Meiosis 6.1, 6.2, 6.6

6.1 – Chromosomes & Meiosis Key Concept:Key Concept: Gametes have half the number of chromosomes that body cells have.Gametes have half the number of chromosomes that body cells have.

You have somatic cells and gametes. Somatic Cells:Somatic Cells: Are body cellsAre body cells Make up all cells in body except for egg and sperm cellsMake up all cells in body except for egg and sperm cells Not passed on to childrenNot passed on to children Gametes:Gametes: Are egg or sperm cellsAre egg or sperm cells Passed on to childrenPassed on to children

Your cells have autosomes and sex chromosomes. Somatic cells have 23 pairs of chromosomes (46 total)Somatic cells have 23 pairs of chromosomes (46 total) (1) Autosomes : pairs 1 – 22; carry genes not related to the sex of an organism(1) Autosomes : pairs 1 – 22; carry genes not related to the sex of an organism (3) Sex chromosomes : pair 23; determines the sex of an animal; control the development of sexual characteristics(3) Sex chromosomes : pair 23; determines the sex of an animal; control the development of sexual characteristics (2) Homologous chromosomes : pair of chromosomes; get one from each parent; carry the same genes but may have a different form of the gene (example: one gene for brown eyes and one gene for blue eyes)(2) Homologous chromosomes : pair of chromosomes; get one from each parent; carry the same genes but may have a different form of the gene (example: one gene for brown eyes and one gene for blue eyes)

Somatic cells are diploid; gametes are haploid. Diploid (2n)Diploid (2n) Has two copies of each chromosome (1 from mother & 1 from father)Has two copies of each chromosome (1 from mother & 1 from father) 44 autosomes, 2 sex chromosomes44 autosomes, 2 sex chromosomes Somatic cells are diploidSomatic cells are diploid Produced by mitosisProduced by mitosis Haploid (1n)Haploid (1n) Has one copy of each chromosome 22 autosomes, 1 sex chromosome Gametes are haploid Produced by meiosis

Chromosome number must be maintained in animals.Chromosome number must be maintained in animals. Many plants have more than two copies of each chromosome (can be tetraploid [4n] or hexaploid [6n])Many plants have more than two copies of each chromosome (can be tetraploid [4n] or hexaploid [6n]) Mitosis and meiosis are types of nuclear division that make different types of cells.Mitosis and meiosis are types of nuclear division that make different types of cells.

Mitosis makes identical diploid cells.Mitosis makes identical diploid cells.

Meiosis makes different haploid cells from diploid cells.Meiosis makes different haploid cells from diploid cells.

6.2 – Process of meiosis Key Concept:Key Concept: During meiosis, diploid cells undergo two cell divisions that result in haploid cells.During meiosis, diploid cells undergo two cell divisions that result in haploid cells.

Cells go through two rounds of division in meiosis. Meiosis reduces chromosome number and creates genetic diversity.Meiosis reduces chromosome number and creates genetic diversity.

Homologous chromosomes (sometimes called homologues)Homologous chromosomes (sometimes called homologues) Pair of chromosomesPair of chromosomes Inherit one from each parentInherit one from each parent Carry similar genes (code for different traits)Carry similar genes (code for different traits) Separate during Meiosis ISeparate during Meiosis I Sister chromatidsSister chromatids Duplicates of each otherDuplicates of each other Each half of a duplicated chromosomeEach half of a duplicated chromosome Attached together at the centromereAttached together at the centromere Separate in Meiosis IISeparate in Meiosis II

Meiosis I Occurs after DNA has been replicated (copied)Occurs after DNA has been replicated (copied) Divides homologous chromosomes in four phases.Divides homologous chromosomes in four phases.

Meiosis I (1) Prophase I(1) Prophase I Chromosomes condenseChromosomes condense Homologous chromosomes pair upHomologous chromosomes pair up Nuclear envelope (membrane) breaks downNuclear envelope (membrane) breaks down Spindle fibers formSpindle fibers form

Meiosis I (2) Metaphase I(2) Metaphase I Homologous chromosomes are lined up along the middle of the cell (along the cell equator) by spindle fibersHomologous chromosomes are lined up along the middle of the cell (along the cell equator) by spindle fibers

Meiosis I (3) Anaphase I(3) Anaphase I Homologous chromosomes move apart to opposite sides of the cellHomologous chromosomes move apart to opposite sides of the cell Sister chromatids remain attachedSister chromatids remain attached

Meiosis I (4) Telophase I & Cytokinesis(4) Telophase I & Cytokinesis Spindle fibers fall apartSpindle fibers fall apart Nuclear membranes reformNuclear membranes reform Cytoplasm splitsCytoplasm splits

Meiosis II Divides sister chromatids in four phases.Divides sister chromatids in four phases. DNA is not replicated between Meiosis I and Meiosis II.DNA is not replicated between Meiosis I and Meiosis II.

