Download Mitosis (Cell division) Cells arise from other cells. You don`t

Mitosis (Cell division)
Cells arise from other cells. You don't “spontaneously” make cells.
Until they discovered microbes, people did believe in “spontaneous generation”
Molds, bacteria, etc. would “appear” magically.
Eventually, they discovered that all cells come from a previous cell.
The problem is, how do these “daughter” cells arise? In particular, how do they get all
the information and instructions that tell them what to do (how to survive)?
It turns out that this information is carried on chromosomes. These carry specific
genetic information (DNA) that will determine a lot of things about a cell or an
individual:
Appearance, behavior, physical ability, etc.
So now the problem is that each “daughter” cell will need its own set of
chromosomes.
So, the main thing we'll be concentrating on is what happens to our chromosomes
as we go through cell division.
Prokaryotes (again, much simpler than Eukaryotes) :
Use “binary fission” (= “dividing in half”)[OVERHEAD, fig. 8.2A, p. 127]
The “chromosome” of bacteria is wound up in one long DNA molecule.
This chromosome is duplicated.
One copy will move towards one end of the cell.
Cell elongates as this is going on.
Cell eventually starts to develop a crease or furrow down the side, and splits into
two individuals.
(In comparison, we'll spend ages going through eukaryotic division)
Eukaryotes
Generally, eukaryotes have more than one chromosome.
When the cell is not dividing, these chromosomes are very loosely coiled and known as
“chromatin”
In humans, we have about 30,000 (more recent estimates have revised this
downward to 25,000) genes distributed over 46 chromosomes.
Genes are the specific parts of chromosomes that have information about
a specific trait (e.g., hair color, height, etc.)
This is somewhat surprising, considering some worms have 20,000 genes.
When one of our cells divides, the problem is that each of the resulting
“daughter” cells will need a full set of 46 chromosomes.
This is much more complicated than a bacteria which only has about 3,000 genes
on a single chromosome.
If a eukaryotic cell will divide, these chromosomes will duplicate before division starts.
A cell that starts to divide has already duplicated it's chromosomes.
When cell division starts, the chromatin will start to coil up tightly and become a more
visible “chromosome”.
Each “chromosome” in a dividing cell consists of two identical “chromatids”
[OVERHEAD, fig. 8.4 B, p. 128]
Before duplication, each chromosome is “single”. After duplication, each
chromosome is “double” and has two identical sister chromatids.
The two chromatids from each chromosome will split apart and then form the
chromosomes of each of the daughter cells.
In this way, each resulting cell will get a complete set of chromosomes.
If the daughter cells divide again, this is repeated, and each “single” strand chromosome
will make another, thus making a double stranded chromosome (with two identical
chromatids).
And so on...
Chromatids are held together at the “waist” by a centromere.
In humans, we'd have 46 duplicated chromosomes before a cell starts to divide, and 92
chromatids. Each daughter cell will wind up with 46 single chromosomes.
Cell cycle [OVERHEAD, fig. 8.4, p. 129]
The cell cycle is divided into two phases, which can be subdivided:
Interphase - this is the phase when a cell is doing what it's supposed to
(making proteins, carrying out its functions, etc.). A cell that will divide
again can double in size during this phase (better “double its contents”).
Interphase has three subphases:
G1 - this is when our cell grows and does its thing. If a cell will
not divide again, it can stay in G1 (actually G0).
S - if a cell divides again, it needs some time to make the copies of
its DNA. S is called S for DNA synthesis.
G2 - the cell may not be ready to divide yet. Some more growth
may be needed. The cell does this during G2.
Mitosis - in this phase, the chromosomes are split apart (the chromatids
separate), and divide up amongst two new daughter cells. There are two
parts to mitosis (sometimes occurring at the same time:
Mitosis itself (splitting up chromosome chromatids)
Cytokinesis (splitting the actual cell in two).
Mitosis is very accurate. The error rate is only about 1/100,000.
(Still, when you consider how many cell divisions take place, it could be
better).
Mitosis - the details [OVERHEAD, fig. 8.5, p. 130 - 131]
Important - mitosis is a continuous process. Each of the phases we'll be talking
about leads smoothly and directly into the next.
Interphase (not actually part of mitosis).
Cell is growing, making proteins, etc. Chromosomes are duplicated in
this phase.
