Mitosis And Meiosis

. Overview of Meiosis •

A. The function of meiosis is to produce eggs and sperm for sexual reproduction. o 1. Egg and sperm cells must have half the number of chromosomes as the parent cell so that when put together they will have the same number of chromosomes as the parent cell. 2. Egg and sperm cells are NOT genetically identical to the parent cell.

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B. Summary of Meiosis o 1. First, the genetic material must be replicated so that each daughter cell can be given the appropriate amount of genetic information. This occurs in Interphase. 2. Second, the genetic material must be divided twice so that each of the four daughter cells gets a set of information. 3. Next, all the other materials in the cell must be divided fairly equally between the four daughter cells. This process is called cytokinesis.

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C. The human parent cell begins with 46 chromosomes arranged in 2 sets of 23 each (1 set from Mom and 1 set from Dad). Each daughter cell will end up with 1 set of 23 chromosomes, some members of which may have come from EITHER the "Mom" or "Dad" set of the parent cell. o 1. A cell that normally has two sets (like us) is diploid while a cell that normally has one set (a sperm or an unfertilized egg) is haploid. 2. One parent cell divides into 4 sperm cells OR 1 egg cell + 3 polar bodies

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D. The Genetic Material o 1. First, the genetic material must be replicated so that there is a duplicate set of genetic information to be given to each daughter cell. 2. Second, the genetic material must be divided twice so that each daughter cell gets half the amount of information.

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E. After the genetic material has been replicated and divided, all the other materials in the cell must be divided fairly equally among the four daughter cells. o 1. During egg development, the majority of the cellular materials are conserved in one cell -- the unfertilized egg -- while the remaining three cells are called polar bodies. The polar bodies contain the same amount of DNA as the unfertilized egg but are extremely small and contain virtually no cytoplasm, organelles, etc.

II. Definitions •

A. Centromere: Region of the chromosome where two identical chromatids are held together B. Chromosome: A linear molecule of DNA and its associated proteins. C. Crossing-over: The exchange of genes between the chromatids of two homologous chromosomes which are closely associated during synapsis. D. Cytokineses: The separation of a mother cell into two daughter cells. It includes the division of cytoplasm, cell membrane, and organelles. E. Diploid: A situation in which an organism's chromosomes occur in pairs, one member of each pair having come from each parent. F. Gametes: The haploid sex cells, including sperm and eggs. G. Haploid: A situation in which the cell contains half the number of cells normally found in an interphase cell. H. Non-disjunction: The improper separation of homologous chromosomes during meiosis

III. Stages of Meiosis - IPMATPMAT •

A. Interphase - This is the period of time during which the cell is not ready for mitosis or meiosis. The cell grows in size and replicates its DNA by turning each un-replicated chromosome into a replicated chromosome made up of two identical chromatids.

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1. Chromosomes are extended (not condensed) and therefore are not visible. 2. Chromosomes replicate 

a. Each chromosome is now made up of two identical sister chromatids joined at a centromere. b. There is now twice the genetic information, although the same number of chromosomes as before.

3. Centrioles replicate 4. Nuclear membrane (envelope) is still present. 5. Nucleolus is visible. •

B. Prophase I o 1. Chromosomes condense (to make separation easier) 2. Nucleolus (or nucleoli) disappears 3. Nuclear membrane disappears 4. Centrioles migrate to opposite poles and begin the formation of the spindle apparatus, composed of spindle fibers. 5. Chromosomes become attached to a spindle fiber at their centromeres. 6. Synapsis occurs during this phase ONLY. 

a. Synapsis is the close association, or lining up of homologous chromosomes. b. During synapsis, crossing-over may occur as chromatids from homologous chromosomes cross each other, and breakage and reformation of DNA bonds occurs.

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C. Metaphase I

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1. Homologous pairs of chromosomes align in the "middle" (equator) of the cell in such a way that there are two chromosomes to each spindle fiber.

D. Anaphase I o

1. Homologous pairs separate and the two members of each pair move to opposite poles of the cell. 2. The number of chromosomes in the cell does not change during anaphase I. 3. Non-disjunction could occur at this stage.

E. Telophase I - May occur or cell may go into Prophase II o

1. Nuclear membranes may reform around the replicated chromosomes at the poles of the cell. 2. Spindles disappear 3. The DNA partially de-condenses. 4. Cytokinesis occurs. 

a. Animals: The cell membrane pinches inward from both sides between the two poles until two daughter cells are formed. b. Plants: Plant cells form a region of new membranes and new cell wall material in the middle of the cell which moves outward. This is called a cell plate.

F. Prophase II o

1. The DNA molecules condense (to make separation easier). 2. The nuclear membrane(s), if present, disintegrates. 3. Centrioles migrate to opposite poles and begin the formation of the spindle apparatus.

4. Chromosomes each become attached to a spindle fiber at their centromeres. ** The remaining Meiosis II Phases are identical to mitosis EXCEPT that sister chromatids ARE NOT identical to each other. ** •

G. Metaphase II o

1. Chromosomes align in the "middle" (equator) of the cell.

H. Anaphase II o

1. Centromere of each chromosome splits and one chromatid from each chromosome moves to centrioles at the poles of the cell.

I. Telophase II o

1. Nuclear membranes reform around each group of newly divided chromosomes. 2. Nucleolus (or nucleoli) reappears 3. Spindles disappear 4. Chromosomes extend, becoming invisible 5. Cytokenesis occurs. 

a. Animals: The cell membrane pinches inward from both sides between the two poles until two daughter cells are formed. b. Plants: Plant cells form a region of new membranes and new cell wall material in the middle of the cell which moves outward. This is called a cell plate.