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Mitosis Animation

I found this excellent animation of mitosis today and wanted to make it available here for anyone who wants to clarify their understanding.

 
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Posted by on October 15, 2013 in Uncategorized

 

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A look into cell division

ImageIt’s that time in the general biology semester where we transfer our attention to cell division.  Having already discussed a number of basic principles like the laws of thermodynamics and a touch of chemistry, and cellular functions such as the flow of energy and the flow of information, it’s now time to look at how cells reproduce themselves.

In this chapter we should be recalling all the parts of the cell and accounting for how they get sorted into the developing ‘daughter cells’, and also recall the role of information, in the form of DNA, and how this is apportioned into the daughter. Of course we will spend most of our time focusing on the distribution of DNA, but we should always keep in mind what we know of other structures and organelles.

I previously wrote an essay describing cell division in humans that marries this information with the subject of the next unit, genetics and inheritance. You can find that text here. Therein, I briefly address one of the oddities of eukaryotic cells, the mitochondria. Mitochondria are odd because they live in our cells as strange symbiotes that share their energy with us in exchange for protection and a supply of nutrients. The theory describing this relationship was proposed by Lynn Margulis, and is widely accepted today. A description of her theory can be found here.

Because Mitochondria (and chloroplasts) are pseudo-autonomous cells, they must replicate themselves. A cartoon and some micrographs that illustrate this process have been borrowed from Nature Reviews.

ImageThe process involves an interaction with the Endoplasmic Reticulum, that guides an assembly of molecules that constrict around the Mitochondria eventually effecting its division into to smaller organelles. What this image does not include is the replication and separation of the mitochondria’s own circular DNA, a process that necessarily precedes the actual division of the organelle.

Altogether, there’s a lot to keep in mind when examining cell division. Why is this cell dividing? How are the instructions for life (DNA) being distributed between daughter cells? What does the daughter cell need in order to survive on its own? How do these parts / organelles handle their own division between the cells? And what would happen if any of this went wrong along the way?

 
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Posted by on October 7, 2013 in Education, Uncategorized

 

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Another link with Cell Cycle Information

Here’s another link to resources discussing cell cycle (provided by my student Joe):

http://www.biology.arizona.edu/cell_bio/tutorials/cell_cycle/main.html

 
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Posted by on October 17, 2012 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

Image

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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