Meiosis: Prophase I 

Study the figures in  text indicated by G-11.  From the readings write a description of the events which occur during prophase I of meiosis. 

 Compare your description  of these events with the ones listed below. 
 
 Prophase I  - 
        1. Nuclear membrane pulls apart and becomes part of  the E.R. and the 
            nucleoli  dissolves (disappears). 
        2. Each DNA molecule twists and coils back on itself  to form  rod shaped 
            chromosomes. 
        3. Centrioles separate and  a new centriole develops at the base of each 
            parent centriole  forming  two centriole organizing centers. These centriole 
            organizing centers than migrate to  poles on opposite  sides of the nucleus. 
        4. Spindle apparatus fill form from microtubules. Spindles will form between 
            centromeres  of  each chromosome and  each  centrilole organizing center, 
            spindles will form  between both centriole organizing centers, spindles  will 
            radiate out behind the centrioles and form asters. 
        5. The chromosomes at this time are attracted to one another in the following 
            manner.  Homologous  chromosome pairs are attracted to one another. 
           This process is calle synapsis and the resulting groups of four chromatids 
            are called  tetrads. Locate the three sets of  tetrads  in the figure below. 
            Draw and label this figure in your notes. 

Meiosis: Prophase I, Tetrads 
 
Examine  the diagram above  showing three pairs of chromosomes as they would appear during prophase I.  Each chromosome consists of two chromatids (example - two long red  or two long blue) held together by a centromere.  The two chromatids  bound together by a centromere are called sister chromatids. The original parent chromatid was the DNA molecule  of the parent cell  and  the attached chromatid was formed during the S stage of interphase. Homologous chromosomes (example one long red set and one long blue set) are attracted to one another so that there are four chromatids lined up (tetrads). 
 
6. How many chromosomes are represented in the figure above? 

        a) 0       b) 3      c) 6     d) 12 

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Study the figure below and answer the following question. 
  
7. How many chromatids are represented in this figure? 

         a) 0       b) 3      c) 6     d) 12 
 
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 Study the figure below and answer the following question. 
  
 

8. How many  tetrads are represented in this figure? 

        a) 0       b) 3      c) 6     d) 12 

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Study the two figures below and compare prophase of mitosis with prophase I of meiosis. Write the difference in your notes. 

   compare these two figures 
 
 
 
 
 

Meiosis: Tetrads with Genes 
 
Note the three sets of tetrads in the figure below. The next step to to place genes (using letters -upper case for dominant and lower case for recessive) on the chromosomes and  observe how the genes segregate during meiosis. 
 
 
 
 

In the  figure below genes have been placed on the long (G,T, and R), medium (A) and short (B)  chromosomes. The "O's" represent kinetochores. Set "A" is one of  each homologous pair and set "B" is the other set. The figure depicts three sets of homologous pair as they would appear during prophase I (except the tetrads would scattered and not in a linear arrangement. The four long structures represent one tetrad , four medium structures represent  a second tetrad and the four short structures represent the third tetrad . Draw these tetrads with corresponding genes in your notes. , 
 
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In somatic cells chromosomes usually exist in pairs. One of the pair came from the mother (ex. Set A) and the other from the father (ex. Set B).  Homologous pair refers  to those pair of chromosomes  that  have the same gene sequence; however the genes do not have to be identical.  For example  in this figure the genes are represented by the letters G, T, and R.  If the gene for  length of stems in  plants (tall  or short)  is represented  by the letter  "T" , the upper case (T) represents tall and lower case (t) represents short.  (same for G and R) in this figure. The chromosomes with the genes G, T, and R in set "A" and "B" are homologous because they  have the same gene sequence. Note that in one set the "T"  is upper case and in the homologue the "T" is lower case. 
 

