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Cell Division, Contact Inhibition and Cancer

imagesIn my general biology class we are now reading chapter 5 of Mader et al, on Cell Division. This chapter is a bridge between those chapters on descriptive cell biology and those describing the activities of the cell and how we explain the inheritance of traits from one generation to another.

We focused our attention on Mitosis and Meiosis of diploid Eukaryotic cells and followed how these two types of nuclear division manage the sorting of genetic material into daughter cells ensuring that each cell gets an appropriate set of instructions for life.

The body is comprised of somatic cells that include everything from skin to muscle and nerve cells. These are called somatic because soma comes from the greek word for body.

The other type of cell is the gametic cell, referring to germ-line, or sex, cells.

Each of these type of cells goes through its own type of division in order to end up with the correct amount of genetic material ( or ‘ploidy’) in the resulting cell or organism. Ploidy refers to the number of complete sets of genetic material a cell has. We, humans, are diploid organisms having two sets of genetic material in each cell.

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Mitosis in Diploid Cells

Somatic cells undergo mitosis in a way that maintains the diploid state of cells creating two exact replicas of the parent cell.

This mechanism makes sense, because one skin cell might replace a neighboring skin cell following its demise in order to maintain a confluent layer. We would expect every skin cell to be genetically identical to every other and mitosis delivers just that.

However, if we imagine sex cells that were made from mitosis, these would also be diploid (2n). Then a diploid sperm would fuse with a diploid egg and make a tetraploid offspring. Then that organism would have octaploid offspring and so on. This, of course, does not happen.

ImageSex cells are instead produced by a different kind of division called meiosis. Meiosis is merely a specialized form of mitosis in which the genetic material (ploidy) is ‘halved’. The resulting cells are then haploid (1n or n). As part of the specialization, meiosis occurs in two steps so instead of producing two cells, it produces four (at least theoretically). Also, instead of being identical, each of the resulting sex cells is unique.

But this discussion is supposed to be about cancer, so let’s ignore meiosis for now. I’ve discussed cancer before here, but I just found a couple of good animations that I wanted to include.

The first is an excellent animation on cell cycle and contact inhibition. See how cancer is defined here as the lack of respect for cell-cell signaling, that would otherwise result in a healthy monolayer of cells.

The second, discusses how cancer cells would need to alter their environment in order to get the nutrients they need to survive. It can be tempting to think that cancer cells don’t need these things, but they certainly do.

http://bcove.me/uc5vydod

Once cells mutate in a way that initiates cancer, the constant struggle with the immune system amounts to a selective pressure allowing only the strongest cells to survive. 😦 A brief article describing this battle can be found on the HHMI website. A more thorough treatment of the subject can be found in a freely available review by my friend Dr. Ezekiel Fuentes-Panana.

Here’s one last animation showing a tumor mass producing metastatic cells that leave the mass and migrate to new locations within the body. I’m not that fond of this video, but it does communicate the message I wanted to get across.

Alltogether, cancer cells are those that no longer obey the rules of polite cellular society and continue to reproduce through unchecked mitosis when such division is not in the best interest of the organism as a whole. One way these cells do this is to cease responding to contact inhibition signals. This results in the production of a tumor mass that will need to obtain energy and will often do so by sending out pro-angiogenic signals resulting in new blood vessel formation. As the tumor continues to grow, it may invade neighboring tissue and ultimately even metastasize into the blood or lymph leading to a number of secondary tumors throughout the body.

 
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Posted by on October 14, 2013 in Uncategorized

 

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Jeopardy, coffee and cookies

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Gameboard for tomorrow’s In-class Jeopardy

Now I’ve done it.

I often feel like saying things out loud is a curse against me. I’m expecting to get up early and make some fresh coffee and cookies for tomorrow’s Micro Exam #1 Review.  But, with the curse and all, I do worry…

The one thing I do have going in my favor is a longing for chocolate chip cookies. For some reason I have not been baking as much lately, but I’ve been trying to get back in the habit. The last two weekends I have made bagels though, so perhaps it will happen.

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Bagel Recipe adapted from Laurel’s Kitchen Bread Book

So, tomorrow, after two cancelled classes due to inclement weather:

Micro: Open review of all material, discussion of the next chapter in Vaccinated (hmmm, I’ve lost track of where we are…), quiz on the last chapter of this unit(Ecology, growth and nutrients) and then potential exam question jeopardy.

General Bio: Continue / finish  cell division and cancer (both covered in chapter 5 of the text), then discussion of the latest chapter of Your Inner Fish (again, I’m at a loss. The chapter after teeth…).

I’d get up and check these things, but I am trapped under a large, sleeping cat who is known to be ferocious when disturbed, so I’d rather keep my hands.

