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Cell Reproduction

Cell Reproduction. Mitosis and Meiosis A. Mitosis 1. Produces two cells with identical chromosomes (same genes) 2. Unicellular reproduction, embryo development, growth and cell replacement B. Meiosis

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Cell Reproduction

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  1. Cell Reproduction Mitosis and Meiosis A. Mitosis 1. Produces two cells with identical chromosomes (same genes) 2. Unicellular reproduction, embryo development, growth and cell replacement B. Meiosis 1. Produces nuclei with one half the chromosomes and new genetic combinations (avoids doubling of chromosome number with each generation) 2. Produces gametes only C. Sexual reproduction 1. Always involves both meiosis and fertilization

  2. Eukaryotic Chromosomes A. Chromatin (diffuse) condenses prior to division to visible chromosomes 1. Already duplicated when condense a. Sister chromatids held together by centromere B. Haploid and diploid 1. Chromosomes found in homologous pairs a. identical but may have varying gene messages b. Humans have 46 (23 pairs) 1. 22 homologous pairs (autosomes) 2. X and Y (23rd pair) sex chromosomes 3. Females (XX); Males (XY) c. Pairs separate during reproduction 1. One of each pair into gamete 2. Offspring receives one of each kind from each parent d. Capable of exchanging genetic material during crossing over (Prophase I) e. Resemble each other in size, shape, and hereditary information f. Separated during meiosis (Anaphase I) g. Two paired homologus chromosomes - tetrad

  3. Eukaryotic Chromosomes 2. Diploid (2N) is chromosomes in pairs a. Fertilized egg and all that arise from it are diploid (somatic cells) b. Most normal zygotes c. Alternation of generation in plants: the recognizable fern is the diploid sporophyte 3. Haploid (N) is one of each chromosome pair a. Cells have undergone meiosis b. Germ cells become gametes c. Many lower organisms are normally haploid (some tetraploid)

  4. Fertilization and meiosis Meiosis Purpose: production of sex cells (gametes) for the next generation Result: 2 divisions resulting in 4 daughter cells with 1/2 (haploid) sets of chromosomes Mitosis?

  5. Comparison/contrast with mitosis • both preceded by replication of chromosomes • unlike mitosis- 1.replication is followed by TWO nuclear cell divisions 2.produces FOUR daughter cells instead of two 3.the resultant daughter cells have HALF the number of chromosomes (meiosis is a REDUCTION division) • creates genetic variation

  6. Steps of Meiosis 1. Interphase • chromosomes replicate • each duplicated chromosome consists of two identical sister chromatids attached at their centromeres • centriole pairs in animal cells also replicate into two pairs

  7. Meiosis I (reduces the chromosome number by one-half) 1. Prophase I • chromosomes condense • SYNAPSIS occurs. Homologous chromosomes come together as pairs (forms a tetrad) • in each tetrad, sister chromatids of the same chromosome are attached at centromeres...nonsister chromatids are linked by X-shaped chiasmata (site of crossing over; genetic variation) • spindle forms from microtubules • nuclear envelope/nucleoli disperse

  8. Metaphase I • each synaptic pair is aligned at equator so that the centromeres of homologues point towards opposite poles • homologues are destined to separate towards opposite poles

  9. Anaphase I • sister chromatids remain attached while homologues move towards the opposite pole (different than mitosis where sister chromatids are moved apart)

  10. Telophase I & Cytokinesis • each pole now has a haploid set of chromosomes composed of two sister chromatids attached at the centromere • cytokinesis occurs producing two daughter cells (cleavage furrow in animal cells; cell plate in plant cells) • NO DNA replication occurs before Meiosis II

  11. Meiosis II (similar to mitosis) NO Interphase; NO DNA replication! 1. Prophase II • spindle apparatus forms and chromosomes move towards the metaphase II plate 2. Metaphase II • chromosomes align on metaphase plate • kinetochores of sister chromatids point towards opposite poles 3. Anaphase II • centromeres of sister chromatids separate • individual chromosomes move toward opposite poles of the cell 4. Telophase II & Cytokinesis • nuclei form • cytokinesis occurs producing 4 haploid daughter cells

  12. Humans • Males: meiosis by primary spermatocytes; each secondary spermatocytes then produces 2 sperm • Females: at the end of meiosis, one ovum and 3 polar bodies are produced • Abnormalities: nondisjunction (XXY, 21, 21, 21-Down syndrome)

  13. Meiosis in Males - spermatogenesis

  14. Meiosis in Females - oogenesis

  15. Nondisjunction and Translocation A. Too few or too many chromosomes in gametes 1. Translocation: attachment of all or part of chromosome to another chromosome 2. Nondisjunction: failure of chromosomes to separate B. Down Syndrome 1. Translocation of chromosome #15 to #21, or 2. Nondisjunction with extra #21 C. Nondisjunction of sex chromosomes 1. XO- Turner Syndrome; sterile female 2. XXY- Kleinfelter Syndrome; sterile male 3. XXX- Metafemale; limited fertility 4. XYY- normal male; somewhat taller

  16. Genetic Variations • Independent Assortment – each of the 23 pairs of chromosomes segregates independently – 223 (8,000,000) gametes with different gene combinations can be produced from 1 original cell • Crossing Over – portions of each chromatid are shared – occurs in Prophase I • Random Fertilization – sperm? So, possible combinations is squared (223 x 223 = 64 trillion) • So???? Evolution……….

  17. Sexual vs. Asexual Reproduction • Asexual – clone…..why? A single parent passes copies of its genes to each of its offspring – no fusion of haploid cells (ex. fission or budding) • Clone – genetically identical to its parent • Sexual – haploid gametes combine to form diploid offspring; because both parents contribute DNA, their offspring have traits of both, but are not exactly like either

  18. Life Cycle • The entire span in the life of an organism from one generation to the next • 3 types: Haploid – protists, some fungi and algae Diploid – most animals Alternation of generations – plants, algae, and some protists

  19. Mitosis vs. Meiosis • http://www.pbs.org/wgbh/nova/baby/divi_flash.html

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