Meiosis II (5) Prophase II(5) Prophase II Nuclear envelope (membrane) breaks downNuclear envelope (membrane) breaks down Spindle fibers formSpindle fibers form

Meiosis II (6) Metaphase II(6) Metaphase II Spindle fibers line chromosomes up along the middle of the cellSpindle fibers line chromosomes up along the middle of the cell

Meiosis II (7) Anaphase II(7) Anaphase II Sister chromatids are pulled apart to opposite sides of the cellSister chromatids are pulled apart to opposite sides of the cell

Meiosis II (8) Telophase II & Cytokinesis(8) Telophase II & Cytokinesis Nuclear membranes form around chromosomesNuclear membranes form around chromosomes Chromosomes begin to uncoilChromosomes begin to uncoil Spindle fibers fall apartSpindle fibers fall apart Cytoplasm splitsCytoplasm splits

Mitosis Vs. Meiosis Mitosis One cell divisionOne cell division Homologous chromosomes do not pair upHomologous chromosomes do not pair up Results in diploid cellsResults in diploid cells Daughter cells are identical to parent cellDaughter cells are identical to parent cell Meiosis Two cell divisionsTwo cell divisions Homologous chromosomes pair up (Metaphase I)Homologous chromosomes pair up (Metaphase I) Results in haploid cellsResults in haploid cells Daughter cells are uniqueDaughter cells are unique

Haploid cells develop into mature gametes. Gametogenesis is the production of gametes.Gametogenesis is the production of gametes. Gametogenesis differs between males and females.Gametogenesis differs between males and females. Sperm (spermatogenesis)Sperm (spermatogenesis) Become streamlined and motile (able to move)Become streamlined and motile (able to move) Primarily contribute DNA to the embryoPrimarily contribute DNA to the embryo Males produce over 250 million sperm per dayMales produce over 250 million sperm per day Egg (oogenesis)Egg (oogenesis) Contribute DNA, cytoplasm, and organelles to the embryoContribute DNA, cytoplasm, and organelles to the embryo During meiosis, the egg gets most of the contents, the other 3 cells become polar bodiesDuring meiosis, the egg gets most of the contents, the other 3 cells become polar bodies Females produce a few hundred eggs over a lifetimeFemales produce a few hundred eggs over a lifetime

6.6 – Meiosis & Genetic Variation Key Concept:Key Concept: Independent assortment and crossing over during meiosis result in genetic diversity.Independent assortment and crossing over during meiosis result in genetic diversity.

Sexual reproduction creates unique combinations of genes. FertilizationFertilization RandomRandom Increases unique combinations of genesIncreases unique combinations of genes In humans, the chance of getting any one combination of chromosomes from any one set of parents is one out of 2 23 x 2 23 (which is one out of over 64 trillion combinations)In humans, the chance of getting any one combination of chromosomes from any one set of parents is one out of 2 23 x 2 23 (which is one out of over 64 trillion combinations)

Sexual reproduction creates unique combinations of genes. Independent assortment of chromosomesIndependent assortment of chromosomes Homologous chromosomes pair randomly along the cell equatorHomologous chromosomes pair randomly along the cell equator Increases the number of unique combinations of genesIncreases the number of unique combinations of genes In human cells, about 2 23 (8 million) different combinations could resultIn human cells, about 2 23 (8 million) different combinations could result

Sexual reproduction creates unique combinations of genes. Crossing overCrossing over Exchange of chromosome segments between homologous chromosomesExchange of chromosome segments between homologous chromosomes Increases genetic diversityIncreases genetic diversity Occurs during Prophase I of Meiosis IOccurs during Prophase I of Meiosis I Results in new combinations of genes (chromosomes have a combination of genes from each parent)Results in new combinations of genes (chromosomes have a combination of genes from each parent)

Genetic linkage Chromosomes contain many genes.Chromosomes contain many genes. The farther apart two genes are located on a chromosome, the more likely they are to be separated by crossing overThe farther apart two genes are located on a chromosome, the more likely they are to be separated by crossing over Genetic linkage: genes located close to each other on the same chromosome tend to be inherited togetherGenetic linkage: genes located close to each other on the same chromosome tend to be inherited together