Meiosis is the process of cell division that produces gametes, or sex cells, with half the number of chromosomes as normal body cells. During meiosis, the parent cell's chromosomes are duplicated and then split into four gamete cells. This ensures that when fertilization occurs, the resulting zygote will have the full number of chromosomes. Without meiosis reducing the number of chromosomes in gametes, fertilization would result in far too many chromosomes in the zygote. Errors during meiosis where the gamete ends up with an extra chromosome can lead to genetic disorders like Down syndrome if that gamete is involved in fertilization.

1. Meiosis Cell division for the production of sex cells

2. Why do we need meiosis? Meiosis is necessary to cut the number of chromosomes going into the sex cells by half. Why halve the chromosomes in gametes? At fertilization, the male and female sex cells will provide ½ of the chromosomes each to the new zygote (future baby) so the offspring has genes from both parents

3. Do the math! each somatic cell (normal cell in the body) has 46 chromosomes in 23 pairs this is the proper number for the cell to function correctly more or fewer chromosomes lead to problems or cell death mitosis keeps this number the same each time a cell divides Meiosis takes the number of chromosomes down to 23 (half the full set), so that when an egg is fertilized by a sperm the new zygote has 46 chromosomes Without this the zygote would get 92 chromosomes, which is WAY TOO MANY!

4. Meiosis – What happens when chromosomes don’t split correctly? gamete has extra chromosome Normal! split looks like this side Error! too many chromosomes in these gametes

5. Meiosis error - fertilization If the gamete with the extra chromosome gets fertilized, then the offspring will not be ‘normal’. In humans this often occurs with the 21 st pair – producing a child with Down Syndrome

6. 21 trisomy – Downs Syndrome Can you see the extra 21 st chromosome? Is this person male or female?

7. Meiosis – without error Parent cell – chromosome pair Chromosomes copied 1 st division - pairs split 2 nd division – produces 4 gamete cells with ½ the original number of chromosomes

8. Meiosis Parent cell 4 gametes 1 st division 2 nd division

11. Overview of Meiosis

12. Put the images in the correct order

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