March 18, 2004
A: FROM MENTOR MARTY CHINTALA IN RI
Sarah, cell biology is not easy for many people, and mitosis and meiosis
can be confusing. Chromosomes are duplicated only once in either
mitosis or meiosis (during interphase). Meiosis reduces the genetic
material in a cell to half of the parent cell, because it produces
gametes. The reason the cells need half of their genetic material is
because sperm and egg cells join together during fertilization, and the
resulting zygote then ends up with the proper amount of genetic material
(half from each parent). Mitosis produces cells identical to the parent
cell. Mitosis and meiosis use similar cell machinery and the steps are
basically similar, but there are some differences. Meiosis takes place
in two stages (I and II, with the steps unique to meiosis occurring in
meiosis I) with two successive divisions that produce 4 nuclei instead
of 2 (mitosis produces 2). Mitosis produces 2 cells that are identical
to each other as well as the parent cell, while meiosis produces 4
nuclei that are not necessarily similar and have half of the genetic
material of the parent cell. One way I always remembered how the
processes were different was that during mitosis the duplicated
chromosomes (the things that look like Xs in the diagrams) line up
single file prior to separating and producing the daughter cells (each
cell getting one half of the X). During meiosis, the process is
slightly different, with two Xs side by side above one another. The
first stage separates the paired Xs into 2 different cells, while the
second stage is similar to mitosis in that the Xs line up and half of
the X goes into each cell. I have added some links of sites that might
help you with the whole process- check them out. I find it is easier to
visualize the processes rather than just read about them. Good luck,
hopefully I haven't confused you more, and let me know if I can clarify
anything further.

http://www.guilford.k12.ct.us/~faitschb/sem1.html
http://serendip.brynmawr.edu/sci_edu/waldron/mitosis.html
http://www.pbs.org/wgbh/nova/baby/divide.html
http://biology.clc.uc.edu/courses/bio104/mitosis.htm
http://biology.clc.uc.edu/courses/bio104/meiosis.htm
********************
A: FROM MENTOR MARSHA SEGEBARTH IN INDIANA
Sarah, While both meiosis and mitosis involve cellular reproduction, as in a parent cell producing offspring, only meiosis occurs as part of sexual reproduction (involving the sperm and egg). Mitosis is a type of cell division in which daughter cells receive the exact chromosomal and genetic makeup of the parent cell. It occurs during growth and repair of the cells you normally think of - skin or epidermal cells, muscle cells, etc. There are stages involved in this. The first is prophase in which the nuclear envelope disappears and the chromatin arranges itself into chromosomes. The next stage or phase is metaphase in which the chromosomes line up at the equator. The next phase is anaphase in which the chromosomes (remember, chromosomes contain the genetic information) pull apart and go to opposite ends of the cell. The next phase is telophase during which the nuclear envelope reappears, the cytoplasmic organelles are distributed fairly evenly on both sides of the cell and cytokinesis takes place. Cytokinesis also involves a cleavage furrow forming which essentially pinches the larger parent cell in to two smaller daughter cells. The next phase - and the one which takes up most of the life cycle of the cell - is called interphase. During this phase the cell grows and the components of the cell are reproduced in preparation for starting the whole cycle again. Visualize this entire process as a circle. That is Mitosis. Meiosis involves the first 4 phases, but then it repeats so that by the time you end at Telophase II you have only 1/2 the genetic information in the daughter cells. That is why the sperm and the egg (the only ones that go through meiosis) have only half the genetic information as other cells. Therefore, when they unite, the fertilized egg has the proper amount of genetic information with 1/2 coming from the sperm and 1/2 from the egg. Does this help? You might want to look up the phases of mitosis in a biology book. Also, remember that meiosis only occurs with the sex cells.
*******************
A: FROM MENTOR LESLIE WAITE IN CA
Hi Sarah!
Ah yes, cell division. Life is complex... Mitosis and meiosis are
both ways that cells (or parts of cells) make copies of themselves so
that they can split into two equal cells. Mitosis refers to the
copying of the chromosomes in the nucleus so that there are now two
copies of each chromosome. The nucleus then splits into two nuclei,
each with their own set of chromosomes. Mitosis is followed by
something called cytokinesis, which is where a cell that has
undergone mitosis (and therefore has two nuclei) splits into two
cells, each with one nuclei.

Meiosis is the process that allows a special type of cell, called a
germ cell, to divide into FOUR new cells, each of which have HALF the
normal amount of chromosomes. Creatures that use sexual reproduction
to make their babies have cells that undergo meiosis.

As far as your question of how mitosis can make a copy of itself, I
am not sure what you mean, but I will talk about how DNA copies are
made. The cell is full of little manufacturing machines called
enzymes. Virtually everything that gets done in a cell gets done
because an enzyme did it. Enzymes are proteins, and each one has a
different job in the cell. In the nucleus of a cell, there are
proteins called polymerases, whose job it is to copy DNA. They take
one DNA molecule and make it into two identical copies in the
nucleus. Similarly, the proteins and other cell building blocks that
a cell needs also get made during the cell cycle so that when the
time comes, the nucleus can split into two nuclei, each with all the
bits that they need to carry out all the activities that nuclei need
to do. Once two nuclei are made, the cell itself splits into two
cells, each taking one nucleus with it, along with the other things a
cell needs to be a cell.

In Meiosis, there is also a copying of DNA that happens, and a
splitting of the nucleus into two nuclei, and the cell into two
cells. BUT in Meiosis, there is a second division that reduces the
number of chromosomes by half. How? Well, each and every cell in your
body (except eggs) has two sets of chromosomes. One set came from
your biological mother, and one set came from your biological father.
So even though there are 23 types of chromosomes, each type has a
"mom" chromosome and a "dad" chromosome. in the first meiotic cell
division, copies are made of both types of chromosome, and the two
daughter cells each inherit both chromosomes. But in the second
meiotic cell division, there is no copying of the DNA. Each daughter
cell from this cell division only inherits one of the two possible
chromosomes. you wind up with only one set of chromosomes in each of
the four final daughter cells.

Hope this overview is helpful, and can get you on the path to
understanding your lectures and textbook. Write back with more
questions if you still can't make sense of things.

Good luck!
Leslie