Expert: Dana Krempels, Ph.D. - 5/15/2007

Question
QUESTION: How many genes undergo DNA replication before mitosis?
ANSWER: Dear Julie,

When a cell prepares for mitosis, *all* the DNA in the nucleus is replicated.  Depending on what species you're talking about, that could be anywhere from about 1000 genes (in a bacterium) to about 30,000-60,000 for a human (the jury's still out on how many genes are present in a typical mammal) to 400,000 (in the case of some plants, such as ferns).

The easiest answer to your question is:  ALL of them.  :)

Hope that helps.

Dana

---------- FOLLOW-UP ----------

QUESTION: How many of the two attached chromosomes go to each end of the cell during anaphase of mitosis?
How many of the two attached chromosomes go to each end of the cell during the first cell division of meiosis?


Answer
Juuuulieeee....

Are you using me to answer your homework questions?  I need you to read your textbook and find out the answers on your own from this point, or you won't really be learning anything much.

Take a look at the diagram in your textbook under the mitosis section, or use the diagram here:

http://www.bio.miami.edu/dana/250/mitosis.jpg

You can watch a narrated movie of mitosis here:

http://www.bio.miami.edu/dana/mov/mitosis.mov

Look at the picture of anaphase.  Notice that there is a chromosome there that looks like an "X" in metaphase.  That is actually two identical (newly copied) chromosomes stuck together at the centromere.

At anaphase, one of each of these goes to each new cell.  (That is, if things go normally.  Sometimes there's a problem, and one new cell gets two copies of the chromosome, and the other gets none!  This can cause both cells to die, or have serious problems.)

The first cell division of *meiosis* is the reduction division.  Look at the diagram of meiosis and compare the configuration of the metaphase chromosome in mitosis and meiosis:

http://www.bio.miami.edu/dana/250/meiosis.jpg

and even better, a movie on meiosis here:

http://www.bio.miami.edu/dana/mov/meiosis.mov

You'll see that at the first meiotic division, TWO sister chromatids (replicated chromosomes) travel together to each new cell.  At the second division, ONE member of each of the sister chromatic pairs goes to each new cell. This means at the end of meiosis, every gamete has HALF the DNA of the cell that originally divided to produce it.

I hope that helps you understand mitosis and meiosis a little better.  Now you can ask me questions ONLY when you're really stuck.  You have to do the rest of your homework on your own, by reading the book and studying the diagrams.  :)

Good luck!

Dana