Hey, that’s against the rules!!!” various ages When you play a game of volleyball, you have to play by the rules. You have to use a certain type of net of a given size and it must be a certain height above the ground. The court has to be a certain size, and it needs to be marked so you know where the boundaries are. You can’t carry the ball, you can’t reach over the net, and you can’t hit the ball out-of-bounds. Each team is allowed to hit the ball a maximum of three times before they must send it over the net, you’re not allowed to let the ball touch the ground, and you have to serve from behind the line.

DNA Structure and Function

Our cells and our bodies also need to play by the rules, and unlike a game of volleyball, they don’t have the option of “cheating.” Our DNA (which stands for deoxyribonucleic acid) is the “rulebook,” the genetic code material that guides and controls everything else that our cells, and therefore our bodies, do.

DNA is a polymer, a string of similar subunits called monomers. In the case of DNA, the monomers are a type of chemical called nucleic acids, and there are four kinds of those used in building DNA. Those four are called adenine, thymine, guanine, and cytosine, but geneticists often refer to them as A, T, G, and C. Think of them as a kind of 4-letter alphabet, and then think of all the words of various lengths, sentences, and paragraphs you could make from just those four “letters.” That’s what our DNA does.

The string of nucleic acids that make up a strand of DNA are divided into groups of 3, and each of those groups is called a codon. Each codon codes for one, very specific, amino acid. Each codon can only code for exactly one amino acid (so there is no ambiguity) – for example, the codon TTC only codes for the amino acid lysine. However, many of the 20 amino acids are coded for by several codons (so there is redundancy) – for example, AAT, GAA, and GAT all code for leucine.

For example, suppose that, somewhere on one of an organism’s chromosomes, the DNA sequence T-A-C-C-T-G-A-A-A-A-C-T happened to occur, and suppose we wanted to figure out what kind of a protein that code would make. The first thing we would need to do is group those nucleic acids into groups of 3 to see what the codons are, so that would become TAC-CTG-AAA-ACT.

It turns out, the DNA does not directly code right to amino acids, but there is an intervening chemical called RNA (which is chemically similar to DNA). One of the “rules” is that, in DNA, A and T always pair with each other, and C and G always pair with each other. However, the RNA doesn’t have thymine, but rather, another, slightly different nucleic acid called uracil (symbolized by U), so wherever there’s an A on the DNA, that will pair with a U on the RNA.

Thus, if the DNA code is
TAC-CTG-AAA-ACT
That will correspond to an RNA code of
AUG-GAC-UUU-UGA

By now, geneticists have figured out what codons code for what amino acids. On the Biol. 104 DNA page (link below) is a copy of a chart (edited from a similar one in their textbook) that allows us to “look up” that RNA code to find the corresponding amino acids.

Thus, if the RNA code is
AUG——GAC——UUU——UGA
That will correspond to the amino acid sequence
start-aspartic acid-phenylalanine-stop

Of course, once biologists “cracked the code,” then they wanted to manipulate and change it, and now, it’s up to you as consumers and voters to decide, morally, ethically, is genetic engineering a “good” or a “bad” thing? Thus, it’s really important, even though you are not biology majors, that you read and learn as much about this as you can!

Oh, in case you’re interested, in many of the cases mentioned above, “they” is Monsanto (the company that also brings you Saccharin, Aspartame, Roundup, PCBs, Agent Orange, and Bovine Growth Hormone [rBGH] in your milk, as well as genetically-engineered corn, cotton, canola, soy, tomatoes, and strawberries, just to mention a few). If you have the time, go to their Web site and read how they pat themselves on the back for all the “good” things they’re doing. However, if you’d like the real story, do a Google search and see what everyone else is saying.


Background Information

Links to Related Information on Our Web Server

The following Web pages contain information related to DNA and genetic engineering.

Bio Lecture DNA
Information on DNA structure and replication, and genetic engineering
Dr. Fankhauser’s Genetic Engineering Web Page
Information on the possible dangers associated with genetic engineering

Your Assignment
Genetics “Practice”

There will be only one, combined assignment for this week’s topics (mitosis and meiosis, genetics, and DNA). Thus even though this will appear on each of those three pages to remind you, you only need to do it once. Genetics is one of those things that just needs lots of practice to “get it.” The grading criteria for this assignment are given below, and you should also refer to those as you work on the assignment. A total of 32 points is possible.

