Problem: Our cells have 46 chromosomes—so why don’t fertilized eggs have 92 chromosomes?
Answer: Meiosis
Meiosis Cell division to produce gametes What are gametes? Egg and sperm!
How does meiosis solve the problem of too many chromosomes? The number of chromosomes is cut in half 46 to 23 Your chromosomes are in pairs: one from mom and one from dad. One from each pair goes into each gamete
In humans: Mitosis Cell division to produce 2 identical daughter cells Each cell has 46 chromosomes: 2 sets (one from mom, one from dad) Diploid (2n): 2 sets of chromosomes Meiosis Cell division to produce genetically different gametes Each gamete has 23 chromosomes: 1 set (about half from mom, half from dad) Haploid (1n): 1 set of chromosomes
Diploid and haploid Our body cells (somatic cells) are diploid Our sex cells are haploid
Meiosis Sex cells divide to produce gametes (sperm or egg). Gametes have half the # of chromosomes.
Fertilization n=23 egg 2n=46 zygote The fusion of a sperm and egg to form a zygote. A zygote is a fertilized egg n=23 egg sperm n=23 2n=46 zygote
Meiosis has two cell divisions Meiosis I: Cuts chromosome number in half. Meiosis II: sister chromatids separate
Meiosis I Two unique things happen that create genetic variation – the reason for diversity within a species.
Unique thing #1. Crossing Over Crossing over: segments of nonsister chromatids break and reattach to the other chromatid. Now the chromatids are not alike—genetic variation
First, homologous chromosomes come together sister chromatids from mom sister chromatids from dad Link together
Crossing Over - variation nonsister chromatids Variation: now all 4 chromatids are different
What does crossing over produce? 4 genetically different chromatids!
Unique thing #2. Independent assortment The random assortment of genes you got from your mom and your dad into gametes. Kind of like shuffling a deck of cards
Happens during Metaphase I metaphase plate OR metaphase plate
Meiosis II Similar to mitosis sister chromatids separate
So what do we have at the end of meiosis? 4 genetically different haploid gametes
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