Meiosis & Sexual Reproduction

Aaaargh! I’m seeing double! Cell Division Mitosis – Asexual Reproduction produce cells with same information identical daughter cells exact copies clones same number of chromosomes same genetic information Aaaargh! I’m seeing double!

Asexual Reproduction Single-celled eukaryotes yeast Paramecium Amoeba Simple multicellular eukaryotes Hydra budding What are the disadvantages of asexual reproduction? What are the advantages?

+ How About The Rest of Us? 46 46 92 What if we did, then…. What if a complex multicellular organism (like us) wants to reproduce? We use sexual reproduction joining of egg + sperm Do we make egg & sperm by mitosis? NO What if we did, then…. 46 + 46 92 egg sperm zygote Doesn’t work!

How Do We Make Sperm & Eggs? We make them by MEIOSIS For humans, this involves reducing 46 chromosomes  23 chromosomes 46 23 23 zygote 46 meiosis egg 23 46 23 fertilization sperm

Meiosis & Mitosis Mitosis to make copies of cells growth repair Development Meiosis to make gametes sperm & egg

Starting Meiosis The whole goal of meiosis is to make sex cells. A diploid (all chromosomes) cell divides to make four haploid (half chromosome) daughter cells.

How do you get these chromosomes? Starting Meiosis The DNA in the diploid cell is stored in homologous chromosome pairs 23 Homologous Pairs – 46 Individual Chromosomes Homologous means “the same” How do you get these chromosomes?

Chromosomes These homologous chromosomes carry genetic information for the same traits One chromosome in the pair comes from mom, the other from dad. DAD MOM gene for eye color gene for eye color blue green gene for hair color gene for hair color brown black

egg sperm

Meiosis 1 In Meiosis I, the nucleus divides and produces two new cells. Each new cell has one duplicated chromosome from each similar pair.

Meiosis 2 In Meiosis II, the nuclei divide and the chromatids separate, producing four cells with half the number of chromosomes of the original nucleus.

new gametes made by offspring The Value of Meiosis Allows for genetic change over time meiosis introduces genetic variation gametes of offspring do not have same genes as gametes from parents New combinations MAY be more beneficial than others. This causes EVOLUTION. Consider the greater variation with 23 pairs of chromosomes = mixing and matching from Dad variation from Mom offspring new gametes made by offspring

How does this explain: family resemblance & differences! Why are the kids so similar to the parents but not exact? Michael & Kirk Douglas Baldwin brothers Martin & Charlie Sheen, Emilio Estevez

Any Questions?? What are the advantages of asexual reproduction? What are the DISadvantages of asexual reproduction? Any Questions?? What are the advantages of sexual reproduction? What are the DISadvantages of sexual reproduction?