Meiosis Ch. 11-4

Asexual Reproduction Offspring can be reproduced from one parent Budding (sponges) Regeneration (sea stars) Grafting (in trees and plants) Binary Fission/Mitosis (unicellular animals) In this situation, offspring are genetically identical to their parent

Sexual Reproduction Produces offspring by union of gametes Gamete: a sex cell (like a sperm or egg) Allows for genetic variability Allows for evolution to occur Variety of genes available to an organism allows it to adapt to various situations.

Meiosis Cell division process that takes place only in gametes DNA copies during interphase Cell divides once (meiosis 1) Cell divides again (meiosis 2) Result is 4 haploid cells

Meiosis

Meiosis 1 Prophase 1 DNA coils up Spindle appears Cell Membrane Centrioles Prophase 1 DNA coils up Spindle appears Nucleolus & Nuclear Membrane disappear Chromosomes line up with their partner Crossing over can occur Spindle (forming) Tetrad Nucleus (fading)

Synapsis Happens during Prophase 1 Synapsis: Two like chromosomes line up beside one another They are now called tetrads

Crossing Over Happens during Prophase 1 When synapsis occurs, chromosome arms wrap around one another Sometimes arms break and join the opposite chromosome.

Crossing Over Crossing Over exchanges DNA between like chromosomes The result of crossing over is Genetic Recombination and increased genetic variability This leads to evolution…

Metaphase 1 Spindle is fully formed Centrioles Cell Membrane Spindle is fully formed Tetrads line up across the center of the cell Their arrangement on the spindle is random!! Spindle (complete) Tetrad **Notice – there is NO nucleus present here!

Anaphase 1 Tetrads are pulled to opposite ends of the cell Centrioles Chromosomes Tetrads are pulled to opposite ends of the cell Chromosomes are randomly separated! Separating Spindle

Independent Assortment Chromosomes are randomly lined up and pulled apart during meiosis. Which daughter cell receives which chromosomes is not controlled, so it is called independent assortment. The result is increased genetic variability!

Independent Assortment

Telophase 1 Cell divides into two new cells The cells are diploid Centrioles Chromosomes Cell divides into two new cells The cells are diploid Cleavage Furrow

Meiosis 2 Meiosis 2 proceeds much like mitosis. DNA does not copy before prophase 2 The end result is 4 haploid sex cells

Prophase 2 Spindle fibers appear Nucleus and Nucleolus are not visible Centrioles Spindle fibers appear Nucleus and Nucleolus are not visible Chromosomes

Metaphase 2 Spindle is completely formed Centrioles Spindle is completely formed Chromosomes line up across the center of the cell Chromosomes

Anaphase 2 Spindle separates chromosomes Spindle (separating) Centrioles Spindle separates chromosomes Chromosomes are pulled to opposite ends of the cell. Chromatids

Telophase 2 Spindle disappears Chromosomes relax Cytokinesis occurs Cell Membrane Centrioles Spindle disappears Chromosomes relax Cytokinesis occurs Nuclear membrane & Nucleolus reform Nucleus Cleavage Furrow Chromatin (haploid #)

Mitosis vs. Meiosis

Mitosis vs. Meiosis

Gametogenesis Gametogenesis: formation of gametes Spermatogenesis: makes sperm in the testes Oogenesis: makes eggs in the ovaries (ova)

Spermatogenesis Meiosis occurs in the testes Result is 4 haploid sperm All 4 sperm will mature and grow tails

Oogenesis Meiosis occurs in the ovaries Cytoplasm divides unequally in the cells Result is 1 haploid ova and 3 polar bodies The ova will mature The polar bodies will disintegrate and die

The End