Download Modeling Mitosis and Meiosis

Primary
Life Science
Modeling Mitosis and Meiosis
Materials:
2 short (2-3“) pieces of one color chenille sticks
2 short (2-3”) pieces of 2nd color chenille sticks
2 longer (5-6 “) pieces of 3rd color chenille sticks
2 longer (5-6”) pieces of 4th color chenille sticks
5 pieces of yarn- approx. 8-10” long
Procedure:
1. There are differences between plant and animal cell mitosis—plants lack centrioles (the little
dots by the poles in animal cell mitosis pictures), and cytokinesis is accomplished in plants by a
new cell plate growing across the equator instead of the cytoplasm pinching into a cleavage
furrow. The cell plate will be visible in the onion root tip lab.
2. Books have diagrams of mitosis and meiosis but usually only illustrate animal cells. This bag
contains materials to illustrate Plant Cells as well.
3. Have the teacher check each stage before moving to the next stage of mitosis or meiosis.
4. (I make a check list of each stage and circulate among the lab tables to sign off each student
on correctly arranged cells before he/she finishes models of each stage.)
MITOSIS: The objective is to make 2 cells identical to the original.
•
Use 2 short chenille sticks (sister chromatids/chromosome) of one color and 2 other short
sticks of 2nd color: these 4 =1 pair of chromosomes already replicated
•
Use 2 longer chenille sticks of 3rd color and 2 longer sticks of 4th color = 2nd pair of
chromosomes
•
5 pieces of yarn about 8-10 “ in length
1. On the table in front of you, make a big circle (12-14”) with 4 pieces of yarn.
2. PROPHASE- (Prepare) Scatter the Xs of the chenille sticks around the cell. This is the longest
phase of mitosis.
3. Designate the poles of the cell at your right and left edges of the circle.
4. METAPHASE (Middle) Line chromosomes up in the middle in a vertical line across the equator.
This is the shortest phase of mitosis.
5. ANAPHASE (Apart) Slowly pull the 2 chromatids of each chromosome apart from each other
towards the poles.
1
6. TELOPHASE ( making Two cells) As chromatids approach the poles, pinch in the cytoplasm at the
equator by pinching in the yarn to make a furrow – Animal Cell
7. For Plant Cell TELOPHASE ( making Two cells), lay the 5th piece of yarn on the equator, as well
as the yarn pieces around each new cell – Cell Plate
8. To make DAUGHTER CELLS, either pinch the furrow closed by moving the yarn together for
Animal Cell, or complete the yarn across the equator to meet the yarn around the outside for
Plant Cells.
9. This process can take anywhere from 30 min. to several hours.
•
See the accompanying photos for each stage—both Animal Cell and Plant Cell
•
For Plant Cell Mitosis, the cytokinesis is accomplished by a new CELL PLATE forming at the
center of the equator and then spreading to the edges, so put last yarn piece right along
vertical equator line and slowly extend it until it reaches both sides = DAUGHTER CELL
MEIOSIS (Reduction Division): The objective is to make cells with half the number of
chromosomes of the original- only one of each homologous pair in the resulting 4 cells
called gametes.
1. There are 2 divisions:
a.
1st separates the homologous pairs from each other
b. 2nd separates chromatids from each other
2.
Have 2 short chenille sticks of 1 color and 2 short sticks of another color, 2 long sticks
of 3rd color and 2 long sticks of 4th color, and 5 pieces of yarn.
3.
To identify the stages in the 2 divisions, there will be PROPHASE I, METAPHASE I,
ANAPHASE I, TELOPHASE I, DAUGHTER CELLS I, and PROPHASE II,
METAPHASE II, ANAPHASE II, TELOPHASE II, and DAUGHTER CELLS II.
MEIOSIS 1:
1.
PROPHASE I - Remake the big yarn circle and put the 2 pairs of chromosomes inside—BUT with
a difference from Mitosis—put the short pairs next to each other and the long pair next to each
other.
