Meiosis Unit 3 Part II – The Cell
What is Meiosis A form of cell division happening in sexually reproducing organisms. Consists of 2 consecutive cell divisions (meiosis I and meiosis II). Produces 4 sex cells. Distributes maternal and paternal chromosomes evenly and randomly.
Where does Meiosis occur Meiosis makes sex cells. For women it creates egg cells in the ovaries. For men it produces sperm cells in the testes.
Why do we need Meiosis? • Meiosis is necessary to halve the number of chromosomes going into the sex cells. • Why halve the chromosomes in gametes? • At fertilization the male and female sex cells will provide ½ of the chromosomes each – so the offspring has genes from both parents.
Terms to Know • Homologous Chromosome corresponding (matching) chromosomes from a female and male parent. • Diploid Cell (2n) – a cell that contains both sets of homologouschromosomes. • Haploid Cell (n) – a cell that contains one set of homologous chromosomes. • Gene – segment of DNA that codes for a trait. • Replicated Chromosome – a chromosome that has doubled it’s DNA. • Non-Replicated Chromosome – a chromosome that has split and contains only one copy of DNA.
What does Meiosis accomplish? Meiosis takes a cell with two copies of every chromosome (diploid) and makes cells with a single copy of every chromosome (haploid). 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 • Same steps as Mitosis, but it divides twice to produce 4 daughter cells instead of 2. • Daughter cells are not identical but similar. • Meiosis I • PMAT • 2 similarcells • Meiosis II • PMAT • 4 similar cells
Interphase/Prophase I • Interphase • Cells duplicate their DNA. • Prophase I • sister chromatids pair up forming a homologous chromosome. • Crossing Over • When two chromosomes exchange portions of their chromatids. • Occurs during Prophase I.
Metaphase I • Metaphase I • Spindle fibers attach to the centromeres on each chromosome.
Anaphase I • Anaphase I • Spindle fibers pull the homologous chromosomes toward opposite ends of the cell.
Telophase I • Telophase I • Meiosis I results in 2 haploid (n) daughter cells. Each with half the number of chromosomes as the original cell. • Still replicated chromosomes.
Prophase II • Prophase II • 2 haploid (n) daughter cells. Each with half the number of chromosomes as the original cell.
Metaphase II • Metaphase II • The chromosomes line up in a similar way to the metaphase stage of mitosis.
Anaphase II • Anaphase II • The sister chromatidsseparate and move toward opposite ends of the cell.
Telophase II • Telophase II • Meiosis II results in four haploid (n) daughter cells. • Daughter cells are non-replicated.
Female vs. Male Oogenesis egg production Spermatogenesis Sperm Production
Fertilization The haploid sperm (carrying 1 set of chromosomes) combines with the haploid egg (carrying 1 set of chromosomes).