  1. Mitosis
    1. Read through the Biol. Lecture Web pages on mitosis and meiosis to become familiar with those processes, how they are the same, and how they differ.
    2. Find/collect a group of “similar” but distinguishable objects such as coins, pieces of string or yarn, socks, or whatever is handy. These will be used to represent the chromosomes in the nucleus of a cell. Also, obtain several, longer pieces of string or yarn to represent cell and nuclear membranes.
    3. Make a cell. Use a longish piece of string to make a circle to represent the cell membrane. Use a shorter piece to make a smaller circle inside to represent the nuclear envelope. This organism will have 6 chromosomes (3 from the father and 3 from the mother). For this you will need 3 pairs of something; for example a pair each of black, red, and blue socks or a pair each of pennies, nickles, and dimes (or whatever is handy that will suit the purpose). Put these 6 “chromosomes” into the nucleus of the cell.
    4. Just before mitosis happens, the chromosomes replicate, but the halves (called sister chromatids) stay attached. Simulate this by stacking 6 more identical objects (well, come as close as you can...) on top of the existing 6 “chromosomes”. For example, stack another penny on top of each of the two existing pennies, another nickle on top of each of the two existing nickles, etc.
    5. In prophase of mitosis, one thing that happens is that the nuclear envelope disintegrates. To demonstrate this, remove the string that’s the nuclear envelope and set it aside.
    6. In metaphase, all the chromosomes line up along the “equator” of the cell. Line up your 6 “chromosomes” (each with its partner still on top) in a row (single-file) across the middle (“equator”) of your cell.
    7. In anaphase, the halves of the chromosomes separate and travel to opposite poles of the cell. For each of your 6 “chromosomes,” now is the time to separate the partners. For each of the 6 stacks of 2, move one of the two items to the “north pole” of the cell and one to the “south pole” of the cell. When you’re done, each pole should have a collection of 6 objects/“chromosomes” identical to the 6 with which you began.
    8. In telophase, the nuclear envelopes re-form and the cell divides into two. First, find a piece of string with which to form a circle around each of the two groups of “chromosomes” to show the nuclear envelope re-forming. Then, near the “equator” of the cell, pinch/poke/move the string that represents the cell membrane in toward the center until the cell is divided into two. Optionally, you could replace that one string with two separate ones to remind yourself that you now have two separate cells.
    9. Congratulations! You have done mitosis.
  2. Meiosis
    1. OK, now try meiosis... Make another cell just like the previous one. Give it a cell membrane and nuclear envelope, again, as well as the same 6 chromosomes.
    2. As above, just before meiosis happens, the chromosomes replicate, as they do in mitosis, so add the matching halves back on top, again.
    3. In prophase I of mitosis, one thing that happens is that the nuclear envelope disintegrates. To demonstrate this, remove the string that’s the nuclear envelope and set it aside. Something else, very important, happens during prophase I: the chromosomes pair up. Move your “chromosomes” around so that the matching ones are next to each other. For example, put the two stacks of pennies (or the two stacks of black socks) next to each other, the two stacks of nickles next to each other, etc.
    4. In metaphase I, the chromosomes line up along the “equator” of the cell, again, but this time still in their pairs. Line up your 3 pairs of “chromosomes” (each with its partner still on top) in a row (double-file, side-by-side) across the middle (“equator”) of your cell.
    5. In anaphase I, the pairs of chromosomes separate and travel to opposite poles of the cell. For each of your 3 pairs of “chromosomes,” now is the time to separate the pairs. Keeping the partner halves still stacked together, move one whole stack from each of the 3 pairs to the “north pole” of the cell and one to the “south pole” of the cell. When you’re done, each pole should have 3 stacks of objects/“chromosomes,” one of each of the kinds with which you began.
    6. In telophase I, as before the nuclear envelopes re-form and the cell divides into two. Similar to what you did above, re-form the nuclear envelopes and divide the cell into two. When you have the cell membrane all the way “divided,” go ahead and substitute two pieces of string for the one, to represent the fact that you now have two, separate cells.
    7. This time, however, you’re not done yet. There is another cell division yet to go. Once again, in prophase II, the nuclear envelopes disintegrate, so remove those from both of the cells.
    8. In metaphase II, the chromosomes line up along the “equator” of the cell, in single-file, similar to what happened in mitosis. In each of your cells, once again, line the 3 “chromosomes” up, single-file, along the equator of that cell.
    9. In anaphase II, you finally get to separate your stacks of “sister chromatids.” From each of your stacks, move one partner to the “north pole” and one to the “south pole” of that cell. Between the two cells, you should now have a total of 4 groups of 3 items.
    10. In telophase II, once again, the nuclear envelopes re-form. You’ll now need 4 pieces of string so you can make a circle around each of the new nuclei. Also, in each cell, once again, pinch in the middle to form 2 cells out of each one, then (optionally) replace each of those cell membranes with 2 separate cell membranes (strings) for each of the new daughter cells. When you are done, you should end up with 4 daughter cells, each with 3 chromosomes.
