Lecture 4 (5/27/09): When Guy Ritchie meets Madonna

In the Lecture 3 Blog ("When Darwin meets Madonna"), I talked quite extensively about how biologists are obsessed with survival and reproduction. I spent much of the Lecture 3 Blog talking about the unique ways that biologists talk about the reproduction of DNA (which they call "replication" or "duplication"), the reproduction of genes (which they also call "replication" or "duplication"), the reproduction of chromosomes (which they also call "replication" or "duplication"), and the reproduction of cells (which they call "cell division"). When discussing cell division, I talked extensively about mitosis and I promised to give a similar treatment to meiosis. If you didn't read the Lecture 3 Blog, I suggest you go back and do so now.

MEIOSIS...the preparation of cells that can make a new organism

As was the case with mitosis, meiosis is a term that we can think of as something that can happen during cell division. Whereas mitosis was an important step--along with DNA replication and cytokinesis--involved in the production of TWO identical daughter cells from ONE mother cell, meiosis is an important step involved in the (eventual) production of FOUR new daughter cells. However, EACH of the FOUR new daughter cells--sometimes called gametes--contain only HALF of the genetic material found in the ONE original mother cell!

Did what I just said make any sense to you at all? If so, great, you're ahead of the game. If not, don't worry, but read on...

In order to figure out what the heck I just wrote in the paragraph above, it might help for us to talk once again about Madonna. This time, however, we also need to talk about Madonna and her ex-husband, Guy Ritchie.

How was Guy & Madonna's (biological) son Rocco 'made?'

If we were to look inside of a single body cell (or "soma" cell) taken from Madonna, how many chromosomes would you be able to see if they were condensed, separated from one another, and visible?

The answer is 46 chromosomes, which is the normal number of chromosomes found in human body cells. How many chromosomes would you be able to see if you were to look inside of a single body cell of ex-husband Guy Ritchie?

The answer, again, is 46 chromosomes. Now, imagine that Madonna and Guy were to have a child. They would each have to contribute one of their cells, right? What if they both contributed one of their body cells--the ones with 46 chromosomes--in the form of an egg cell (from Madonna) and a sperm cell (from Guy)? When these two 46-chromosome cells meet to form a single fertilized egg cell (aka. a zygote) this hypothetical fertilized cell would have 92 total chromosomes! 46 from Madonna's egg cell and 46 from Guy's sperm cell, right?

Well, Guy & Madonna did actually make a zygote by joining one of his sperm cells and one of her egg cells sometime in 1999. How do we know this? Because in 2000 Madonna gave birth to Rocco John Ritchie. However, if we took a single body cell from (now 9 yr. old) Rocco and counted the total number of chromosomes inside the cell nucleus, we would find 46 (not 92!)--which is the same number of chromosomes as both his mother's and father's normal body cells. How could this be?

Well, this is actually possible through the 'magic' of the process called meiosis! Take a single 46-chromosome human cell and divide it up into some 'daughter' cells that each have 23 chromosomes...and Voila!...you can then combine them with some other human's 23-chromosome cell (through sexual intercourse or perhaps by way of some type of medical procedure) to form a single 46-chromosome cell.

When it comes to meiosis, the devil is definitely in the details...

One of the things that gives student fits when learning meiosis is trying to memorize all of the different steps involved in taking a single 46-chromosome 'mother' cell (not as in mom!) become 4 new 'daughter' cells (not as in female!) each with 23 chromosomes in them. That's right, meiosis doesn't just take a 46-chromosome mother cell and make TWO 23-chromosome daughter cells from it, it makes FOUR 23-chromosome daughter cells or gametes. How does this happen?

During Lecture 3, Professor S gave you a couple of slides during his lecture that illustrates how this reduction of the total number of chromosomes found in the gametes (or daughter cells) is accomplished. One of these slides is shown to the right.

One of the things you should notice in this slide is that the way a single mother cell can produce 4 new gametes with half of the mother cells original chromosome number is to divide twice! (These two divisions are indicated by the purple horizontal bars in the diagram.)

But if we just started dividing cells without messing around with the total numbers of chromosomes present in the cell, we would end up with something rather odd. Lets go back to one of Madonna's 46-chromosome body cells to help illustrate this point...

If Madonna's 46-chromosome body cell divided into two new daughter cells--without altering the total number of chromosomes in the cell--she would produce 2 new daughter cells each with 23 chromosomes, right? If EACH of these two 23-chromosome daughter cells then divided into two new daughter cells themselves--and again, without altering the total number of chromosomes in the cell--she would produce a total of 4 daughter cells (2 daughter cells from each of the first 2 daughter cells) EACH with 11-1/2 chromosomes...oooh, that can't be good.

If one of these 11-1/2 chromosome daughter cells was one of the egg cells that she was hoping to join with Guy Ritchie's sperm cells, not only would she be cutting one of the chromosomes in half (who wants 1/2 of a whole chromosome?), but she would also be contributing only 1/4 of the total chromosomes--i.e., the total genetic material--that her future son Rocco would need in order to have 46 chromosomes in his own body cells!

How do you think Madonna's and Guy Ritchie's bodies deal with this chromosome-number problem?

Chromosome replication/duplication before cell division...

As you can hopefully go back and actually see in the slide above, there is a replication/duplication of the chromosomes inside of the 'mother' cell prior its FIRST division...as you can see above, the single red-colored chromosome replicated itself, and so did the blue-colored chromosome.

