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Meiosis: the process by which we make gametes (“sex cells”) – sperm and eggs.

We’ll follow the pathway on the right side of this diagram.

Meiosis: start again at the middle top of the diagram (“Parent cell”)

In this cell, how many chromosomes are there in total?        4

How many blue chromosomes are there?                             2

Why are there 2 of them?                                                       There is 1 from each parent

How many red chromosomes are there?                              2

Why are there 2 of them?                                                       There is 1 from each parent

– – – –

In human cells, how many chromosomes are there in total?                        46

Why are they found in pairs?                                                  There is 1 from each parent

In human cells, how many chromosomes are from the mom?         23

In human cells, how many chromosomes are from the dad?           23


Meisos: Prophase
Instead of four single chromosomes, what do we have now?           4 pairs of chromosomes. Originals were duplicated

What do we call the set of the original and copy?                              Tetrad  (tetra is Greek for “four”)

What is the total number of chromosomes right now?                      4 x  = 8

How is this different from prophase in mitosis?                                 In mitosis all four sets floated apart.

But here in meiosis they are stuck together in pairs.

– – – – –

In a human cell, what would the total number of chromosomes be?            46 x 2 = 92

Meiosis: Metaphase
Again we see a spindle apparatus form, just like in Mitosis.

One tetrad attaches to the spindle on one side.  Other tetrad attaches to spindle on the other side.
But in meiosis we don’t want exact copies of the parent cell.  We want all the daughter cells to be different!

So the chromosomes start crossing over – works like shuffling a deck of cards.

Let’s look at crossing over in more detail:

Meiosis crossing-over

Meiosis: Anaphase and Telophase (In the main diagram, the images in the mid-right)

Tetrads have split apart. Half of them moved to the left, half moved to the right.

Parent cell split apart into two daughter cells.

Result? We have 2 new cells.  Each is a shuffled / mixed-up copy of the parent.

Each has the same number of chromosomes as the parent cell.

But we’re not done. We’re trying to make gametes (sperm and eggs), right?

Do they have the same # of chromosomes as regular body cells? Nope.

Gametes always have half the number of chromosomes!  So how do we do this? We need one more series of cell divisions.

Meiosis II: the final round of cell division

Now we end up with four daughter cells. Each has:

*  shuffled genes (all different from the parent)

* half the normal number of genes


Now for the final results!

{ from

http://campus.udayton.edu/~INSS/Dillon230/LECTHELP-2/2spermatogenesis_vs.htm }

meiosis sperm eggs


(Sperm formation)

(Ovum formation)

bullet  involve meiosis
bullet  produces 4 haploid daughter cells


bullet  Each “daughter cell” develops into a sperm
bullet One daughter cell becomes the ovum;
the three polar bodies die
bullet  Process begins at puberty
(Sperm are all “fresh”)
bullet Process begins before birth!
(Ova are as old as female.)
bullet  Process ends at death
bullet Process ends at Menopause
bullet Time to complete:  ~ 7  weeks
bullet Time to complete:  13 – 50 years
bullet  Billions produces at a time
bullet One ovum (oocyte) matures monthly

Learning Standards

2016 Massachusetts Science and Technology/Engineering Curriculum Framework

HS-LS3-1. Develop and use a model to show how DNA in the form of chromosomes is passed from parents to offspring through the processes of meiosis and fertilization in sexual reproduction.

HS-LS3-2. Make and defend a claim based on evidence that genetic variations (alleles) may result from (a) new genetic combinations via the processes of crossing over and random segregation of chromosomes during meiosis, (b) mutations that occur during replication, and/or (c) mutations caused by environmental factors. Recognize that mutations that occur in gametes can be passed to offspring.

Disciplinary Core Idea Progression Matrix – “Nearly every cell in an organism contains an identical set of genetic information on DNA but the genes expressed by cells can differ. In sexual reproduction, genetic material in chromosomes of DNA is passed from parents to offspring during meiosis and fertilization. “

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