    11. Congratulations! You have done meiosis and you now have 4 eggs or sperm.
  3. Fertilization
    1. Don’t get rid of your eggs/sperm just yet! To do this the “official” way, you could go through the whole process of meiosis with a different “parent” so that you end up with 4 sperm from one parent and 4 eggs from the other parent. However, to simplify things, from the 4 you have sitting there, now, pick one to be an “egg” and one to be a “sperm,” and think of them as having come from different parents.
    2. If necessary (if they're a distance apart), the sperm will have to “swim” over to where the egg is, until they are touching. That might be easier to do if you slide the nucleus and chromosomes onto a sheet of paper so you can move it as one unit. To make the next part easier, you might want to reposition the cell membranes of the egg and the sperm cells so the loose ends of the strings meet where the egg and sperm are touching.
    3. Now, the whole sperm nucleus (just the nucleus, not the whole cell) has to go inside the egg cell, leaving its cytoplasm and cell membrane behind. If you previously placed the nucleus on a sheet of paper, just slide the whole thing over, into the egg cell, and as close to the egg nucleus as you can get it. When that step is complete, both nuclei should be within the egg cell, and the egg cell membrane should be “closed” all around (the hole where the sperm nucleus entered closes up).
    4. Then, the sperm and egg nuclei unite, so put all of the chromosomes from both into one nucleus (and set aside the spare string). How do the number and types of chromosomes compare with what you started with before meiosis?
    5. Congratulations! You have just conceived a baby!
  4. Think about and summarize How are mitosis and meiosis similar, and how are they different? Where in a person’s body does mitosis happen, and where does meiosis happen? In what way(s) are meiosis and fertilization the “opposite” of each other?
  5. Genetics
    1. Genes are located on chromosomes. Thus, as the chromosomes move around in meiosis and segregate into the daughter cells, they carry with them all of the chromosomes located on them. For example, in the meiosis demonstration you just did, suppose the first set of “chromosomes” (the pennies?) contained the gene for eye color. If the individual was heterozygous for eye color, one chromosome would carry a B allele for “brown”, and the other chromosome would carry a b allele for “blue”. When the chromosomes replicate, they make an exact copy of themselves, so the coins/socks/yarn you stacked on top of each other would carry the same allele as each other. Suppose the nickles (the second pair of “chromosomes”) carry a gene for tongue-rolling, but if the individual is heterozygous there, too, one nickle would have an R allele (for “rolling”) while the other one would have an r allele (for “non-rolling”). Suppose the third pair of “chromosomes” (the dimes?) contain a gene for ability to taste a certain kind of test paper called PTC paper, and suppose, again, that the individual is heterozygous for this gene, too. Then one dime “chromosome” would carry a T allele (for “taster”) and the other would carry a t allele (for “non-taster”).
    2. If it helps you to visualize what’s going on, here, set up the meiosis demonstration, again, but this time, go ahead and label the “chromosomes” with the appropriate genes they contain. When you get to metaphase I and you’re lining up the pairs of chromosomes in the center of the cell, don’t be concerned whether all of the B, R, and T alleles are on the same “side” or not, because in “real life” it’s pretty much a 50:50 chance for each pair which one will line up on the “north” side and which will line up on the “south” side. This means that when the chromosomes do their first division in anaphase I, it’s a 50:50 chance for each pair which one will wind up at whichever pole of the cell. Thus, for example, if you end up with BrT at the “north pole,” that’s just one possible example of what might happen in real life.
    3. Understanding how genetic crosses work is best accomplished by working practice problems and Punnett squares. Spend time working with the Genetics Practice Problems Web page until you feel comfortable working these kinds of problems.
    4. When you submit your work for this assignment, the data-submission Web page will automatically generate several genetics problems which you will be asked to work out.
  6. DNA Do a Web search to find out more information on one kind of genetically-modified organism (GMO) and summarize what you found out. Terms for which to search might include “GMO”, “genetic engineering”, “genetically-modified”, “frankenfood”, “monoclonal antibody”, “rituxan”, “rituximab”, “epratuzumab”, “galiximab”, “campathin”, “alemtuzumab”, and/or “roundup-ready”, etc. Make sure you tell what organism was modified, what gene(s) were inserted, where those genes came from (what other organism), and the reason why somebody thought it would be a good idea to do that. What are some of the advantages or benefits that proponents claim will result from this, and what are some of the disadvantages or problems that opponents claim will come as a result of this. Based on what you’ve found out, would you say that, ethically/morally, this is a “good” thing or a ”bad” thing? Give scientific reasons to justify your point-of-view.
  7. At this point, if you are a registered student, you should submit your work.

