The Cell Cycle and Mitosis Cell Cycle Interphase Mitosis Cytokinesis.
Meosis and mitosis
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Transcript of Meosis and mitosis
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Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Chapter 10LectureSlides
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10.1 Discovery of Meiosis
• Gametes are reproductive cells (eggs and sperm) that contain half the complement of chromosomes found in somatic cells the gametes fuse to form a new cell called a
zygote, which contains two complete copies of each chromosome
• the fusion of gametes is called fertilization, or syngamy
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10.1 Meiosis
• The formation of gametes must involve some mechanism to halve the number of chromosomes found in somatic cells if not the number of chromosomes would
double with each fertilization meiosis is a process of reduction division in
forming gametes• this ensures a consistent chromosome number
across generations
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10.1 Discovery of Meiosis
• Meiosis and fertilization constitute a cycle of sexual reproduction
• Somatic cells have two sets of chromosomes making them diploid
• Gametes have only one set of chromosomes, making them haploid
Figure 10.1 Diploid cells carry chromosomes from two parents
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10.1 Discovery of Meiosis
• Some organisms reproduce by mitotic division and do not involve gametes this is called asexual reproduction an example is binary fission in prokaryotes
• Other organisms are able to reproduce both sexually and asexually for example, strawberry plants flower (sexual
reproduction) and send out runners (asexual reproduction)
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10.2 The Sexual Life Cycle
• In sexual reproduction, haploid cells or organisms alternate with diploid cells or organisms
Figure 10.3 Three types of sexual life cycles
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10.2 The Sexual Life Cycle
• In animals, the cells that will eventually undergo meiosis are reserved early on for the purpose of reproduction these cells are referred to as germ-line cells
and are diploid like somatic cells but only the germ-line cells will undergo
meiosis to produce haploid gametes
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Figure 10.4 The sexual life cycle in animals
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10.3 The Stages of Meiosis
• Meiosis involves two divisions, meiosis I and meiosis II meiosis I separates the homologues in a
homologous pair DNA is replicated only before meiosis I meiosis II separates the replicate sister
chromatids when meiosis is complete, the result is that
one diploid cell has become four haploid cells
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Figure 10.7 How meiosis works
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10.3 The Stages of Meiosis
• Meiosis I is traditionally divided into four sequential stages
1. Prophase I • Homologues pair up and exchange segments
2. Metaphase I• The paired homologous chromosomes align on a central plane
3. Anaphase I• Homologues separate from the pairing and move to opposite
poles
4. Telophase I• Individual chromosomes gather at each of the two poles
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10.3 The Stages of Meiosis
• During prophase I, homologous chromosomes line up together as a pair crossing over occurs between two nonsister
chromatids of homologous chromosomes• the chromatids break in the same place, and
sections of chromosomes are swapped• the result is a hybrid chromosome
the pairing is held together by the cohesion between sister chromatids and the crossovers
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Figure 10.5 Crossing over
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10.3 The Stages of Meiosis
• During metaphase I, the orientation of the homologous chromosome pairs is a matter of chance each possible orientation of which homologue
in a homologous pair faces which pole results in gametes with different combinations of parental chromosomes
this process is called independent assortment
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Figure 10.6 Independent assortment
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10.3 The Stages of Meiosis
• In anaphase I, the chromosome pairs separate and individual homologues move to each pole
• In telophase I, the chromosomes gather at their respective poles to form two chromosome clusters
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Figure 10.8 Meiosis (just Meiosis I)
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10.3 The Stages of Meiosis
• After meiosis I, a brief interphase occurs where there is no replication of DNA
• Meiosis II follows and is basically a mitotic division of the products of meiosis I except that the sister chromatids are non-
identical because of crossing over in meiosis I
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10.3 The Stages of Meiosis
• Meiosis II is also divided into four stages
1. Prophase II• new spindle forms to attach to chromosome clusters
2. Metaphase II• spindle fibers bind to both sides of the centromere and individual
chromosomes align along a central plane
3. Anaphase II• sister chromatids move to opposite poles
4. Telophase II• the nuclear envelope is reformed around each of the four sets of
daughter chromosomes
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Figure 10.8 Meiosis (Meiosis II only)
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10.4 How Meiosis Differs from Mitosis
• Meiosis has two unique features not found in mitosis synapsis
• this is the process of drawing together homologous chromosomes down their entire lengths so that crossing over can occur
reduction division• because meiosis involves two nuclear divisions but
only one replication of DNA, the final amount of genetic material passed to the gametes is halved
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Figure 10.9 Unique features of meiosis
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Figure 10.10 A comparison ofmeiosis and mitosis
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10.5 Evolutionary Consequences of Sex
• Sexual reproduction has an enormous impact on how species evolve because it rapidly generates new genetic combinations
• Three mechanisms help produce this variety1. Independent assortment
2. Crossing over
3. Random fertilization
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Inquiry & Analysis
• Is there a difference in the rate at which microtubules are synthesized during interphase and metaphase?
• When are the microtubules of the spindle assembled?
Are New Microtubules Made During Cell Division?