Lecture 5 (5/29/09): Why Can't We Be Friends?...Meiosis & Punnett Squares finally shake hands

BE FOREWARNED...This post will likely fry your brain, but in a good way when all is said and done. More importantly, it will be that final preparation you need to get ready for Monday's exam...

What do you see in the image to the right of these words?

What's that? A Punnett Square? Yes, I thought so.

Don't get me wrong. I see what you're seeing, but I also see something else.

In fact, I see lots of other things. And if you're going to do well in this course, you need to start seeing things in this image in addition to a Punnett Square filled with letters.

In this Blog, I intend to walk you through a list of things that I see when I look this image. My hope is that together we amble at a comfortable pace and distance, but if not, you should feel free to use the Comments feature at the bottom of this post.

From letters to 'bowling pins'...

When I look at the Punnett Square I see bowling pins. Well, at least that's how some students describe what they think chromosomes look like. You need to know that the A's and the a's in the Punnett Square represent alleles and that alleles are variants or versions of genes. Where are genes found? On chromosomes. In the image I've included on the left (click on it to make it bigger), the blue things are the chromosomes and the small white bands on each of the blue things represent a region of the chromosome called a gene. The capital (A) and lower case (a) letters let you know that some of the genes are different from one another--that is, they are alleles.

What I see may not seem like a big deal to you at first, but I think learning to look at Punnett Squares in this particular way is very important. Here's why: If you can look at a Punnett Squares and see chromosomes, then you can then connect Punnett Squares to a bunch of other images or representations that have been presented in lecture. Watch how I do this...

Getting to Know Parent 1

If we focus our undivided attention on Parent 1 (shown inside the red circle below at left), we can ask a simple question: Where did Parent 1's two chromosomes--one with the A allele and one with the a allele come from? But first, maybe we should take a step back and ask: Exactly what are these two images?

Well, the chromosome on the left (the one with the A allele) is a gamete, and so is the chromosome on the right (the one with the a allele). Here, then, is a question for you: What process did Professor S talk about in lecture that makes gametes? That's right, you knew it, it's meiosis.

If these two chromosomes actually represent two gametes (which, by definition, are reproductive cells with half of the total number of chromosomes of the parent cell they came from), then you and I should be able to somehow connect the images that Professor S showed us in class when he was lecturing about meiosis.

We will do this in just a minute, but first, I hope you're scratching your head as you're reading this because you think something is just not right. How many gametes are shown for Parent 1 in the red circle above? Two gametes. How many gametes are typically produced when a parent cell goes through the full process of meiosis (including both meiosis I & II)? Four gametes. If four gametes are produced by meiosis and Punnett Squares usually only show two gametes for a single parent, then where did the other two gametes go? Why aren't they included in Punnett Squares too? To begin to address this strange omission, we'll need to start combining images of Punnett Squares with images of meiosis.

Punnett Square? Meet meiosis...

To the right is one of the images Professor S showed you during his lectures about meiosis (click on it if you want to make it bigger). The only thing I've done to it is cropped it to remove some of his writing, and then I've added some black A's and a's so that we can keep track of the alleles through all the various stages of meiosis. What's nice about this image is that, in addition to the letters, the blue and red colors can help us keep track of the alleles too. Which just now makes me think of an exam tip:

• Exam tip: Perhaps you should bring a couple of different colored pens or highlighters to Monday's exam, then you could color-code any meiosis diagrams you are given or have to draw yourself. They'll help you keep track of alleles too.

Lets walk through this diagram in order to 'mine' it for all it's worth...I'm going to start at the very top--above the 2st purple bar--and then work my way down to the production of the (count 'em) four gametes. By the end of this exercise (if you can't see it for yourself already) you should be able to explain why scientists only include two gametes per parent when they draw Punnett Squares for basic monohybrid crosses instead of four.

Step One...DNA or chromosome replication

One way to talk about the process that this parent cell is undergoing above the 1st purple bar (cropped and enlarged below at left) is as DNA or chromosome replication. In the language of meiosis, the small white arrow between the top and bottom cell counts as Interphase I, when the DNA in the parent cell makes an exact copy of itself (you could argue that the bottom cell is actually Prophase I). It's important that we do some serious term work here before the exam...

Learning the Language of Interphase I of Meiosis

How do biologists talk about the red and blue 'bowling pins' in the first cell? Well, they say that the parent cell contains a "homologous pair" of chromosomes. If you just look at the letters/colors (i.e., the alleles) you might be confused by this term. Homo- means "same" or "similar" and the two chromosomes in that first cell clearly have different alleles (different letters/colors): Shouldn't they be called a 'heterologous pair' or something like that?

