CELL DIVISION OBJECTIVES: Describe the main events of the cell cycle. Describe the structure of chromosomes. Explain what happens during the four phases of mitosis.

Splitting up a cell What do you think would happen if this cell suddenly split in half?? Cells must prepare for cell division!

Getting the DNA ready Chromosomes – structures in the nucleus that contain DNA and proteins. Chromsomes are not visible except during cell division! Before cell division, each chromosome is copied! chromatids centromere

The Cell Cycle Cell Cycle – the series of events that cells go through as they grow and divide. 2 major parts: interphase cell division

Interphase Interphase – period of time when the cell is not dividing. Three phases: G1 phase: cell is growing and doing its job. S phase: chromosomes are copied. G2 phase: cell prepares for division.

Cell Division (M phase) Cell division is divided into 2 parts: mitosis – the division of the nucleus cytokinesis – the division of the cytoplasm Mitosis has 4 phases: prophase metaphase anaphase telophase

Mitosis phase 1: Prophase First and longest phase of mitosis Chromosomes become visible Centrioles move to opposite sides of the cell and organize the spindle: a fanlike structure that separates the chromosomes. Nuclear membrane breaks down to open the nucleus.

Mitosis phase 2: metaphase Lasts only a few minutes Chromosomes line up in the center Spindle attaches to the centromeres on the chromosomes

Mitosis phase 3: anaphase Centromeres split, allowing the sister chromatids to separate Spindle pulls the chromatids apart

Mitosis phase 4: telophase Final phase of mitosis Chromosomes unravel Nuclear membranes form around each set of chromosomes to form two nuclei Spindle breaks apart

cytokinesis The cytoplasm pinches together until two new cells are formed.

Mitosis & Cytokinesis