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Meiosis

Biology

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Objectives:

  • Students will understand the main outcome of meiosis
  • Students will be able to describe the basic process of meiosis
  • Students will be able to explain why organisms use meiosis

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What is Meiosis?

  • Meiosis is necessary for sexual reproduction
  • Meiosis only occurs in cells that have two copies of each chromosome
  • Meiosis creates four new cells from one original cell
  • The new cells only have ONE copy of each chromosome

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Identifying Chromosomes

  • Each type of chromosome has its own genes
  • Chromosomes often look different from each other under a microscope
  • Chromosomes are numbered (except for sex chromosomes)
  • Humans have 23 different chromosomes, but are paired so we end up with 46 (23 from mom and 23 from dad)

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Vocabulary

  • Cells that have two different versions of each chromosome - one from each parent - are called “diploid” or 2n
  • Cells that have only one version of each chromosome are called “haploid” or n

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More Vocabulary

  • Sister chromatids are exact, identical copies of the same sequence of DNA that are attached to each other
  • When sister chromatids split, they form two (nearly) identical strands of DNA
  • Homologous chromosomes are two unique versions of the same chromosome – they have the same genes, but potentially different versions of these genes

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Meiosis

  • Before meiosis, DNA is replicated so each chromosome consists of two sister chromatids
  • During meiosis 1, homologous chromosomes separate (daughter cells are haploid)
  • During meiosis 2, sister chromatids separate

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Outcome of Meiosis:

  • Original cell was diploid (2n)
  • Four new cells are all haploid (n)
  • Daughter cells are genetically unique:
    • Homologous chromosomes are randomly separated so each cell has some chromosomes from each parent
    • Crossing-over creates new genetic combinations that are different from original chromosomes
  • New cells are (or can divide mitotically to become) gametes – they can fuse during sexual reproduction to form a new 2n cell

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The Take-Home:

  • Meiosis results in FOUR cells from one original parent cell
  • Each new cell has half the genetic material
  • Each new cell has ONE copy of each chromosome
  • This is critical for sexual reproduction!