Unit 6: Meiosis Handouts · Unit 6: Meiosis Handouts Name_____ Mrs. Canale LE 2013-2014 [Canale LE] Page 2 2. Parent cell ... Well, just like in mitosis, there are some spindle

[Canale LE] Page 1

Unit 6:

Meiosis

Handouts

Name____________________

Mrs. Canale LE

2013-2014

[Canale LE] Page 2

Meiotic Cell Division

Purpose Outcome

Process

1.

1. 2. Example:

1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11.

12

Daughter

Cells

Parent cell

# of chromosomes

Meiosis - __________________________

__________________________________

__________________________________

__________________________________

[Canale LE] Page 3

Diploid (2n)=_______________________________________________

Monoploid (n)= _____________________________________________

Meiosis (Gametogenesis) Spermatogenesis Oogenesis What is it?

Meiotic Divisions

First Division

Second Division

Where does it happen? # of cells produced? Chromosome # of

daughter cells (diploid

(2n) or monoploid (n))

Questions:

1. How many parent cells does the process of meiosis start with? _____

2. Is the parent cell in meiosis diploid or monoploid? _______________

3. Are the daughter cells in meiosis diploid or monoploid? ___________

4. Why might this be?

____________________________________________________

Sperm cells

Ovum Polar bodies

Parent cells

Diploid (2n)

[Canale LE] Page 4

Meiosis

Adapted from: http://www.sciencecases.org/mitosis_meiosis/mitosis_meiosis.asp

The Case of the Dividing Cell: Mitosis and Meiosis in Cellular

Court Part I—The First Day of Testimony

Narrator

Judge

Counselor Oocyte

Counselor Liv

Mr. Nuclear Membrane

Jury Members

Mr. Sperm

Narrator: Hear ye. Hear ye. The Honorable Judge Cellular now presiding. All rise. The judge in his black robes

came silently into the courtroom and slid into his high-backed chair.

Judge: Ahem, let us see now. This is the case of the State vs. Egg Cell Number 6624223. The defendant (the

Egg Cell) is charged with being an undesirable mutant. The penalty is death. This is the most serious of matters.

It requires our undivided attention. Let us have the opening statements. The prosecution first.

Counselor Liv: Yes, thank you, Your Honor. I am a liver cell, one of about 100 trillion cells living in a human

named Martha. It is essential that we all live together with one purpose—Martha’s survival. If Martha doesn’t

survive, all the cells die—I, the rest of the liver cells, the kidney cells, the muscle cells, the nerve cells, and all

of the others that make up this human being.

We would not be here except for the fact that we all are descendants of other organisms, and we have relied

on sexual reproduction. In humans, like Martha, the cells in her ovary produce some unusual cells called eggs.

And in Martha’s husband there are some cells called sperm. If these two cells fuse together they will produce a

fertilized egg that may grow up to be another colony of cells we call a baby.

Now the point of all of this is that in order to produce a perfect baby, the sperm and the egg must be perfect. This brings me to the central issue of this crime. The defendant on the stand today is one of Martha’s cells. We all love Martha—some of us have lived with her for years—but as we will show, she has been alive for 43 years and this has led to trouble. Many of her eggs are flawed. This is exactly the case with our defendant, Egg Cell Number 6624223. SHE IS NOT PERFECT. She should be destroyed. She is disfigured by having an extra chromosome 21. This leads to Down Syndrome. We believe that you will see the wisdom of this solution by the end of the trial and trust in your good judgment.

Judge: Thank you, Counselor Liv. Now, Counselor Oocyte, would you give your opening remarks?

Counselor Oocyte: Thank you, Your Honor. Cells of the jury, you have heard the remarks of my colleague. She

argues that my client is flawed. My client is unusual, but that does not mean she is flawed or damaged. In every

part of Martha’s body there are unusual cells. Some cells in the lining of her digestive tract and liver, perhaps

Counselor Liv herself, have extra chromosomes. These cells are all living healthy lives and contributing to

Martha’s welfare. There is hardly a reason to get upset with my client who has a single extra chromosome 21.

We will demonstrate to the jury that death is hardly a reasonable remedy, and that, in fact, no crime has been

committed at all.


Judge: Thank you. Counselor Liv, you may call your first witness.

Counselor Liv: Your Honor, I call Mr. Nuclear Membrane of the skin to the stand.

Counselor Oocyte: I object Your Honor. Mr. Nuclear Membrane cannot serve in this capacity, since he is an

expert in the topic of cell division that occurs in skin cells ONLY. This is not the same as what occurs in sex

cells. The skin cells divide by mitosis. The sperm and eggs are produced by meiosis—an altogether different

process.