Well, the reason why the two chromosomes are called a homologous pair is because the (big) A and the (little) a are alleles for the SAME gene (e.g., hair color). So the term "homologous pair" refers to the fact that the two chromosomes have regions on them that code for the SAME gene (in this case, hair color). Each 'bowling pin' is a single chromosome and together the blue and red 'pins' make a homologous pair of chromosomes. What about the cell at the bottom? What kind of language do biologists use for the chromosomes in that cell?

There now appears to be FOUR chromosomes, but two of the blue ones appear to be connected. Biologists call this blue 'X' a single "replicated chromosome" and they call each half of the blue 'X' a "sister chromatid." Same thing with the red 'X': it too, is a single replicated chromosome made up of two sister chromotids. I know this way of talking makes it sound like there are only TWO chromosomes present in the cell (instead of the FOUR that we can clearly see!), but biologists get away with this accounting scheme because they use the phrase "replicated chromosome." Is there a homologous pair of chromosomes still present in this bottom cell? Your textbook says, yes, and biologists say that there is a "homologous pair of replicated chromosomes" present in the bottom cell. Since I've bolded and italicized certain terms in this paragraph, can you see the subtle change in the language of Interphase I? From one perspective there are FOUR chromosomes present after DNA replication; from another perspective there are two chromsomomes present, i.e., a pair of replicated homologous chromosomes. 2 ("a pair") x 2 ("replicated chromosomes") = 4, right?

Learning the Language of VARIATION during Interphase I of Meiosis

Now, in yesterday's Blog ("Learning the Language of Variation") I dealt extensively with how biologists talk about variation of things like DNA, genes, chromosomes, and cells. Because nearly all of these terms have thus far been present in our discussion of Interphase I above, I ask you the following question: Does the replication of DNA during Interphase I lead to any new genetic variation? Well, unless there's some sort of copying error, then "No." DNA replication should produce NO new variation in the DNA. We could also say that DNA replication should produce NO new variation in the genes--or the alleles. We could also say that DNA replication should produce NO new variation in the chromosomes. In other words, by the end of Interphase I all the same 'stuff' is there (all the same A's and a's), but there's just twice as much of it!

Step Two...Meiosis I

So we now have a single parent cell that contains twice the number of its normal number of chromosomes. In other words, it started with 2 chromosomes (a homologous pair of chromosomes) and now it has 4 (a homologous pair of replicated chromosomes). The next stage of development for this cell that is now packed with additional genetic material is to undergo cell division. This cell division is shown below at left (again, cropped and enlarged from the image up farther up the page).

Whether you realize it or not, the purple bar in this image is actually filling in for almost all of the stages of meiosis I. The single representation above the purple bar could be considered early Prophase I, and the representations below the bar could be considered the end of Telephase I (and cytokinesis). What about all of the other meiosis I phases inbetween? Well, like all representations, this image hides some things from sight while other things are more obvious. Before we address what is missing in this image, lets look in the other direction: What is obvious in this image?

Learning the Language of Meiosis I

Well, in this image you can see right away what's happening in terms of the number of cells and the number of chromosomes (as well as the genetic information, i.e., the alleles, on these chromosomes) before and after meiosis I. Before meiosis I, how many cells are there? ONE parent cell. After meiosis I, how many cells are there? TWO 'daughter' cells or TWO gametes. Before meiosis I, how many chromosomes are there? There are FOUR chromosomes (OK, OK...there is a homologous pair of replicated chromosomes). After meiosis I, how many chromosomes are there? There are still FOUR chromosomes, but now there are only 2 chromosomes per cell! In biologist-speak, they would say that the "homologous pair of replicated chromosomes" have separated and each new gamete now contains a single "replicated chromosome."

Learning the Language of VARIATION of Meiosis I

Just like I did when discussing Interphase I above, I now ask you a similar question: Can the steps in meiosis I lead to any new genetic variation? Yes or No? Think for a minute and look over your notes...I'll wait.

You should have emphatically answered, "YES, the steps of meiosis I can lead to genetic variation!" However, I'm afraid that if I try to explain WHY within this post we'll lose the 'flow' of the overall story I'm trying to tell. So, if you're wondering why you should have emphatically answered , "YES!", then follow a link to a brief but important tangent-story ("Meiosis & Variability") and then come back and continue with this one. You won't regret it...

Step Three...Meiosis II

So, we now have two gametes that each contain the same total number of chromosomes as the original parent cell (TWO!). The next stage of development for these two gametes is to undergo yet another cell division. This cell division is shown below at left (like before, it's cropped and enlarged from the image farther up the page).

