Each centriole is composed of
nine triplets of microtubules
Centrioles arranged in a circle.CellSpindle
fibersCentromere
ChromatidsMicrotubule
tripletsSection 3 Mitosis and Cytokinesis
128 CHAPTER 6Chromosomes and Cell Reproduction
Chromatid Separation in Mitosis
Every second about 2 million new red blood cells are produced in
your body by cell divisions occurring in the bone marrow. These cells
have received the signal to divide. The cells continue past the G 2 phase
and enter into the last two phases of the cell cycle—mitosis and
cytokinesis. During mitosis the nucleus divides to form two nuclei,
each containing a complete set of the cell’s chromosomes. During
cytokinesis the cytoplasm is divided between the two resulting cells.
During mitosis, the chromatids on each chromosome are physi-
cally moved to opposite sides of the dividing cell with the help of
the spindle, shown in Figure 8. are cell structures made up
of both centrioles and individual microtubule fibers that are
involved in moving chromosomes during cell division.Forming the Spindle
Animal cells usually have one pair of centrioles, with the centrioles at
right angles to each other. During the G 2 phase of the cell cycle, the
centriole pair is replicated so that the cell has two pairs of centrioles
as it enters the mitotic phase. When a cell enters the mitotic phase,
the centriole pairs start to separate, moving toward opposite poles of
the cell. As the centrioles move apart, the spindle begins to form.
Centrioles and spindle fibers are both made of hollow tubes of pro-
tein called microtubules. Each spindle fiber is made of an individual
microtubule. Each centriole, however, is made of nine triplets ofSpindlesObjectives
Describethe structure and
function of the spindle
during mitosis.
Summarizethe events
of the four stages
of mitosis.
Differentiatecytokinesis in
animal and plant cells.
Key Terms
spindleThe spindle, made up of centrioles
and spindle fibers, helps move
chromosomes apart during mitosis.Figure 8 The spindle4B 6E4B 6E4B 6E