Since Sci wrote so recently about the preservation of somatic cells and gametes for species regulation, she thought it might be a good idea to run through some basic concepts. REALLY basic. Like the stuff you had in high school and forgot. That kind of basic. She was also inspired in this post by reading so recently about HeLa cells, and how they allowed scientists to make great strides in the deeper understanding of mitosis and the dysregulations that occur.
Why, you ask? Because basic is important, and because mitosis is PARTICULARLY important. Mitosis, when it happens, and how it happens is behind a lot of the things that concern many people today, things like aging and cancer, and who isn’t concerned about those? And also, mitosis means lots of pretty pictures!
Sci wanted to cover mitosis from the original guy who started it all, Walther Flemming (yes, it’s spelled ‘Walther’, though the English translation is Walter), but unfortunately all the best original documents are in German. Sci doesn’t have too much German (ok, she doesn’t have ANY), but if someone is willing to play translator for me (GrrlScientist? I know you’re working on your German! :)) I’d love to play around with the original work!
As it is, we’re just gonna go through it, with lots of pretty pictures and lots of pointing arrows. And a lot of explanations.
Your first picture: My-tosis
(Sci-s toses. Note the penguins and snowflakes in honor of this year’s Snowpocalypse)
So what is mitosis? Mitosis comes from the Greek word for “thread”. This first sounds like it doesn’t make any sense, until you think about what Walther Flemming saw when he looked in his microscope in 1878 (well, ok, he probably did it in 1877 or 1876, but he PUBLISHED it in 1878, and therefore that’s the date everyone cares about). The threads he was looking at were bundles called chromosomes, conglomerations of DNA and histones, all bunched together for cell division.
Now, most people think of mitosis as being just the process of one cell dividing into two. This actually isn’t QUITE true. Mitosis is what happens when a cell divides its chromosomes up into two separate nuclei, but it’s not the splitting of the cell itself. The cell splitting is called cytokinesis, where the cell divides in half, dividing up all of its organelles and splitting the cell membrane in two, creating two distinct cells.
So to really get a good picture of this, we’re going to start off with some basic cell structures and go from there.
(A cartoon of a typical animal cell. Sci sees the cutouts around this and thinks that it would be REALLY AWESOME to have a detailed cell/molecular biology coloring book. She already had two for neuroanatomy and they are SO much fun!)
You can see the various organelles (“little organs”, and yeah, they basically are) above have been labeled. I’m going to try to find links to some simple explanations of what these organelles are, because I cannot help but notice that the explanations on this from wikipedia are not exactly basic (“The position at which the contractile ring assembles is dictated by the mitotic spindle. This seems to depend upon the GTPase RhoA, which influences several downstream effectors (such as the protein kinases ROCK and citron) to promote myosin activation”…ok, that reads like the lit, not like wikipedia, step it up, geeks…) So here we go:
Cell Membrane: holds the cell in a bag. Animal cells are surrounded by just a membrane, while plant cells have a membrane and a cell wall. The cell membrane is made of lipids (these are chemicals that we usually think of as making up fats) strung together with their opposite ends pointing towards each other, like this:
(As an extra point, the outer parts of each lipid are hydrophilic, meaning they don’t mind being in contact with water, while the inner parts are hydrophobic, which means “water fearing”. The hydrophobic ends will thus clump together to avoid water, creating globs and layers like that seen here. It also means that things trying to get into the cell will either have to be hydrophobic enough to get through the formed membrane, or they will have to go in through channels and transporters.)
This is an example membrane, but trust me when I say the actual result is FAR more complicated, and full of tons of receptors and lipid rafts and chains and transporters and STUFF. Very, very important stuff. But that’s a story for another time. For now, the membrane is just what holds the cell to a certain size and protects it from the outside world.
Lysosomes: These contain some hefty acids, and are the garbage disposal of the cell. Because they are acidic, they are capable of breaking down anything the cell doesn’t want, like worn out organelles and engulfed viruses or bacteria. They also play a big role in cell death.
The Nucleus: (pronounced “NUKE-lee-us”, if I hear “Nuke-you-Lus” someone will get smacked) Surrounded by its own membrane, this is where the action happens. The nucleus contains all the genetic material (that’s DNA, or RNA in some cases) of the cell. All commands for production of proteins, cell reproduction, cell death, or basically anything else come from the nucleus.
Nucleolus: This is an area inside the nucleus that is very important in the production of ribosomes, which are very important for the creation of proteins. But they are a story for another time.
Nuclear Membrane: the membrane surrounding the nucleus. The nucleus is SO VIP that you have to have a special tag just to get into the membrane. No valet parking, though.
Vacuole: These are little organelles which do lots of jobs, varying from holding waste to exporting specific substances, depending on the type of cell and what they are needed for.
Mitrochondrion: Everyone likes to call these the powerhouse of the cell. They use some rather complicated cell cycles to create the cell’s energy, which is in the currency of ATP.
Golgi Apparatus: This is Sci’s favorite organelle, because it’s got a great name. The Golgi apparatus does the packaging of molecules and directing them to their target locations, whether that be in the cell or to destinations unknown.
Ribosomes: These are made in the nucleolus, but are little organelles in themselves, and are part of the incredibly important translation of genetic material to protein.
