1. The Cell Cycle Biology B/DNA and the Cell Cycle

2. 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.

3. Preparing for Cell Division As cells prepare for division the DNA that was loose in the nucleus folds into chromosomes.

4. 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.

5. 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

6. Interphase

7. 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

8. 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.

9. Prophase

10. Metaphase Metaphase lasts only a few minutes. The centromeres of the chromosomes attach to the spindles in the center of the cell.

11. Metaphase

12. 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.

13. Anaphase

14. 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.

15. Telophase

16. 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.

17. Cytokinesis