Whether you realize it or not, the purple bar in this image is actually filling in for almost all of the stages of meiosis II. The two gametes above the purple bar could be considered early Prophase II, and the representations below the bar could be considered the end of Telephase II (and cytokinesis). What about all of the other meiosis II phases inbetween? Once again, like all representations, this image hides some things from sight while other things are more obvious. Before we address what is missing in this image, lets look in the other direction: What is obvious in this image?

Learning the Language of Meiosis II

In this image, you can see right away what's happening in terms of the number of cells and the number of chromosomes (as well as the genetic information, i.e., the alleles, on these chromosomes) before and after meiosis II. Before meiosis II, how many cells are there? TWO 'daughter' cells or gametes. After meiosis II, how many cells are there? FOUR 'daughter' cells or FOUR gametes. Before meiosis II, how many chromosomes are there? There are FOUR chromosomes, but only 2 chromosomes per gamete. In biologist-speak, they would say that each gamete contains a "single replicated chromosome" (they also might say that the blue X is a "single replicated chromosome made up of two sister chromotids"...same for the red X). After meiosis II, how many chromosomes are there? There are still FOUR chromosomes, but now there is only ONE chromosome per gamete! In biologist-speak, they would say that each "single replicated chromosome" has separated and each new gamete now contains a single "chromosome."

Learning the Language of VARIATION of Meiosis II

Just like I did when discussing meiosis I above, I now ask you a similar question: Can the steps in meiosis II lead to any new genetic variation? Yes or No? Think for a minute and look over your notes...I'll wait.

Once again, you should have emphatically answered, "YES, the steps of meiosis II can lead to genetic variation!" However, I'm afraid that if I try to explain WHY within this post we'll lose the 'flow' of the overall story I'm trying to tell. So, if you're wondering why you should have emphatically answered , "YES!", then follow a link to a brief but important tangent-story ("Meiosis & Variability") and then come back and continue with this one. Again, you won't regret it...

"Are We There Yet?" Are Punnett Squares & Meiosis ready to hold hands?

Almost, we need to focus on one last cropped and enlarged image before we introduce the two. The image below at left shows the final FOUR gametes produced from meiosis. Each of the four gametes has how many chromosomes? ONE, that's right (the gametes are haploid cells).

What I want you to notice now are the letters (i.e., alleles). How many different types of alleles do you see? Your answer should have been, "Two." There are two types of letters, (big) A and (little) a. OK, so there are four total letters, but there are only two types of letters. Now I need you to see something else that isn't there...sperm and egg cells. If these were, say, human gametes, we would call each of them sperm cells (if they were made by a male) or egg cells (if they were made by a female).

During sexual reproduction, only one sperm cell gets to fertilize an egg cell. Lets say all four of the gametes in the picture above were from a male--lets call him Parent 1. How many different types of sperm cells did he just produce through meiosis? TWO, not four. How many different types of sperm cells could he give to a human female during sex? TWO, (big) A or (little) a, right? Are we now ready to finally combine the image of meiosis with the image of the Punnett Square? I hope so...because my fingers are getting tired and my brain is now fried trying to help you do well on tomorrow's exam...

Finally, Two Important Images Meet...

If you've carefully followed this post, the image below should now be somewhat 'intuitive' to you, whereas before reading this post I expect that many of you never thought to put these two different images together (click on it to make it bigger). If you never thought to put these two images together, that's unfortunate: They were made for each other, but many students tend to (incorrectly) think of them as two different "topics" that Professor S lectured about on different days. Nothing could be farther from the truth!

There is almost always a kind of 'flow' of ideas in college science courses. Unfortunately, sometimes this flow doesn't get communicated well by instructors, and sometimes students ignore when the flow is being explained to them. I have tried in this post to teach you to 'see' things that aren't there at first glance. I have tried to show you that one way of understanding the concepts in this course is to move images around, to hold them next to one another, to superimpose them over the top of each other, in other words, to interrogate them until you begin to see new things.

For many of you, this will prove to be an entirely new way of studying for a science exam, but I think in the end it will be well worth your time spent. In fact, if you find some new ways to do what I did today to other images presented in the course and/or in the textbook, I would love to hear about it at any point during the semester. And so too, I'm sure, would your fellow students...perhaps you could start your own Blog and make a few posts for them. Just in thinking about how to craft this lengthy post, I know I learned a ton of new things about meiosis today...I hope you have too.

Learning the Language of "Variation"...

