Summary

This document provides an overview of meiosis, a type of cell division that produces gametes. It explains the process of meiosis and its importance in sexual reproduction. Also, details about the phases of meiosis are included.

Full Transcript

Meiosis Meiosis (greek word) = “reduction” a specialized form of cell division that produces reproductive cells, such as plant and fungal spores, sperm and egg cells. Without meiosis, the chromosome number would double with each generation, and sexual reproduction would...

Meiosis Meiosis (greek word) = “reduction” a specialized form of cell division that produces reproductive cells, such as plant and fungal spores, sperm and egg cells. Without meiosis, the chromosome number would double with each generation, and sexual reproduction would not be possible. x 46 Organisms that reproduce Sexually are made up of two different types of cells. 1. Somatic Cells are “body” cells and contain the normal number of chromosomes ….called the “Diploid” number (the symbol is 2n). Examples would be … skin cells, brain cells, etc. 2. Gametes are the “sex” cells and contain only ½ the normal number of chromosomes…. called the “Haploid” number (the symbol is n)….. Sperm cells and ova are gametes. n = number of chromosomes in the set… so….2n means 2 chromosomes in the set…. Polyploid cells have more than two chromosomes per set… example: 3n (3 chromosomes per set) Gametes The Male Gamete is the Sperm and is produced in the male gonad the Testes. The Female Gamete is the Ovum (ova = pl.) and is produced in the female gonad the Ovaries. Homologous Chromosomes Pair of chromosomes (maternal and paternal) that are similar in shape and size. Homologous pairs (tetrads) carry genes controlling the same inherited traits. Each locus (position of a gene) is in the same position on homologues. Humans have 23 pairs of homologous chromosomes. 22 pairs of autosomes 1 pair of sex chromosomes The process by which ”gametes” (sex cells) , with half the number of chromosomes, are produced. During Meiosis diploid cells are reduced to haploid cells Diploid (2n) → Haploid (n) If Meiosis did not occur the chromosome number in each new generation would double…. The offspring would die. ⦿ TWO DIVISION: n=23 human sex cell sperm n=23 n=23 2n=46 n=23 diploid (2n) n=23 haploid (n) n=23 Meiosis I Meiosis II 14 Haploid n=23 (1n) human egg sex cell n=23 2n=46 Polar Bodies (die) diploid (2n) n=23 Meiosis I Meiosis II 15 ⦿ “Reduction phase/division” ⦿ SYNAPSIS - means “fastening together”; the pairing of homologous chromosomes ⦿ CROSSING OVER - exchanging of genetic information by homologous chromosome ⦿ The centromeres joining these chromatids remain intact throughout the entire division, rather than splitting as in mitosis. ⦿ Keep in mind that Meiosis I reduces the number of chromosome from Diploid (2n) to Haploid (n) Interphase I Similar to mitosis interphase. Chromosomes replicate (S phase). Each duplicated chromosome consist of two identical sister chromatids attached at their centromeres. Centriole pairs also replicate. ⦿ Cell division that reduces the chromosome number by one-half. (Reduction phase/division) ⦿ Four phases: a. Prophase I b. Metaphase I c. Anaphase I d. Telophase I Prophase I 18 ⦿ Longest and most complex phase (90%). ⦿ Chromosomes condense. occurs - Homologous chromosomes come together to form a tetrad. is two chromosomes or four chromatids (sister and non- sister chromatids). 19 Homologous chromosomes sister chromatids Tetrad sister chromatids 20 Cohesins - connects two sister chromatids Synaptonemal complex - connects two homologous chromosomes (tetrads) ⦿ Crossing over may occur between non- sister chromatids at sites called chiasmata. ⦿ Crossing over: segments of nonsister chromatids break and reattach to the other chromatid. ⦿ Chiasmata (chiasma) are where chromosomes touch each other and exchange genes (crossing over.) ⦿ Causes Genetic Recombination 21 Crossing Over creates variation (diversity) in the offspring’s traits. nonsister chromatids Tetrad chiasmata: site variation of crossing over Nucleus & Nucleolus disappear Spindle forms Chromosomes coil & Synapsis (pairing) occurs Tetrads form & Crossing over Occurs spindle fiber centrioles aster fibers TETRAD 25 OR Homologs line up at equator or metaphase plate 26 Homologs separate. Sister chromatids remain attached at their centromeres. 27 ⦿ Each pole now has haploid (n) set of chromosomes. ⦿ Cytokinesis occurs and two haploid daughter cells are formed. cytokinesis 28 ⦿ No Interphase II. (Interkinesis- Short pause between Meiosis I and Meiosis II) ⦿ No DNA Replication (no s phase) ⦿ Remember: Meiosis II is similar to mitosis 29 ⦿ Same as Prophase in mitosis ⦿ Nucleus & nucleolus disappear ⦿ Chromosomes condense ⦿ Spindle forms 30 Same as Metaphase in mitosis Chromosomes (not homologs) line up at equator 31 ⦿ Same as Anaphase in mitosis ⦿ SISTER CHROMATIDS separate 32 ⦿ Same as Telophase in mitosis. ⦿ Nuclei and Nucleoli reform, spindle disappears ⦿ CYTOKINESIS occurs. ⦿ Remember: FOUR HAPLOID DAUGHTER cells are produced. ⦿ Called GAMETES (eggs and sperm) 1n Sperm cell fertilizes 1n egg to form 2n zygote 33 Non-disjunction Non-disjunction is the failure of homologous chromosomes, or sister chromatids, to separate during meiosis. Non-disjunction results with the production of zygotes with abnormal chromosome number. Non-disjunctions usually occur in one of two fashions The first is called Monosomy, the second is called Trisomy. If an organism has Trisomy 18 it has three chromosomes in the 18th set, Trisomy 21, three chromosomes in the 21st set. If an organism has Monosomy 23 it has only one chromosome in the 23rd set. Common Non-disjunction Disorders Down’s Syndrome – Trisomy 21 Turner’s Syndrome – Monosomy 23 (X) Kleinfelter’s Syndrome – Trisomy 23 (XXY) Edward’s Syndrome – Trisomy 18 Amniocentesis An Amniocentesis is a procedure a pregnant woman can have in order to detect some genetics disorders, such as non-disjunction. Amniocentesis Amniotic fluid withdrawn COMPUTATION (Part 1) Remember these concepts in MEIOSIS: 1. One germ cell = 4 daughter cells 2. Number of chromosomes = reduces into half 1. A diploid cell containing 20 chromosomes would, at the end of meiosis, produce cells containing how many chromosomes? 2. A cell containing 40 chromatids at the beginning of meiosis would, at its completion, produce cells containing how many chromosomes? COMPUTATION (Part 2) Remember these concepts in MEIOSIS (Gametogenesis): In spermatogenesis: 1. One germ cell = 4 sperm cells In oogenesis 1. One germ cell = 1 egg cell 2. One germ cell = 3 polar bodies (Number of chromosomes = reduces into half) 1. Three germ cells that underwent spermatogenesis produces how many productive sperm cells? 2. Three germ cells that underwent oogenesis produced how many egg cells? 3. 15 germ cells that underwent oogenesis produced how many polar bodies? 4. 5 germ cells containing 46 chromosomes each underwent spermatogenesis. How many chromosomes will be present in each sperm cell?

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