The Cell CycleBiology B/DNA and the Cell Cycle

Limits to Cell Growth As living organisms grow, so do their cells. As cells get larger, eventually they must

divide for two reasons: DNA overload: As a cell gets large, its DNA

cannot hold all the information necessary for the cell to run properly.

Movement of materials: Materials have to travel too far to get from the cell membrane to the nucleus. The cell becomes inefficient.

Preparing for Cell Division As cells prepare for division the DNA that

was loose in the nucleus folds into chromosomes.

The Cell Cycle At any given time in a cell’s life it is either

dividing or not dividing. The periods of time when cells are not

dividing are called Interphase. The periods of time when cells are dividing

is referred to as the M phase.

Interphase There are three important things that

happen during Interphase: Cell Growth DNA Replication (in preparation for cell division) Preparation for the M phase

Interphase usually lasts longer than the M phase

Interphase

M Phase The “M” in M phase stands for Mitosis,

which is the process of cell division. Mitosis is divided into four phases that

happen one after the other: Prophase Metaphase Anaphase Telophase

Prophase Prophase is the longest of the phases of

mitosis. Chromosomes appear during prophase. Centrioles (two tiny organelles in animal

cells) move away from the nuclear envelope and towards opposite ends of the cell and a spindle (fiber) forms between them

The nuclear envelope breaks down.

Prophase

Metaphase Metaphase lasts only a few minutes. The centromeres of the chromosomes

attach to the spindles in the center of the cell.

Metaphase

Anaphase Sister chromatids (that make up the

chromosomes) separate and move along the spindles to opposite sides of the cell.

The chromatids now become individual chromosomes

Anaphase ends when the chromosomes stop moving.

Anaphase

Telophase Telophase is the final phase of mitosis The chromosomes unfold into loose

groupings of DNA. Nuclear envelopes re-form around the DNA

at each end of the cell. The spindle breaks down.

Telophase

Cytokinesis There is one last part to the M phase and

the completion of cell division. During cytokinesis the cytoplasm splits to

form two separate cells. In animal cells, the cytoplasm actually splits

off. In plant cells a cell plate forms in the middle

of the dividing cell.

Cytokinesis