Prophase
Nucleus starts to disintegrate, chromatin coils up into chromosomes (each
chromosome will be double stranded with two chromatids), nucleoli
disappear. In animal cells, centrioles appear at the center of each
centrosome (theses are connected to long spindles made up of
microtubules)
This is a preparatory phase - things are getting ready.
Prometaphase (not always considered a distinct phase)
Nucleus disappears completely. Mcirotubules hook into the
chromosomes. Microtubules from opposite centrosomes hook into sister
chromatids (chromatids on the same chromosome). Chromosomes are
moved to the center of the cell.
Metaphase
Spindle (microtubules going from one centrosome to another) are fully
formed. Chromosomes are now lined up in the middle off our cell. Some
microtubules are attached to sister chromosomes, some are not.
Anaphase
Chromosomes are split apart. Each chromatid is now considered a
chromosome.
Microtubules that chromosomes are now attached to shorten,
pulling chromosomes to opposite ends of the cell.
Other microtubules lengthen, forcing the ends of the cell apart.
Telophase
Cell elongation continues. Two nuclei begin to form (re-form) at opposite
ends of our cell. Chromosomes begin to unwind and become less distinct
(they're reverting to chromatin). Spindle disappears.
Cytokineses
This is when the cell membrane pinches in and splits our cell in two,
resulting in two new daughter cells, each with its own (complete) set of
chromosomes.
This phase can happen almost anytime, but usually starts sometime
during telophase (obviously, it can't be complete too soon, or our
chromosomes would not be done yet).
This is different in plant and animal cells [OVERHEAD, fig. 8.6 A & B,
p. 132]:
Animals - like described.
Plants - because of the cell wall, the cell membrane can't just
pinch in. Vessicles filled with cell wall material show up during
telophase, lign up in the middle, and start to build a new cell wall.
This eventually fuses with the existing cell wall (&
membrane) and completely separates the daughter cells.
[OVERHEAD, not in book]
Factors and control mechanisms affecting cell division
Different things will control the cell cycle and determine whether or not a cell will divide
[OVERHEAD, fig. 8.7A & B, p. 133]:
Anchorage dependence - some cells will not divide unless they're on a solid surface.
The bottom of a petri dish is often sufficient, though biologists infer that really
this means they must be in contact with some part of the body.
Density dependent inhibition
Some cells will stop growing when there are too many of them. As cells come
into contact with one another, division stops.
Clearing a space will often start division again
Book mentions this is similar to what happens when you cut yourself
(you've cleared a space, cells will grow until the cut is filled in).
Growth factor
Proteins (there are several) that will stimulate cells to divide.
Adding more growth factor gets things started again.
Checkpoints [OVERHEAD, fig. 8.8A, p. 134]
Working together with some of these growth factors are check points.
As our cell goes through it's phases, each phase must complete in such a
way so that the next one will get started. Things don't necessarily happen
“automatically”.
Stages in the cell cycle where the cell determines whether or not to proceed to the
next step in our cell cycle.
For example, a cell that will not divide again (many muscle and nerve
cells, for example), stop at a “checkpoint” in G1. They stay in G1 (though
now we call it G0).
Very often, it's the G1 checkpoint that is controlled by growth factors.
[OVERHEAD, fig. 8.8B, p. 134] - often, signals from the rest of the
body determine if a cell will proceed past a checkpoint
Signal transduction!
The other checkpoints are also important, as the cell can not enter the next phase
until all the conditions have been met.
Ensures that the cell has everything prepared to successfully enter the
next phase.
Checkpoints out of control
Sometimes the checkpoints don't work. Some of the control mechanisms don't work
correctly.
Normally, cells that begin to multiply excessively are destroyed by the immune system.
But, if for some reason, they are not --> tumor
If these cells spread to the rest of the body --> malignant tumor
Essentially, at this point we call it a cancer.
Cells often spread though the blood or lymphatic systems
Cancers are named for the part of the body in which they originate.
Can be subdivided further:
Carcinomas (from tissues covering or lining the inside of organs.
Sarcomas (arise in connective tissues such as bone or cartilage)
Leukemias / lymphomas (from tissues giving rise to blood cells).
Cancer cells do not have the normal regulatory signals that stop cell growth.
Other cancer cells may produce excessive amounts of growth factor.
Cancer can be treated with:
Radiation (damages cancer cells more than regular cells)
Chemotherapy (often attacks various parts of the cell cycle)
Often used when cancer is more widespread
Also affects other cells that divide rapidly (hair follicles, for
example).
Surgery (removing affected tissue).