Study the figure below and answer the following question. 
_ 

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9. How many homologous  pair  of chromosomes are in the figure above? 

           a) 1           b) 3        c) 6         d) 12 
 
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Study the figure below and answer the following question. 
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10. Which chromosomes are homologous? 

 a) U and V :  S and T                                  c) S and U : T and V 
 b) T and S  :  V and U                                 d) none of these 
 
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Study the figure below and answer the following question. 

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11.  Identify the tetrads. 
   a) S and U : T and V                             c)  S, U, T, and V 
   b) T and S  :  V and U                           d)  none of these 
 

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Meiosis: Metaphase I 
 
    Metaphase I -  This is an easy stage to identify.  The chromosomes (tetrads) are aligned  along the equator  of the cell. This accomplished by the spindles that are attached to  its kinetochore. Study behavioral objective 22 and the figures included in the readings indicated by G-11. 
 

Study the figure below and answer the following question. 

12. How many kinetochores are present on each centromere? 

        a) 0       b)2      c)  6     d)  12 

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Study the figure below and answer the following question. 
 
 
 13.  Describe the arrangement  of the alignment of chromosomes along the  "equator"  in relation to  the centrioles. 
      a. The equator  runs parallel  to a line running between the two  centriole 
          organizing  poles. 
      b. The equator runs perpendicular to  a line running between the two centriole 
          organizing poles. 
      c. The equator connects the two centriole organizing poles. 

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 In the  figure below genes have been placed on the long (G,T, and R), medium (A) and short (B)  chromosomes. The "O's" represent kinetochores. Set "A" is one of  each homologous pair and set "B" is the other set.  The figure  depicts  three sets of homologous pair as they would appear during metaphase I. Note that they are lined up along the equator with homologous chromosomes paired on top of one another. The four long structures represent one tetrad , four medium structures represent  a second tetrad and the four short structures represent the third tetrad . Draw these tetrads with corresponding genes in your notes. 

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 Meiosis: Anaphase I 
    Anaphase I -  Homologous  chromosomes are pulled  to opposite poles  of the cell.  The chromosomes    consist of two sister chromatids which remain attached at the centromere (compare this to mitosis).   One of each homologous  pair with attached  chromatids  are pulled by the spindles  which are attached to the kinetochore located on the centromere. Study behavioral objective 22 and the figures included in the readings indicated by G-11. 

Study the figure below and answer the following question. 

  
14. How  many  chromosomes of each homologous pair  are being pulled to each  centriole organizing pole? 

           a) 1       b) 2      c) 3     d) 6 

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 In the  figure below genes have been placed on the long (G,T, and R), medium (A) and short (B)  chromosomes. The "O's" represent kinetochores. Set "A" is one of  each homologous pair and set "B" is the  other set.  The figure  depicts  three sets of  homologous pair as they would appear during anaphase I. Note that homologous chromosome pairs are being pulled apart (indicated by the vertical lines). The four long structures represent one tetrad , four medium structures represent  a second tetrad and the four short structures represent the third tetrad . Draw these tetrads with corresponding genes in your notes. 

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      Meiosis: Telophase I 
 
   Telophase I -  When the chromosomes reach the poles  the following  events occur: 

    1. Nuclear membrane and nucleoli  reform. 
    2. Chromosomes uncoil  and untwist to reform chromatin. 
    3. Spindles  break down and disappear. 
    4. Centrioles  remain  next to the nucleus. 

 Study behavioral objective 22 and the figures included in the readings indicated by G-11. 
 

Study the figure below and answer the following question. 
 

15.  How many chromosomes are present in each of these cells? 

                      a) 3       b) 6       c)  12     d) 24 

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16.   The chromosomes at this stage consist of two sister chromatids held together by a centromere. 

                      a) true            b) false 

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Study the figure below. At the end of telophase I there are two cells. Each cell contains only one of each homologous pair (one long, one medium, and one short). The chromosomes are still in their duplicated form (two chromatids per chromosome). At this point the  two nuclei are haploid. 
 
