The Plant Cell Cycle

Chapter 3

• Nucleus• DNA• Replication• Cytoskeleton

The plant cell

NUCLEUS

MITOCHONDRIUM

CHLOROPLASTCHLOROPLAST

CHLOROPLAST CHLOROPLAST

MITOCHONDRIUM

MITOCHONDRIUM

MITOCHONDRIUM

CYTOSOL

Notes: 1) cytosol is the same as cytoplasm 2) not all of the plant cell structures and organelles are shown

Cell wall

Plasma membrane

Fig. 3-8, p. 37

0.2 µm1 µmnuclear envelope

one pore

lipid bilayer facingthe nucleoplasm

nuclearenvelope

lipid bilayer facingthe cytoplasm pore complex that

spans both bilayers

Nucleus

-Ovoid or irregular shaped

-Surrounded by double membrane (nuclear envelope)

Structure of Phospholipids and Glycolipids

• Hydrophilic

• Hydrophobic

Structure of Phospholipids and Glycolipids

Central dogma of Molecular Biology

+

DNA

REPLICATIONTRANSCRIPTION

TRANSLATION

RNA

mRNA

protein

Ribosome

Early Prophase cell

Chromosome

Nucleolus(rDNA, rRNA, ribosomal proteins, etc…)

nuclearenvelope

Nucleolus: production center for ribosomes

Structure of DNA

Fig. 3-9a, p. 37

A nucleosomeconsists of part ofa DNA moleculelooped twicearound a coreof histones (chromosomal proteins).

core ofhistonemolecules

Fig. 3-9b, p. 37

Immerse achromosome insaltwater and itloosens up to abeads-on-a-stringorganization. The“string” is oneDNA molecule.Each “bead” isa nucleosome.

Fig. 3-9c, p. 37

At a deeperlevel of structuralorganization, thechromosomalproteins and DNAare organized asa cylindrical fiber (again a helical structure).

Fig. 3-9d, p. 37

At times when a chromosome is mostcondensed (during Mitosis), the chromosomal proteinsinteract, which packages loops of alreadycoiled 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)

Fig. 2-18, p. 26old new new old

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

Fig. 3-15, p. 43

some cellsleave the

cycle pre-DNA synthetic phase or gap phase

DNAsynthesis

phaseInterphaseMitosis (M)

cytokinesis

telophase

anaphase

metaphase

prophase

premitosis phasesome cellsleave the

cycle

CELL

CYCLE

G1

G2

S

Note: Interphase includes G1, S and G2 phases

Fig. 3-16, p. 44

control point

G2

G1

M

S

control point

Principal Control Point Hypothesis

Fig. 3-18, p. 45

microtubulesnucleus

cell wall

cytoplasm

band ofmicrotubules

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.

A B C

Mitosis

• Four phases– Prophase– Metaphase– Anaphase– Telophase

Fig. 3-19, p. 46

Early Prophase Late Prophase Metaphase Late anaphase Telophase

cellplate

spindlefiber

pole

kinetochorechromosome

nucleolus

nuclearenvelope

Mitosis

a b c d e

Fig. 3-17, p. 45

The roles of microtubules in mitosis

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.