Module 5 Lesson 2 - 2

Lesson 2 - Mitosis

The Cell Cycle

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Recall that the cell cycle refers to the moment a cell is created until it divides into two daughter cells. A cell cycle involves two phases: interphase (growth) and mitotic phase (division).

Interphase

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Read pages 553 - 555

Interphase is a period during cell cycle when the cell is not dividing. It is growing, performing its normal cell functions, and replicating the genetic material in preparation of the mitotic phase. Most cells spend much of their time in interphase.

G1 Phase

During G1 stage of interphase, the cell is growing rapidly, producing proteins, and performing its intended function. Muscle or nerve cells may remain healthy and functioning at this stage for a long time. Most somatic cells reach a point at which they move to the S phase.

S Phase

In synthesis phase, or S phase, the cell duplicates its DNA. A chromosome replicates itself  by pulling apart the DNA helix and then building the missing portion on each side. DNA replication is covered in detail in Module 7.

After replication, the chromosome holds the copy. These doubled chromosomes do not contain any new genetic material. Rather, they are identical copies of each other. While together, they are known as sister chromatids and are joined together by a regional structure called the centromere until anaphase of mitosis.

In anaphase, the chromatids  pull apart and go to separate daughter cells. This occurs for each chromosome contained in the cell. For example, in the human body cell,  46 chromosomes duplicate or replicate themselves. At anaphase of mitosis, one chromatid of each of these 46 chromosomes goes to each daughter cell.

G2 Phase

After the DNA has been doubled successfully, the cell enters G2 phase. Here, the cell continues to fulfill its role in the body. Nearing the end of interphase, the cell prepares for the mitotic phase by storing energy and building proteins and other structures needed for cell division.

Mitotic Phase

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The mitotic phase is shorter than interphase and involves two processes: mitosis and cytokinesis.

Mitosis

Mitosis is one of two types of cell division. Mitosis is the type of cell division by which one cell divides into two daughter cells that are identical to the parent cell. Mitosis is used to develop a multicellular organism from a zygote. It is used to develop new cells to be used by an organism to grow or to replace worn tissues.

Mitosis is simple cell replication. Begin with one parent cell and end with two complete daughter cells, nearly identical to the original. If the cell being considered is a unicellular organism, then this kind of division is known as binary fission. 

Mitotic division in multicellular organisms is responsible for growth, development, and repair. Organisms must grow through cell division and not by increasing the cell size. Increasing the cell size decreases the ratio of the surface area of cell membrane to cytoplasmic volume. The surface area of the cell membrane reaches a limit where it is too small to support the necessary movement of nutrients and waste. Therefore, a cell must stop growing at a certain point and divide to add new cells.

The advantage of mitosis (and binary fission) is in speed and energy requirements. Only one cell is needed to start, and from that many thousands can result. This is why some bacterial infections can spread so rapidly. The disadvantage of this kind of replication is that daughter cells are genetically identical to the parent cell. Therefore, little or no variation exists in the population, and it may be susceptible to environmental changes such as the use of drugs to treat bacterial infection.

However, some bacteria mutate very rapidly and develop a resistance to drugs. These are called super bugs, and they are a high concern in the medical community. In Unit D, you will study the factors that determine the success of populations surviving or why they might not be successful in surviving leading to a decrease in their population size.


Mitosis is an orderly process that divides a cell's chromosomes carefully. These chromosomes were copied precisely in S Phase so that each one has the identical genetic content.

4 Phases of Mitosis

Mitosis can be divided into four distinct phases:

1. Prophase
   • Chromosomes condense (shorten by coiling), becoming visible under the microscope.
   • Centrioles migrate to opposite sides of the cell, and the nuclear membrane dissolves.
   • Spindle fibres grow from each centriole and attach to the centromere of each chromatid pair.

   
   

2. Metaphase
   • Chromatid pairs still joined at the centromere line up along the middle of the cell.

   
   

3. Anaphase
   • Chromatids are pulled apart by shortening of the spindle fibres (microtubules). One complete set of chromosomes is pulled to each pole.

   
   

4. Telophase ( the opposite of what happens in Prophase)
   • Chromosomes arrive at the poles and begin to decondense.
   • Spindle fibers disappear.
   • Nuclear membrane reforms.
     

Can you think of an easy mnemonic for I and PMAT to help you memorize the process of mitosis?  Your can read more about these phases on pages 557 and 558 of your textbook.

Cytokinesis

Cytokinesis is the last step in cell division. After mitosis is complete, cytokinesis divides the cytoplasm and therefore the cell physically into two daughter cells that are genetically identical to the parent cell. The cytoplasm and nucleus are divided into two daughter cells.