Cell-DivisionMeiosis.pdf

Meiosis – A Source of Distinction Why do you share some but not all characters of each parent? What are...

Meiosis – A Source of Distinction Why do you share some but not all characters of each parent? What are the rules of this sharing game? At one level, the answers lie in MEIOSIS meiosis. Reduction-Division Genetic Recombination 1 Meiosis Meiosis žThe form of cell division by which žSex cells divide to produce GAMETES, with HALF the number of GAMETES (sperm or egg). CHROMOSOMES, are produced. žGametes have HALF the # of žDIPLOID (2n) ® HAPLOID (n) chromosomes. žMeiosis is SEXUAL reproduction. žOccurs only in GONADS (testes or žTWO divisions (MEIOSIS I and MEIOSIS ovaries). II). ž Male: SPERMATOGENESIS -sperm ž Female: OOGENESIS - egg or ova 3 4 Spermatogenesis Oogenesis Haploid n=23 n=23 (1n) human human egg sex cell sperm sex cell n=23 n=23 n=23 2n=46 2n=46 Polar Bodies n=23 (die) diploid (2n) n=23 diploid (2n) n=23 haploid (n) n=23 Meiosis I Meiosis II Meiosis I Meiosis II 5 6  Interphase I Interphase I ž Similar to mitosis interphase. ž CHROMOSOMES (DNA) replicate in the S phase ž Each duplicated chromosome consist of two žNucleus and nucleolus visible. identical SISTER CHROMATIDS attached at their Nucleus CENTROMERES. chromatin ž CENTRIOLE pairs also replicate. cell membrane nucleolus 7 8 Meiosis I (five phases) Prophase I žLongest and most complex phase (90%). žCell division that reduces the žChromosomes condense. chromosome number by one- half. žSynapsis occurs - Homologous chromosomes come together žFive phases: to form a tetrad. a. Prophase I žTetrad is two chromosomes b. Prometaphase I or four chromatids (sister and c. Metaphase I Prophase I non-sister chromatids). d. Anaphase I e. Telophase I 9 10 Prophase I - Synapsis Non-Sister Chromatids-HOMOLOGS Homologs contain DNA that codes for the same Homologous chromosomes genes , but different versions of those genes Genes occur at the same loci sister chromatids sister chromatids 11 Tetrad 12 Homologous Chromosomes Features of Meiosis žPair of chromosomes (maternal and paternal) that are similar in shape and size. Meiosis includes two rounds of division – meiosis I and žHomologous pairs (tetrads) carry GENES meiosis II. controlling the SAME inherited traits. žEach locus (position of a gene) is in the During meiosis I, homologous chromosomes (homologues) become closely associated with each other. This is same position on homologues. LOCI synapsis. žHumans have 23 pairs of homologous Proteins between the homologues hold them in a chromosomes: synaptonemal complex. a. First 22 pairs of autosomes b. Last pair of sex chromosomes 13 14 Homologous Chromosomes eye color eye color locus locus hair color hair color locus locus Paternal Maternal 15 16 Crossing Over Features of Meiosis žCrossing over may occur between non-sister chromatids at sites called Crossing over: genetic recombination between non-sister chiasmata. chromatids -physical exchange of regions of the chromatids žCrossing over: segments of chiasmata: sites of crossing over nonsister chromatids break and reattach to the other chromatid. The homologues are separated from each other in anaphase I. žChiasmata (chiasma) are where chromosomes touch each other and exchange genes (crossing over.) žCauses Genetic Recombination 17 18  Genetic Recombination nonsister chromatids Tetrad chiasmata: site variation 19 20 of crossing over Sex Chromosomes MEIOSIS I Meiosis I Homologs separate XX chromosome - female XY chromosome - male 21 22 Prophase I is divided into 5 Prophase I distinctive sub-stages: Nucleus & Nucleolus disappear Leptotene – The chromosomes begin to Spindle forms condense Chromosomes coil & Synapsis (pairing) occurs Zygotene – Synapsis begins with a synaptonemal Tetrads form & Crossing over Occurs complex forming between homologous chromosomes spindle fiber Pachytene – Crossing over of genetic material centrioles occurs between non-sister chromatids aster Diplotene – Synapsis ends with disappearance of fibers synaptonemal complex; homologous pairs remain attached at chiasmata Diakinesis – Chromosomes become fully condensed and nuclear membrane disintegrates prior to metaphase I 23 24 TETRAD  Prometaphase I Metaphase I Attachment of spindle fiber microtubules to Shortest phase the kinetochore proteins at the centromeres Tetrads align on the equator. Kinetochore microtubule Independent assortment occurs – chromosomes separate randomly causing GENETIC RECOMBINATION 25 26 Metaphase I Anaphase I žHomologous chromosomes separate and move towards the poles. žSister chromatids remain OR attached at their centromeres. Homologs line up at equator or metaphase plate 27 28 Anaphase I Homologs separate 29 30  Telophase I Telophase I žEach pole now has haploid (1n) set of chromosomes. žCytokinesis occurs and two haploid daughter cells are formed. cytokinesis 31 32 Meiosis II ž No Interphase II or very short MEIOSIS II ž No DNA Replication žRemember: Meiosis II is similar to Sister Chromatids mitosis Separate 33 34 Prophase II Prometaphase II žSame as Prophase in mitosis žSame as Prophase in mitosis žChromosomes condense žNucleus & nucleolus disappear žSpindle forms žSpindle fibers are fully formed žKinetochore formation 35 36  Metaphase II Anaphase II Same as Metaphase in mitosis žSame as Anaphase in mitosis žSISTER CHROMATIDS separate Chromosomes (not homologs) line up at equator 37 38 Telophase II Telophase II žSame as Telophase in mitosis. žNucleus and Nucleolus reform, spindle disappears žCYTOKINESIS occurs. žRemember: FOUR HAPLOID DAUGHTER cells are produced. žCalled GAMETES (eggs and sperm) 1n Sperm cell fertilizes 1n egg to form 2n zygote 39 40 Karyotype Karyotype žAn organized picture of the chromosomes of a human arranged in pairs by size from largest to smallest. žPairs 1-22 called AUTOSOMES žLast pair are SEX CHROMOSOMES Male - XY Female - XX 41 42  Boy or Girl? The Y Chromosome “Decides” Fertilization žThe fusion of a sperm and egg to form a zygote. žA zygote is a FERTILIZED EGG sperm n=23 n=23 egg 2n=46 zygote Y chromosome X chromosome 43 Boy or Girl? The Y Chromosome “Decides” 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. Common Non-disjunction Disorders The first is called Monosomy, the second is called Trisomy. If an organism Down’s Syndrome – Trisomy 21 has Trisomy 18 it has three Patau Syndrome – Trisomy 13 chromosomes in the 18th set, Trisomy Turner’s Syndrome – Monosomy 23 (X0) 21…. Three chromosomes in the 21st set. Klinefelter’s Syndrome – Trisomy 23 (XXY) If an organism has Monosomy 23 it has only one chromosome in the 23rd set. Edward’s Syndrome – Trisomy 18 Down Syndrome(Trisomy 21) Patau Syndrome(Trisomy 13) 49 50 Turner’s Syndrome(Monosomy 23) Klinefelter’s Syndrome(Trisomy 23) 51 52 Edwards Syndrome(Trisomy 18) 53 54