Sunday, June 2, 2013

Science Sunday: Cell Division—Mitosis

Posted by: Rebekah

“I will praise thee; for I am fearfully and wonderfully made: marvellous are thy works; and that my soul knoweth right well.”
                                                                        Psalm 139:14

There is no doubt that living cells are indeed a masterwork. God designed each cell so that it could duplicate itself; hence, cells are able to constantly replenish their numbers.

But how do cells accomplish such a feat?

Well, eukaryotic cells all have two important phases in their cycles—interphase and the mitotic phase (a.k.a. M phase).

Interphase is further divided into shorter sub phases called G 1, S, and G2, all of which are characterized by the production of proteins and of cytoplasmic organelles (e.g. mitochondria and endoplasmic reticulum); thus, the cell grows in all three sub phases. It is important to note, however, that DNA synthesis (the duplication of chromosomes) occurs only in the S phase (Reece 277).  

But what about the mitotic phase? Well, this is where mitosis comes in.

The mitotic (M) phase is divided into two parts: mitosis and cytokinesis. Mitosis is then further divided into prophase, prometaphase, metaphase, anaphase, and telophase. For simplicity’s sake, however, I have chosen to integrate prometaphase into prophase (as seems to be a common custom).

A few months ago, I found this real nice webpage, and I thought it would be a nice idea to share these pictures of whitefish blastula with everyone: they perfectly illustrate the different stages of mitosis.

1)    Prophase
Chromosomes become condensed; each chromosome consists of two sister chromatids joined at their centromeres (click here for a picture); each chromatid develops a kinetochore at its centromere (a kinetochore is a specialized protein structure); a mitotic spindle forms, which consists of two centrosomes and of microtubules which extend from each centrosome (these microtubules act as ropes that eventually move chromosomes around) (Reece 278).


Do you see the condensed chromosomes?

2)    Metaphase
At the beginning of metaphase, centrosomes (please don’t confuse centrosome for centromere) are found at opposite ends of the cell; chromosomes align at the metaphase plate (please note that the metaphase plate is more of a conceptual idea, and is not actually a cellular structure); microtubules at opposite ends of the cell attach to sister chromatid kinetochores (these specific microtubules are called kinetochore microtubules) (Reece 279-280).


The alignment of chromosomes is seen very well in these pictures.

3)    Anaphase
Each sister chromatid pair parts suddenly; each separated sister chromatid is now called a daughter chromosome; as kinetochore microtubules shorten, daughter chromosomes move toward opposite ends of the cell; as nonkinetochore microtubules lengthen, the cell elongates; each opposite end of the cell now has the same amount of identical chromosomes (Reece 279).

Can you see the chromosomes moving apart?

4)    Telophase
Two daughter nuclei form; nuclear envelopes form; nucleoli reappear; chromosomes become less condensed; remaining microtubules are depolymerized (Reece 279).

You can now see that the end result will eventually be two identical daughter cells.  

Cytokinesis (the final part of the mitotic stage) is the division of the cytoplasm. It is characterized by a cleavage furrow in animal and human cells, and by a cell plate in plant cells; thus, separating the two daughter cells (Reece 279-282).

Well, that's about it for now. I hope you enjoyed!



                                              Picture Credits

All pictures were obtained from Professor Ken Marr’s web page at Green River Community College. Here is the link.

                                                Works Cited

Reece, Jane B., et al. Campbell Biology. 9th Global ed. “The Mitotic Phase Alternates with Interphase in the Cell Cycle.” Boston: Pearson, 2011. 277-282. Print.

WARNING: Due to several reasons, I do NOT recommend Campbell Biology for your
homeschool. However, due largely impart to its prevalent use in colleges and universities (and even Wikipedia), I chose to use it as my primary source.