Gulf Medical University L3 & L4 Meiosis I & II Lecture Notes PDF

L3 & L4 MEIOSIS I & MEIOSIS II Dr. Preetha J Shetty October 8, 2024 www.gmu.ac.ae COLLEGE OF Learning objectives: Define meiosis Spermatogenesis Oogenesis Fertilization First meiotic division (Meiosis I) and their differ...

L3 & L4 MEIOSIS I & MEIOSIS II Dr. Preetha J Shetty October 8, 2024 www.gmu.ac.ae COLLEGE OF Learning objectives: Define meiosis Spermatogenesis Oogenesis Fertilization First meiotic division (Meiosis I) and their different stages Second meiotic division (Meiosis II) and their stages Non-disjuction of chromosomes and their disorders Genetic consequences of meiosis  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.  Fertilization The fusion of a sperm and egg to form a zygote. zygote A zygote is a fertilized egg sperm n=23 n=23 egg 2n=46 zygote  Meiosis is 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.  Meiosis Meiosis is Two cell divisions (called meiosis I and meiosis II) with only one duplication of chromosomes.  Meiosis in males is called spermatogenesis and produces sperm. Meiosis in females is called oogenesis and produces ova.  Spermatogenesis Secondary Spermatocyte n=23 human sex cell sperm Primary Spermatocyte n=23 n=23 2n=46 Secondary Spermatocyte haploid (n) n=23 diploid (2n) n=23 4 sperm cells are produced from each n=23 primary spermatocyte. meiosis I meiosis II Oogenesis *** The polar bodies die… only one ovum (egg) is produced from each primary oocyte. 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.  Interphase I Nucleus and nucleolus visible. chromatin nuclear membrane cell membrane nucleolus Meiosis I (four phases) Cell division that reduces the chromosome number by one-half. Four phases: a. Prophase I b. Metaphase I c. Anaphase I d. Telophase I  Prophase I Longest and most complex phase. 90% of the meiotic process is spent in Prophase I Prophase I has been further subdivided into the following five phases based on chromosomal behaviour: 1. Leptotene 2. Zygotene 3. Pachytene 4. Diplotene 5. Diakinesis  Prophase I 1. Leptotene: Chromosomes condense. 2. Zygotene: At this stage, homologous chromosomes begin to pair closely along their entire length. This process of pairing is known as Synapsis. 3.Pachytene: This is the stage characterized by crossing-over events. Crossing over generally occurs in areas of syneptonemal complexes. Such crossing over are called expected crossing over..  Prophase I - Synapsis Homologous chromosomes sister chromatids sister chromatids Tetrad Crossing Over creates variation (diversity) in the offspring’s traits. nonsister chromatids Tetrad chiasmata: site variation of crossing over 4. Diplotene In this phase of cell division chaismata are formed due to the migration of DNA strands from one DNA molecule to another DNA molecule in reciprocal manner (The Chiasmata (chiasma) are the sites of crossing over). over) This is called homologous recombination. 5. Diakinesis: The final stage of meiotic prophase-I is diakinesis. This is marked by terminalisation of chiasmata. In this stage the chromosomes reach maximal condensation. Crossing Over is one of the Two major occurrences of Meiosis (The other is Non-disjunction) Prophase I spindle fiber centrioles aster fibers  Metaphase I Shortest phase Tetrads align on the metaphase plate. INDEPENDENT ASSORTMENT OCCURS: 1. Orientation of homologous pair to poles is random. 2. Variation 3. Formula: 2n Example: 2n = 4 then n = 2 thus 22 = 4 combinations Metaphase I OR metaphase plate metaphase plate Anaphase I Homologous chromosomes separate and move towards the poles. Sister chromatids remain attached at their centromeres. Anaphase I Telophase I Each pole now has haploid set of chromosomes. Cytokinesis occurs and two haploid daughter cells are formed. Telophase I Meiosis II No interphase II (or very short - no more DNA replication) replication Remember: Meiosis II is similar to mitosis Prophase II same as prophase in mitosis Metaphase II same as metaphase in mitosis metaphase plate metaphase plate Anaphase II same as anaphase in mitosis sister chromatids separate  Telophase II Same as telophase in mitosis. Nuclei form. Cytokinesis occurs. Remember: four haploid daughter cells produced. gametes = sperm or egg Telophase II  Non-disjunction Non-disjunction is one of the Two major occurrences of Meiosis. (The other is Crossing Over) 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 numbers…… remember…. An abnormal chromosome number (abnormal amount of DNA) is damaging to the offspring. 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 (X0) Kleinfelter’s Syndrome – Trisomy 23 (XXY) Edward’s Syndrome – Trisomy 18 Genetic consequences of Meiosis Reduction of the chromosome number Segregation of alleles Random assortment of the homologs Additional shuffling of the genetic material by crossing over. SUMMARY Somatic cells have diploid set of chromosomes and gametes have haploid set Of chromosomes. Meiosis in males is known as spermatogenesis resulting in the production of sperms Meiosis in females is known as oogenesis resulting in formation of ovum. Meiosis has two divisions: Meiosis I and Meiosis II but the replication of DNA happens only once. Meiosis I is known as reduction division. Meiosis II is similar to mitosis. Both the Meiotic divisions are broadly classified into 4 phases, prophase, metaphase, anaphase and telophase. But Prophase I of Meiosis I is further subdivided into 5 phases Depending on the behavior of the chromosome. Crossing over and non-disjunction are two major events occurring  DISCLAMER The contents of this presentation, can be used only for the purpose of a Lecture, Scientific meeting or Research presentation at Gulf Medical University, Ajman. www.gmu.ac.ae