Counselor Liv: I am not calling Mr. Nuclear Membrane as an expert in meiosis but as one on mitosis. I wish to

make clear the difference between the two.

Counselor Oocyte: Then I withdraw my objection. Go ahead, this should be

interesting.

Counselor Liv: Mr. Membrane, please describe the simple cell division that occurs

in the skin.

Mr. Membrane: First, I want to stress that the normal skin cell divides

repeatedly in its life. Skin cells are always being wiped away when Martha’s hands

rub against anything. We have a busy time keeping up with the normal wear and

tear of everyday living.

When Martha’s cells get ready to divide, the genetic information

(chromosomes) has to be copied. If you’re going to make another

cell just like yourself, you’ve got to make sure that the next cell

knows what to do. The DNA in the chromosomes tells the cell what

proteins to make. In order to do that, you must send the right

information. So the cell has to make a copy of the instructions in

the DNA— the genetic material.

Counselor Liv: Thank you for your testimony. Counselor Oocyte, the witness is yours for

questioning.

Counselor Oocyte: Thank you, Judge. Now just to be sure I have this right, Mr. Nuclear

Membrane, you disappear during this process and reappear at the end?

Mr. Membrane: That’s right. I have to sort of break apart so that the chromosomes can move to

the center of the cell. They couldn’t do this if I were in the way, could they?

Counselor Oocyte: No, I don’t suppose they could, Mr. Nuclear Membrane. Then the next time

you see anything is when you reassemble around the two new sets of chromosomes on opposite


sides of the cell. So everything else you have told us about in between these two times is mere

hearsay, isn’t it?

Mr. Membrane: Well, not hearsay exactly. I know what happens in my own cell.

Counselor Oocyte: Let’s move on to another question, shall we? Tell me, when the DNA makes a

copy of itself, is the copy always perfect?

Mr. Membrane: Well, you know… most of the time it is. It needs to be…

Counselor Oocyte: Isn’t it true that many cells have ‘imperfect copies’ of DNA during mitosis and

things aren’t disastrous? Sometimes these mutations turn out well. You wouldn’t say it’s always a

reason to destroy the cell, would you? It depends upon the situation, doesn’t it?

Mr. Membrane: Yes, I guess so.

Counselor Oocyte: No further questions.

Judge: Counselor Liv, please call your next witness.

Counselor Liv: Yes, Your Honor. Mr. Sperm will you please take the stand. He comes from

Martha’s husband Sam. He has been involved with the meiosis process ever since Sam’s puberty.

Now, sir, I believe you were explaining the cell division process that occurs in the testes. It is

different than in the skin and other parts of the body. Right?

Mr. Sperm: That is correct, Counselor. The cell division in the sex glands doesn’t lead to two

identical cells like we find in mitosis. No, indeed—it leads to four sperm cells, each absolutely

unique. Quite special.

Counselor Liv: How so, Mr. Sperm?

Mr. Sperm: Let me remind you that all cells in the human body start out with 46 chromosomes,

but actually these are not really 46 completely different chromosomes. No sir. They are two sets

of 23 chromosomes; one set has come from Sam’s mother and the other set from his father. Put

them together and you have Sam’s 46 chromosomes all jumbled up in the nucleus running the

business of his cells.

Now, you will remember when Mr. Nuclear Membrane testified, he said that during mitosis all of

the chromosomes moved to the center of the cell in a line. Well, the chromosomes also line up

during meiosis, but THEY-LINE-UP-IN-A-DIFFERENT-WAY! In fact, everything is different

when we sex cells do it! We are the only ones in the body that go through meiosis. We have a

special job to do; we have to make sperm and eggs. All the other cells go through mitosis; they

are simply making clones (exact copies) of themselves. We, however, are out to make something

unique! It is rather clear, isn’t it… how special we are?


Looking at my diagrams, you will see how the chromosomes line up during meiosis. Just look

at that! See, it isn’t random at all. There aren’t 46 chromosomes in a row. No sir, there are 23

pairs of chromosomes in a row, although I can only show two pairs in Exhibit A. But look how Sam’s

chromosomes from his mom have paired up with his dad’s set; the number 1s are together, and 2s,

and 3s and so on.

Judge: I need a little clarification here for my poor judge’s brain. Are you

saying that when sperm are going to be made, the cell starts off in pretty

much the same way as when a skin cell or a muscle cell is going to be

made? That is, each chromosome’s DNA first makes a copy of itself and

the nuclear membrane disappears?

