The Process of Cell Growth & Division

Prokaryotic vs Eukaryotic Cells  Remember that prokaryotic cells are:  Smaller than Eukaryotic Cells  Simpler than Eukaryotic Cells  It turns out that their DNA is also:  Shorter than Eukaryotic Cells  3,000 vs 300,000 genes long  Organized into only 1 strand, while Eukaryotic cells its organized into many  1 chromosome in a bacterium vs 46 in a human

A closer look at Eukaryotic DNA throughout the life of the cell…  Day-to-day the DNA is coiled around histone proteins into threads called chromatin.  Before dividing the DNA replicates (makes an identical copy of itself). The copy attaches to the original forming somewhat of an X like structure. These copies are called chromatids.  When dividing the DNA coils even further into short rods called chromosomes. These are so dense they can be easily seen through a microscope.

The Process of Cell Growth & Division

Growth (Interphase)  The period of cell growth is called interphase.  Interphase is composed of 3 parts: G1, S, G2

G1: Gap 1  Cell grows bigger and makes proteins needed for life.

 Cells can live their entire life growing. An example of this is nerve cells in humans. They never progress past this part of the cell cycle.  Most cells, though, will divide (reproduce) at some point. Why would cells divide?  Survival (they have become too big)  DNA overload  Cell Transport overload  Benefit of the larger organism  Grow  Heal  If a cell is going to divide it will continue through this sequence…

S Phase: DNA Synthesis  During the S phase, the cell replicates its DNA.  Remember the cell needs 2 complete sets of DNA to divide into two daughter cells, each with a complete copy of DNA.

G2: Gap 2  Cell grows even bigger and makes proteins needed for two cells!  Once this is complete the cell is ready for division!

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Division  Division of the DNA  Prokaryotes divide in a process called Binary Fission.  Eukaryotes divide in a process called Mitosis.

Binary Fission (Prokaryotes)  DNA (chromosomes) segregate and move to opposite sides of the cell.

Mitosis (Eukaryotes)  More complicated – must divide the entire contents of the nucleus. For a human cell that is 46 chromosomes!  The cell MUST make sure that the DNA is divided evenly so that EACH new cell has the correct instructions for life.  Because it is so important there are multiple steps…

The Four Steps of Mitosis 1. Prophase 2. Metaphase 3. Anaphase 4. Telophase Mitosis is described as a series of steps. The four steps are: The acronym PMAT may help you remember the phases in order!

Step 1 - Prophase  During prophase the nucleus prepares for division. There are 4 major events that occur in prophase. These events are outlined below:  The DNA condenses and coils into chromosomes.  The nuclear membrane disappears.  Two organelles called centrioles move to opposite ends of the cell.  Threadlike spindle fibers stretch across the cell making a football- shaped network of fibers. This is a picture of a cell in prophase.

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Step 2 - Metaphase  There are 2 major events that occur in metaphase. These events are outlined below:  The replicated chromosomes line up at the center of the cell.  Each centromere becomes attached to a spindle fiber. This is a picture of a cell in metaphase.

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Step 3 - Anaphase  During anaphase the double stranded chromosomes in the center of the cell divide. There are 3 major events that occur in anaphase. These events are outlined below:  Each centromere divides.  The sister chromatids separate.  Then, the separate chromatids begin to move away from each other toward the opposite ends of the cell. This is a picture of a cell in anaphase.

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Step 4 - Telophase  Telophase is the last step in mitosis. There are 4 major events that occur in telophase. These events are outlined below:  Centrioles and spindle fibers start to disappear.  The chromosomes stretch out and become harder to see.  A nuclear membrane forms around each mass of chromosomes.  A new nucleolus appears in each new nucleus. This is a picture of a cell in telophase.

Division of the Cell (Cytokinesis) In both Prokaryotes and Eukaryotes it is called cytokinesis. Prokaryote AnimalPlant Eukaryote Differs between

Cytokinesis in Animal Cells Cell’s membrane pinches inward creating a cleavage furrow. Eventually the two sides touch resulting in two separate daughter cells.

Cytokinesis in Plant Cells Cell’s membrane CAN NOT pinch inward because of the rigid cell wall. Instead, a cell plate forms between the two nuclei and slowly grows into a new wall separating the two daughter cells

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The Cell Cycle As you study, focus on learning the steps of the Eukaryotic Cell Cycle (right hand side)!

S phase G1 phase G2 phase

Cell Cycle in Onion Cells interphaseprophasemetaphase anaphaseTelophase / cytokinesis

Can You Find Cells in the Different Phases of the Cell Cycle?

The End