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Published byByron Lindsey Modified over 9 years ago
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If parents have two copies of each chromosome, how do they pass on only one to their offspring?
MEIOSIS is the process of cell division that decreases the number of chromosomes to make sperm or egg cells Original cell Sperm or egg cells
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The purposes of Meiosis:
Split the total number of chromosomes in half so that only ONE chromosome from each pair is inherited from each parent. Produce variation in offspring by randomly sorting the chromosomes.
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Meiosis Diploid (2n) (n = # of pairs of chromosomes) Haploid (n)
Original cell Diploid (2n) (n = # of pairs of chromosomes) Sperm or egg cells (GAMETES) Haploid (n) (single chromosomes)
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Fertilization Gametes: Zygote: (haploid sex cells) (diploid embryo)
Sperm Egg Gametes: (haploid sex cells) Zygote: (diploid embryo) Fertilized egg
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The Cell Cycle – “life” of the cell
4 stages in cell life: G1 – Growth, making proteins, doing work S – Synthesis – making a copy of DNA G2 – Growth (more) getting ready for cell division M – Mitosis or Meiosis – cell division
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Mitosis Produces two identical daughter cells
Each daughter cell has the same kind and number of chromosomes as the original parent cell Mitosis takes place AFTER DNA replication
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Interphase (Resting stage)
Chromosomes cannot be seen Chromosomes duplicate and double in number How do the chromosomes duplicate?
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nuclear membrane disappearing
Prophase nuclear membrane disappearing chromatids centromere Chromosomes become visible Each chromosome consists of two identical chromatids homologous chromosomes
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Metaphase Centrosome (made of 2 centrioles)
Chromosomes line up at the equator (centre of the cell) Spindles are formed to attach to the centromere of each chromosome
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Anaphase Sister chromatids separate as individual chromosomes
They move apart towards the opposite poles
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nuclear membrane forming
Telophase nuclear membrane forming Chromosomes gradually disappear Nuclear membrane is formed around each set of chromosomes
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Can you identify the different stages of mitosis from the diagram below?
anaphase interphase telophase metaphase prophase
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Meiosis Same steps as Mitosis: Prophase, Metaphase, Anaphase, Telophase Meiosis happens TWICE!!! Metaphase I, Anaphase II, etc.
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Some Vocab Homologous Chromosomes: chromosomes that contain the same genes, but may have different alleles (one is from mom, one is from dad) Sister Chromatids: identical copies of a single chromosome connected by a centromere
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Meiosis 1
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Close-up of Metaphase I
Crossing over: segments of DNA are exchanged between homologous chromosomes
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Law of Independent Assortment
It is random where the chromosomes line up! Mom can be either side, Dad either side.
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Meiosis II
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Close up of Metaphase II
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Summary of Meiosis Segregation of alleles: one chromosome in each of the 4 cells: only one copy of the allele Creates 4 haploid daughter cells Meiosis Video
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Comparison of Mitosis and Meiosis
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Mitosis vs. Meiosis Mitosis Meiosis Purpose
To produce new cells for 1) growth 2) replacement of old or damaged cells To produce new gametes for sexual reproduction Start with 1 Diploid somatic cell (i.e., skin cells, liver cells, muscle cells, lung cells, etc.) 1 Diploid germ cell End with 2 identical Diploid somatic cells 4 haploid gametes Number of cell divisions that occur 1 2
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Demonstration of mitosis and meiosis
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Mendel’s Laws related to Meiosis
Law of Segregation Law of Independent Assortment
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Pop bead modeling of meiosis
Follow directions from teacher Use pop beads to model each step in animation with your teacher Draw each step on your worksheet Meiosis step 1 Meiosis step 2 Meiosis step 3
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