What occurs during the phases of meiosis? Sections 8.1 2 to 8.18

Homologous Chromosomes Somatic Cell Body cells All cells excepts the sperm and the egg (gametes/sex cells) Chromosome Number for Humans 46 23 pair Homologous Chromosomes 2 chromosomes that are the same length, shape, centromere position and code for the same genetic information 1 from each parent

TECHNIQUE 5 µm Pair of homologous replicated chromosomes Centromere Fig. 13-3b TECHNIQUE 5 µm Pair of homologous replicated chromosomes Centromere Figure 13.3 Preparing a karyotype Sister chromatids Metaphase chromosome

Autosome vs. Sex Chromosome Autosomes Chromosomes that are the same in males and females Pairs 1 through 22 on a karyotype Sex Chromosomes Chromosomes that are different in males and females Chromosomes that determine sex Pair 23 on a karyotype Female – XX Male XY

Diploid vs. Haploid Diploid Cells that have 2 sets of genetic information 2 sets of homologous chromosomes Designated by “2n” In humans 2n= 46 Normal body cells Haploid Cells that have 1 set of genetic information Only one of each homolog Designated by “n” In humans n=23 Gametes

Gametes vs. Zygote Gametes Sex Cells Sperm or Egg Haploid Zygote Fertilized egg Diploid

Meiosis Gamete formation Like mitosis, meiosis is preceded by the replication of chromosomes 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 – also called Reduction Division (MI = reduction / MII = division) Each daughter cell has only half as many chromosomes as the parent cell – starts with “2n” cell ends with four “n” cells

Prophase I Nuclear membrane breaks down Nucleolus disappears Chromatin condenses into chromosomes Synapsis occurs Homologous chromosomes come together (forming a tetrad) Crossing over can occur

Metaphase I Homologous pairs line up at the metaphase plate Spindle fibers attach to the kinetechores

Anaphase I Homologous chromosomes are pulled apart Note: centromeres DO NOT split

Telophase I and Cytokinesis Double chromosomes are at the poles Nuclear membrane reforms Nucleolus reappears Cytoplasm Separates Note: at this point there are 2 genetically unique haploid cells - REDUCTION

Prophase II Nuclear membrane breaks down Nucleolus disappears Chromatin condenses to chromosomes Note: no synapsis Note: haploid

Metaphase II Chromosomes move to the metaphase plate Note: single file like mitosis except these cells are haploid

Anaphase II Sister chromatids pulled apart Note: centromere splits just like anaphase of mitosis only with haploid cells

Telophase II and Cytokinesis Chromosomes are at the poles Nuclear membrane reforms Cytoplasm divides Note: produces 4 genetically unique haploid cells

Gamete formation in Animals

Gamete formation in Plants

How does meiosis differ from mitosis?

Replicated chromosome Fig. 13-9a MITOSIS MEIOSIS MEIOSIS I Parent cell Chiasma Chromosome replication Chromosome replication Prophase Prophase I Homologous chromosome pair 2n = 6 Replicated chromosome Metaphase Metaphase I Anaphase Telophase Anaphase I Figure 13.9 A comparison of mitosis and meiosis in diploid cells Telophase I Haploid n = 3 Daughter cells of meiosis I 2n 2n MEIOSIS II Daughter cells of mitosis n n n n Daughter cells of meiosis II

Mitosis vs. Meiosis Fig. 13-9b SUMMARY Meiosis Mitosis Property DNA replication Number of divisions Occurs during interphase before mitosis begins One, including prophase, metaphase, anaphase, and telophase Synapsis of homologous chromosomes Does not occur daughter cells and genetic composition Two, each diploid (2n) and genetically identical to the parent cell Role in the animal body Enables multicellular adult to arise from zygote; produces cells for growth, repair, and, in some species, asexual reproduction Occurs during interphase before meiosis I begins Two, each including prophase, metaphase, anaphase, and telophase Occurs during prophase I along with crossing over between nonsister chromatids; resulting chiasmata hold pairs together due to sister chromatid cohesion Four, each haploid (n), containing half as many chromosomes as the parent cell; genetically different from the parent cell and from each other Produces gametes; reduces number of chromosomes by half and introduces genetic variability among the gametes Figure 13.9 A comparison of mitosis and meiosis in diploid cells

Independent Assortment of Chromosomes Homologous pairs of chromosomes orient randomly at metaphase I of meiosis In independent assortment, each pair of chromosomes sorts maternal and paternal homologues into daughter cells independently of the other pairs Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

Possibility 1 Possibility 2 Two equally probable arrangements of Fig. 13-11-3 Possibility 1 Possibility 2 Two equally probable arrangements of chromosomes at metaphase I Metaphase II Figure 13.11 The independent assortment of homologous chromosomes in meiosis Daughter cells Combination 1 Combination 2 Combination 3 Combination 4

Crossing Over Crossing over produces recombinant chromosomes, which combine genes inherited from each parent In crossing over, homologous portions of two nonsister chromatids trade places Crossing over contributes to genetic variation by combining DNA from two parents into a single chromosome

Recombinant chromosomes Fig. 13-12-5 Prophase I of meiosis Nonsister chromatids held together during synapsis Pair of homologs Chiasma Centromere TEM Anaphase I Figure 13.12 The results of crossing over during meiosis Anaphase II Daughter cells Recombinant chromosomes

Chiasma The microscopically visible site where crossing over has occurred between chromatids of homologous chromosomes during prophase I