Cell Division

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Cell Division ___________________________________________________________________________________

Mitosis 

Characteristic way in which somatic (non-sex) cells divide o

A parent cell will copy of all its internal components, divide them equally, and then split in half to form 2 daughter cells

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The daughter cells formed are identical to each other

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In single-celled, eukaryotic organisms this is the way that they form new individuals

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In multi-cellular organisms, mitosis is used for growth and repair of damaged tissue o

Ex. 

When an egg is fertilized, it undergoes mitosis to form a large mass of cells that will eventually be known as the embryo

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When you cut your hand, dead cells are replaced by new ones that come from the division of undamaged cells

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Mitosis is tightly controlled by the cell cycle to ensure that mitosis happens only when it is needed

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A normal cell completes the cycle with-in 24 hours

Main steps during mitosis: 1. Interphase: DNA is replicated along with organelles and other cellular components and the cell prepares for division. During the replication process, the DNA changes from one double helix (unreplicated) to two double helices (replicated). This change occurs with-in the nucleus where the DNA is normally housed The two helices in the replicated chromosomes are joined to one another in the special region of the chromosome called the centromere – at this point, the two double helices are called are sister chromatids

2. Prophase: (preparation phase) The DNA recoils and the chromosomes condense; the nuclear membrane disappears, and the mitotic spindles begin to form.

3. Metaphase: (organizational phase). The chromosomes line up the middle of the cell with the help of spindle fibers attached to the centromere of each replicated chromosome.

4. Anaphase: (separation phase). The chromosomes split in the middle and the sister chromatids are pulled by the spindle fibers to opposite poles of the cell.

5. Telophase: The chromosomes, along with the cytoplasm and its organelles and membranes are divided into 2 portions. 6. Cytokinesis The actual splitting of the daughter cells into two separate cells is called cytokinesis and occurs differently in both plant and animal cells as is illustrated below. Animal cells: The cell pinches in on both sides to form a cleavage furrow. This will gradually move toward the center to split the parent cell into 2 daughter cells.

Plant cells: The cell forms a cell plate, which starts in the center and moves towards the outer edges to split the parent cell into two daughter cells.

Meiosis:

Meiosis is used in sexual reproduction for the formation of gametes (egg and sperm cells). It creates genetic diversity. The gametes that are produced as an end result of meiosis are haploid cells, they contain half the genetic material of the parent cell. This will ensure that when gametes combine during fertilization that the new zygote will contain the normal amount of chromosomes instead of double the genetic material. Meiosis I: Just like in mitosis, during INTERPHASE, the DNA is replicated before the cell undergoes the division process. 1. Prophase I: The chromosomes coil up and condense and the nuclear membrane disappears. The cell prepares for separation. During this process, the chromosomes of each homologous pair lie so close to one another that the arms can get tangled up. The lining up of homologous chromosomes in pairs is known as synapsis. This can lead to exchange of genetic material between the homologous pairs known as crossing-over. This will serve to create genetic diversity. 2. Metaphase I: The homologous pairs line up in the center of the cell. During this time, the chromosomes obey the law of independent assortment – this says that each homologous pair arranges itself independently of the others. 3. Anaphase I: The homologous pairs are separated and pulled opposite poles of the cell by the spindle fibers. 4. Telophase I: The parent cell separates into 2

daughter cells with the division of the cytoplasm, organelles, and membranes. The new daughter cells now have the number of chromosomes of the parent cell but the chromosomes are still replicated. The two daughter cells are no longer identical to the parent cell or each other. Meiosis II: This is essentially the same as mitosis – we have to split the replicated chromosomes so they are no longer in a replicated state. Remember that this is taking place now in both of the two new daughter cells we have just created! Prophase II: This stage resembles that of mitotic prophase – the chromosomes recoil and condense and any nuclear membrane that has formed will now disappear. Metaphase II: The chromosomes line up in the middles attached to the spindle fibers and prepare for separation – again, remember this is happening in two different cells simultaneously.

Anaphase II: The sister chromatids are pulled apart to opposite ends of the cells by the spindle fibers.

Telophase II: The cells split into 2 daughter cells, and cytokinesis occurs. The

end result is 4 haploid daughter cells that are not identical to one another.