Meiosis

Cell division for the production of sex cells

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

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!

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

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 21st pair – producing a child with Down Syndrome

21 trisomy – Downs Syndrome

Can you see the extra 21st chromosome?

Is this person male or female?

Meiosis – without errorParent cell – chromosome pair

Chromosomes copied

1st division - pairs split

2nd division – produces 4 gamete cells with ½ the original number of chromosomes

Meiosis Parent cell

4 gametes

1st division

2nd division

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Overview of MeiosisOverview of Meiosis

Put the images in the correct order

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