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Meiosis and Sexual Life Cycles In this topic we will examine a second type of cell division used by eukaryotic cells: meiosis. In addition, we will see how the 2 types of eukaryotic cell division, mitosis and meiosis, are involved in transmitting genetic information from one generation to the next during eukaryotic life cycles. (^2)
Objective # 1
List, describe, diagram, and
identify the stages of meiosis.
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Objective 1
Only diploid cells can divide by meiosis.
We will examine the stages of meiosis in a diploid cell where 2N = 6
Meiosis involves 2 consecutive cell divisions. Since the DNA is duplicated only prior to the first division, the final result is 4 haploid cells:
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Objective 1
DNA duplication during interphase
Meiosis I
Meiosis II
Overview of meiosis in a cell where 2N = 6
After meiosis I the cells are haploid.
Objective 1, Stages of Meiosis Prophase I: ¾ Chromosomes condense. Because of replication during interphase, each chromosome consists of 2 sister chromatids joined by a centromere. ¾ Synapsis – the 2 members of each homologous pair of chromosomes line up side-by-side to form a tetrad consisting of 4 chromatids: (^6)
Objective 1, Stages of Meiosis
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Objective 1, Stages of Meiosis Prophase I: ¾ During synapsis, sometimes there is an exchange of homologous parts between non-sister chromatids. This exchange is called crossing over.
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Objective 1, Stages of Meiosis
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Objective 1, Stages of Meiosis Prophase I: ¾ the spindle apparatus begins to form. ¾ the nuclear membrane breaks down:
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Objective 1, Stages of Meiosis (2N=6)
Prophase I
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Objective 1, Stages of Meiosis
Metaphase I:
¾ chromosomes line up along the equatorial plate in pairs, i.e. homologous chromosomes remain paired.
¾ spindle microtubules attach to the kinetochores on each centromere
¾ orientation of each pair of chromosomes is random and independent of the other pairs (independent assortment): 12
Objective 1, 4 Possible Metaphase I Arrangements:
Metaphase I
*We will follow this arrangement
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Objective 1, Stages of Meiosis
Metaphase II:
¾ chromosomes line up along the equatorial plate
¾ spindle microtubules attach to the kinetochores on each centromere:
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Objective 1, Stages of Meiosis (2N=6)
Metaphase II
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Objective 1 Stages of Meiosis
Anaphase II:
¾ centromeres split and each former chromosome becomes 2 separate chromosomes
¾ spindle microtubules contract pulling the 2 members of each pair of chromosomes (that were formerly sister chromatids) to opposite poles of the cell: (^22)
Objective 1, Stages of Meiosis (2N=6)
Anaphase II
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Objective 1, Stages of Meiosis
Telophase II:
¾ nuclear envelope reforms around the 4 sets of daughter chromosomes
¾ chromosomes uncoil
¾ cytokinesis divides the 2 daughter cells into 4, genetically unique, haploid cells:
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Objective 1, Stages of Meiosis (2N=6)
Telophase II
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Objective # 2
Describe the processes of
independent assortment and
crossing over and explain their
biological significance.
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Objective 2 Independent assortment and crossing over increase genetic variability among the daughter cells produced during meiosis. If there are n pairs of chromosomes in the original cell, independent assortment produces 2n^ possible chromosome combinations in the daughter cells:
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Objective 2
Independent assortment produces 2n
possible chromosome combinations :
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Objective 2 In addition, because of crossing over, each chromosome involved in meiosis winds up with a different combination of genes. Therefore, an almost limitless number of gene combinations are possible in the daughter cells produced during meiosis:
Objective 2
With just 1 crossover, the number of possible gene combinations increases from 8 to 16!
Objective # 3
Compare mitosis with meiosis and explain the importance of each.
