By the end of prophase — see the discussion of prometaphase on the previous page — the nuclear envelope has entirely vanished and the chromosomes have condensed (they are tightly coiled and easily visible under a light microscope). In addition, the microtubules of the spindle apparatus have attached to the centromeres at their kinetochores. The centrosomes are now at opposite ends ("poles") of the cells.
|
Chromosomes (blue), kinetochores (pink),
microtubules (green). Other parts of the
cell are unstained and therefore invisible.
|
Now, during metaphase — the second stage of mitosis in the eukaryotic cell cycle — the chromosomes, pulled by the spindle fibers, line up along the middle of the cell, halfway between the centrosomes on an imaginary plane called the metaphase (or equatorial) plate. The chromosomes are now maximally condensed.
In mitosis, individual replicated chromosomes, each composed of two sister chromatids, move to the equatorial plate during this step (whereas during the first division of meiosis, pairs of replicated chromosomes (tetrads) line up at this stage).
|
 |
Compare this simplified diagram with
the photomicrograph at left.
|
|
