Cellular Reproduction

 

“Life comes from Life”- The Theory of Biogenesis 

1. Myths and superstitions prevail in the 1600’s generating the idea of spontaneous generation.  This is the idea that life spawns from nonliving air, soil, and water.  Experiments preformed in those days were less than scientific.
2. 1668- Francesco Redi performed a controlled experiment that proved flies arose from other flies, and not from rotting meat.
3. 1864-Around the time that the Cell Theory was being formulated, Louis Pasteur confronted the idea of spontaneous generation at the microbe level;  “Where did microbes, too small to see, come from?”  Using a flask with a curved “S” neck, Pasteur showed that “invisible” microbes floating in the air could reproduce when the conditions are favorable.  And indeed those microbes arise from other microbes.

 

The Continuance of Life-

1. Continuation of a Multicellular Organism (SELF)-
1. Mitosis- cells divide to create identical cells for growth, maintenance, and repair of a multicellular organism. The cell splits in two:

                                                               i.      The parent cell is the original cell.

                                                             ii.      The daughter cells are the identical copies created when the parent cell divided.

 

2. Continuation of the Species-
1. Sexual reproduction-the creation and union of reproductive cells from two separate organisms to increase variation in a species.

                                                               i.      Meiosis- cells divide to produce unidentical reproductive cells.

                                                             ii.      Gametes- reproductive cells that can not grow into a new organism without joining with another gamete.

 

2. Asexual reproduction- the production of offspring without the creation or union of gametes.

                                                               i.      Binary Fission- the creation of identical bacterial cells (prokaryotes) from an original parent cell.  Simpler than mitosis, a bacterium copies its one ring-like chromosome and distributes them to the two new daughter cells.

 

                                                             ii.      Spore Formation- the creation of spores, which are reproductive cells capable of developing into a new organism on their own. Found in fungi, mosses, and ferns.

 

Why are Cells so Small?

1. Both a cell’s volume and surface area grow as the cell grows.  However, a cell’s volume or contents grow at a faster rate than the growth of the surface area. As a result, the nutritional demands of the cell’s volume soon outgrow its ability to transport enough nutrients and wastes across a membrane.  Consequently, the cell stops growing before it reaches that point or it will starve.

 

Cell Specialization- Levels of Organization in a Multicellular Organism

1.       In multicellular organisms, body cells become dependent on one another to survive.  Specialized cells develop to take on different roles necessary for life.  For instance, there are different types of cells for the roles of respiration, obtaining nutrients, elimination of wastes, and communication.

2.       What at one time a cell could do on its own now needs the cooperation from other cells to survive.  Thus, as organisms became more complex different levels of organization developed:

1.       Cells- lowest level specialized for a specific function.

2.       Tissue- a grouping of the same type of specialized cells

3.       Organ- A grouping of different tissues working together to achieve a specific purpose. There are only four different types of tissue (ex. Nervous tissue), that are combined in different ways to produce organs.

4.       Organ system- All organs that function together to achieve a common objective.  Ex. The mouth, esophagus, stomach, small and large intestines all work together to obtain nutrition.

5.       Organism- All organ systems working together.

 

The Cell Cycle-In Multicellular Organisms

Cell Cycle- Cells divide, new cells are born, and then after a time of growth and development they finally divide starting the process anew.   Steps: (1.) Interphase (2.) Mitosis (3.) Cytokinesis

(From the beginning of one interphase to the beginning of the next interphase) 

 

1. Interphase - period of cell growth and development.  Many cells of the body remain in this phase and do not reproduce (Ex. Nerve cells) or reproduce only when it is needed.  If cell reproduction is to occur, then a replica (exact copy) of the cell’s DNA is made during interphase.  These identical sets of DNA will be distributed to the two new daughter cells during cell division.

 

2. Mitosis - the process of organizing the cell’s chromosomes, so they can then be equally distributed to the two new daughter cells.  It is the beginning of cellular division.

The Stages of Mitosis (PMAT)

1.       Prophase – chromosomes become visible, nuclear envelope disappears

2.      Metaphase – chromosomes line up in the middle of the cell

3.      Anaphase – Each chromosome splits in half and moves to opposite ends of the cell. (Sister Chromatids separate)

Two Sister Chromatids make up a chromosome.  One of the sister chromatids is the replica, or copy, of the DNA that was produced during interphase.

4.      Teloplase –Two nuclei form.  A nuclear envelope forms around each complete set of DNA.

 

3. Cytokinesis – ends cell division.  It is the physical separation of the cytoplasm into two new daughter cells.

 

When the cell cycle breaks down, problems arise - Cancer.

1. Uncontrolled cellular division causes Cancer.  Typically, most cells in the body do not divide unless it is for repair or maintenance.   A certain protein called cyclin is responsible for the timing of the cell cycle.  If a cell were no longer able to produce cyclin, then the cell would divide uncontrollably.  These “cancer” cells continue to divide crowding out healthy tissue.  Cancer “kills” when the mass of cancer cells interferes with the proper functions of vital organs.