Mitosis: One cell, two cell, old cell, new cell
You’re probably not aware of it, but you’re losing your skin right now. While you go about
your daily business, you’re dropping thousands of skin cells onto your clothes, your
computer keyboard, your friends, everything around you. You are one massive skin cell
shedder.
So why aren’t you skinless?
Because, thankfully, the cells underneath your outer layers of skin are busily making
copies of themselves to replace cells that are lost. They’re so prolific at it that you’re
wearing a completely different skin than you were just a couple of months ago. You’ve
got a whole new set of red blood cells, too, and a fresh stomach lining. In fact, most
organs in your body go through some sort of regular cell refreshing. It’s the way your
body keeps things running smoothly, healing up, and fending off the aging process.
Divide and conquer
The machine behind all this new cell generation is a process called mitosis. Put simply,
mitosis is the act of one cell nucleus (called the parent) dividing into two new nuclei
(called daughters), each with a set of DNA strands identical to that of the parent. Looking
at it more deeply, though, mitosis involves a complex set of molecules and cell
structures that only appear during this time.
Mitosis is part of a longer series of events called the cell cycle. The cell cycle is the
orderly sequence of events in which a cell duplicates its contents and divides into two
new cells. The first part of the cycle is preparation for mitosis: the cell grows,
accumulates nutrients, and, most importantly, copies all its chromosomes, so that there
are two identical sets of DNA. This process takes up most of the cell cycle. During
mitosis, the nucleus of the cell splits in two, with each new nucleus getting a set of
chromosomes. The rest of the cell divides in a process called cytokinesis, so that when
the whole sequence of events ends, there are two cells, each with an exact replica of the
parent cell’s DNA.
All cells that are animal, plant, or fungi must reproduce and make more of themselves.
Mitosis assures the right genetic complement when making one cells into two, and it’s
the process by which a single fertilized egg becomes a multicellular organism. Many of
the mechanisms and structures involved in mitosis are similar, or even the same, from
one organism to the next. This means that mitosis is one of the most ancient processes
among living things, and that its elements have been conserved through 3.5 million
years of evolution.
Step by step
When researchers talk about the cell cycle, they often think of it as a series of steps,
each with different things happening. Each step is characterized by a set of specific
goings-on.
Interphase is the long phase of the cell cycle before mitosis, when the cell does its prep
work for division, copying its DNA. During this phase, the chromosomes are diffuse and
indistinct, and fill the cell nucleus.
In prophase, the chromosomes crunch up into structures called chromatids, that are
dense enough to see with a light microscope. The membrane around the cell nucleus
breaks apart, and you can begin to see “fibers,” called microtubules, crossing the cell.
These will become the mitotic spindle.
To imagine metaphase, think of the cell as a globe. The spindle fibers line the
chromatids up in two rows, like tiny people standing along the cell’s “equator.” Each row
is an identical set of chromatids, and the two rows together are arranged as if each
“person” was facing its twin.
During anaphase the spindle fibers, which are attached to the chromatids, pull each pair
of twins apart, dragging an identical set to each of the poles. Before this phase, the cell
has gone through a process to make sure its DNA was copied correctly, and if it wasn’t,
mitosis is delayed and the cell will try to repair itself. Anaphase is the point of no return:
once the chromatids have separated, they’re going to move toward their respective
poles.
Telophase is sort of a “clean-up” phase: the chromatids arrive at their poles and new
nuclear membranes form around them. They un-scrunch, and are no longer visible
under the microscope. The spindle also disappears.
Cytokinesis is the final step, and can happen at the same time as telophase; the cell
pinches itself in the middle until, voila, two new cells!
Timing is everything
How does a cell know when to divide? That’s an important question that lots of scientists
are researching, but there are no simple answers. What is known is that cells get signals
that tell them to start the cell cycle, usually in the form of proteins. Those signals can be
triggered by a number of different situations. An injury is one example: if you fall and
scrape your knee, you’ve broken skin cells. The nearby cells will get a message: “We
need more of you to replace these damaged cells.” This is how the healing process
occurs. In other circumstances, changes in the levels of a hormone, or even
environmental factors like temperature, can trigger the signals for cell division.
There are also a host of compounds that monitor mitosis as it’s happening, doublechecking that the DNA copies are correct and that each new cell receives the right
complement of chromosomes. Cells also get signals not to divide, often when they
encounter other cells like themselves. This keeps the body from making too many cells
of a given type.
Too much of a good thing
Most of us don’t think much about cell division happening in our bodies—until it goes
wrong. Cancer can be a form of cell division or mitosis gone awry. One important reason
to understand what keeps mitosis under control is to better understand what’s happened
when that control is lost. Sometimes cell nuclei manage to divide and foster new cells
even if their DNA copies aren’t quite right. Or a cell with DNA damage might divide and
divide and divide without paying attention to signals to stop, and therefore replicate a
mistake in its genetic material. Such out-of-control cell divisions can produce a huge
number of cells in a short amount of time. When there are DNA errors involved, this
overload of mitosis and cell division can lead to tumors and cancer.
The vast majority of the time, though, your cells (and the cells of other organisms) follow
all their instructions correctly, replacing lost or dead cells with great efficiency. Which is
quite an impressive feat when you consider the billions of DNA components that must be
copied for each division, and with extreme precision required to get those copies right.
Not to mention that this exacting mechanism is carried out continuously, unnoticed by
you, day in and day out, keeping you alive.