with unreplicated chromosomes

LE 13-8ab Sister chromatids Chiasmata Spindle Centromere (with kinetochore) Metaphase plate Homologous chromosomes separate Sister chromatids remain attached Microtubule attached to kinetochore Tetrad MEIOSIS I: Separates homologous chromosomes PROPHASE I METAPHASE I ANAPHASE I Homologous chromosomes (red and blue) pair and exchange segments; 2n = 6 in this example Pairs of homologous chromosomes split up Tetrads line up

LE 13-8b Cleavage furrow MEIOSIS II: Separates sister chromatids PROPHASE II METAPHASE IIANAPHASE II TELOPHASE I AND CYTOKINESIS TELOPHASE II AND CYTOKINESIS Sister chromatids separate Haploid daughter cells forming Two haploid cells form; chromosomes are still double During another round of cell division, the sister chromatids finally separate; four haploid daughter cells result, containing single chromosomes

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings A Comparison of Mitosis and Meiosis Mitosis conserves the number of chromosome sets, producing cells that are genetically identical to the parent cell Meiosis I reduces the number of chromosomes sets from two (diploid) to one (haploid), producing cells that differ genetically from each other and from the parent cell The mechanism for separating sister chromatids is virtually identical in meiosis II and mitosis

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Three events are unique to meiosis, and all three occur in meiosis l: Tetrads, Synapsis and crossing over in prophase I: Homologous chromosomes physically connect (synapsis) and exchange genetic information (at chiasmata) At the metaphase plate, there are paired homologous chromosomes (tetrads), instead of individual replicated chromosomes At anaphase I, it is homologous chromosomes, instead of sister chromatids, that separate and are carried to opposite poles of the cell resulting in haploid daughter cells.

LE 13-9 Propase Duplicated chromosome (two sister chromatids) Chromosome replication 2n = 6 Parent cell (before chromosome replication) Chromosome replication MITOSISMEIOSIS Chiasma (site of crossing over) MEIOSIS I Prophase I Tetrad formed by synapsis of homologous chromosomes Tetrads positioned at the metaphase plate Metaphase I Chromosomes positioned at the metaphase plate Metaphase Anaphase Telophase Homologues separate during anaphase I; sister chromatids remain together Sister chromatids separate during anaphase Daughter cells of meiosis I Haploid n = 3 Anaphase I Telophase I MEIOSIS II Daughter cells of mitosis 2n2n 2n2n n Sister chromatids separate during anaphase II n nn Daughter cells of meiosis II

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Process/ Characteristic MitosisMeiosis DNA replicationDuring interphase S phase During interphase S phase Sets of DivisionsOneTwo Synapsis and crossing over (chiasmata) Do not occurOccurs when homologous pairs form tetrads in prophase I Daughter cells, genetic composition Two Identical to parent cell Four haploid, different from parent cell and each other Role in animal bodyProduces daughter cells with same chromosome number (2n or n) as parent cell. For growth, tissue repair or reproduction Reduce chromosome number to half (n) in gametes

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Review Qs: 1. What is the main role of meiosis? 2. How are the terms chiasma, tetrad and synapsis related to each other? 3. Where does genetic recombination occur in meiosis? I and/or II and phase(s)? 4. In which phase does the chromosome number go from 2n to n? 5. In which phase do homologous chromosomes separate? 6. In which phase do sister chromatids separate?