2.3.1 Mitosis
The goal of mitosis is to maintain the complete number of chromosomes during cell div-
ision. Mitosis is a highly ordered process, because chromosome loss during cell division
would be detrimental to the adult plant. Mitosis can be broken into five basic steps, each
defined by the organizational state of the chromosomes (Fig. 2.7).
The chromosomes are in the relaxed state throughout most of the life of the cell, called
interphase, which is the period of cellular life when the cell grows and prepares its chromo-
somes for cell division. During the synthesis phase (S phase) of interphase, the chromo-
somes replicate their DNA and form the sister chromatids. As the cell enters mitosis, the
chromosomes condense into the tightly wound state and the nucleus breaks down, which
are characteristics ofprophase. The chromosomes appear in a disorganized mass that can
be seen under the light microscope. The cellular machinery that performs the actual
work of cell division involves a group of proteins called themitotic spindle apparatus,
but we will focus on the state of the chromosomes during mitosis in this chapter. As the
chromosomes become organized along the middle of the cell, they entermetaphase.
During metaphase, the chromosomes line up at the center of the cell with each of the
sister chromatids on opposite side of themetaphase plate. The centromere sits directly
on the middle line, and is broken in half and pulled to the opposite ends of the cell
duringanaphase. The chromosomes appear as small V shapes, with the centromere
pulled to the opposite poles with chromosome arms lagging behind. During this phase,
the cell transiently has a 4Nchromosomal number, because the centromeres between the
sister chromatids are broken, producing two chromosomes. When the chromosomes
reach the opposite ends of the cell, the nuclear membranes re-form, which characterizes
telophase. At this point, the two sister chromatids from all the chromosomes have been sep-
arated from one another, and the cell can divide by a process calledcytokinesisinto two
daughter cells that have the exact same DNA. During mitosis, the chromosomes are
Figure 2.8.Mitosis and the two steps of meiosis differ from one another by the arrangements of the
homologous chromosomes prior to cell division.
2.3. MITOSIS AND MEIOSIS 31