Meiosis 2023 - PDF

MEIOSIS DR/ Eman Ramadan Abdel-Hamid Lecturer of Medical Genetics FOMSCU BASIC CONCEPTS: HOMOLOGOUS CHROMOSOMES  PLOIDY Diploid (2n) Cell Haploid (n) Cell 46 chromosomes 23 chromosomes 23 pairs of chromosomes Somatic cells...

MEIOSIS DR/ Eman Ramadan Abdel-Hamid Lecturer of Medical Genetics FOMSCU BASIC CONCEPTS: HOMOLOGOUS CHROMOSOMES  PLOIDY Diploid (2n) Cell Haploid (n) Cell 46 chromosomes 23 chromosomes 23 pairs of chromosomes Somatic cells gametes Objectives (ILOs): Meiosis: ❑ Definition ❑ Stages( meiosis I & meiosis II) ❑ Genetic consequences of meiosis ❑ Comparison between Meiosis& Mitosis ❑ Oogenesis & Spermatogenesis DEFINITION Meiosis is the cell div ision process unique to germline cells that produces haploid gametes from diploid precursor cells. Meiosis involves one round of DNA replication followed by tw o rounds of cell division: meiosis I & meiosis II. STAGES 1- Meiosis I:(Reduction division) During which homologous chromosomes separate. Haploid cells with duplicated chromosomes are formed from a diploid cell. 2- Meiosis II: During which sister chromatids separate. 4 Haploid cells with unduplicated chromosomes are formed.  Stages of Meiosis Meiosis I Meiosis II Prophase I Prophase II Leptotene stage Zygotene stage Pachytene stage Metaphase II Diplotene stage Diakinesis Anaphase II Metaphase I Telophase II Anaphase I Cytokinesis Telophase I Cytokinesis  MEIOSIS I Interphase I Before meiosis I starts, the cell enters an interphase stage, during which DNA synthesis occur.  MEIOSIS I Prophase I Leptotene Zygotene Pachytene Diplotene Diakinesis  MEIOSIS I Prophase I: Leptotene lepto-: narrow, fine Tene: band, ribbon Chromatin begins to condense & chromosomes become visible.  MEIOSIS I Prophase I: Zygotene Synapsis (the process of pairing of homologous chromosomes) Zygo-: union, fusing begins. A synaptonemal complex begins to form, holding each pair of homologous chromosomes together. Bivalents= Tetrads= paired homologues.  MEIOSIS I Prophase I: Pachytene Pachy-: thick Synapsis is complete. Crossing over (recombination) occurs: The breakage and exchange of homologous segments of non- sister chromatids from paternal and maternal members of a pair of homologous chromosomes.  MEIOSIS I Prophase I: Diplotene Diplo-: double The synaptonemal complex starts to disappear. The homologous chromosomes start to separate, except at the chiasmata.  MEIOSIS I Prophase I: Diakinesis Dia-: through, across Kinesis: motion Chromosomes reach maximum condensation. Nuclear membrane starts to break down. MEIOSIS I  MEIOSIS I Metaphase I The nuclear membrane disappears. A spindle has formed. Paired chromosomes are aligned along the equatorial plane.  MEIOSIS I Anaphase I Homologous chromosomes are drawn to opposite poles of the cell (disjunction). The 23 pairs of homologous chromosome are assorted independently of one another → original paternal and maternal chromosome sets are distributed into random combinations.  MEIOSIS I Telophase I & Cytokinesis I Telophase I: Each haploid set of chromosomes groups at one pole of the cell. Cytokinesis I: A cleavage furrow forms, dividing the cell into two daughter cells, which enter interphase. MEIOSIS I  MEIOSIS II Interphase II - Before meiosis II starts, the cell enters a brief interphase, with NO S phase.  MEIOSIS II Prophase II - Each chromosome consists of one pair of sister chromatids held together at the centromere.  MEIOSIS II Metaphase II - Chromosomes are aligned at the equatorial plane.  MEIOSIS II Anaphase II - Sister chromatids separate and are pulled to opposite poles.  MEIOSIS II Telophase II & Cytokinesis Telophase II: One member of each pair of homologous chromosomes is present at each pole. Cytokinesis: A cleavage furrow forms, dividing each cell into two daughter cells. MEIOSIS II Genetic Consequences of Meiosis 1. Reduction of the chromosome number from diploid to haploid. 2. Segregation of alleles so that only one member of the original gene pair is included in each gamete. 3. Generation of genetic diversity by two mechanisms: a. Recombination as a result of crossing over. b. Independent assortment of homologous chromosomes. Comparison of Mitosis and Meiosis GAMETOGENESIS The process of formation of haploid gametes from diploid precursor cells. Gametogenesis: Spermatogenesis Oogenesis SPERMATOGENESIS SPERMATOGENESIS Starts at puberty & continues throughout life. Process takes ~ 64 days. 200 million sperm/per ejaculate. OOGENESIS Ova develop from oogonia in the ovarian cortex during prenatal life. Each oogonium is the center of a developing follicle. By 3rd month of prenatal development, oogonia have begun to develop into primary oocytes. Primary oocytes start meiosis I, and are arrested in prophase I. OOGENESIS Primary oocytes are arrested in prophase I from before birth till puberty. Starting from puberty, once a month, individual follicles begin to grow & mature, and one is ovulated. Primary oocytes OOGENESIS The primary oocyte completes meiosis I, giving the secondary oocyte, and the first polar body. Meiosis II begins during ovulation but is arrested in metaphase II. Meiosis II is completed only if fertilization occurs. The second polar body is formed. There is a period of arrest between start of meiosis I and start of meiosis II Prolonged arrest of primary oocyte in prophase I higher risk of nondisjunction with advanced maternal age Higher risk of fetal chromosomal abnormalities with advanced maternal age