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The Plant Cell Cycle
Chapter 3
• Nucleus
• DNA
• Replication
• Cytoskeleton
The plant cell
Cell wall
CHLOROPLAST
CHLOROPLAST
MITOCHONDRIUM
NUCLEUS
MITOCHONDRIUM
MITOCHONDRIUM
CYTOSOL
CHLOROPLAST
CHLOROPLAST
MITOCHONDRIUM
Notes: 1) cytosol is the same as cytoplasm
2) not all of the plant cell structures and organelles are shown
Plasma membrane
one pore
Nucleus
-Ovoid or irregular
shaped
-Surrounded by
double membrane
(nuclear envelope)
nuclear envelope
1 µm
0.2 µm
lipid bilayer facing
the nucleoplasm
lipid bilayer facing
nuclear
the cytoplasm
envelope
pore complex that
spans both bilayers
Fig. 3-8, p. 37
Structure of Phospholipids and
Glycolipids
• Hydrophilic
• Hydrophobic
Structure of Phospholipids and
Glycolipids
Central dogma of Molecular
Biology
DNA
TRANSCRIPTION
REPLICATION
RNA
TRANSLATION
Ribosome
mRNA
protein
+
Nucleolus: production center for ribosomes
nuclear
envelope
Nucleolus
(rDNA, rRNA,
ribosomal
proteins, etc…)
Chromosome
Early Prophase cell
Structure of DNA
A nucleosome
consists of part of
a DNA molecule
looped twice
around a core
of histones
(chromosomal
proteins).
core of
histone
molecules
Fig. 3-9a, p. 37
Immerse a
chromosome in
saltwater and it
loosens up to a
beads-on-a-string
organization. The
“string” is one
DNA molecule.
Each “bead” is
a nucleosome.
Fig. 3-9b, p. 37
At a deeper
level of structural
organization, the
chromosomal
proteins and DNA
are organized as
a cylindrical fiber
(again a helical
structure).
Fig. 3-9c, p. 37
Fig. 3-9d, p. 37
At times when a
chromosome is most
condensed (during
Mitosis), the
chromosomal proteins
interact, which packages
loops of already
coiled DNA into a
“supercoiled” array (two
additional helical
organizations).
Such a tightly packaged
DNA molecule is more
easily moved around
compared to when it
would be a long “loose”
thread (important during
cell division)
old
new
new
old
Fig. 2-18, p. 26
Cytoskeleton
• Structures in cytoskeleton
– Microtubules
– Motor proteins
– Microfilaments
• Specialized proteins connect microtubules and
microfilaments to other organelles
– Connections thought to coordinate many cell processes
Cell Cycle
some cells
leave the
cycle
pre-DNA synthetic phase
or gap phase
G1
cytokinesis
telophase
CELL
DNA
Interphase synthesis
phase
S
anaphase
metaphase
prophase
some cells
leave the
cycle
Mitosis (M)
CYCLE
G2
premitosis phase
Note: Interphase includes G1, S and G2 phases
Fig. 3-15, p. 43
Principal Control Point Hypothesis
control point
G1
M
S
control point
G2
Fig. 3-16, p. 44
A
B
microtubules
nucleus
C
band of
microtubules
cell wall
cytoplasm
Fig. 3-18, p. 45
Formation of the preprophase band of microtubules in meristematic cells:
A) Section at right angles to the plane of the future cell plate shows a cross section of
microtubules.
B) Section in the plane of the future cell plate shows microtubules encircling the
nucleus.
C) Three-dimensional drawing of A and B.
Mitosis
• Four phases
– Prophase
– Metaphase
– Anaphase
– Telophase
Mitosis
a
d
c
b
e
cell
plate
nuclear
envelope
nucleolus
pole spindle
fiber
chromosome
kinetochore
Early Prophase
Late Prophase
Metaphase
Late anaphase
Telophase
Fig. 3-19, p. 46
The roles of microtubules in mitosis
Fig. 3-17, p. 45
Visualization of tubulin (component of microtubules) during mitosis.
Cell division in Plants versus Animals
• Animal cells do not have a cell wall
Because plant cells have cell walls, the direction of cell division is a
major determinant of overall plant development (cells are caught in a
rigid structure and cannot reorganize after cell division has been
completed). The direction/orientation of cell division is controlled by
plant hormones.
Cell division in Plants versus Animals
• Animal cells do not form a preprophase band (PPB)
• During Telophase:
- in plants: a cell plate is formed at the start of cytokinesis (direction
of cell plate formation depends on hormone action via effects on
PPB formation)
- in animals: the cell pinches in the middle to form two cells, no cell
plate is formed.