For those of you who followed the Lecture 4 Blog ("When Guy Ritchie meets Madonna"), you know that I closed my post by making the following statement about "variation": Biologists frequently use this term whenever they talk about both the reproduction and the survival of living things.

In this post, I want to talk a little bit more about how (and why) biologists use the term "variation." One reason I want to do this is because in Lecture 4 I heard Professor S talk a great deal about variation when discussing meiosis. However, I also want to talk a little bit about variation and scale because when biologists talk about variation, the way they talk about it often depends on what scale--or what size 'thing'--they're talking about. For most students, this ability to talk about variation in so many different ways can be confusing.

So, lets first look at how biologists talk about variation in small things...

Does DNA show variation?

Sure it does. All we have to do is compare one strand of DNA to another strand and ask ourselves, 'What makes two strands of DNA different?'

If you look at the image on the left (an image from Lecture 2) you might guess that one way in which strands of DNA can vary (or show variation) is by having different sequences of nucleotides (remember, in DNA the nucleotides are abbreviated by the letters A, T, G, and C).

Another way that one strand of DNA could vary from another is in terms of its length. In other words, one strand of DNA could be longer than another strand. With DNA, a longer length is simply having more numbers of nucleotides.

So, there are at least two ways in which one DNA strand can vary from another strand. What about something at a little bit larger scale, like a gene?

Do genes show variation?

Sure they do. If you followed the Lecture 3 Blog ("When Darwin meets Madonna"), you already know that genes and DNA are similar concepts, but just at different scales: Genes are certain length segments or sequences of DNA. So, lets quickly compare two genes and see how they might show variation.

One way in which one gene can vary from another gene is by having different sequences of nucleotides. So, for example, one gene might have a nucleotide sequence such as ATTGCC and another gene might have a nucleotide sequence ACCGTT. If these two genes were actually variations of a gene for some trait like hair color, you could then say that these two variations were two different alleles for the hair color gene.

Another way that one gene could vary from another is in its length. In other words, one gene could be a 6-nucleotide sequence like ACCGTT, and another gene could be a 9-nucleotide sequence like ACCGTTAAA. Doesn't this sound almost exactly like how we talked about variation in DNA (i.e., sequence and length)? OK, what about something at a little bit larger scale, like a chromosome?

Do chromosomes show variation?

Sure they do. Again, if you followed the Lecture 3 Blog, you already know that chromosomes and genes and DNA are similar concepts, but just at different scales: Chromosomes contain multiple genes and genes are certain length sequences of DNA, therefore chromosomes are ALSO certain length sequences of DNA! The relationship between DNA, genes and chromosomes is easily grasped by simply looking at one of my favorite images--the image to the right.

Although it isn't labeled, most of you know that the chromosome is the X-shaped thing at the far right of the image. It is a depiction of a condensed chromosome (chromosomes don't always look like this!), but you can see that if you were to unwind or unravel the condensed chromosome it is simply made from a long strand of DNA! Scientists often separate segments of this long strand of DNA into things called genes, and you and I talked above about how the DNA itself can be separated into things called nucleotides (or base pairs). The letters A, T, G, and C don't appear in this image, but I'm sure you can imagine where they might be if they were included in it--they would be the (almost) horizontal 'rungs' of the (nearly) vertical 'ladder.'

So, when we ask the question, 'Do chromosomes vary?' or 'How do chromosomes vary?' we can answer it similar to how we answered the variation question about both genes and DNA. Chromosomes can vary from one another in terms of the number of genes they contain and they could also vary from one another in terms of the types of genes they contain. Now, how would you rephrase this terms of DNA (or nucleotides)? Try it...

Maybe something like: Chromosomes can vary from one another in terms of the total number of nucleotides they contain and they could also vary from one another in terms of the sequences of the nucleotides they contain. OK, what about something at a little bit larger scale, like a cell?

Do cells show variation?

Sure they do. From your previous science courses, you may have heard of (or seen under a microscope) different human body cells that were categorized as brain cells, skin cells, muscle cells, bone cells, and/or blood cells. This is one way that cells show variation: different types of human body cells can not only look different, but they can also do different things. In other words, humans--and other multicellular organisms--have many different cell 'types' (i.e., cell structures) each of which have specialized 'jobs' (i.e., cell functions).

But I want to draw your attention briefly to a slightly different way of talking about cell variation. I want to draw your attention instead specifically to the chromosomes--and the genes, and the DNA--inside of human cells. Here are some questions for you...

QUESTION #1: Do each of the human body cells vary in terms of their chromosome number?

