by
Clyde Freeman Herreid
Department of Biological Sciences
University at Buffalo, State University of New York
The jury filed back into the courtroom, hardly visible to the spectators seated on the benches. Their tiny prokaryotic bodies seemed to bounce along to some inner music. It had been three days of testimony and the jury had now become comfortable with the routine. Once seated, they started whispering among themselves and court reporters could catch snatches of their conversations:
“God, I feel so small…. I know what you mean…. What do you think about that mitochondrion that testified? He looked sort of cool. Look at all of that stuff in their bodies. I mean really… to see all of those membranes inside. I mean, it grosses me out. I really shouldn’t be prejudiced, after all they have invited us here, but it really is a bit too much…. Shh, here comes the judge.”
“Hear ye. Hear ye….”
With preliminaries over, Judge Cellular peered over his glasses and stated, “I do hope we can move this trial along more rapidly, councilors. We really don’t have to burden the jury with hearing from every organelle in the eukaryotic galaxy; surely the testimony is quite adequate to show how mitosis works. And, no, we don’t have to hear from the plant cell witnesses. I’m sure we all understand it sufficiently well. Now, Councilor Liv, at our last session we left off with the witness for the prosecution having been sworn in. Was it Mr. Spermatocyte?”
“Yes, Your Honor. He is a primary spermatocyte and the defense has already stipulated that they will accept him as an expert witness for the process of meiosis. You will recall that he comes from Martha’s husband Sam and his testes. He has been involved with the meiosis process ever since Sam’s puberty.”
“Very well. Mr. Spermatocyte, please come back to the stand. Come on sir, don’t hang back. No need to swear you in again. Let’s get on with it, Ms. Liv.”
“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?”
“That is quite correct, Councilor. The cell division in the sex glands doesn’t lead to two identical cells like we typically find in mitosis. No, indeed—it leads to four sperm cells, each absolutely unique. Quite special. Quite.”
“How so, Mr. Spermatocyte?”
“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.”
“But if it is the case that all cells in Sam’s body have the same set of chromosomes, how come the cells act differently? Why do skin cells act like skin and nerve cells act like nerves and not visa versa. I should think that…”
“I object to this line of questioning, Your Honor. This is immaterial and irrelevant to the case.”
“Overruled, Ms. Oocyte. It is necessary for the prokaryotic jury to understand how the cells in the colony differ from one another even though they have the same set of instructions. You may answer the question, Mr. Spermatocyte.”
“Gladly. You see, even if all cells have the same chromosomes and the same genes, not all of the same genes are turned on! A muscle cell has only the genes for being a muscle working, while the skin has only skin genes working. That’s all there is to it.”
“That’s fine Mr. Spermatocyte. Now continue with what happens when the sperm cells are formed.”
“Certainly. 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. We are certainly not your run–of–the–mill sort of cells. All the other cells go through mitosis; they are simply making clones of themselves. We, however, are out to make something unique! It is rather clear, isn’t it… how special we are? Let me illustrate what happens when the sperm are formed. Naturally, I have brought my own set of diagrams along. They show what happens in my human, ‘Sam.’
“You might know.”
“I’ll have none of that nonsense in my courtroom, Ms. Oocyte. The bailiff will set the diagrams up on the easel for you, Mr. Spermatocyte.”
“Very well. I’ll continue. Now you really have to notice this. Take a look at how the chromosomes line up during meiosis. Just look at that, won’t you! 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 momma have paired up with his dad’s set; the number 1s are together, and 2s, and 3s and so on. And if you look closely you can see that each chromosome is composed of two chromatids, just exactly like we saw for the skin cell demonstration.”
“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 during Interphase?”
“That’s absolutely correct, Your Honor.”
“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 down the line? Why, if I looked close enough, I would see the chromatids. In fact, they would look like teams of four chromatids.”
“That’s it, Your Honor! You have it precisely!”
“And, so…? Please explain the relevance of that arrangement to me and the jury. I can see them puzzling along with me.”
