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Lecture

The Living Body 3 - Meiosis.docx

4 Pages
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Department
Natural Science
Course Code
NATS 1610
Professor
Barbara Czaban

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NATS 1610 WINTER TERM L22 – Forming the Living Body – Meiosis Meiosis involves TWO cell divisions and results in the formation of 4 haploid gametes • Keeps chromosome numbers constant through generations − Haploid gametes – sperm, egg cell − Fertilization restores diploid number • Increases genetic variability in the population through genetic recombination − Crossing over − Independent assortment • Failure of chromosomes to separate during meiosis creates cell with extra or missing chromosomes − Nondisjunction Meiosis divides the chromosomes number in half • Germ Cells – diploid (2n) − Found in gonads – ovary, testis, anther − Each cell has a pair of homologous chromosomes − Formed by mitosis − Undergo meiosis to produce haploid gametes • Gametes – haploid (1n, or just n) − Each has one member of each homologous pair − Sex cells – egg (ovum), sperm − Formed by meiosis − Participate in fertilization • Fertilization − Union of gametes restores diploid number in zygote Meiosis halves the chromosome number A. In meiosis I, each duplicated chromosome in the nucleus pairs with its homologous partner B. Homologous partners separate. The still-duplicated chromosomes are packaged in tow new nuclei C. Sister chromatids separate in meiosis II. The now unduplicated chromosomes are packaged into four new nuclei Overview of Meiosis • 2 cell divisions • Results in 4 haploid daughter cells − DNA replication during interphase forms two copies of each chromosome, and the copies remain attached at the centromere (called sister chromatids) − During meiosis I, homologous chromosomes separate into two haploid cells, each containing one member of each homologous pair − During meiosis II, sister chromatids separate and four haploid daughter cells are formed that will develop into gametes o *Note: the number of homologous chromosomes pairs (diploid number) differs in different species of organisms Summary of Meiosis I – separates homologues • Duplicated homologues pair, align, separate – Results in two HAPLOID daughter cells • 2 chromatids/chromosomes – Reduction Division 1) Prophase I: Homologous chromosomes condense, pair up, and swap segments. Spindle microtubules attach to them as the nuclear envelope breaks up. − Prometaphase I occurs before Metaphase I begins 2) Metaphase I: The homologous chromosome pairs are aligned midway between spindle poles 3) Anaphase I: The homologous chromosomes separate and begin heading towards the spindle poles − Motor proteins − Each homologue still consists of 2 chromatids 4) Telophase I: Two clusters of chromosomes reach the spindle poles. A new nuclear envelope forms around each cluster, so two haploid n
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