Instant Notes: Plant Biology

Section B – Structure

B6 Cell division

Cell division in plants occurs in meristems (Topic C1) and involves two parts:

mitosisin which the chromosomes are replicated and sorted into two nuclei,

andcytokinesisin which the cell wall, cytoplasm and organelles divide. In

dormant meristems, the cells rest in G 0 phase. When conditions are correct, the

cell begins the processes leading to division. The entire cycle may be considered

as four phases, G 1 ,S, G 2 andM(Fig. 1).

InG 1 phasethe cell doubles in size and new organelles and materials needed

for two cells are formed. During this phase, the nucleus migrates to the center of

the cell and is surrounded by a sheet of cytoplasmic strands called the phragmo-

somethat bisects the center of the cell at the plane across which it will divide.

The phase ends with the G 1 /S checkpoint. The process can stop at this point (see

Cell cycle control, below), or proceed to S phasein which DNA and associated

nuclear proteins are replicated. At the end of S phase the cell contains two full

copies of its genetic information. It proceeds to G 2 phasewhen the chromo-

somes begin to condense and structures required for division form. A distinct

band of microtubules (Topic B1), the pre-prophase band, forms around the

cytoplasm in a ring where the edge of the phragmosome lay, again predicting

the plane of cell division. At the end of G 2 , the cell has to pass another check-

point (G 2 /M) at which stage, if conditions are suitable, it enters M phasein

which the cell divides.

The cell cycle

Key Notes

Cell division occurs in stages: interphase G 1 – cell enlarges, nucleus

migrates to center, protein synthesis; interphase S – DNA replication

occurs; interphase G 2 – preprophase band and structures of mitosis form,

chromosomes condense. In M-phase mitosis, chromosomes replicate and

divide to form two daughter nuclei. In M-phase cytokinesis, the

cytoplasm divides and a cell plate and new cell walls form.

The cell cycle has two check-points: G 2 /M and G 1 /S. Progression through

the cycle is controlled by cyclins that are synthesized and degraded

through the cycle and activate cyclin-dependent protein kinases (CDPKs).

Meiosis occurs to produce haploid cells. It involves an extra round of cell

division. The first phase, prophase to anaphase I, results in exchange of

DNA between the pairs of chromosomes followed by their separation

with both chromatids present. The second phase (metaphase II to

telophase II) is a mitosis resulting in separation of the chromatids and the

formation of four haploid cells.

Related topics The plant cell (B1) Pollen and ovules (D2)

Nucleus and genome (B5) Features of growth and

Meristems and primary tissues development (F1)

(C1)

The cell cycle

Cell cycle control

Meiosis