“Yes, indeed. But I must digress a moment. I need to…”
“I do object, object, object! What is this, a biology lesson? Surely, we have listened patiently about all of this coming and going long enough. We all get it. Mitosis is different than meiosis. Isn’t that sufficient? Let’s move the trial along. My client is different than other little gametes. We have granted that. She is on trial for her life! Why are we listening to this lecture about sperm? My client is an egg cell, for God’s sake!”
“Sit down, Ms. Oocyte. I will have no further outburst in my courtroom or I will cite you for contempt. Do you understand?”
“Yes, Your Honor. But this is extremely frustrating, and I am sure that the jury is as disturbed as I. Just look at them. My objection still stands. What is your ruling, if you please?”
“Ms. Oocyte, your point is well made. Councilor Liv, are you going to connect this up in some way? Why are we talking about sperm instead of eggs?”
“The formation of sperm and eggs are essentially the same, Your Honor. But it is easier to explain if we first look at the simpler sperm formation. We will make the connection in a moment.”
“Very well, then. Continue with your testimony, Mr. Spermatocyte. I will withhold my ruling for the present.”
“Thank you. The point I was making is that when these pairs of chromosomes lie side by side they touch at many points. That’s when they sometimes swap parts, and you can see that represented too in Exhibit B. Pieces of Sam’s mother’s chromosome #7 can exchange pieces with Sam’s father’s #7. It’s quite like a mix and match affair. You see it gives variability to the chromosomes. This is called ‘crossover’ by the geneticists.”
“Does it occur often?”
“Oh, absolutely! In fact, in humans at every meiotic tetrad (that’s what the four chromatids are called) there are about 10 crossovers. So by the time that crossover is finished, the individual chromatids can hardly be thought of as mother’s or father’s any more. They are a hybrid of the two.”
“Let us now return to the main point, Mr. Spermatocyte. What is the result of this peculiar arrangement of the chromosomes?”
“Well, just like in mitosis, there are some threads that are formed in the cell that attach to the chromosomes. These are the spindle fibers. They act to pull the sets of chromosomes apart—look at Exhibit C. This time though the chromatids of the chromosomes won’t be separated. Only the pairs of mother and father chromosomes are separated.”
“Do you mean that all of the mother’s chromosomes go one way and the father’s go another and end up in different cells?”
“Oh, dear me, no. Did I say that? No, no…. Oh, I see where the trouble is! I didn’t make a crucial point, and the diagram might be slightly misleading. When the father and mother’s chromosome pairs lie together that doesn’t mean that they line up so that all of the mother’s are on one side and the father’s are on the other. Nothing could be further from the truth. No, they can be on either side. It is just chance that the mother’s #10 is on the right side of the cell and the father’s is on the left. It could have just as easily been the opposite. So the end result is that it is simply a mixture. The only thing that counts is that each side gets a complete set of chromosomes.”
“That’s this variability point again. It sounds like each cell is going to be a pretty unique collection of genes. What happens next? Complete sets of chromosomes are on each side of the cell; does that mean that the cell is going to divide?”
“You are quite correct, Ms. Liv. Two cells are formed each with its own collection of 23 chromosomes, as represented in Exhibit D. But wait, things are not over. The chromosomes still have their chromatids. Remember them? Now the cells go to work to pull these apart.”
“An excellent explanation, Mr. Spermatocyte. I surmise that you are about to tell me that they now go through the same steps as mitosis—that the chromosomes line up in the center of the cell and then the spindle fibers pull the chromatids apart so that….”
“Hold it, hold it, I object. Your Honor, I’ve been patient long enough. Who is the witness here and who is the attorney? It’s a cute routine that they’re putting on, but come on now, this is clearly leading the witness.”
“I agree, Councilor Oocyte. Objection sustained. Ms. Liv. Please do not lead the witness. Rephrase your statements.”
“All right. Mr. Spermatocyte, please tell us what happens next.”
