Cell Growth and Division

Cell Growth and Division

• Cells divide instead of continuously growing, due to the difficulty of moving sufficient nutrients and waste across the cell membrane in larger cells.
• The rate of exchange depends on the surface area to volume ratio.
• The volume increases much faster than the surface area as the cell grows.
• This causes a reduction in the surface area to volume ratio, making it harder to get enough oxygen and nutrients in, and waste products out.
• DNA can't meet the growing cell's needs once it grows too large.

Surface Area to Volume Ratio

• To calculate the ratio, divide the surface area by the volume.
• Volume increases more quickly than surface area.
• This decreases the surface area to volume ratio.
• This creates problems for the cell in taking in necessary materials and removing wastes.

Cell Division

• Cells divide to help overcome the problem of the surface area to volume ratio.
• Cell division produces 'daughter' cells.
• Daughter cells are smaller than the original cell.
• This solves the problem of material exchange, which allows the cell to function effectively.

Eukaryotic Cell Division

• Eukaryotic cells divide in two stages:
  • Mitosis: division of the nucleus
  • Cytokinesis: division of the cytoplasm

Replication and Chromosomes

• Replication of DNA occurs before cell division.
• Each chromosome is actually two identical sister chromatids.
• Sister chromatids are connected by a centromere, usually located near the center of the chromosome.
• Chromosomes are not visible except during cell division. When not dividing, chromosomes exist as chromatin.

The Cell Cycle

• A series of events a cell goes through to grow and divide.
• Periods of "in-between" time are called interphase.
• The cell grows, prepares for division, and then divides to form two daughter cells, each then repeating the cycle.
• Four phases in the cell cycle.

Events of the Cell Cycle

• G1 phase: Cell growth, increase in size and synthesizing new proteins and organelles.
• S phase: Chromosome replication (synthesis).
• G2 phase: Organelles and molecules needed for cell division are produced.
• M phase: Mitosis and cytokinesis; interphase is the phase between divisions.

Mitosis Stages

• Prophase: Chromosomes condense and become visible. DNA replication results in sister chromatids. Centrioles move to opposite poles. The nucleolus disappears. Spindle fibers form and attach to chromosomes. The nuclear membrane breaks down.
• Metaphase: Spindle fibers connect to the centrioles and chromosomes align along the equator of the cell.
• Anaphase: Centromeres split, allowing sister chromatids to separate. Chromatids move to opposite poles, creating a 'V' shape.
• Telophase: Spindle fibers disperse, nuclear membrane forms around each set of chromatids. Nucleoli reform. End of nuclear division. A cleavage furrow forms during cytokinesis that divides the cell into two. New daughter cells form.

Regulating the Cell Cycle

• Multicellular organisms carefully control cell growth and division.
• This allows for replacement of damaged cells or cells that have worn out, and ensures the appropriate number and size of cells as needed for an organism.
• Cells will continue to divide until they touch other cells.
• This 'stop' signal regulates cell growth and division. Mechanisms for this control are in place in our bodies.

Uncontrolled Cell Growth (Cancer)

• Cell growth is highly controlled. When this control isn't working correctly, problems occur.
• Cancer is a disorder in which cells lose the ability to control their growth.
• Uncontrolled division makes masses of cells called tumors.
• Tumors can damage surrounding tissue.
• Cancer cells can break off and spread throughout the body, disrupting activities.
• This can lead to serious medical problems and even death.

Causes of Cancer

• Smoking (tobacco)
• Radiation exposure
• Viral infections
• Genetic defects

Cancer as a Cell Cycle Disease

• Cancer is a disease of the cell cycle.