Cell Size and the Cell Cycle Chapter 5, Section 1 In your textbook

Cell Size and Cell Division  Most cells are between um in diameter (1 micrometer = 1 x m)  Some bacterial cells are only several nanometers in diameter (1 nm = 1 x m)  Largest known cell = yolk of an ostrich egg, at 8 cm in diameter

Why do Cells Divide?  to replace old / damaged cells  to help the organism grow  to prevent individual cells from getting too large

Why Can’t a Cell Become Really Large?  Substances need to get in and out of the cell at a reasonable rate to keep the cell supplied with nutrients and prevent wastes from building up  These substances typically move through the cytoplasm by diffusion; as the distance they must travel increases, diffusion becomes less efficient (it takes longer)

…Too Large (cont’d)  The surface-area-to-volume ratio must be high enough to allow diffusion to occur fast enough; as a cell grows, its volume increases faster than its surface area

…Too Large (cont’d)  There must be enough DNA present to program the cell’s activities and keep them running; in some larger cells (like Pelomyxa – a giant amoeba), multiple nuclei are present

The Cell Cycle

Before cell division can take place…  a cell must replicate its genetic material so that the daughter cells created during division have the same amount of genetic material as the parent cell.

What happens during the cell cycle? The cell cycle is the continuous series of events that occur during a cell’s “lifetime”. It consists of four phases: (1) G1 – First “Gap” Phase During this phase, the cell grows and prepares for division. Mainly, its cytoplasm is increasing in volume.

Cell Cycle (cont’d) (2) S – “Synthesis” Phase During this phase, the cell’s DNA is replicated. (3) G2 – Second “Gap” Phase During this phase, the cell continues to grow and prepare for division. Mainly, its organelles are being replicated.

What happens during the cell cycle? (cont’d) These first three phases (G1, S, and G2) are often grouped together and called interphase.

What happens during the cell cycle? (cont’d)  M – Mitosis During this phase, the nucleus and the nuclear material divides  Cytokinesis During this phase, the cytoplasm divides.