Question

How do cells know every chromosomes are attached to the spindle fibers, and they are attached in the middle. How do cells know they are ready for anaphase?
help plezzzzzzzzzzzzzzzzzzz

Answer 1

This is a good question flyqueen, brings me back to the BIO-121 days of college!
It is a bit hard to explain, so you should also read this wikipedia article (link below)

http://en.wikipedia.org/wiki/Cell_cycle

The basic answer is a variety of signal proteins and the corresponding receptors which may only be present during the correct times (most likely right at the end of growth or the current phase. Just like most processes in the animal body, things happen when the signal to happen is there. For example, during interphase, the cell grows. It will continue to grow until the unique and correct protein becomes available, which will then bind to the receptor signaling the cell that it is ready to go on to prophase. Then the same things happens again when the nucleoli has dissapeared, and the sister chromatids are visible, and the signal to continue to metaphase will execute.

That wass actually the simpler explanation actually, but as you will see in the wiki article, picture a "cell cycle clock" mechanism. Shaped like a clock, the needle will go with time around the center, as the cell grows and performs necessary duties. But at specific points along the way, there are "checkpoints."
These are like traffic lights which default on RED. They only turn green to let the cell continue once it has been properly inspected and, through the correct signals, it has grown correctly and is ready for the next step, whether that be to continue growing, or to divide.
If the signal is not there, or the cell has grown incorrectly, it will not be able to continue around the clock. It will either be sent to the "non-dividing" phase (G0), or will perform aptosis (cell suicide).
The whole cell division cycle is run in this way, stopping and going, and double-checking to ensure correct cell division.

This is how a cell would know if the chromosomes are attached to the spindle fibers, as well as when it is ready to enter/exit anaphase.

Everything happens for a reason, and there must be a reason for something to happen : )
Hope this helps! any questions feel free to ask! good luck!

Answer 2

Ry is right. In case you are looking for more info however, i always did love the cell cycle, so here's my 10 cents on the matter:

The mitotic spindle is just a network of microtubules. They attach to the sister chromosomes in the center of a cell during the M-A checkpoint (thats the checkpoint that you are interested in). These sister chromosomes are held together by a protein complex called the kinetocore (composed of cohesins and mad/bub proteins plus some others i believe). Once the microtubules are secured to the chromosomes via proteins called cohesins, mad and bub proteins "let go" of a protein called Cdc20, which activates APC (anaphase promoting complex). This complex ubiquinates (that means it marks the protein to be destroyed) securin. Securin binds to and inhibits a protein called seperase. So when APC ubiquinates securin, seperase is released and can now cut up the kinetocore, releasing the sister chromosomes to be pulled to opposite ends of the cell.

APC also ubiquinates some cdk and cyclin proteins, clearing the way for progression to anaphase.

Answer 3

And you have to be aware that several times the cell makes a mistake, especially in the case of the mititic spindles, that is how polyploids are made, like cells in Down syndrome.