..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.
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 MeiosisHow 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.
1 comment:
this is a real life saver
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