You CAN'T see this same replication/duplication step in the slide at the left--another slide used in lecture--because the chromosome replication/duplication step isn't there! Which brings me to my first point...

Meiosis is NOT the replication/duplication of the genetic material in the nucleus--meiosis IS the separation and distribution of already replicated/duplicated genetic material in the nucleus into four 'daughter' nuclei (nuclei = the plural of nucleus).

If you read yesterday's Blog, you can see that I just tried to keep the same form as I did when talking about mitosis so you can see the similarities and differences between the two processes. Here's how I wrote about mitosis yesterday:

• Mitosis is NOT the replication/duplication of the genetic material in the nucleus--mitosis IS the separation of already replicated/duplicated genetic material in the nucleus into two identical 'daughter' nuclei (nuclei = the plural of nucleus). ONLY when other important parts of the cell divide--later during cytokinesis--can we then form two new 'daughter' cells from the original 'mother' cell.

Can you now see some of the important differences between mitosis and meiosis?

• Mitosis results in two daughter cells that are identical in total chromosome number to the mother cell.

• Meiosis results in four daughter cells (or gametes) that are NOT identical in total chromosome number to the mother cell--in fact, they have exactly half the total chromosomes of the original mother cell.

• Both mitosis and meiosis require the genetic material inside the mother cell to replicate/duplicate PRIOR TO the separation and distribution of them into their new daughter cells.
  

Just as I did with mitosis, here are some mathematical forms to consider that builds on the ideas up above and in Lectures 3 & 4...

• 1 mother cell + DNA replication + meiosis I = 2 daughter cells each with the same total number of chromosomes found in the mother cell

AND...

• 2 daughter cells (from meiosis I) + meiosis II = 4 daughter cells each with half the number of chromosomes found in the mother cell
  

Here is a narrative form to consider that also builds on the ideas above...

• If you want to produce (four) daughter cells that each have half of the genetic material of (one) mother cell...FIRST you need to duplicate/replicate the genetic material (via DNA replication)...SECOND, you need to separate and distribute the genetic material so as to form two new daughter cells each with the same amount of genetic material as the original mother cell...THIRD, you then need to separate and distribute the genetic material of the two new daughter cells AGAIN...so as to form a total of four new daughter cells each with half of the genetic material of the original mother cell.
  

• Since we're forming new cells in meiosis by dividing a mother cell (remember, not as in mom!) into two daughter cells (remember, not as in male/female!) and THEN dividing the two daughter cells into two more daughter cells for a total of four gametes, you would expect that some type of cytokinesis is happening too, since, for example, new cell membranes must be formed for each of the new cells.

What else happened in Lecture 4?

Recall that I've been making a big deal of how biologists view the world with Darwinian glasses. In other words, biologists frequently view all life on Earth with two things in sharp focus: survival and reproduction. In both my Lecture 3 and 4 Blogs, I've focused mainly on how biologists talk about the reproduction of things, for example, DNA, genes, chromosomes, cells, and even people (Madonna's egg cell + Guy Ritchie's sperm cell = Lil' Rocco John Ritchie).

Can you now see in these blue words that I've been walking you 'up' in scale from smaller things to bigger things? I hope so...because I told you back in the Lecture 1 Blog ("Concepts, Principles & Models Oh My!") that scientists are love to talk about things at many different scales (often simultaneously).

In the next few weeks, I'm going to be talking about the reproduction of things on even larger scales...for instance, we'll talk more about the reproduction of whole organisms, and we'll also talk about the reproduction of things like traits in groups of similar organisms (i.e., populations). When we talk about the reproduction of traits in populations over time (adding another scale into the mix of ideas!) we'll start talking about something called evolution, which is a theory about changes in the traits of groups of similar organisms (or species) over time.

What about SURVIVAL...when are we going to talk more about that idea?

We already have!!! In Lecture 4, Professor S also started talking with you about something called "variation" (variation = difference), which he discussed when presenting you with the nitty-gritty details about meiosis. He said that meiosis makes new cells that have different genetic material than the original mother cells. I don't know if you caught it, but at one point he said that genetic variation can be "beneficial" for cells and/or organisms. What he could have said was that genetic variation can be beneficial for the survival of cells and/or organisms (some organisms are single cells).

OK, so here are a few questions for you to consider: What in the heck does Professor S mean when he uses the term "variation" and what in the world does it have to do with survival? What does the term "variation" have to do with reproduction? Also, can we talk about variation with respect to some of our now-familiar scales? For example...

• Is there variation in DNA?
  
• Is there variation in genes?
  
• Is there variation in chromosomes?
  
• Is there variation in cells?
  
• Is there variation in organisms?
  
• Is there variation in populations of organisms?
  

Here are a couple of additional questions that some of you may be ready to start thinking about...

• How does mitosis promote or prevent variation?
  
• How does meiosis promote or prevent variation?
  
• How does sexual reproduction promote or prevent variation?
  
• How does asexual reproduction promote or prevent variation?

These questions (and more) will likely be the questions that motivate the way I approach some the next few Blogs, because when he started talking about "variation" in Lecture 4 Professor S opened a massive (and interesting!) can of worms that you and I will need to look inside of, stick our hands in, and begin feeling our way around...