Instant Notes: Plant Biology

auxin and cytokinin (Topic F2), are known to initiate the cell cycle. Auxin stimu-

lates DNA replication, while cytokinin initiates the events of mitosis. The cell

cycle is also controlled by the activity of cell proteins called cyclinsandcyclin-

dependent protein kinases(CDPKs; a kinase is an enzyme which will phospho-

rylate another protein). One group of cyclins, the G 1 cyclins, are manufactured

by the cell in G 1 and activate CDPKs which stimulate DNA synthesis at the G 1 /S

control point. If sufficient G 1 cyclins are not formed, the cell will not progress to

S. Having passed this point, the G 1 cyclins are degraded and a new family of

cyclins, the mitotic or M cyclinsare produced. These activate a second set of

CDPKs which permit the cell to pass the G 2 /M control point into mitosis (Fig. 3).

Whereas animal cells which pass G 1 /S are committed to undergo division, plant

cells are not. This means that many plant cells continue to replicate DNA

without dividing. This is known as endoreduplication, which is shown by more

20 Section B – Structure

Nucleoplasm

Nucleolus

Nuclear

envelope

Chromosome

Spindle fiber

Pole

Chromatids

Chromatids

Centromere with

attached

kinetochores

Phragmoplast

Nucleolus

Chromosome

Cell plate

Daughter

chromosomes

Nuclear

envelope

(a) Early prophase (c) Late prophase

(e) Anaphase

(b) Mid-prophase

(d) Metaphase (f) Telophase

Fig. 2. Stages of mitosis (see also Table 1).

M-cyclins activate

CDPK

stimulate mitosis

M-cyclins

synthesized

G 1 cyclins

degrade

G 1 cyclins

synthesized

G 1

G 2

G 1 cyclins:

activate CDPK,

stimulates DNA synthesis

M-cyclins

degrade

G 2 /M

G 1 /S

M

S

Fig. 3. Cell cycle control by the synthesis and breakdown of cyclins and their binding to

cyclin-dependent protein kinases (CDPK).