Mr. Sperm: That’s absolutely correct, your Honor.

Judge: And what you’re telling me I see in Exhibit B is that the

chromosomes move to the center of the cell, but this time they’re lining

up with the mother’s and the father’s side by side. That is, I could look

inside and see the two #2 chromosomes next to each other and a little

further along I could see the two #16’s and so on?

Mr. Sperm: That’s it, Your Honor! You have it precisely!

Judge: And, so…? Please explain the importance of that arrangement to

the jury and me.

Counselor Oocyte: I object, object, object! What is this, a biology lesson?

Surely, we have listened patiently long enough. We all get it. Mitosis is

different than meiosis. Isn’t that enough? Let’s move the trial along. My

client is on trial for her life! Why are we listening to this lecture about sperm? My client is an egg

cell!

Judge: Sit down, Counselor Oocyte. I will have no further outbursts in my courtroom. Do you

understand?

Counselor Oocyte: Yes, your Honor. But this is extremely frustrating. My objection still stands.

Judge: Counselor Oocyte, your point is well made. Counselor Liv, why are we talking about sperm

instead of eggs?

Counselor Liv: The formation of sperm and eggs are essentially the same, Your Honor. But it is

easier to explain if we first look at the sperm formation. We will make the connection in a

moment.

Judge: Very well, then. Continue with your testimony.

Exhibit A

Exhibit B


Mr. Sperm: Thank you. Well, just like in mitosis, there are some spindle

fibers that are formed in the cell that attach to the chromosomes.

They pull the sets of chromosomes apart—look at Exhibit C. This time

though only the pairs of mother and father chromosomes are separated.

It’s important that each side gets only one complete set of

chromosomes (monoploid (n) number). Now there’s only one #1, and one

#2, etc.

Counselor Liv: It sounds like each cell is going to be a pretty unique

collection of genes. What happens next? One complete set of

chromosomes is on each side of the cell; does that mean that the cell is

going to divide?

Mr. Sperm: You are quite correct, Counselor Liv. Two

cells are formed each with its own collection of 23

doubled chromosomes, as represented in Exhibit D. But

wait, things are not over. Now the cells go to work to

pull the sister chromatids apart into single chromosomes.

Counselor Liv: An excellent explanation, Mr. Sperm. I

assume that you are about to tell me that they now go

through the same steps as mitosis

Counselor Oocyte: Hold it, hold it, I object. Your Honor, I’ve

been patient long enough. Who is the witness here and who is

the attorney?

Judge: I agree, Counselor Oocyte. Objection sustained.

Counselor Liv: All right. Mr. Sperm, please tell us what happens

next.

Mr. Sperm: Well, you are correct. In Exhibit E you see the

nuclear membrane dissolves again and the chromosomes line up

in the middle of the cell, just like in mitosis.

Then in Exhibit F the spindle fibers attach to their chromatids

and Exhibit G shows the fibers pulling them apart. This way

each side of the cell gets a one complete set of instructions.

Then, the cytoplasm divides. That’s it. Meiosis is finished.

If everything is perfect, each cell at the end has one complete

set of instructions (monoploid (n) number of chromosomes).

Each sperm has 23 chromosomes. And, of course, there are

four of them produced.

Exhibit C

Exhibit G

Exhibit F

Exhibit D

Exhibit E


Counselor Liv: After all, that really is the

basic idea of meiosis, isn’t it? To have a

normal cell with 46 chromosomes (diploid (2n)

number) duplicate and then instead of dividing

once, it divides twice. This makes four cells,

each with only one set of chromosomes

(monoploid – n).

Judge: Your witness, Counselor

Oocyte.

Counselor Oocyte: Fine, now let’s get the picture of what happens in the egg. In the formation of

sperm, meiosis leads to four cells, each with a set of chromosomes. I take it that these four cells are

called sperm when the process is finished. Sperm, is that right?

Mr. Sperm: Absolutely. They are sperm with great wonderful tails and great potential, I must say.

In fact, I’ve got some more fascinating diagrams…

Counselor Oocyte: No! I mean, no that’s quite all right. I think we’ve seen quite enough of those.

Please continue.

Mr. Sperm: Oh. Okay, well, in the formation of the egg, exactly the same process happens. One of

those four, rather like you, Counselor Oocyte, will eventually form an egg. The only real difference is

that three of the cells will be tiny and one will be huge, rather like your client there. The tiny ones

just disintegrate. The huge one is the egg and it has all of the nutrients and cytoplasm saved for

itself.

