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Warm Up. Locate your notes on mutations Be prepared to share what you learned about mutations with the class. . Meiosis Part I. Monday; March 19, 2012. Essential Questions. How can mitosis and meiosis each contribute to the production of offspring with varying traits?

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warm up
Warm Up
  • Locate your notes on mutations
  • Be prepared to share what you learned about mutations with the class.
meiosis part i

Meiosis Part I

Monday; March 19, 2012

essential questions
Essential Questions
  • How can mitosis and meiosis each contribute to the production of offspring with varying traits?
  • How can sexual reproduction (meiosis) result in a great variety of possible gene combinations and contribute to natural selection?
what is meiosis
What is Meiosis?
  • Meiosis – Produces gametes containing half the number of chromosomes as the parent cell
  • Occurs only in the sex organs
  • Gametes – male (sperm) and female (egg) sex cells produced by specialized body organs
  • Humans:
    • Body Cell – 46 chromosomes
    • Sex Cell (Gamete) – 23 chromosomes
importance of meiosis
Importance of Meiosis
  • Without Meiosis, all offspring resulting from fertilization would have twice the amount of genetic information as its parents.
  • If too much genetic information is present within the nucleus, it can have adverse effects on the offspring.
importance of meiosis1
Importance of Meiosis
  • Meiosis solves this issue by producing gametes with half the amount of genetic information as the parent.
  • Allows for sexual reproduction and therefore, genetic variation
crossing over
Crossing Over
  • Crossing over – when genetic material is exchanged due to the tight pairing of homologous chromosomes
  • Occurs during Prophase I (in Meiosis I)
  • 2 – 3 crossovers per pair of chromosomes
  • Results in new combinations of alleles and genetic variation
  • Allele – alternative forms of a gene, and therefore a trait
  • Consist of two separate divisions
    • Meiosis I
      • Starts with 1 Diploid (2n) cell and results in 2 Diploid (2n) cells
    • Meiosis II
      • Starts with 2 Diploid (2n) cells and results in 4 Haploid (n) cells
        • Males = 4 Haploid sperm cells
        • Females = 1 Hapoid egg cell and 3 haploid polar bodies
phases of meiosis i
Phases of Meiosis I

1. Interphase

2. Meiosis I

A. Prophase I (Crossing over occurs)

B. Metaphase I

C. Anaphase I

D. Telophase I

  • Begins with a single 2n parent cell that is located in the sex organ
  • DNA is replicated resulting in a cell with 4 times the amount of chromosomes (4n)
  • After replication, two identical sister chromatids are present
prophase i
Prophase I
  • Chromosomes and spindle fibers are present
  • Homologous chromosomes come together, matched gene by gene, to form a tetrad.
  • Tetrad – consist of two homologous chromosomes paired tightly together
  • Homologous chromosomes – paired chromosomes with genes for same traits arranged in the same order
metaphase i
Metaphase I
  • Centromeres of each chromosome become attached to a spindle fiber
  • Spindle fiber pulls the tetrad into the middle of the nucleus
  • Homologous chromosomes are lined up side by side as tetrads
anaphase ii
Anaphase II
  • Homologous chromosomes separate and move to opposite ends of the cell
  • Centromeres holding the sister chromatidsdo not split
  • Each new cell receives only one chromosome from each homologous pair
telophase i
Telophase I
  • Spindle fibers beak down
  • Chromosomes uncoil
  • Cytoplasm divides to yield two new cells
  • Resulting cells only have half the genetic information of original cell from each homologous pair
meiosis i
Meiosis I
  • Beginning of Meiosis I
    • 1 Diploid (2n) Parent cell, located in the sex organ, was present
  • End of Meiosis I
    • 2 Diploid (2n) Daughter cells, located in the sex organ, is present
    • These 2 Diploid Daughter Cell will divide again during Meiosis II
  • Meiosis Video