Modeling Meiosis

Essential Question:How do our cells determine who we are?

Today: Modeling Meiosis

Learning target: Describe 3 key differences between mitosis and meiosis.

Background Information:Today we are going to do a step by step simulation of meiosis. We will be working with the chromosomes of a Puffle, an imaginary organism…...

1. If the Puffle has a diploid chromosome number of four, what will the haploid chromosome number be? __________________2. Place a large paper plate in the center of your work space. Imagine that this is one sex cell in a Puffle. The boundary of the plate is the cell membrane. 3. You are going to create a diploid nucleus containing two pairs of chromosomes. First, divide each of your four balls of colored clay in half. 4. Take one-half of each ball and roll it between your hands to form four elongated, snakelike chromosomes. Make the red and blue chromosomes as long as your index finger. Make the green and yellow ones half that length. Do the same thing with the other half of your clay.

2

Large paper plate

5. Remember that a chromosome is made of tightly coiled strands of DNA. Within each chromosome there are many, many genes. The chromosomes within each pair are said to be homologous, meaning similar but not necessarily the same. Homologous chromosomes contain the same genes but not necessarily the same alleles. For instance, two homologous chromosomes might contain the gene which codes for eye color, but the alleles might be different--like blue eye allele on one chromosome and brown eye allele on the other.

Blue eyeallele

Brown eye allele

HomologousChromosomes(1 from mom, other from dad)

Table II -- Imaginary Traits of the Puffle Genes on PAIR 1 Fur Color Fur Type Green Chromosome G (green fur) c (straight fur)Yellow Chromosome g (yellow fur) C (curly fur) Genes on PAIR 2 Eye Color Eyelash lengthBlue Chromosome B (blue eyes) L (long eyelashes)Red Chromosome b (red eyes) l (short eyelashes)

6. We are going to examine four traits in the Puffle, specified by four genes. Remember that genes are segments of chromosomes which code for proteins that cause observable traits (phenotypes). Table II describes some traits of our imaginary creature and their locations on each chromosome.    

Table II -- Imaginary Traits of the Puffle Genes on PAIR 1 Fur Color Fur Type Green Chromosome G (green fur) c (straight fur)Yellow Chromosome g (yellow fur) C (curly fur) Genes on PAIR 2 Eye Color Eyelash lengthBlue Chromosome B (blue eyes) L (long eyelashes)Red Chromosome b (red eyes) l (short eyelashes)

7. Label the location of each gene by cutting out the paper labels provided and sticking them on to the clay as shown above. Cut the paper as close to the letters as you can.

g

C c

G b

l

B

L

g

C c

G b

l

B

L

INTERPHASE 8. During interphase, each chromosome is replicated by making another complete set of DNA. Simulate this replication by creating a matching chromosome (same shape and color) using the other half of your clay. Label each chromatid with genes so they are exact copies. 9. Connect sister chromatids together at the centromere (center point) by pinching them together as shown below:

10. How many chromatids are present at this stage?_________ How many chromosomes? ____________. Each chromosome is made of _______ chromatids.

84

g

C

g

C c

G

c

GB

L

B

L

b

l

b

l

centromere

chromatid chromatid

2

PROPHASE I11. Pair up each newly replicated chromosome with its homologous partner, forming two tetrads (2 pairs of chromosomes, total of 4 chromatids).

12. Now that homologous chromosomes are near each other, crossing-over can occur. In each tetrad, place one chromatid over another. Then trade alleles so the colors are mixed.

lL

bB

lL

bBB b

L l

B b

L l

cC

Gg g G

C ccC

Gg g G

C c

METAPHASE I 13. Imagine that the nuclear membrane has now broken down in your imaginary Puffle cell. Line the two tetrads (pairs of chromosomes) end to end across the center of the cell. Attach a piece of yarn stretching from the centromere (center point) of each chromosome to the end of the cell nearest to it. What do the pieces of yarn represent? ____________________

centromereLarge paper plate

yarn

14. In a real cell, would the red chromosome always line up on the same side as the green chromosome? Why or why not? _______________________________________________ __________________________________________________________________________

spindle fibers

No. Chromosomes line up in random order, like shuffling cards.

ANAPHASE I

15. Separate each tetrad and use the spindle fibers to move the homologous chromosomes to opposite poles of the cell as shown below:

TELOPHASE I 16. Cytokinesis (cell division) occurs to form two daughter cells. These daughter cells are not haploid or diploid, but some kind of intermediate stage. Arrange your clay chromosome models on two large paper plates as shown below:

17. Are the two daughter cells genetically the same or different? Explain. _________________________________________________________________________________________________________________________________________________

Different. Chromosomes got mixed up and separated.

g g

B B

L l L l

bb cC

GG

cC

MEIOSIS IIPROPHASE II & METAPHASE II 18. Line up the chromosomes in each cell end to end along the center line. Attach spindle fibers (yarn) to the centromeres (center points) as shown below:

centromere

yarn

ANAPHASE II 19. Separate the two sister chromatids and move them to opposite poles as shown below:

20. What does the yarn represent? What’s its function in real cells? _______________________________________________________________________ _______________________________________________________________________

Spindle fibers. Pulls chromatids apart.

TELOPHASE II 21. Cytokinesis occurs again, producing a total of four daughter cells. You can show this by separating the 8 sets of chromosomes on to 4 small paper plates as shown below:

22. Are these cells diploid or haploid? How do you know ? _____________________ _______________________________________________________________________ 23. Are the four daughter cells genetically the same as each other or different?_________ Are they genetically the same as the parent cells or different? ___________________ Are these body cells or gametes (reproductive cells)?___________________________ _ 24. Write the genotype and phenotype beneath each of the daughter cells shown in #21.

Each have half as many chromosomes as parent cell

Gametes (reproductive cells)

gCBL gcBl GCbL Gcbl

differentdifferent

haploid

Cleaning up: Please remove the letter labels from the clay and separate the clay back into four different color balls so that the next class can use them. Thank you!

Yellow, straight furblue eyesshort lashes

Green, curly furred eyeslong lashes

Green, straight furred eyesshort lashes

Yellow, curly furblue eyeslong lashes

Table II -- Imaginary Traits of the Puffle Genes on PAIR 1 Fur Color Fur Type Green Chromosome G (green fur) c (straight fur)Yellow Chromosome g (yellow fur) C (curly fur) Genes on PAIR 2 Eye Color Eyelash lengthBlue Chromosome B (blue eyes) L (long eyelashes)Red Chromosome b (red eyes) l (short eyelashes)

6. We are going to examine four traits in the Puffle, specified by four genes. Remember that genes are segments of chromosomes which code for proteins that cause observable traits (phenotypes). Table II describes some traits of our imaginary creature and their locations on each chromosome.