 
  In the  figure below genes have been placed on the long (G,T, and R), medium (A) and short (B)  chromosomes. The "O's" represent kinetochores. Set "A" is one of  each homologous pair and set "B" is the  other set.  The figure  depicts  three sets of  homologous pair as they would appear during telophase I. Note that homologous chromosome pairs are separated into two cells indicated by the double line. Draw these tetrads with corresponding genes in your notes. 

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Note that the chromosomes (3)  in each cell (separated by double line) are haploid (one of each pair present) and are not genetically alike. 
 

 
Meiosis: Prophase II 
 

Meiosis II  is a normal mitotic division. The stages will be exactly like the mitotic  stages studied earlier, except  the two mother cells (prophase I) will be haploid and the end result will be four haploid daughter cells. 

Examine the figure below. The two cells at the right would correspond to the cells  at  prophase II. During the second meiotic division, there will be no DNA replication prior to prophase II. 
 
 

                "A"                                     "B" 

        The two cells  above the letter "B" represent the two mother cells during prophase I. 
 
        1.  Nuclear membrane pulls apart and becomes part of  the E.R. and the 
             nucleoli  dissolves (disappears). 
        2.  Each DNA molecule twists and coils back on itself  to form  rod shaped 
             chromosomes. 
        3. Centrioles separate and  a new centriole develops at the base of each 
            parent centriole forming  two centriole organizing centers. These centriole 
            organizing centers than migrate to poles on  opposite sides of the nucleus. 
        4. Spindle apparatus fill form from microtubules. Spindles will form between 
            centromeres  of each chromosome and  each  centrilole organizing center, 
            spindles will form  between both centriole organizing centers, spindles  will 
            radiate out behind the centrioles and form asters. 

 Study behavioral objective 22 and the figures included in the readings indicated by G-11. 
 

Examine  the diagram below  showing three pairs of chromosomes as they would appear during prophase II. Each chromosome consists of two chromatids  held together by a centromere.  The two chromatids  bound together by a centromere are called sister chromatids. At this point there is only one of each homologous pair present; therefore the nuclei are haploid. 

Study the figure below and answer the following question. 

17.   How many chromosomes per cell are represented in this figure? 

                          a) 0       b) 3      c) 6     d) 12 
 
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18.   How many chromatids per cell  are represented in this figure? 
 
                           a) 0       b) 3      c) 6     d) 12 
 
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19.   How many centromeres per cell are represented in this figure? 
 
                a) 0       b) 3      c) 6     d) 12 

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                                             The figures above may also represent prophase II. 
 
During prophase II of meiosis  the homologous chromosomes are not in pairs. See the figure above. 
There are only three chromosomes per cell. 

In the figure below genes are added to the chromosomes.  In this figure the genes are represented by the letters G, T, and R.  If the gene for  length of stems in  plants (tall  or short)  is represented  by the letter  "T" , the upper case (T) represents tall and lower case (t) represents short.  (same for G and R) in this figure.  Note that set "A"  chromosomes is in one cell and set "B" is in a different cell (separated by the double line).  Each cell has a haploid number of chromosomes (the chromosomes are not in pairs). 

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20.  How many homologous  pair  of chromosomes  per cell are in this figure? 

               a) 0      b)  2    c) 3     d)  6 
 

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21. How many chromosomes are present in each cell? 

                a) 0      b)  2    c) 3     d)  6 

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Meiosis: Metaphase II 

This is an easy stage to identify.  The chromosomes are aligned  along the equator  of  the  cell. This accomplished by the spindles that are attached to  its kinetochore. Using the figures in the text compare metaphase of mitosis, metaphase I of  meiosis, and metaphase II of meiosis.  There are no homologous chromosomes located in the metaphase II stage.  Draw these three stages in your notes. Include three sets of chromosomes. In the figure below genes have been added to the chromosomes. At this stage their are two cells ("A and B"). Each cell contains three chromosomes (L, M and N). These  chromosomes are not paired. 

