Sexual Reproduction Parents: 2 Offspring: different Cell division: MEIOSIS 46 23 23 MEIOSIS Produces gametes: SPERM and EGG 23 23 23 23

Why meiosis? FERTILIZATION: 23 + 23 = 46 EGG (haploid) HALF the normal # of chromosomes SPERM (haploid) HALF the normal # of chromosomes ZYGOTE (diploid) normal # of chromosomes (2 of each)

Meiosis PROS CONS Genetic variation The MOST IMPORTANT benefit!!! Not as fast as asexual reproduction (can’t produce as many offspring) Need to find a mate PROS Genetic variation The MOST IMPORTANT benefit!!!

Cells in your body have a chromosome set (all 46)- they are called diploid  Gamete cells (sperm and eggs) only have half (23)  - they are called haploid

Genetic Variation in Meiosis Recombination Crossing Over Homologous chromosomes switch pieces

Genetic Variation in Meiosis INDEPENDENT ASSORTMENT Homologous pairs of chromosomes are “sorted” differently each time a gamete is formed Sperm from the same man and eggs from the same woman can have different combinations of genes because of this independent assortment! Example: What are the possible gametes that will be produced from this diploid cell? HINT: Each gamete will get one “S” and one “T”

3 Sources of Genetic Variation in Meiosis CROSSING OVER (prophase I) 2. INDEPENDENT ASSORTMENT (metaphase I) 3. RANDOM FERTILIZATION

Mitosis Includes only 1 cycle of cell division Occurs in body cells (somatic cells) No crossing over of chromosomes Results in two genetically identical diploid cells Metaphase: 1 duplicated chromosome per spindle fiber Meiosis Includes 2 cycles of cell division (meiosis I and meiosis II) Occurs only in sex cells (gametes) Chromosomes cross over in Prophase I Results in four genetically different haploid cells Metaphase I: 2 duplicated chromosomes per spindle fiber

Meiosis and Sexual Reproduction: Practice Which of the following types of cells would experience mitosis, meiosis, or both? Write ME for meiosis, MI for mitosis.   ____A pollen cell of a plant ____A leaf cell of a plant ____A root cell of a plant ____The cells in the petal of a flower ____Stem cells ____Kidney cells ____Liver cells ____Testes cells ____Ovary cells ____Tree bark cells