2.4 The Cell Cycle

The Cell Cycle The cell cycle is the sequence of events from one cell division to another Made up of 3 phases: Interphase Cell division Mitosis Cytokinesis

PHASE 1: Interphase 90% of the time, most of the cells in your body are not dividing. Instead, they are growing and working. cell makes copies of each organelle in the cytoplasm when it gets big enough it will make copies of its chromosomes.

Each chromosome and its copy are called sister chromatids Each sister chromatid carries identical instructions for the functions of the cell Joined at a region in the center called centromere.

At some point, the cell gets too large to function efficiently and must divide. The original cell (parent cell) divides into 2 genetically identical cells (daughter cells) in PHASE 2: CELL DIVISION

PHASE 2: Cell Division is made up of mitosis During mitosis, the nuclear material divides. The result is 2 nuclei in one cell. During PHASE 3: cytokinesis, the cytoplasm and the rest of the organelles divide. begins before mitosis is finished.

Mitosis is a continuous process divided into 4 stages: Prophase (P) Metaphase (M) Anaphase (A) Telophase (T)

Mitosis: Overview

Prophase: Chromosomes coil and condense into visible threads The nuclear membrane breaks down and disappears. Sister chromatids become joined at or near the center & look like an X

Spindle forms between the centriole pairs. In animal cells, the pair of centrioles has also been duplicated (in interphase) each pair moves to opposite poles of the cell. Spindle forms between the centriole pairs. Plant cells do not have centrioles, but they do form a spindle.

Metaphase: Sister chromatids attach to the completely formed spindle fibers. Paired chromosomes line up along equator

Sister chromatids are pulled apart by the spindle Anaphase: Sister chromatids are pulled apart by the spindle Each pair splits at the centromere (becomes a chromosome). One chromosome from each of the previous pairs moves to the opposite pole of the cell.

The events of prophase happen in reverse: Telophase: The events of prophase happen in reverse: Nuclear membranes form around the 2 new sets of chromosomes. Spindle disappears and chromosomes uncoil and become invisible again

This phase ends when the chromosomes have returned to the interphase stage. Cytokinesis begins

PHASE 3: Cytokinesis Begins at the end of mitosis, during telophase. Divides the cytoplasm into two daughter cells.

In animals, the cell membrane indents or pinches inwards. In plants, the cell forms a cell plate which starts in the center and moves towards the outer edges to split the cell in two.