here
were people of all ages and sizes playing volleyball together.
Thus some people’s bodies had already grown a lot, while others were still
in the process of growing. Some people’s bodies were mature
enough to produce sperm or eggs, and from those, new babies. Each baby
begins as one cell, formed by the union of an egg and a sperm, and from that, over
the span of many years, an adult-sized person, comprised of many cells,
is formed. All along the way, many new
cells are formed to repair damage or replace those which have worn off.
Mitosis and Meiosis
Each cell in a person’s body contains an identical copy of
that person’s DNA, the genetic code material that is the blueprint/instructions
for growing and maintaining that person’s body. Thus, as new cells are
formed, each one must also contain one, exact copy of those same instructions.
DNA is a long, stringy molecule made up of various combinations and
repetitions of four possible subunits, sort-of like a four-letter alphabet
from which all kinds of words of various lengths can be made. Depending on
the order in which those four “letters” (the four possible subunits) occur,
depending on what “word” they “spell,” will dictate, will code for, specific
amino acids in a specific sequence, and therefore a particular protein
molecule (so, a long-enough strand of DNA to code for one protein molecule
would be analogous to a “word”). A gene is that portion of a DNA
molecule which codes for one protein. However, just like we can put a bunch
of words together to make a sentence, similarly, genes for a variety of
proteins/traits can be grouped together (can follow one another) on a strand
of DNA. A whole strand of DNA, containing or comprised of genes for many
traits/proteins, is called a chromosome.
The nucleus of each cell in the body of a human contains 46
chromosomes. We get 23 of those from our father and 23 from our mother. The
chromosomes from our father match those from our mother, like 23 pairs of
socks, in that they contain genes for the same traits at the same locations
in the DNA sequence. For example, perhaps the third gene on chromosome #1
from our father and #1 from our mother both code for eye color (though, it is
possible that one might have instructions for “make blue eyes,” while the
other has instructions for “make brown eyes”). Thus, we can say that we have
two sets of chromosomes (where each “set” consists of 23 chromosomes)
or that we have 23 pairs of chromosomes.
Mitosis is the process of “regular” cell division by which one cell
divides into two cells, each of which is genetically identical to the original
cell. Mitosis is the process by which our bodies grow, as more and more cells
are made, and mitosis is also the process by which wounds heal, by which our
skin replaces worn off cells, and by which “worn-out” cells are, in general,
replaced by newer, younger cells.
To illustrate this a bit more simply, let’s invent a
fictitious organism which has 5 pairs of chromosomes (one set of 5 from the
father, one set of 5 from the mother). To distinguish among these, let’s
represent the five possible chromosomes by a star, a heart, a triangle, a
circle, and a square. Again, one set of five came from the father (the blue
ones) and one set of five came from the mother (the pink ones), but both the
mother and the father gave a “star,” a “heart,” etc. Just before mitosis
happens, as the cell is getting ready to divide, an exact copy of each
chromosome is made (the second step in the diagram), and initially, that
copy stays attached to the matching copy from which it was made. During the
process of mitosis (the third step in the diagram), one copy of each
chromosome is pulled to opposite sides of the cell. Then, finally, the cell
divides in half (the fourth step in the diagram), and each new cell gets one
complete copy of all the chromosomes. Notice that, at the end of the process,
each daughter cell is, genetically, an exact copy of the original
cell.
Meiosis is a special kind of cell division that is used to make gametes
(eggs or sperm, depending on the person’s sex). Note: women only make eggs
and men only make sperm. Gametes must contain only one
set of chromosomes, so that when a sperm fertilizes an egg, the condition
of having two sets will be restored. As an example of how this works, let’s
use a “cell” with the same “chromosomes” as
above.
Thus, while we’re starting with a total of ten chromosomes,
we must end up with only five, and those five must make up one set (pick one
from each pair). To accomplish this, as the chromosomes are making copies of
themselves, they also pair up in their matching pairs (like organizing your
sock drawer?). Just like you normally can’t tell a right-foot sock from a
left-foot sock, so also you really can’t tell the mother’s from the father’s
chromosome in a pair, and just like when you grab two matching socks and
fold them, together, it’s totally random which chromosome from each pair will
end up on whichever “side” (second and third steps in the diagram). Once the
chromosomes have replicated and lined up, that is followed by a series of
two cell divisions. In the first division, the matching copies of
each chromosome stay together, but the pairs are pulled apart, and
the cell divides to form two daughter cells (steps four and five in the
diagram), each of which has only one set of chromosomes.
In the second cell division (steps six and seven), the matching copies of
each chromosome separate, and when the cells divide, this results in a total
of four daughter cells.
Background Information
Links to Related Information on Our Web Server
The following Web pages contain information related to
the processes of mitosis and meiosis.
- Bio Lecture Mitosis
- Information on the process of cell division by which general body cells replicate
- Bio Lecture Meiosis
- Information on the process of cell division by which sex cells with half as many chromosomes are produced
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.
- Mitosis
- 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.
- 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.
- 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.
- 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.
- 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.
- 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.
- 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.
- 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.
- Congratulations! You have done mitosis.
- Meiosis
- 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.
- As above, just before meiosis happens, the chromosomes replicate,
as they do in mitosis, so add the matching halves back on top, again.
- 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.
- 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.
- 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.
- 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.
- 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.
- 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.
- 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.
- 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.
- Congratulations! You have done meiosis and you now have 4 eggs or
sperm.
- Fertilization
- 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.
- 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.
- 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).
- 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?
- Congratulations! You have just conceived a baby!
- 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?
- Genetics
- 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”).
- 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.
- 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.
- 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.
- 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.
- 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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times since 15 Nov 2006.