2
During PROPHASE I of Meiosis, CROSSING OVER can occur. It is a random process in which
pieces of 2 homologous chromosomes switch places with each other. (The grass is always
greener on the other chromosome!) This leaves the homologs with a different setup of genes
and introduces variation.
2. METAPHASE I – Line up the pairs of chromosomes at the equator- again the poles are on your
right and left edges of the circle. All 4 short chromosomes are across from each other and all 4
long chromosomes are across from each other.
3. ANAPHASE I- Each chromosome (made of 2 chromatids) pulls apart from the other chromosome
in its pair. 1 short chromosome goes to the right and other short goes to the left. Same for the
long chromosomes.
HOW EACH PAIR SEPARATES HAS NOTHING TO DO WITH OTHER PAIR!!
4. TELOPHASE I – the yarn (cell membrane) starts to pinch in at the equator as the chromosomes
move to the poles.
5. DAUGHTER CELLS I- the yarn/cell membrane completely closes and the 2 new cells move apart
MEIOSIS 2:
6. Now we continue the difference. There is a 2nd division but no DNA replication in INTERPHASE
this time. The cells go straight to PROPHASE II.
7. PROPHASE II - No exchanging parts between chromosomes this time.
8. METAPHASE II – Sister chromatids line up in the middle at the equator in both new cells.
9. ANAPHASE II – Sister chromatids pull apart and move towards the poles.
AGAIN, HOW EACH CHROMOSOME MOVES, HAS NOTHING TO DO WITH OTHER CHROMOSOME
MOVES.
10. TELOPHASE II - The yarn starts pinching in at the equator in both cells to make 4 new cells.
11. DAUGHTER CELLS II – Close the yarn completely and separate the 4 new cells from each other.
Each new cell has only 1 short chromosome and 1 long chromosome. This cutting of the number
of chromosome in half produces Gametes, the sex cells needed for sexual reproduction.
•
Refer to accompanying photos for the different stages.
3
•
There is no DNA replication prior to PROPHASE II. Each chromosome is already made up of
2 chromatids.
•
As the DAUGHTER CELLS II are formed, each has only 1 of each pair of chromosomes. These
cells are the sex cells, gametes, and will join to produce a fertilized cell with the normal
number of pairs of chromosomes restored.
•
Again, if Plant Cell Meiosis, use extra yarn piece to make cell plate in 1st and 2nd divisions.
•
Meiosis is vital in sexual reproduction as the number of chromosomes must be maintained.
Extra copies are a lethal mutation in most cases. Students are probably familiar with Down
Syndrome individuals- they have 1 extra chromosome usually in pairs 19 -22, which are very
small chromosomes. The result can have devastating physical and mental problems, and
that is with just on extra chromosome. A duplicate set would be lethal.
•
Some plants will spontaneously make polyploid cells (more than the normal number of
chromosome pairs) and there are even some polyploid animals, but this is rare.
VARIATIONS:
1.
If and how crossing over occurs, chromosomes change from their original state.
2.
In Anaphase I, which chromosome in a pair goes to which pole has no effect on how the other
chromosome in the pair goes. Huge variation occurs here. Since we have 23 pairs of
chromosomes, how pair 1 separates has no influence over pair 2 or 3 or …HUGE variation
introduced at this stage.
3.
Which male gamete meets which female gamete to produce the offspring also introduces a
great deal of variation.
4.
Randomly occurring mutations can also inject variations. (not all mutations are harmful).
5.
Astronomical odds against exact copies! The certainty that no one in the universe was/will be
created just like you –priceless!
4
Prophase
Metaphase
Prophase I
Prophase I with
crossing over
Metaphase I
2 Daughter Cells
1/Prophase II
Plant Cell
Telophase
with partial
Cell Plate
MITOSIS
Plant
Daughter
Cells
Interphase
Anaphase
Animal
Daughter Cells
Animal Cell
Telophase
with cleavage
Metaphase II
Anaphase II
Anaphase I
Telophase II
MEIOSIS
5
Telophase I
4 Daughter
Cells=Gametes