• NO! All normal human body cells should have IDENTICAL copies of each of the chromosomes that were originally found in the fertilized egg cell or zygote. Why? Because all of the human body cells resulted from mitosis of the original zygote and mitosis supposedly makes exact copies of all 46 human chromosomes (23 from mom and 23 from dad).

QUESTION #2: Do each of the human body cells vary in terms of the number and types of genes found on the 46 chromosomes in a cell?

• NO! All normal human body cells should have IDENTICAL copies of each of the genes that were originally found on the 46 chromosomes of the fertilized egg cell or zygote. Why? Because all of the human body cells resulted from mitosis of the original zygote and mitosis supposedly makes exact copies of all genes on each of the 46 human chromosomes.

QUESTION #3: Do each of the human body cells vary in terms of the total number and sequences of nucleotides found on each of the 46 chromosomes in a cell?

• NO! All normal human body cells should theoretically have IDENTICAL numbers and sequences of nucleotides that were originally found on the 46 chromosomes of the fertilized egg cell or zygote. Why? Because all of the human body cells resulted from mitosis of the original zygote and mitosis supposedly makes exact copies of the strands of DNA (and thus the nucleotide sequences).

Now wait one minute...If all human body cells have exactly the same number of chromosomes (46), exactly the same number and types of genes, and exactly the same total numbers and sequences of nucleotides in the DNA, a huge problem arises...

If all human body cells have exactly the same number of chromosomes, exactly the same number and types of genes, and exactly the same total numbers and sequences of nucleotides in the DNA, how in the heck do we end up with highly specialized body cells like as brain, skin, muscle, bone, and blood cells that not only look different, but they also do different things?

• A POSSIBLE EXPLANATION: Could it be possible that each cell--despite having exactly the same "genetic material"--can actually 'activate' some regions of DNA while at the same time 'deactivate' others? In other words, is it actually possible that regions of DNA (i.e., genes) can somehow be turned 'on' and 'off' by the cell? (Biologists might phrase this same idea as the following question: Is human cell specialization a function of gene regulation?)
  

The idea of gene regulation is actually not an idea that we will go into in any great detail in BS110, but those of you who have yet to take BS111 will likely hear lots more about it (for those of you who have already taken BS111, maybe you now have a better sense of why you were hearing about it).

A VERY IMPORTANT POINT: There is at least one other way that variation at the cellular level is talked about by biologists! When Professor S presented meiosis to you in class this past week, he talked a great deal about how "genetic variation can occur during meiosis." In this post, I have not dealt directly with this issue in my discussion of cells and variation. However, it will be something that I do address in the Lecture 5 Blog "Why Can't We Be Friends? Meiosis & Punnett Squares finally shake hands" which will be published on Sunday (5/31). In the meantime, you can wade your way through your notes for ideas about "crossing over" and "independent assortment."

Concluding thoughts...

The main reason why I let today's Blog wander into the world of variation is because in BS110 we will continue to talk about "variation" in living things. And, more importantly, we will continue to talk about it at different scales. Today, I started explaining how biologists talk about variation at smaller scales and I moved you (too slowly?) in the direction of how they talk about variation at larger scales.

In the process, notice how many different terms I used in talking about variation: DNA, nucleotides, genes, alleles, chromosomes, cells. In the coming weeks, you will talk more about variation at even larger scales, such as variation within individual organisms and also variation within entire populations or communities of organisms. In these discussions, you will likely hear terms used such as genotype, phenotype, heterozygous, homozygous, traits, characters, characteristics, frequency, probability, adaptation, natural selection and evolution--these are all terms that are commonly used by biologists when talking about variation at larger scales.

However, the very terms I just listed are also terms that have intimate connections with ideas in biology that I've been talking a lot about in my Blogs lately: survival and reproduction. For example, this past week Professor S used some of the terms I listed above when recently lecturing on meiosis, Punnett Squares, and Mendel's rules of inheritance. In this class, whether you realize it or not, each week we are edging 'up' in scale to larger and larger units in our investigations into living things.

In addition, whether you realize it or not, we are also edging ever closer to learning how to see the world of living things through a pair of Darwinian glasses. In other words, in the coming weeks it will become harder and harder for you to look at living things and to NOT think about them as engaged in a constant 'battle' or competition for things like food, water, and (at least in the case of sexually reproducing organisms) for mates/sex.

My next Blog...(which I'll publish on Sunday afternoon): Lecture 5 (5/29/09): "Why Can't We Be Friends?...Meiosis & Punnett Squares finally shake hands". In that Blog, I'll do some specific Exam 1 prep regarding a bunch of important ideas presented in Lectures 4 & 5.

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...