Cellular Metabolism and Reproduction: Mitosis and Meiosis 75
Centrioles
Nucleolus
Nucleus
Nuclear
membrane
Cell
membrane- interphase 2a. early prophase 2b. middle prophase
2c. late prophase 3. metaphase 4a. early anaphase4b. late anaphase 5. telophase interphase(^) ®
Learning
Cengage ©
Figure 4- 11 An illustration of interphase and the stages of
mitosis.
pinched-in area of the chromosome where a disk of pro-tein
called the kinetochore (kye-NEE-toh-kor) is found. In
animal cells, the centriole pair begins to move apart to the
opposite poles of the cell forming a group of micro-tubules
between them called the spindle fibers. In plant cells, a
similar group of spindle fibers forms even though there are
no centrioles. As the centrioles move apart to the opposite
poles of the cell, they become surrounded by a cluster of
microtubules of tubulin that radiate out-ward looking like a
starburst. This starburst form is called the aster. The
spindle fibers form between the asters. As these fibers
form, they push the centrioles to the opposite ends of the
cell and brace the centrioles against the cell membrane. At
this time, the nuclear membrane breaks down and its
components become part of the endoplas-mic reticulum.
The nucleolus is no longer visible.
Each chromosome has two kinetochores, one for each
sister chromatid. As prophase continues, a group of
microtubules grow from the poles to the centromeres of the
chromosomes. The microtubules attach the kineto-chores to
the poles of the spindle. Because these micro-tubules
coming from the two poles attach to opposite sides of the
centromere, they attach one sister chromatid to one pole
and the other sister chromatid to the other, thus ensuring
separation of the sister chromatids, each one going to a
different daughter cell.
Metaphase II.
Metaphase is the second stage of mitosis and begins when
the sister chromatids align themselves at the center of the
cell. The chromosomes are lined up in a -circle along the
inner circumference of the cell called