Name_________________________
When sex cells form, the nucleus divides in a different way from what we saw in Mitosis. Remember that body cells have chromosomes in pairs. When a cell in an ovary or a testis divides to make sex cells, the pairs of chromosomes separate. One chromosomes from each pair goes into each new nucleus. So each egg or sperm has just half the number of chromosomes as a body cell. This kind of division is called MEIOSIS.
In a fruit fly, there are 8 chromosomes in a body cell. After mitosis, there are still 8 chromosomes in a body cell. But after meiosis, there are just 4 chromosomes in each sex cell that forms, one chromosome from each pair. In your body cells, there are 46 chromosomes. After mitosis, there are still 46 chromosomes in a body cell. After meiosis, there are just 23 chromosomes in each sex cell (egg or sperm) that forms.
What each part of the lab represents:
1. One bead -
Yellow string of beads -
Red string of beads-
2. Magnet -
1. Place your “chromosomes” on your desk.
2. Use chalk to make a large circle on your desk so that your desk is round like an ANIMAL cell.
 How many cells are we starting with? _______
 The cell is DIPLOID / HAPLOID. (circle one) o
How do you know?_______________________________________________________
3. Draw a nucleus and put the chromosomes inside.
 How many chromosomes are we starting with? _______
 How many chromosomes came from DAD? _______ How many are from MOM? _______
Before mitosis starts,
takes place and a copy is made of each chromosome.
4. Get another bag of beads and REPLICATE your DNA. You now have 4 chromosomes with 2 strands.
 How many chromosomes are in the cell now? _______
The two short red chromosomes that are attached to each other are a pair.
 What are these chromosomes called? ______________________________
The pair of long red chromosomes is related to the pair of long yellow chromosomes.
 What are these pairs of chromosomes called? ______________________________

In real life, the following things would happen at the start of prophase I:
1. chromatids coil and pairs attach to each other (Tetrads)
2. nuclear membrane begins to disappear
3. centrioles appear and begin to move to opposite sides of the cell
5. On your desk, erase the nuclear membrane and draw 2 centrioles near where the nucleus was.
Draw a TETRAD in the box below. Label the: centromere and chromatids.
 What is the job of the centrioles? ______________________________________________
6. Now draw everything that you see on the desk.
The following things would happen in metaphase I:
1. centrioles move to opposite ends of the cell
2. chromatids line up IN PAIRS at the center (“equator”) of the cell
3. spindle fibers form and attach to a centriole and a centromere
7. On your desk, move the pennies to opposite sides of the “cell.” Line up the chromosomes IN PAIRS on the
“equator.” Draw spindle fibers with chalk that go from the pennies to the centromeres. Draw exactly what you see on your desk below.
 What is the job of the spindle fibers? _____________________________________
 How does this cell look different than a cell in metaphase of MITOSIS?
__________________________________________________________________

The following things would happen in anaphase I:
1. homologous chromosomes pull apart and move toward the centrioles
2. spindle fibers shorten
8. On your desk, shorten the spindle fibers and separate the homologous chromosomes. Move the chromatids to opposite sides of the cell. Draw exactly what you see on your desk below.
 How many chromosomes will be in the cell to the right?_________ The cell to the left?_________
 How many chromosomes were in the mother cell? __________
The following things would happen in telophase I:
1. cell membrane would pinch in the middle, making two cells
2. centrioles and spindle fibers disappear
3. cytoplasm and organelles will divide between the two cells (CYTOKINESIS)
9. On your desk, redraw the cell membrane to pinch in the middle. Take the pennies off the table. Draw exactly what you see on your desk below.
 How many cells are there now? ____________
 How many chromosomes did we start with?_________
 How many chromosomes do we end up with in each cell? __________
 These cells are DIPLOID / HAPLOID (circle one) .

In real life, the following things would happen at the start of prophase II:
1. centrioles appear again and begin to move to opposite sides of the cell
2. spindle fibers return
10. On your desk, put two pennies near where the nucleus used to be. Draw exactly what you see on your desk in the space below.
 How many cells are we starting with? ______
The following things would happen in metaphase II:
1. chromatids line up in single file at the equator of the cell
2. spindle fibers attach to a centriole and a centromere
11. On your desk, move the pennies to opposite sides of the “cell.” Line up the chromosomes in single file on the equator. Draw spindle fibers with chalk that go from the pennies to the centromeres. Draw exactly what you see on your desk below.
 Does this cell look like cells do during MITOSIS? ________ Why? _______________
_____________________________________________________________________

The following things would happen in anaphase II:
1. sister chromatids pull apart and move toward the centrioles
2. spindle fibers shorten
12. On your desk, shorten the spindle fibers and separate the homologous chromosomes. Move the sister chromatids to opposite sides of the cell. Draw exactly what you see on your desk below.
 How many chromosomes will be in the cell to the right?______ The cell to the left?_________
 How many chromosomes were in the mother cell? __________
The following things would happen in telophase II:
1. cell membrane would pinch in the middle, making two cells
2. centrioles disappear
3. cytoplasm and organelles will divide between the two cells (CYTOKINESIS)
4. nuclear membrane will reappear
13. On your desk, redraw the cell membrane to pinch in the middle. Take the pennies off the table. Draw exactly what you see on your desk below.
 How many cells are there now? __________ How many cells did we start with?_______
 How is this different than MITOSIS?_________

1. In the space, write T for “true” or F for “false” for each sentence.
____ a. The chromosomes of body cells are paired (diploid).
____ b. The process by which sex cells are made is called meiosis.
____ c. A human body cell has 23 chromosomes.
____ d. Every living organism has the same number of chromosomes.
____ e. A sex cell has the same number of chromosomes as a body cell.
____ f. A sex cell has twice the number of chromosomes as a body cell.
2. A frog sperm cell has 13 chromosomes. How many chromosomes does a frog body cell have? _________
3. Circle the word that makes each sentence TRUE. a. Sperm cells form from the process known as MITOSIS / MEIOSIS. b. Skin cells form from the process known as MITOSIS / MEIOSIS. c. The number of chromosomes in a cell is the same after MITOSIS / MEIOSIS takes place. d. An egg cell with just 1/2 as many chromosomes as a body cell forms during MITOSIS / MEIOSIS.
4. Each body cell of a corn plant has 20 chromosomes. How many chromosomes are there in a sperm cell of corn? ________ How many are in an egg cell? ________
5. What do chromosomes do? __________________________________________________________
At FERTILIZATION, an egg and a sperm cell join. A human egg cell has 23 chromosomes. A human sperm cell has 23 chromosomes. When the sperm fertilizes the egg, the ZYGOTE (fertilized egg) that forms has 46 chromosomes. The same thing happens when a fly sperm fertilizes a fly egg. The fly egg and sperm each have
4 chromosomes. When they join, there are 8 chromosomes.
The zygote of a fly, human, or any other living thing is a body cell. It has pairs of chromosomes. In a human zygote, there are 23 pairs, or 46 chromosomes. When this zygote divides, each cell in the embryo that develops is a body cell. All the divisions of the zygote into other body cells are by mitosis.
6. What happens to the number of chromosomes after FERTILIZATION? _________________________
7. What kind of cells are in the embryo? ___________________________