Transcript Meiosis
Meiosis By Tibor Cemicky What is Meiosis? • Meiosis is a two-part cell division process in organisms that reproduce sexually • Occurs in Humans, animals and plants. Never in prokaryotic organisms such as bacteria. • Meiosis is used in sexual reproduction of organisms to combine male and female genes, to create a new, biological organism. • There are two stages of meiosis: meiosis I and meiosis II • At the end of the meiotic process, four haploid daughter cells are produced. Each contains halve the chromosomes of the parent cell. Diploid vs. Haploid Cells • Diploid: In a diploid cell, there are 2 copies of each chromosome, making for a total of 40+ chromosomes. These chromosomes are not identical, they have the same genes arranged in the same sequence however they usually do not have the same number of alleles. Therefore they are not identical but homologous. • Haploid: In a haploid cell, there's only one copy of each chromosome, therefore there are only 20+ chromosomes. Stages of Meiosis • Meiosis 1: – – – – Prophase 1 Metaphase 1 Anaphase 1 Telophase 1 • Meiosis 2: – – – – Prophase 2 Metaphase 2 Anaphase 2 Telophase 2 Process of Meiosis Prophase 1 => Metaphase 1 => Anaphase 1 => Telophase 1 => Prophase 2 => Metaphase 2 => Anaphase 2 => Telophase 2 Prophase 1 Prophase 1 => Metaphase 1 => Anaphase 1 => Telophase 1 => Prophase 2 => Metaphase 2 => Anaphase 2 => Telophase 2 • Nuclear envelope breaks up and disappears • Chromosomes become visible as they shorten, coil, and thicken • Each chromosome is composed of two sister chromatids (attached at centronome) containing identical genetic information • Homologous chromosomes (chromosomes containing sequences for the same genes) become associated along their entire lengths (they synapse), to form a structure known as a tetrad or homologous pair (contains 2 chromosomes and 4 chromatids) • During synapsis, the two chromosomes in the homolog pair exchange segments of DNA in a process called crossing over. As a result, the gene combinations on a chromosome can be changed. Example: If one chromatid of a chromosome initially contained genes for brown eyes and brown hair, after crossing over, it could contain genes for blue eyes and brown hair, by taking the gene for blue eyes from one of the chromatids of the other chromosome. Metaphase 1 Prophase 1 => Metaphase 1 => Anaphase 1 => Telophase 1 => Prophase 2 => Metaphase 2 => Anaphase 2 => Telophase 2 • Nuclear membrane dissapears completely • Tetrads (homologous pairs) move to a plane called the "metaphase plate“ halfway between the two poles of the cell • Spindle fibers attach to different chromosomes in each homologous pair at the centromeres Anaphase 1 Prophase 1 => Metaphase 1 => Anaphase 1 => Telophase 1 => Prophase 2 => Metaphase 2 => Anaphase 2 => Telophase 2 • The cell elongates • Two chromosomes 1 from each chromosome pair (tetrad) are pulled toward opposite poles as the microtubules of the spindle apparatus shorten Telophase 1 Prophase 1 => Metaphase 1 => Anaphase 1 => Telophase 1 => Prophase 2 => Metaphase 2 => Anaphase 2 => Telophase 2 • The chromosomes reach the poles • At each pole, now, there is a complete haploid set of chromosomes consisting of one member of each homologous pair (tetrad) • Nuclear membrane reforms around the daugher nuclei • Cytokinesis occurs and the cell splits into 2 daughter cells Prophase 2 Prophase 1 => Metaphase 1 => Anaphase 1 => Telophase 1 => Prophase 2 => Metaphase 2 => Anaphase 2 => Telophase 2 • Meiosis 2 begins. It is very similar to mitosis. • Chromatids are no longer identical because they mixed during Prophase 1 (crossing over) • Nuclear envelope breaks down • Spindle apparatus begins growing outward from the centrosomes Metaphase 2 Prophase 1 => Metaphase 1 => Anaphase 1 => Telophase 1 => Prophase 2 => Metaphase 2 => Anaphase 2 => Telophase 2 • Chromosomes line up at the metaphase II plate at the cell's center • Spindle fibers bind to both sides of the centromeres (region of found near the middle of a chromosome where two sister chromatids come in contact) Anaphase 2 Prophase 1 => Metaphase 1 => Anaphase 1 => Telophase 1 => Prophase 2 => Metaphase 2 => Anaphase 2 => Telophase 2 • Spindle fibers shorten and separate the sister chromatids which move toward the opposite cell poles. Telophase 2 • Distinct nuclei form at the opposite poles • Cytokinesis occurs, the two cells divide to form 4 haploid daughter cells • Nuclear envelopes form in each new daughter cell • Now there are four daughter cells each with one half the number of chromosomes of the original parent cell. • Meiosis is now complete Mitosis vs. Meiosis Purpose of Meiosis and Mitosis in eukaryotic organisms: •Both Meiosis and Mitosis are found in complex organisms which reproduce sexually including Humans. •Identical versions of cells can be created through Mitosis. Mitosis may therefore be used for growth and reparation of damaged organs and tissues. •Meiosis is a special process reserved only for the creation of the egg and sperm cells.