Chapter 13 Meiosis

Heredity Genes Are the units of heredity Are segments of DNA Each gene in an organism’s DNA has a specific locus on a certain chromosome

Pairs of Chromosomes A diploid cell Has 2 copies of each of its chromosomes—therefore 2 copies of each gene In a human, diploid cells (somatic) have 46 chromosomes (2n = 46)

Pairs of Chromosomes A haploid cell Has only 1 copy of each of its chromosomes— therefore 1 copy of each gene In a human, a haploid cell (sex cells: egg & sperm) has 23 chromosomes (n = 23)

Sexual Reproduction In sexual reproduction Fertilization and meiosis Two parents give rise to offspring that have unique combinations of genes inherited from the two parents Fertilization and meiosis Alternate in sexual life cycles A life cycle Is the generation-to-generation sequence of stages in the reproductive history of an organism

Multicellular diploid Human Life Cycle Key Haploid (n) Diploid (2n) Haploid gametes (n = 23) Ovum (n) Sperm Cell (n) MEIOSIS FERTILIZATION Ovary Testis Diploid zygote (2n = 46) Mitosis and development Multicellular diploid adults (2n = 46)

Pairs of Chromosomes Homologous chromosomes Are the two chromosomes composing a pair Have the same genes Pair of homologous chromosomes

Pairs of Chromosomes Sex chromosomes Are distinct from each other in their characteristics Are represented as X and Y Determine the sex of the individual, XX being female, XY being male

Pairs of Chromosomes A karyotype Is an ordered, visual representation of the chromosomes in a cell

Chromosomes in Dividing Cell In a cell in which DNA synthesis has occurred All the chromosomes are duplicated and thus each consists of two identical sister chromatids Key Maternal set of chromosomes (n = 3) 2n = 6 Paternal set of chromosomes (n = 3) Two sister chromatids of one replicated chromosome Centromere Two nonsister chromatids in a homologous pair Pair of homologous chromosomes (one from each set)

Meiosis Meiosis is a form of cell division Meiosis That reduces the number of chromosome sets from diploid to haploid Meiosis Takes place in two sets of divisions, meiosis I and meiosis II

Overview of Meiosis Interphase Meiosis I Meiosis II Pair of homologous chromosomes in diploid parent cell Overview of Meiosis Duplicated pair of homologous chromosomes Chromosomes duplicate Sister chromatids Diploid cell with duplicated chromosomes Meiosis I 1 Homologous chromosomes separate Figure 13.7 Overview of meiosis: how meiosis reduces chromosome number. Haploid cells with duplicated chromosomes Meiosis II 2 Sister chromatids separate Haploid cells with unduplicated chromosomes

Stages of Meiosis Meiosis I Meiosis II Reduces the number of chromosomes from diploid to haploid Meiosis II Produces four haploid daughter cells

Telophase I and Cytokinesis Prophase I Metaphase I Anaphase I Centrosome (with centriole pair) Sister chromatids remain attached Sister chromatids Chiasmata Centromere (with kinetochore) Spindle Metaphase plate Cleavage furrow Homologous chromosomes separate Homologous chromosomes Fragments of nuclear envelope Figure 13.8 Exploring: Meiosis in an Animal Cell Microtubule attached to kinetochore Each pair of homologous chromosomes separates. Two haploid cells form; each chromosome still consists of two sister chromatids. Duplicated homologous chromosomes (red and blue) pair and exchange segments; 2n  6 in this example. Chromosomes line up by homologous pairs.