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22. How many chromatids per chromosome in cell "A" or cell "B"? 

                    a)  0    b)  2     c)   3       d)  4 
 
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23. Are the chromosomes in cell "A" genetically like the chromosomes in cell "B"? 

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Meiosis:  Anaphase II 
 
 

In each cell (cells "A" and "B") sister chromatids are pulled  to opposite poles  of the cell. Each cell has three chromosomes (L,M, and N).  Up to this  time each chromosome consists of two sister chromatids.   The  centromeres  "O" split  and  the chromatids  are pulled by the spindles  which are attached to the kinetochore located on the centromere. This is represented by the vertical line Note that the chromosomes are  not in pairs like they were during mitosis. 

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24.  How  many chromatids are being pulled to each  centriole organizing pole? 

                     a) 2      b) 3         c)  6          d)  12 
 
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 Meiosis: Telophase II 

This stage is the opposite of prophase. When the chromosomes reach the poles  the following  events occur: 

    1. Nuclear membrane and nucleoli  reform. 
    2. Chromosomes uncoil  and untwist to reform chromatin. 
    3. Spindles  break down and disappear. 
    4. Centrioles  remain  next to the nucleus. 

The cells under "A" represent prophase II chromosomes and the cells under "B" represent telophase II chromosomes. Study the figure below and answer the following questions. 
 
          A                            B 
 
          "A"                         "B" 

25.  How many chromosomes are present in each of these cells (B)? 

    a) 2      b) 3         c)  6          d)  12 

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        "A"                           "B" 
26.   The chromosomes at this stage (B) consist of two sister chromatids held together by a centromere. 

                      a) true            b) false 
 
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The figure below represents the four cells formed by meiosis along with their genetic make-up. Each cell contains three chromosomes (L,M and N). These chromosomes are not in pairs; therefore the nuclei are haploid. 

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Note that two of the cells contain a "B" and two "b". 
 
 

Study behavioral objective  23, 24 and 25. In your notes write out a definition contrasting  between diploid and haploid cells; somatic and sex cells. Diploid cells contain homologous pairs  of chromosomes.  One of each homologous chromosome pair  was furnished by the mother and the other by the father.  Haploid cells  contain one half the number of chromosomes as a diploid  since they contain only one of each  homologous  chromosome pair.  Haploid cells are formed  as the end result of meiosis.   Only diploid cells  can under go meiosis  however both diploid and haploid cells can undergo mitosis. In higher plants and animals  meiosis occurs in special organs (sex organs). The haploid cells that are the end result of meiosis will develop into sex cells (gametes). Male gametes are called sperm and female gametes are called eggs or ovum.  In contrast to sex cells  the cells that make up the rest of an organism are called somatic cells. Usually in higher plants and animals  somatic cells are diploid. 
 
27. How  many  chromosomes are in your somatic cells? 

                a) 12      b) 23      c) 24      d)  46    e)  48 
 
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28. How many chromosomes in your somatic cells did you receive from your mother? 

                 a)  12      b)  23      c)  24     d)  46    e)  48 

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29. How many chromosome pair are the  in  the human sperm cell? 

                a)  0       b)  23      c)  30     d) 46 

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30.  Which cells contain  homologous pairs of chromosomes? 

         a)  egg        b) sperm      c) gametes     d) human bone cell 
 
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 Meiosis provides a means for chromosomes to segregate out individually from one another. Genes located  on separate chromosomes  will also segregate out independently from one another.  However,  genes located on the same chromosome can not segregate independently  from one another.  For example during Prophase I above  the genes " G,t, and R" were  located on the same chromosome and "G,T and R"  on its homologue. How do these genes (genes located on the same chromosome) segregate out from one another, or will the gene G always be linked to t and r ? Linked genes  may be switched with its homologue by a process called crossing over. This process occurs during Prophase I of meiosis. The next web page  will explain crossing over and show that  linked genes on one chromosome may be  switched with the genes on its homologue.  Click here to go to  the next web page.
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