Grading Criteria

1.   Mitosis and Meiosis:
2 — The student clearly demonstrated that (s)he knows the difference between mitosis and meiosis
1 — The differences between mitosis and meiosis were included and was at least partially correct
0 — Mitosis and meiosis were incorrectly distinguished from each other or the distinction between the two was not included
2 — The student, obviously, went beyond the minimum requirements of the assignment
1 — The student adequately completed the assignment
0 — The student completed considerably less of the assignment than what was required
2.   Genetics Problems (for each problem × 3 problems):
2 — The male gametes (sperm) were correctly specified (genotypes) and placed
1 — The genotypes and/or placement of the sperm were partially incorrect
0 — The genotypes and/or placement of the sperm were wrong or missing
2 — The female gametes (eggs) were correctly specified (genotypes) and placed
1 — The genotypes and/or placement of the eggs were partially incorrect
0 — The genotypes and/or placement of the eggs were wrong or missing
2 — The genotypes of the offspring were correct
1 — The genotypes of the offspring were partially incorrect
0 — The genotypes of the offspring were wrong or missing
3.   Genetically-Modified Organism:
2 — Thorough/complete information on the chosen GMO was included
1 — Adequate information on the chosen GMO was included and was at least partially correct
0 — Information on the chosen GMO was too sketchy or absent or was incorrect
2 — An ethical point-of-view was included and was backed up by thoroughly-researched facts
1 — The student’s point-of-view was included, but was backed up only by personal opinions/beliefs and/or partially incorrect or skimpy facts
0 — The student did not include his/her ethical point-of-view, or there was no justification given for how/why that opinion was reached
2 — The student, obviously, went beyond the minimum requirements of the assignment
1 — The student adequately completed the assignment
0 — The student completed considerably less of the assignment than what was required
4.   Overall:
2 — The grammar, English usage, punctuation, and spelling were very good
1 — The grammar, etc. were OK
0 — The grammar, etc. were poor
2 — It is evident that the student used much insight, thoughtfulness, and critical thinking when completing this assignment
1 — The student adequately thought about the assignment – there was, perhaps, a bit of “fuzzy thinking” in a couple places
0 — The assignment gives the appearance of being “slapped together” just to get it done, with little evidence of thoughtfulness
Total Possible:
32 — total points

Copyright © 2006 by J. Stein Carter. All rights reserved.
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