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).
- 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.
- 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).
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?)
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.
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