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Meiosis Sex Cell Formation

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Mitosis Versus Meiosis We have learned that MITOSIS is the DIVISION of SOMATIC cells (Body cells) Purpose is for organism GROWTH Retain the same number of chromosomes (2n) through out all divisions In order to produce GAMETES (sex cells), we must go through a different process MEIOSIS Sometimes thought of as REDUCTION division Why? MITOSIS- produces two IDENTICAL daughter cells, each also identical to the original parent cell

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Homologous Chromosomes We inherit half of our DNA from each parent The combination of our fathers and mothers genes give us our own unique set of characteristics Somatic cells contain genetic material that is half from mom and half from dad (2n) HOMOLOGOUS chromosomes are two sets of chromosomes that control the same genes One homolog from mom, one from dad

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Meiosis and Chromosome Number The number of chromosomes in any given organisms cells is defined by the chromosome number N Diploid number (2n): cells that contain both sets of homologous chromosomes Aka: ALL somatic body cells Haploid number (n): cells that contain 1 set of inherited DNA AKA: ALL Sex cells (gametes)

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Karyotype: display of chromosomes This is a HUMAN karyotype. There are 23 pairs of DNA Each pair consists of one version from mom, and one version from dad (total=46 chromosomes)

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Meiosis Sex Chromosomes the 23rd chromosome, determines gender Two Forms: X Females have XX Y Males have XY

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Meiosis- reduction of chromosome number MEIOSIS is the formation of gametes by reducing a diploid cell (2n) into FOUR genetically different haploid cells (n) *Remember- mitosis divided one cell into 2 identical cells this this is quite different In males: formation of haploid sperm cells In females: formation of haploid egg cells

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Meiosis- 2 sets of division Meiosis I: separation of homologous chromosomes Each homolog contains an attached copy (sister chromatids) Meiosis II: separation of sister chromatids This stage looks identical to Mitosis

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Meiosis I Prophase I Protein cause homologous chromosomes to stick together along with their length Tetrads the paired chromosomes Now 4 chromatids Chromosome #: 2n 1 set from mom 1 set from dad 1(mom)+1(dad)= 2n

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Meiosis I Prophase I Crossing Over 2 nd new step; tetrads exchange genetic material This process introduces unique, new traits. This is one way, other than mutations, organisms can acquire NEW traits

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Meiosis I Metaphase I Tetrads move to the middle of the cell and line up across the spindle. Notice now chromosomes line up NEXT to their homologous pair (2 lanes)

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Meiosis I Anaphase I Homologous chromosomes separate and migrate to opposite poles Sister chromatids migrate together Each chromosome is made up of two copies

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Meiosis I Telophase I The chromosomes arrive at poles Each pole has a haploid daughter nucleus because it only has one set of chromosomes

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Meiosis I Cytokinesis Form two daughter cells Chromosomes in each daughter cell are still duplicated (double in number) Chromosome #: n Because moms set and dads set were separated, now you have haploid number

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Meiosis II Prophase II In each haploid daughter cell, spindle forms Nuclear envelope disappears

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Meiosis II Metaphase II Chromosomes line-up in the middle of cell Spindle attaches to centromeres

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Meiosis II Anaphase II Sister chromatids separate and move to opposite poles

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Meiosis II Telophase II & Cytokinesis Chromatids arrive at poles Now individual chromosomes Nuclear envelope reforms Cytokinesis splits cells

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Meiosis Finished Produced four DIFFERENT haploid daughter cells

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Genetic Variation How chromosomes line- up and separate at is a matter of chance So the chromosomes that end up in the resulting cells occur randomly Four combinations possible

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Closure- Did you get it?? 1.What are two sets of chromosomes that control the same genes called? Homologous chromosomes 2.What are homologous chromosomes doing when they are crossing over? Exchanging genetic material 3.What is the chromosome number at the beginning of meiosis I? at the END of meiosis I? At the end of Meiosis II? 2n (diploid), n (haploid), n (haploid)

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Bellringer- Name that phase Name each phase that is either being described or the picture depicts Ready??