Telophase II and Cytokinesis Prophase II Metaphase II Anaphase II During another round of cell division, the sister chromatids finally separate; four haploid daughter cells result, containing unduplicated chromosomes. Sister chromatids separate Haploid daughter cells forming Figure 13.8 Exploring: Meiosis in an Animal Cell

Prophase I Prophase I typically occupies more than 90% of the time required for meiosis 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 regions where crossing over occurred

Metaphase I In Metaphase I, tetrads line up at the metaphase plate, with one chromosome facing each pole Microtubules from one pole are attached to the kinetochore of one chromosome of each tetrad Microtubules from the other pole are attached to the kinetochore of the other chromosome

Anaphase I In Anaphase I, pairs of homologous chromosomes separate One chromosome moves toward each pole, guided by the spindle apparatus Sister chromatids remain attached at the centromere and move as one unit toward the pole

Telophase I & Cytokinesis In the beginning of Telophase I, each half of the cell has a haploid set of chromosomes; each chromosome still consists of two sister chromatids Cytokinesis usually occurs simultaneously, forming two haploid daughter cells In animal cells, a cleavage furrow forms; in plant cells, a cell plate forms No chromosome replication occurs between the end of meiosis I and the beginning of meiosis II because the chromosomes are already replicated

Prophase II In Prophase II, a spindle apparatus forms In late Prophase II, chromosomes (each still composed of two chromatids) move toward the metaphase plate

Metaphase II In Metaphase II, the sister chromatids are arranged at the metaphase plate Because of crossing over in Meiosis I, the two sister chromatids of each chromosome are no longer genetically identical The kinetochores of sister chromatids attach to microtubules extending from opposite poles

Anaphase II In Anaphase II, the sister chromatids separate The sister chromatids of each chromosome now move as two newly individual chromosomes toward opposite poles

Telophase II & Cytokinesis In Telophase II, the chromosomes arrive at opposite poles Nuclei form, and the chromosomes begin de- condensing Cytokinesis separates the cytoplasm At the end of meiosis, there are four daughter cells, each with a haploid set of unreplicated chromosomes Each daughter cell is genetically distinct from the others and from the parent cell

Genetic Variation Genetic variation Is the raw material for evolution by natural selection Natural selection results in the accumulation of genetic variations favored by the environment

Mechanisms of Variation Meiosis provides various mechanisms for variation by: Crossing over Independent assortment Random fertilization Mutations

Crossing Over Crossing over produces recombinant chromosomes Combines DNA from each parent Homologous portions of two nonsister chromatids trade places Crossing over increases genetic variation by combining DNA from two parents into a single chromosome

Crossing Over Prophase I of meiosis Nonsister chromatids held together during synapsis Pair of homologs Chiasma Crossing Over Centromere TEM Anaphase I Figure 13.11 The results of crossing over during meiosis. Anaphase II Daughter cells Recombinant chromosomes

Independent Assortment In independent assortment Each pair of chromosomes sorts its maternal and paternal homologues into daughter cells independently of the other pairs There are 2n possibilities of arrangements (i.e. 8.4 million in humans)

Independent Assortment 2n = 4 so 2n is 22 = 4 possible combinations Possibility 1 Possibility 2 Two equally probable arrangements of chromosomes at metaphase I Metaphase II Figure 13.10 The independent assortment of homologous chromosomes in meiosis. Daughter cells Combination 1 Combination 2 Combination 3 Combination 4 2n = 4 so 2n is 22 = 4 possible combinations

Random Fertilization Don’t forget random fertilization! Adds to genetic variation: any sperm can fuse with any ovum (unfertilized egg) Each gamete has 8.4 million possible chromosome combinations Produces a zygote with any of about 70 trillion diploid combinations

Review of Mitosis vs. Meiosis Prophase Duplicated chromosome (two sister chromatids) Chromosome replication Parent cell (before chromosome replication) Chiasma (site of crossing over) MEIOSIS I Prophase I Tetrads formed by synapsis of homologous chromosomes Metaphase Chromosomes positioned at the metaphase plate Tetrads Metaphase I Anaphase I Telophase I Haploid n = 3 MEIOSIS II Daughter cells of meiosis I Homologues separate during anaphase I; sister chromatids remain together Haploid Daughter cells of meiosis II n Sister chromatids separate during anaphase II Anaphase Telophase Sister chromatids separate during anaphase 2n Diploid daughter cells of mitosis 2n = 6 Diploid 2n = 6