Meiosis
Meiosis How is meiosis going to be different from mitosis? Meiosis is part of sexual reproduction How is sexual reproduction different from asexual reproduction?
Grab a book pg. 273 List your observations Discuss: What looks different?
What are the goals of Meiosis? Make cells for reproduction Reduce chromosome numbers Create genetic variation
Making cells for reproduction Which cells go through meiosis? Gametes Egg cells and sperm cells
Reducing chromosome numbers How many chromosomes do humans have? 46
Reducing chromosome numbers That s 2 sets of chromosomes with 23 chromosomes in each set Why do we have 2 sets of chromosomes? One set from mom and one set from dad
Chromosome numbers of various organisms Organism Number of Chromosomes Penicillium 1-4 Saccharomyces (yeast) 18 Mosquito 6 Housefly 12 Garden Pea 14 Corn 20 Adder s tongue fern 1,262 Frog 26 Human 46 Orangutan 48 Dog 78
Adders Tongue Fern
Reducing chromosome numbers There is a name for cells that have 2 sets of chromosomes Diploid Humans are a diploid species
Reducing chromosome numbers Why do we need to reduce the chromosome number? Basic math! Fertilization requires 2 cells with only 1 set of chromosomes 1 set + 1 set = 2 sets 23 + 23 = 46
Reducing chromosome numbers There is a name for cells that have 1 set of chromosomes Haploid Human gametes are haploid
Reducing chromosome numbers Haploid gametes fuse during fertilization and form a zygote
Creating genetic variation How can this happen? Does it create new genes or new DNA? No it just results in new combinations of both parents genes New combinations = diversity Why is genetic diversity important?
Creating genetic variation 1. Crossing over 2. Independent assortment 3. Fertilization of random cells
Creating genetic variation Crossing over: Homologous chromosomes come together and trade segments
Creating genetic variation What is a homologous chromosome? Chromosomes that carry the same genes 1 from each set
Creating genetic variation Are they exactly the same? NO! They do not have the exact same DNA sequence. Where did they come from?
Creating genetic variation Trading segments mixes the DNA, creating new combinations
Creating genetic variation Independent assortment When homologous chromosomes line up, they line up independently (randomly) They do not have a preset pattern of how they line up
What do you mean random? Lots of possible combinations!
Creating genetic variation Random alignment of chromosomes creates new combinations
Creating genetic variation Fertilization of random cells Sexually reproducing organisms do not get to choose which cells get to fuse during fertilization Random cells fuse together creating new combinations
Meiosis 1 Time for Meiosis! DNA condenses into chromosomes Nucleus dissolves and disappears Centrioles produce spindle fibers Crossing over occurs between homologous chromosomes Chromosomes line up in pairs, randomly arranged Homologous chromosomes are separated into 2 sets 2 nuclei come back Cell divides, forming 2 haploid daughter cells
Time for Meiosis! Meiosis 2 Nucleus dissolves and disappears (in both cells) Chromosomes line up Sister chromatids are separated 2 nuclei come back Cells divide (both cells) 4 total daughter cells are produced
Meiosis 1: First step DNA coils up into chromosomes Nucleus disappears
Crossing Over Homologous chromosomes come together and trade segments
Crossing Over Cont.
Homologous pairs of chromosomes line up randomly in the middle of the cell
Homologous chromosomes separate into 2 sets
2 nuclei come back Cell splits in half forming 2 haploid daughter cells
Meiosis 2 We do it all again but just slightly different Don t forget all the steps of meiosis 2 are happening in 2 cells at the same time!
Nucleus disappears Meiosis 2
Chromosomes line up How is this different from the first time?
What happens next? Sister chromatids are separated
2 nuclei come back Cells split in half 4 total daughter cells are produced
Fertilization Then what?
Lets review How is meiosis different than mitosis?
Comparing sexual and asexual reproduction Advantages and disadvantages of both