Smooth ER and Rough ER: ER stands for Endoplasmic reticulum. This is a large organelle set which helps in the storage, packing, and processing of molecules such as proteins, fats, or anything else. The difference between smooth ER and rough ER is that rough ER is studded with ribosomes, making it look bumpy. Smooth ER is more important in storage, while rough ER is more important in processing and packaging.
Cytoplasm: The interior of the cell. It is not nearly so empty as it appears. It’s not only full of organelles, it’s also full of water, lots of small molecules to maintain pH balances, and a ton of structural elements which give the cell its shape.
Centrosome: This is going to be a big important organelle coming up. Usually it is used as an organizing center for microtubules which provide the cell with structure, but during mitosis it will become part of the organizing force creating the mitotic spindle. We’ll get to that.
ONWARD TO MITOSIS
So now we’re organized. The main action in the next few shots is going to be in the nucleus, with the chromosomes being the main players and the centrosome getting some big directorial roles. Let’s do this.
Interphase “The between-phase”
(plant cells in interphase, via Wisconsin’s Botany dept.)
This phase is the preparation for cell division, and is also the phase in which the cell will spend most of its life. The idea of “interphase” can also be broken up in to smaller phases of the cell’s life, the G1, S, and G2 phases, which include growth and the doubling of the genome (in S phase). With the right signals given for moving on, it’s all systems go.
Prophase “‘the before-stage”
In animal cells (plant cells also have a PREprophase which is part of dividing a cell when you have a cell wall), cells go straight into prophase.
Normally, your DNA is loosely clumped together, spooled around histones and in the form of chromatin. During prophase, these chromatin clump together and form choromosomes. Remember that the DNA has already replicated, and so each chromosome has a sister, which is clumped together with it. These are clumped together in pairs, each pair is called a chromatid, which seems like a great name for a small grasshopper. Looks like this:
(Via same site as before, these guys have some lovely pics)
Prometaphase “the before-the-middle-stage”
And here’s where the centrosomes enter the picture. First the membrane around the nucleus dissolves, and the centrosomes, which have been hanging out near the nucleus, forage outward to the poles of the cell (cue awesome adventurous music, I’m thinking something with kettledrums), one on each opposite side. They then send out microtubules (cell structure components).
These microtubules are going to connect to kinetochores, which are the protein bundles which hold each set of chromatids together in that iconic “X” shape:
But the full prometaphase looks like this:
That’s prometaphase, stained in gorgeous colors that I want on my wall as art (why don’t people sell more of these as art, srsly??) The centromeres are in yellow, the microtubules reaching out are in red, and the chromosomes are in blue.
Metaphase“The middle phase”
This is the build up! The microtubulues have attached themselves to the kinetochores on the chromosomes, and the tension involved (think of molecular tug of war) has everyone line up on the midline. BANANAS…UNITE!!!
(Does this not look completely AWESOME!?! Via UNC)
Anaphase “the up-stage” (weird Greek, not entirely relevent)
Annnnnnd…SPLIT! When everyone is lined up on the midline and all of the microtubules are attached to the chromosomes, and the tension is just TOO MUCH TO HANDLE, the microtubules shorten, and the chromosomes split in half. I think watching this happen in a microscope in real time (which Sci has never seen) might make me cheer. A lot. BANANAS…SPLIT!!!
(From molecular expressions at FSU)
Telophase “The end phase”
Already we have reached the end times. The now split chromosomes have split and head toward the centromere poles that were set up in prometaphase. The nuclear membrane reforms, this time in two separate places around each centromere, closing in on the two clumps of chromsomes on either side. These are called daughter nuclei.
And now we break to….GO BANANAS, GO GO BANANAS.
(Ok, I got that out of my system. Every time I think about mitosis I think of the bananas song. Chromosomes…UNITE on the midline! Chromosomes…SPLIT to the poles! Go, DNA, go go DNA! Ok, I’m a geek. But now you will think of the bananas song when you think of mitosis, TOO, and I will no longer be alone!!! Also, I made Mr. SiT do the chant with me complete with movements. My work here is done.)
Cytokinesis “Do the locomotion”
Cytokinesis usually occurs usually along with telophase, but is technically a separate process and gets its own name. This is when the cytoplasm of the cell splits, along with all the organelles and everything else, the cells pinch in the middle, and two new daughter cells remain.
So now we’re through (we’ve gone bananas). But what is the significance of mitosis? First off, when it occurs. If a cell gets the wrong signals, or overcomes certain stopgaps to constant division, it will divide constantly. When a cell divides constantly, grows constantly, and manages to avoid the immune system’s assassins (things which can occur via many routes), you end up with a tumor, and if it goes further you end up with metastasizing cancer. Pretty important. A deep understanding of mitosis and the regulators driving cell division has allowed scientists to look into what might cause the dysregulations behind cancer, to try and get to the heart of the problem.
And what about aging? Individual cells will not live and divide forever (unless they are cancer cells). Normal cells can’t survive in vitro past about 50 divisions, after which they enter senescence. This may play a role in parts of human aging, as our cells become less able to replace themselves, but can also play an important role in things like wound healing and repair. So mitosis is important. It’s…bananas.
And last but not least, if you want to see it all in action, go to Cells Alive! to see some freakin’ sweet animation of mitosis, complete with explanations at each level. That site is lovely.