Sexual Reproduction and Meiosis Chapter 11 1

Sexual life cycle Made up of meiosis and fertilization Diploid cells Somatic cells of adults have 2 sets of chromosomes Haploid cells Gametes have only 1 set of chromosomes Allows offspring to inherit genetic material from 2 parents

In most animals, diploid state dominates Life cycles of sexually reproducing organisms involve the alternation of haploid and diploid stages Some life cycles include longer diploid phases, some include longer haploid phases In most animals, diploid state dominates Zygote first undergoes mitosis to produce more diploid cells multicellularity Later in the life cycle, some of these diploid cells undergo meiosis to produce haploid gametes (sperm & eggs)

Meiosis DNA replicates (chromosome # doubled) Meiosis I Stages: Prophase I – synapsis occurs Metaphase I – homologous chromosome pairs line up (4 chromosomes) Anaphase I – homologous chromosomes seperate Telophase I Results in 2 cells, but still diploid number Meiosis II Stages Prophase II Metaphase II Anaphase II – sister chromosomes seperate Telophase II Results in 4 cells, haploid number, one of each homologous pair in every cell

Features of Meiosis Meiosis includes 2 rounds of division Meiosis I and meiosis II Each has prophase, metaphase, anaphase, and telophase stages Synapsis (crossing-over) During early prophase I Homologous chromosomes become closely associated (eventually swapping DNA) Includes formation of synaptonemal complexes Formation also called tetrad or bivalents

First meiotic division is termed the “reduction division” Results in daughter cells that contain one homologue from each chromosome pair No DNA replication between meiotic divisions Second meiotic division does not further reduce the number of chromosomes Separates the sister chromatids for each homologue Meiosis II is very much like mitosis (but half the DNA)

Prophase I Chromosomes coil tighter and become visible, nuclear envelope disappears, spindle forms Each chromosome composed of 2 sister chromatids Synapsis (crossing-over) Homologues become closely associated Crossing over occurs between nonsister chromatids Nonsister chromatids remain attached at chiasmata Chiasmata move to the end of the chromosome arm before metaphase I

Crossing over Occurs between nonsister chromatids Allows the maternal and paternal homologues to exchange chromosomal material (recombination) Alleles of genes that were formerly on separate homologues can now be found on the same homologue Chiasmata – site of crossing over Contact maintained until anaphase I

Metaphase I Terminal chiasmata hold homologues together following crossing over Microtubules from opposite poles attach to each homologue (not each sister chromatid) Homologues are aligned at the metaphase plate side-by-side Orientation of each pair of homologues on the spindle is random (This sets up independent assortment)

Many random combinations of the homologous pairing of 3 chromosomes Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Many random combinations of the homologous pairing of 3 chromosomes

Anaphase I Microtubules of the spindle shorten Chiasmata break Homologues are separated from each other and move to opposite poles Sister chromatids remain attached to each other at their centromeres Each pole has a complete haploid set of chromosomes consisting of one member of each homologous pair Independent assortment of maternal and paternal chromosomes occurs

Telophase I Nuclear envelope re-forms around each daughter nucleus Sister chromatids are no longer identical because of crossing over (prophase I) Cytokinesis may or may not occur after telophase I Meiosis II occurs after an interval of variable length

Meiosis II Resembles a mitotic division But with a haploid rather than diploid cell Prophase II: nuclear envelopes dissolve and new spindle apparatus forms Metaphase II: chromosomes align on metaphase plate Anaphase II: sister chromatids are separated from each other Telophase II: nuclear envelope re-forms around 4 sets of daughter chromosomes; cytokinesis follows

Prophase II Following a brief interphase, with no S phase, meiosis II begins. During prophase II, a new spindle apparatus forms in each cell, and the nuclear envelope breaks down. In some species the nuclear Envelope does not re-form in telophase I, removing the need for Nuclear envelope breakdown.

Metaphase II In metaphase II, chromosomes consisting of sister chromatids joined at the centromere align along the metaphase plate in each cell. Now, kinetochore microtubules from opposite poles attach to kinetochores of sister chromatids, as in mitosis.

Anaphase II When microtubules shorten In anaphase II, sister chromatids are pulled to opposite poles of the cells, as in mitosis.

Telophase II The nuclear membranes re-form around four different clusters of chromosomes. After cytokinesis, four haploid cells result. No two cells are alike due to the random alignment of homologous pairs at metaphase I and crossing over during prophase I.

Final result of Meiosis Four cells containing haploid sets of chromosomes In animals, these cells develop directly into gametes In Spermtogenesis: 4 sperm In Oogenesis: 1 egg, 3 polar bodies In plants, fungi, and many protists, haploid cells then divide mitotically Produce greater number of gametes

Errors in Meiosis Nondisjunction – failure of chromosomes to move to opposite poles during either meiotic division Aneuploid gametes – gametes with missing or extra chromosomes Most common cause of spontaneous abortion in humans

Meiosis vs. Mitosis Meiosis is characterized by 4 features: Synapsis and crossing over Sister chromatids remain joined at their centromeres throughout meiosis I Kinetochores of sister chromatids attach to the same pole in meiosis I DNA replication is suppressed between meiosis I and meiosis II

Meiosis I Mitosis Metaphase I Crossovers and sister chromatid Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Meiosis I Mitosis Metaphase I Crossovers and sister chromatid cohesion lock homologues together. Microtubules connect to the kinetochores of sister chromatids so that homologues are pulled toward opposite poles. Metaphase Homologues do not pair; kinetochores of sister chromatids remain separate; microtubules attach to both kinetochores on opposite sides of the centromere. Anaphase I Anaphase Microtubules pull the homologous chromosomes apart, but sister chromatids are held together at the centromere. Microtubules pull sister chromatids apart.

Homologous chromosomes pair; synapsis and crossing over occur. Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. MEIOSIS I Prophase I Metaphase I Anaphase I Telophase I Parent cell (2n) Paternal homologue Chromosome replication Homologous chromosomes pair; synapsis and crossing over occur. Paired homologous chromosomes align on metaphase plate. Homologous chromosomes separate; sister chromatids remain together. MITOSIS Prophase Metaphase Anaphase T elophase Homologous chromosomes Chromosome replication Maternal homologue Two daughter cells (each 2n) Homologous chromosomes do not pair. Individual homologues align on metaphase plate. Sister chromatids separate, cytokinesis occurs, and two cells result, each containing the original number of homologues.

each containing half the original number of homologues. Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. MEIOSIS II Prophase II Metaphase II Anaphase II Telophase II Four daughter cells (each n) Chromosomes align, sister chromatids separate, and four haploid cells result, each containing half the original number of homologues.