10.2 – Meiosis Sexual Reproduction - PowerPoint Presentation

10.2 – MeiosisSexual Reproduction Function: creates gametes (sperm, egg, pollen) for sexual reproduction .

Meiosis: Cell division for sexual reproduction. Males - testes Females - ovaries Sex Cells

Homologous Chromosomes A normal human body cell contains 46 chromosomes in 23 pairs23 from mother; 23 from father Each pair of chromosomes are called HOMOLOGOUS CHROMOSOMES In a homologous pair, one chromosome is from the mother and one is from the father

Haploid vs. Diploid Haploid cell – a cell with only one copy of each chromosome ( 23 total in humans) ½ the number of chromosomes as the parent or normal body cells.n  haploid; n = 23 Diploid cell – a cell with 2 copies of each chromosome (46 total; 23 pairs in humans) 2n  diploid; 2n = 46

Diploid All body cells are diploid (2n ) Skin, liver, bone, lung, etc. Mother  23 chromosomesFather  23 chromosomesBody cells are called SOMATIC cellsDiploid body cells divide via mitosis to form two identical diploid daughter cells46 chromosomes total

Haploid Sex cells are called GAMETES (sperm and egg) Gametes join to form a ZYGOTE sperm + egg = zygote Gametes are haploid (n) Sperm = nEgg = nn + n = 2n

Meiosis Meiosis is the process of making haploid gametes from a diploid cellEach haploid gamete will have HALF the genetic material as the diploid parent cell 2n  n

MeiosisMeiosis happens in two stages: Meiosis IMeiosis IIThere are two cell divisions 4 gametes are created

Phases of Meiosis 1st Division Prophase I Metaphase I Anaphase I Telophase I 2nd Division Prophase II Metaphase II Anaphase II Telophase II CytokinesisInterphase

Interphase During Interphase:Cell grows in sizeDNA is duplicated Nuclear membrane Chromatin

Prophase I During Prophase I:Chromatin condenses into chromosomes Nuclear membrane dissolves Spindle formsHomologous chromosomes pair up and CROSSING OVER occurs Crossing Over: homologous chromosomes pair up and randomly exchange genetic information during Prophase 1

Mistakes in Meiosis Genetic Recombination : AKA Crossing Over Occurs during meiosis, chromosomes actually cross over each other, genetic material gets switched !This leads to genetic variability! Can be an endless number of different chromosomes possible!

Spindle Homologous Chromosomes (Crossing Over)

Metaphase IDuring Metaphase I: Homologous chromosomes are pulled to the middle of the cell by the spindle

Spindle Homologous Chromosomes

Independent AssortmentChromosomes line up randomly during metaphaseThere is a 50% chance of the maternal or paternal chromosomes going to either side of the cell

= MOTHER = FATHER

Anaphase I During Anaphase I:Homologous chromosomes are pulled towards opposite ends of the cell by the spindle

Spindle Chromosomes

Telophase I During Telophase I:Nuclear envelope reformsSpindle dissolvesCell stretches and cytoplasm divides ( cytokinesis )

Chromosomes

NO Interphase! There is NO Interphase between Meiosis I and Meiosis II Each gamete must receive HALF of the parent’s genetic material so no duplication is necessary

Prophase II During Prophase II:Nuclear membrane dissolvesSpindle re-forms Meiosis II is the division of the two diploid cells resulting from Meiosis I 4 haploid cells will be created

Chromosomes Spindle

Metaphase IIDuring Metaphase II: Chromosomes are pulled to the middle of the cell by the spindle

Chromosomes Spindle

Anaphase II During Anaphase II:Sister chromatids are separated and pulled to opposite ends of the cell by the spindle

Spindle Chromosomes

Telophase IIDuring Telophase II Nuclear membrane re-formsSpindle dissolvesCells stretch and prepare for cytokinesis

Chromosomes

CytokinesisThe cytoplasm divides resulting in a total of 4 unique haploid cellsAll gametes are different because of crossing over during Prophase I and independent assortment during Metaphase I & II

Meiosis: IPMAT cytokinesis; 2 nd phase of PMAT producing 4 (haploid-single) daughter cells I II Prophase Synapsis occurs and crossing over of chromosomes sameMetaphase Pairs of chromosomes line in the plates 2 celled-pair of chromosomes line in the plates AnaphaseHomologous pairs separateSister chromatids separatesTelophaseDivision occursDivision occurs

Genetic Diversity Meiosis increases genetic diversity because every sperm or egg is a little bit different. No two offspring are exactly the same Crossing Over Independent Assortment Genetic diversity causes some organisms to be more fit to survive and reproduce ( evolution)

Meiosis and Gametes How does meiosis create genetic diversity? 1) crossing over 2) random assortment of chromosomes and later … 3) random pairing of gametes

Nondisjunction Nondisjunction - an error in Meiosis when homologous chromosomes fail to separate properly during Anaphase I or IIThe resulting gametes will have an extra chromosome or be missing a chromosome 3 copies instead of 2 1 copy instead of 2 Most zygotes will not survive

Nondisjunction during Meiosis II Nondisjunction during Meiosis I

Non-Disjunction In humans, a person with an extra 21st chromosomes will have Down’s Syndrome

Down Syndrome Trisomy – when an organism has 3 copies of a chromosome where it should only have two Trisomy is caused by nondisjunction Trisomy of the 21st chromosome results in DOWN SYNDROME3 copies of the 21 st chromosome

Trisomy 21 causes Down Syndrome

Mitosis vs. Meiosis Makes EXACT copiesNo new genetic diversity2n  2n (diploid  diploid) Happens in body (somatic) cellsOne cell division Asexual Makes UNIQUE gametes Increases genetic diversity 2n  n (diploid  haploid)Happens in gamete producing cells (GONADS: testes and ovaries)Two cell divisionsSexual

= Sexual Reproduction Diploid Cell – full set of chromosomes Same as Mitosis 2 daughter cells are identical to parent cell. Chromosomes do NOT duplicate here. Each cell divides- chromosomes separating randomly! The Result: 4 daughter cells with ½ the number of chromosomes needed for an individual. Haploid Cells –½ set of chromosomesAlso called GametesEx. Sperm, egg, pollen

Eggs are created Sperm are created Resulting Cell when 2 gametes fuse = Fertilization

Mitosis vs. Meiosis Mitosis Meiosis Function Type of Reproduction # of Cells Produced Type of Cell Produced # of Cellular Divisions # of Chromosomes in Daughter cells

Mitosis vs. Meiosis Mitosis Meiosis Function Create new cells for growth & replace old cells Create gametes for sexual reproduction Type of Reproduction Asexual Sexual # of Cells Produced 2 4 Type of Cell Produced Diploid Haploid # of Cellular Divisions 1 2 # of Chromosomes in Daughter cells Same as parent cell ½ as parent cell

somatic cell - undergoes mitosis, non-sex cell, diploid, identical with the parent germ cell - undergoes meiosis, sex-cell, haploid, non-identical with the parent, brings variety of genetic informationA germ cell is a reproductive cell, like an ovum, pollen, sperm, or spore. It is made in special tissues and contains the haploid chromosome number, which, in humans, is 23. "Somatic cells" includes all the other cells; somatic cells are "body cells" (SOMA=body). They are diploid; human somatic cells have 46 chromosomes