“Well, you definitely are correct. In Exhibit E you see the nuclear membranes again disolving, and in Exhibit F the chromosomes are lining up in the middle of the cell, just like in mitosis and it doesn’t matter how they get into a line as long as they do it. Then 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 complete set of instructions. Then, the cell divides. That’s it. Meiosis is finished. If everything is perfect, each cell at the end has one complete set of instructions. Each sperm has 23 chromosomes. And, of course, there are four of them produced. In the first stage of meiosis we produced two cells, and then in the second stage each of these divided again and produced two. That makes four, doesn’t it?
“After all, that really is the basic idea of meiosis, isn’t it? To have a normal cell with 46 chromosomes go through duplication and then instead of dividing only once, it divides twice. This makes four cells, each with a set of chromosomes instead of dividing only once and having a cell with two sets of chromosomes, and that’s what you see in Exhibit H.”
“Yes, indeed.”
“Your witness, Councilor Oocyte.”
“I hardly know where to begin. This explanation is so confusing that I don’t wonder that the jury is baffled about what is going on here. You are telling us that meiosis leads to four cells, not two like we saw previously in mitosis. Right? And you seem to be suggesting that, with all of this crossover and mixing of chromosomes, each sperm cell is unique. If that is so, am I correct in saying that one variation is just as normal as the next?”
“Well, I’m not sure I would say that…. I guess I would say that some cells are more normal than others. If there is a mutation, I would say that cell is, uh…, suspect.”
“Why is that?”
“Because most mutations decrease the chances that a sperm will survive. Most mutations are bad.”
“But not all. Right?”
“Right.”
“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 really called sperm when the process is finished. Sperm, is that right?”
“Absolutely. They are sperm with great wonderful tails and great potential, I must say. In fact, I’ve got some more fascinating diagrams…”
“No! I mean, no, that’s quite all right. I think we’ve seen quite enough of those. Please conitnue.”
“Oh. Okay, well, in the formation of the egg, exactly the same process happens. The primary oocyte, rather like you, Ms. Oocyte, will eventually form an egg. You will go through the two steps of chromosome shuffling and at the end four cells will be formed. 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 are called polar bodies and they just disintegrate. The huge one is the egg and it has all of the nutrients and cytoplasm saved for itself. See for yourself how loaded the defendant is with nu….”
“PLEASE, Mr. Spermatocyte confine your remarks to the general situation and refrain from making personal asides…. Now, one more question, sir. Is it true that many egg cells that develop in an older woman like Martha don’t go through exactly the same steps? That lots of times there are differences?”
“You are definitely correct. Older men and women seem to have more mutations and…”
“That will do for the moment, Mr. Spermatocyte. Now, tell me what were the events that have lead to the special chromosome pattern that we see in the defendant, Egg Cell Number 6624223?”
“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. It is a simple mistake…”
“PLEASE, don’t refer to it that way as there are thousands of women that produce these eggs each year.”
“I object to Councilor Oocyte leading the witness. If the witness says it is a mistake, then that is what he means. He has been certified as an expert witness, Your Honor.”
“Objection sustained.”
“Please, tell us then how this situation occurs to millions of women each year.”
“It is 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 now have two cells with 23 chromosomes with chromatids. Then the trouble starts. When the chromosomes line up again in the center, this time for the chromatids to separate, all goes well except that chromosome 21 doesn’t always separate completely. If this happens to the egg, then it will end up with an extra copy of 21.”
“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.”
“Yes. The condition is called trisomy and 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.”
“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. Most importantly, there was a surprise character witness for the defense, a population geneticist who was a prokaryote. Here was a bacterial cell speaking about the value of mutations in the course of evolution. Councilor Liv tried to discredit him, but only succeeded in alienating the jury, according to some accounts. Unfortunately, the final decision of the jury is not recorded as the courthouse files of the proceedings have been destroyed in a mysterious fire that some blame on an anarchist movement that claimed that eukaryotic cells are nothing more than a symbiotic collection of prokaryotes. So we are left with mere speculation as to the verdict in this trial.
Faithfully recorded by Researcher E. coli #53623.
Originally published at http://www.sciencecases.org/mitosis_meiosis/mitosis_meiosis2.asp
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