CH 10.2 Meiosis

MeiosisMeiosis is cell division that produces gametes with half the number of chromosomes as a body cell

MeiosisDiploid cell: has 2 of each kind of chromosomeAll body cells, except gametes, are diploid cells (such as liver or lung cells)When they divide, this is called mitosis

Haploid cell: has 1 of each kind of chromosomeGametes are haploid cells When they divide, this is called meiosis

Why Meiosis?Each type of organism has a specific number of chromosomes (humans have 46 chromosomes) Because each parent passes along a chromosome, there has to be some way for offspring to have the same number of chromosomes as their parents and not double

Why Meiosis?Gametes that are produced are sperm cells for males and egg cells for femalesSexual reproduction: when haploid egg and sperm cells join together to form a diploid zygoteIn humans, each sperm and egg cell has 23 chromosomes (which is half of 46, the normal number of chromosomes) or Sperm (23) + Egg (23) = Zygote (46)

Why Meiosis?For the most part, healthy human zygotes cannot have more than 46 chromosomesOnce formed, zygotes undergo mitosis to grow and develop (asexual reproduction)

Before MeiosisHomologous chromosomes are attached to each otherHomologous chromosomes: 2 of the same type of chromosomeChromosomes copy themselves so that they can be passed along to the daughter cells

Before MeiosisTetrad: 2 homologous chromosomes attached together (= 4 sister chromatids)

Genetic Variation Genetic variation: rearrangement of alleles that can produce completely new combinations (this is essential for evolution to occur)

Genetic VariationCrossing Over- When the homologous chromosomes pair so tightly that a piece of the chromatid breaks off. The piece changes place with a piece from the other homologous tetrad. This scrambles the traits around = genetic variety.

Genetic VariationGenetic Recombination: the reassortment of chromosomes and genetic material, either by crossing over or independent assortment, which increases genetic variety

Genetic VariationBoth crossing over and independent assortment are completely RANDOMso this is why humans can sometimes look completely different from other family members

Meiosis I2 new cells are formedProphase I: *Tetrads condense (2 homologous chromosomes attached together) *Crossing over occurs between chromosomes*Nuclear membrane disappearsCentrosomes move to opposite ends of cell to form spindle fibersMetaphase I: Tetrads line up in the middle of the cell

Meiosis IAnaphase I: Tetrads split and each set of sister chromatids moves to opposite ends of the cell

Telophase I:Cytokinesis (division of the cytoplasm) results in 2 new cells, each with homologous chromosomes

Meiosis II(4 new cells are formed) *LIKE MITOSIS*Prophase II: Centrosomes on the chromosomes form spindle fibers in preparation to be pulled apart

Metaphase II: Chromosomes (sister chromatids) line up randomly in the middle

Meiosis IIAnaphase II: Sister chromatids separate and move to opposite ends of the cell

Telophase II/Cytokinesis: : New nuclei are formed and four daughter cells are produced, each containing ONE chromosome from each homologous pair

Mistakes in MeiosisNondisjunction: when homologous chromosomes dont separate from each other during meiosis

So homologous chromosomes move together into a new gamete giving that gamete too many chromosomes, while the other new gamete is missing the chromosome

Mistakes in MeiosisOrganisms with extra chromosomes can survive

Organisms with too few chromosomes usually do not survive

Mistakes in MeiosisPolyploidy: when organisms have more than the normal number of chromosome setsinstead of 46 chromosomes, they might have double or triple that numberCan be lethal for animals but not plants (plants just usually get bigger)

Conditions that Result from Extra Chromosomes

Examples of NondisjunctionGenotype (X=Female sex chromosome, Y= male sex chromosome)Turners Syndrome XOTrisomy X (metafemales)XXXKlinefelters syndromeXXYJacobs syndromeXYYDown syndromeTRISOMY 21 (3 copies)Pataus syndromeTRISOMY 13Lethal condition in malesOY

Comparing Mitosis and Meiosis

Mitosis MeiosisDNA replicates in interphaseDNA replicates in interphaseONE division of chromosomesSeparate sister chromatidsTWO divisions of chromosomesSeparate homologous chromosomes (meiosis I)Separate sister chromatids (meiosis II)Makes 2 identical DIPLOID cellsMakes 4 HAPLOID cellsChromosome # is identical to parent cellChromosome # is HALF of the parent cellUsed for replacing old cells, growthUsed to make gametes