Counselor Oocyte: Is it true that many egg cells that develop in an older woman like Martha don’t go

through exactly the same steps? Lots of times there are differences?

Mr. Sperm: You are definitely correct. Older men and women seem to have more…

Counselor Oocyte: That will do for the moment, Mr. Sperm. Now, tell me what were the events that

have led to the special chromosome pattern that we see in the defendant?

Mr. Sperm: As I understand it, she has an extra chromosome 21. This can happen in a woman’s egg

as it is developing, especially an older woman.

Counselor Oocyte: Please, tell us then how this situation occurs to millions of women each year.

Mr. Sperm: It’s simple, really. Everything is fine through the first stage of meiosis. The

chromosomes make copies, they line up in the center and the pairs of chromosomes separate. We

have two cells with 23 chromosomes with chromatids. Then the trouble starts. When the

Exhibit H


chromosomes line up again in the center to separate, chromosome 21 doesn’t separate completely like

the rest. If this happens to the egg, it will end up with an extra copy of 21.

Counselor Oocyte: It sounds like if this egg is fertilized by a normal sperm cell with its 23

chromosomes, then the baby that results will have an extra chromosome 21.

Mr. Sperm: Yes. The condition leads to a baby that has Down Syndrome. About one baby in 600

hundred has three copies of 21, but in women over the age of 45, one baby out of 50 will have Down

Syndrome.

Counselor Oocyte: This sounds pretty common to me. It’s hard to imagine that one would want to….

**********************

The transcript of the trial stops here. Missing is the testimony of the remaining witnesses for the prosecution and the all-important defense witnesses. Newspaper accounts of the proceedings indicate that there continued to be fireworks between the two attorneys. Unfortunately, the final decision of the jury is not recorded as the courthouse files of the proceedings have been destroyed in a mysterious fire. So we are left with mere speculation as to the verdict in this trial.

Non-disjunction – failure of

homologous pairs of chromosomes

to separate during meiosis.

This is how Down Syndrome and

some other chromosome

abnormalities happen in sex cells.

Tetrad –

4 sister

chromatids

Canale LE Page 12

Comparison of Mitosis and Meiosis

Characteristic Mitosis Meiosis

Occurs in what type of cell (body cell or sex cell)?

Parent cell is diploid (2n) or monoploid (n)

Number of cell divisions

Number of daughter cells produced

Daughter cells are diploid or monoploid

Daughter cells’ genetic material is identical to or different

from parent cell

Type of cell produced (body cell or sex cell)

“My toe” is part of

my body - so

mitosis happens in

body cells

What do you write

above and below the

“e” to remind of the

type of cell meiosis

happens in?

Canale LE Page 13

Summary of Mitosis and Meiosis Mitosis

1. How many cells do you start out with in Mitosis? ________________________

2. How many cells do you end up with in Mitosis? __________________________

3. How does the size of each daughter cell compare to the size of the original

parent cell? ___________________________________________________

4. How does the chromosome number compare between each daughter cell and the

original parent cell? _____________________________________________

5. Are the cells produced by Mitosis diploid (2n) or monoploid (n) (haploid)?

_____________________________________________________________

6. Mitosis is known as what kind of reproduction? _________________________

7. What type of cells undergoes Mitosis? _______________________________

Meiosis

1. How many cells do you start out with in Meiosis? ________________________

2. How many cells do you end up with in Meiosis? __________________________

3. How does the size of each cell made by Meiosis compare to the size of the

original cell? __________________________________________________

4. How does the chromosome number compare between the cells made by Meiosis

and the original cell? ____________________________________________

5. Are the cells produced by Meiosis diploid (2n) or monoploid (n) (haploid)?

_____________________________________________________________

6. Meiosis is known as what kind of reproduction? _________________________

7. What type of cells undergoes Meiosis? _______________________________

8. Why is it important that gametes be monoploid (n) (haploid)?

_____________________________________________________________

9. What is it called when homologous chromosomes pair with one another during

metaphase I of Meiosis? _________________________________________

10. What structure is formed by the pairing of homologous chromosomes?

____________________________________________________

11. What is it called when homologous chromosomes exchange segments of DNA?

_____________________________________________________________

12. What does crossing over help to provide for an organism and a species?

_____________________________________________________________

Documents

Unit 6: Meiosis Handouts · Unit 6: Meiosis Handouts Name_____ Mrs. Canale LE 2013-2014 [Canale LE] Page 2 2. Parent cell ... Well, just like in mitosis, there are some spindle