Bellwork Draw this in your notes and fill in the phases of mitosis MEIOSIS Unit 6 Terms to Know Homologous Chromosomes corresponding (matching) chromosomes from a female and male parent. -Each pair of homologous chromosomes has genes for the same traits Diploid Cell (2n) a cell with two of each kind of chromosome Haploid Cell (n) a cell that contains one set of the same kind of chromosomes Sexual reproduction reproduction involving the production and fusion of haploid sex cells Gene segment of DNA that codes for a trait Replicated Chromosome a chromosome that has doubled its DNA Non-Replicated Chromosome a chromosome that has split and contains only one copy of DNA. What is Meiosis and why do we need it? A form of cell division happening in sexually reproducing organisms consisting of two consecutive cell divisions (meiosis I and meiosis II). Meiosis produces 4 haploid (n) sex cells Spermmale gamete Eggfemale gamete Meiosis: a kind of cell division, which produces gametes containing half the number of chromosomes as a parents body cell Meiosis Overview Meiosis I: One diploid cell (46 replicated chromosomes/23 pairs) becomes two cells Meiosis II: The two cells become 4 haploid cells These cells are not identical but they are very similar Why half the chromosomes in gametes? At fertilization the male and female sex cells will provide of the chromosomes each so the offspring has genes from both parents Fertilization The haploid sperm (carrying 1 set of chromosomes) combines with the haploid egg (carrying 1 set of chromosomes). The result is a diploid zygote. Female vs. Male Oogenesis egg production Spermatogenesis Sperm Production What does Meiosis accomplish? 1. Meiosis takes a cell with two copies of every chromosome (diploid) and makes cells with a single copy of every chromosome (haploid). 2. Meiosis scrambles the specific forms of each gene that each sex cell (egg or sperm) receives through crossing over and independent assortment. Steps in Meiosis PMAT 2 similar cells 23 chromosomes Separates homologous pairs Haploid PMAT 4 similar cells 23 chromosomes Separates replicated chromosomes Haploid Meiosis I starts diploidMeiosis II starts haploid Same steps as Mitosis, divides twice to produce 4 daughter cells instead of 2. Daughter cells are not identical but similar Interphase Interphase Cells duplicate their DNA G1, S, G2 Prophase I Prophase I sister chromatids pair up forming a homologous chromosome Crossing Over: When two chromosomes exchange portions of their chromatids (genetic material is swapped) Synapsis: the homologous chromosomes line up forming a four part structure called a tetrad. Crossing over during meiosis, increases the genetic variability due to allele combinations. Genetic recombination is a major source of variation among organisms Metaphase I Metaphase I Spindle fibers attach to the centromeres on each chromosome Homologous chromosomes line up together. Anaphase I Anaphase I Spindle fibers pull the homologous chromosomes toward opposite ends of the cell. Telophase I/Cytokinesis Telophase I/Cytokinesis Meiosis I results in 2 haploid (n) daughter cells. Each with half the number of chromosomes as the original cell (23) Still replicated chromosomes. Prophase II Prophase II 2 haploid (n) daughter cells. Each with half the number of chromosomes as the original cell. Interphase does NOT happen again. Same steps as normal prophase. Metaphase II Metaphase II The chromosomes line up in a similar way to the metaphase stage of mitosis. Occurs in BOTH cells. Anaphase II Anaphase II The sister chromatids separate and move toward opposite ends of the cell Telophase II / Cytokinesis Telophase II / Cytokinesis Opposite of prophase occurs, cytoplasm splits. END RESULT: four haploid (n), non-replicated, non- identical daughter cells. Mitosis vs. Meiosis Diploid 2n diploid 2n 46 chromosomes 46 chromosomes Replicated chromosomes Non-Replicated Chromosomes Somatic Cells Creates 2 identical cells 1 trip through PMAT Diploid 2n haploid n 46 chromosomes 23 chromosomes Replicated chromosomes Non-Replicated Chromosomes Gamete cells Creates 4 similar cells 2 trips through PMAT Meiosis Drawing Things to Label: Label each cell as diploid (2n) or haploid (n) Label the chromosome number of each cell Circle and label a homologous pair of chromosomes Circle and label a replicated chromosome. Circle and label a non-replicated chromosome.