 
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Posted by on February 27, 2013 in Uncategorized

 

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An overview of Cell Cycling in Humans

Consider Leonardo here, who is going to be making sperm cells via meiosis. In this figure we’re tracking n number, chromosome number and c number with every division. Note that because we are following human cells – the n number always remains 23. However, the ‘ploidy’ will change between haploid(n) and diploid(2n) and c number will change between 1,2 or 4 depending on ploidy and whether the chromosomes have one or two chromatids.

Once sex cells(sperm) are formed, we see the combination of sperm and egg during fertilization to form a zygote which restores the diploid condition. Then this cell goes into cycle many times over going through mitosis again and again as the organism grown from zygote into a full grown person.

 
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Posted by on October 11, 2012 in Uncategorized

 

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Aside

Cell Division is an essential part of life. However, this means different things to different cells. For single-celled organisms cell division is reproduction – one mother cell divides into two daughter cells. If these single-celled organisms are prokaryotic, then this division happens by the relatively simple process of binary fission. If the organism is eukaryotic, division occurs by a more structured process of mitosis.

Regardless of the differences between these organisms, cell division consists of three core components:

  1. Duplication of DNA
  2. Segregation of DNA (into areas that will become the new cells)
  3. Cytokinesis

In Prokaryotes, this describes the cellular events perfectly. The circular DNA (arguably a chromosome) is duplicated, the two copies of the DNA goes to opposite sides of the ce

ll and cytokinesis divides the cell in half along the midline.

An illustration of a number of bacterial cells undergoing binary fission:

The entire process is outlined below:

In eukaryotes, the same basic process occurs, but because of the differences in how DNA is organized and localized (in a nucleus), differences emerge.

The basic form of eukaryotic cell division is called mitosis. In single celled organisms this process functions as an asexual form of replication, while in multicellular eukaryotes, this form of division serves to add to the cell number (i.e. growth / healing).

Mitosis is actually the division of the nucleus and DNA, this process is accompanied by the division of the cell itself, cytokinesis.

There are four phases of mitosis:

  1. Prophase
  2. Metaphase
  3. Anaphase
  4. Telophase
  5. Cytokinesis overlaps somewhat with Telophase, but continues on afterwards to complete cell division.

Each stage of Mitosis is defined by a number of events describing the organization of the DNA, the state of the nuclear membrane and the development of mitotic spindles that maneuver the chromosomes within the cell. I won’t describe these here as this can be found in any number of other sources.

What I will discuss is the pattern of chromosome number, DNA copy number and the chromosome number.

The chromosome number is referred to as ‘n’.

The DNA copy number is referred to as ‘c’.

 Humans have an n number of 23. What this number tells you is how many pairs of homologous chromosomes an organism has.* This immediately begs the question, ‘what’s a homologous chromosome?”

Homologous chromosomes are those that have the same genes – that is, they code for the same traits even if they do not have exactly the same versions of these genes on each chromosome. For each of the 23 different kinds of chromosomes humans have, each person gets one copy from their mom and one from their dad. So these 23 pairs of chromosomes are 23 pairs of homologous chromosomes.

This 23 number we keep talking about is the n number in humans.  Each species n number may be different, but all the members of that species have the same n number. Further, the n number does not change at any time during the cell’s life

Because we have one chromosome from each of our parents in every one of the cells of our body we are diploid organisms (from the Latin, di – meaning and two ploid – well, ‘ploid’ doesn’t really come from Latin)

All the cells in our body are diploid – except the gametes (sex cells). These cells are formed by a special kind of cell division called meiosis. This type of division is very similar to mitosis, except that it consists of two separate rounds of division and the resulting sex cells have only one of each chromosome type and are therefore called haploid (think ‘half’)

Cells of the body are 2n = diploid.

Sex cells are n = haploid.

This leaves the c number…

The easiest way to determine the c number is to simply count the number of chromatids for each type of chromosome.  Consider the cells below. On the left is a cell in G1 phase of the cell cycle. Then the cell goes through S phase where DNA replication occurs and looks like the cell on the right.

Note that the n number does not change, but the c number doubles when the number of chromatids doubles.

* It’s important to note here that not all organisms have similar patterns of chromosome arrangements as we humans do. In this essay I am referring to humans and organisms that handle their DNA as we do.

Cell Division i…

 
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Posted by on October 10, 2012 in Uncategorized

 

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Boy, could I use a beer

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Gates to the brewery in Plzen, Czech Republic

And if you were in my intro to Biology class today, you are either thinking the same thing or having one as I write this.

Today was chapter 5 of Mader et al: Cell Division. This is the first chapter that I really enjoy teaching. Cell division is at the heart of life, inheritance and evolution and what we will be talking about to a greater or lesser degree for the remainder of the semester.

But sometimes things don’t come out the way I envision. Today was one of those days when nothing came out the way I envisioned.

I’ll write about cell division later. Perhaps after I have that beer.

 

 
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Posted by on October 9, 2012 in Uncategorized

 

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