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Metaphase I

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Prophase I Homologous chromosomes exchange genetic material by crossing over and tetrads form

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Anaphase II What is separating here??? Sister Chromatids

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Telophase I Two nuclei form, each containing a haploid set of replicated chromosomes

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Anaphase I Homologous pairs separate and migrate to opposite ends of the cell

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Comparing mitosis and Meiosis http://highered.mcgraw- hill.com/sites/0072507470/student_view0/ch apter3/animation__comparison_of_meiosis_a nd_mitosis__quiz_1_.html http://highered.mcgraw- hill.com/sites/0072507470/student_view0/ch apter3/animation__comparison_of_meiosis_a nd_mitosis__quiz_1_.html

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Mitosis vs Meiosis: Cell division processes Mitosis Occurs in all growing tissue of organisms Purpose is for organism GROWTH Division of BODY cells (Somatic cells) Examples: hair, blood, muscle, skin, etc. Meiosis Occurs in the Reproductive Organs of organisms Purpose is to create GAMETES (sex cells) for sexual reproduction Examples: Egg cells Sperm cells

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Prophase vs Prophase I MeiosisMitosis Spindle forms Nuclear membrane breaks down Chromosomes condense Chromosome #: 2n (diploid) Spindle forms Nuclear membrane breaks down Homologous chromosomes cross over and form tetrads Chromosome #: 2n (diploid)

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Metaphase vs Metaphase I MitosisMeiosis Chromosomes line up single file Spindle attaches to centromeres Chromosome #: 2n (diploid) Homologous pairs line up side- by-side Spindle attach to centromeres Chromosome #: 2n (diploid)

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Anaphase vs. Anaphase I MitosisMeiosis Sister Chromatids are pulled apart Cell elongates Chromosome #: 2n (diploid) Homologous pairs are pulled apart Cell elongates Chromosome #: 2n (diploid)

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Telophase vs Telophase I (And Cytokinesis) MitosisMeiosis Cytoplasm divides Nucleus forms Two genetically IDENTICAL daughter cells form Chromosome #: 2n (diploid) Cytoplasm divides Nucleus forms 2 different daughter cells form Nuclei is still REPLICATED Chromosome #: n (haploid)

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Meiosis II MitosisMeiosis

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Nuclear Envelope breaks down Spindle Forms

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Sister chromatids Line up single file Spindle attaches to centromeres

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Sister Chromatids Separate Cell Elongates

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Nuclear Membrane reforms Spindle disappears Cytoplasm begins to divide via cytokinesis

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Cytokinesis divides both cells Now, 4 genetically different daughter cells All reduced to haploid (n # of chromosomes) Notice how none are the same

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We Do Activity Visualizing meiosis Take out a piece of paper and number it 1-8 Take out 3 sheets of blank paper Using scrap paper, cut 8 small strips of paper Color 4 strips one color Color 4 another color Follow Ms. Hamadehs directions With your table partner, take out 2 different colored highlighters

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Human chromosomes There are 46 chromosomes (23 homologous pairs) in each somatic cell 22 pairs of autosomes 1 pair of sex chromosomes XX = Female, XY = Male Karyotype - chromosomes are arranged according to shape and size

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Nondisjunction and chromosomal disorders Nondisjunction failure of chromosomes to separate and segregate into daughter cells Nondisjunction may occur during meiosis 1 or meiosis 2 Abnormal number of chromosomes may result

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Normal monosomic gametes Normal meiosis MEIOSIS I MEIOSIS II Results of crossing- over not shown Replicate DNA

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MEIOSIS I MEIOSIS II Replicate DNA Nondisjunction during meiosis I Non-disjunction Disomic gametesNullisomic gametes

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MEIOSIS I MEIOSIS II Replicate DNA Nondisjunction during meiosis II Non-disjunction DisomicNullisomicMonosomic gametes

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EXAMPLES OF NON-DISJUNCTION

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Klinefelter's syndrome, 47, XXY It is the most common sex chromosome disorder and the second most common condition caused by the presence of extra chromosomes Symptoms: -Language impairment -Lanky, youthful build or rounded body type -Low levels of Testosterone and small testicles / Infertile

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Turners Syndrome (X) Common symptoms: Short stature swelling of the hands and feet Broad chest and widely spaced nipples Low hairline Low-set ears Reproductive sterility Increased weight, obesity Small fingernails Characteristic facial features Webbed neck

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What genetic disorder is this? They may have never known

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Down Syndrome 47, XY, +21

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1 in 1,250 births 47 chromosomes XY or XX #21 Trisomy Nondisjunction Down Syndrome