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MEIOSIS A REDUCTION DIVISION Background Info: • Chromosomes are composed of genes – Each c-some has hundreds of genes – Body cells have 46 (23 pairs of) c-somes • Diploid: 2 of each pair: 46 total (body cells) • Haploid: 1 of each pair: 23 total (gametes) • Homologous Chromosomes: a pair of chromosomes that are the same length with the same centromere position – One from each parent Overview: • Occurs in the reproductive organisms of organisms that reproduce sexually. • The c-some number is cut in half through two consecutive divisions: a reduction division. • Each gamete formed will provide half of the information for the offspring Interphase I: • Interphase: same as mitosis – G1:Cell functions normally – S Phase: chromosomes replicate and pair up with their sister chromatid (identical) – G2: spindle apparatus is made, chromatin condenses Prophase I: • Nucleus breaks down • Chromosomes condense • Synapsis occurs: – 2 sister chromatids pair up forming a tetrad made up of 2 pairs. • Crossing over occurs: – The tetrads are tightly held together thus the sisters exchange information. • Spindle forms and moves to the opposite poles Prophase I: Metaphase I: • Chromosomes centromeres attach to the spindle fibers • Homologous chromosomes line up at the equator of the cell Metaphase I: Anaphase I: • Homologous chromosomes separate and move to opposite poles of the cell Anaphase I: Telophase I: • The spindle fiber breaks down • Sister chromosomes uncoil in some species, however they are still attached at the centromere – The sisters may not be identical due to crossing over • The cell divides= 2 Diploid (2n) cells – In some species the nucleus reforms Meiosis II: • Prophase II: No new replication!! – Spindle fibers form in each of the new cells and attach to the centromeres • Metaphase II: – Chromosomes line up at equator • Anaphase II: – Centromeres split, sister chromatids separate and move to opposite poles • Telophase II: – Four nuclei form around the chromosomes, spindle breaks down, cells divide Products of Meiosis: • Four new gametes are formed • Each nucleus contains a haploid number of the chromosomes • Each of the four gametes contains half of the genetic information used to form a zygote.