Cell Growth and Division

Limits to Cell Growth The larger a cell becomes, the more demands the cell places on its DNA. The cell also has trouble moving enough nutrients and wastes across the cell membrane.

Surface to Volume Ratio The cell needs to maintain a high surface to volume ratio. A cell’s volume will increase at a faster rate than its surface area as it grows in size. Cell division occurs to restore a high surface to volume ratio by taking a single cell and dividing it into 2 “daughter” cells.

Surface to Volume Ratio

Cell Division This process requires copying of all the genetic material (DNA) before moving forward. In prokaryotes, a simple process of just dividing the cell’s contents into two parts.

Cell Division In eukaryotes, a 2 part process. Stage 1, the cell’s nucleus is divided by mitosis Stage 2, the cell’s cytoplasm is divided by cytokinesis.

Chromosomes Are made up of DNA, which carries the cell’s coded genetic information, and proteins. Every organism has a specific number of chromosomes. As humans, our expected chromosome number is 46. Chromosomes are only visible by microscope during cell division.

Chromosomes During replication (the copying of each chromosome in the cell), each chromosome is attached to its identical copy. They are called sister chromatids. The sister chromatids are attached at the centromere, which is usually near the middle of the chromatids.

Chromosomes

The Cell Cycle During the cell cycle, a cell grows, prepares for division, and divides to form two daughter cells, each of which then begins the cycle again. The cell cycle is divided into 4 distinct phases. G1, S, G2, and M

G1 Part 1 of interphase. A period of activity in which cells do most of their growing. During this time, cells increase in size and synthesize new proteins and organelles.

S Part 2 of interphase. Chromosomes are replicated and the synthesis of DNA molecules takes place. This stage triggers the completion of the cell cycle.

G2 Part 3 of interphase. The shortest of the 3 stages of interphase. Organelles and molecules required for cell division are produced.

M Mitosis is divided into four phases. Prophase Metaphase Anaphase Telophase Depending on cell type, the process can last a few minutes to several days.

Cell Cycle

Prophase 1st and longest of the 4 stages of mitosis. Centrioles separate and move to opposite ends of the cell. Chromosomes attach to spindle fibers near the centromere of the chromatids. Nucleolus disappears and the nuclear envelope breaks down.

Prophase

Metaphase 2nd of the 4 stages of mitosis. Usually only a few minutes in length. Chromosomes line up across the center of the cell.

Metaphase

Anaphase 3rd of the 4 stages of mitosis. Chromatids split apart from each other at the centromere, causing them to become individual chromosomes. The separated chromosomes move away from each other, towards opposite ends of the cell.

Anaphase

Telophase 4th of the 4 stages of mitosis. Chromosomes begin to disperse into a tangle of dense material, no longer seen as individual units. The nuclear envelope reforms around the two clusters of chromosomes at opposite ends of the cell.

Telophase

Cytokinesis Occurs around the same time as telophase and completes the process of cell division. Differs between plant and animal cells. In animals, a cleavage furrow forms between the two nuclei which is simply a pinching in of the cell membrane. In plants, a cell plate forms between the two nuclei and continues to extend in both directions making a new cell wall between the two sides.

Cytokinesis

Regulating Cell Cycle Cyclins Regulate the timing of the cell cycle in eukaryotic cells as a checkpoint before completing the M phase of the cycle. Cancer is when the body’s cells lose the ability to control growth. Cancer cells do not respond to the signals that regulate growth.