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Meiosis. Chapter 11-4. The Basics. Genes are located on the chromosomes. Each organism must inherit one copy of every gene from both parents. Each organism has 2 complete sets of genes. Those two sets must be separated so that each gamete produced contains just one set of genes.

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meiosis

Meiosis

Chapter 11-4

the basics
The Basics
  • Genes are located on the chromosomes.
  • Each organism must inherit one copy of every gene from both parents.
  • Each organism has 2 complete sets of genes.
  • Those two sets must be separated so that each gamete produced contains just one set of genes.
chromosome number
Chromosome Number
  • Humans have 46 chromosomes.
    • 23 came from your mother
    • 23 came from your father
  • These chromosomes are homologous (same gene). This means that one complete set of genes came from each parent.
  • All 46 chromosomes are present in every human body cell (a.k.a. somatic cell).
  • This is the diploid chromosome number (2 sets).
gametes
Gametes
  • Gametes are sex cells—sperm or egg.
  • Gametes contain only 1 copy of each chromosome.
  • These cells are haploid (1 set).
  • Humans have 23 chromosomes in every gamete.
meiosis5
Meiosis
  • Meiosis is a process of reduction division.
  • The number of chromosomes per cell is cut in half through the separation of homologous chromosomes.
  • 2 distinct stages:
    • Meiosis I
    • Meiosis II
  • By the end of Meiosis II, 4 haploid cells are produced.
meiosis i
Meiosis I
  • Chromosomes are duplicated in interphase.
  • Prophase I
    • Homologous chromosomes pair up
  • Metaphase I
    • Spindle fibers attach to chromosomes to line them up in the middle
    • Crossing over occurs (genes are swapped between homologous chromosomes to increase genetic diversity)
  • Anaphase I
    • Homologous chromosomes are separated
meiosis i results
Meiosis I Results
  • Because of crossing over, the cells that are produced have chromosomes that are different from each other and from the original cell.
  • The cells produced are NOT identical copies like in mitosis.
  • Meiosis I results in 2 haploid daughter cells that are NOT genetically identical.
meiosis ii
Meiosis II
  • The chromosomes are NOT replicated in between Meiosis I and Meiosis II.
  • Prophase II—the 2 haploid cells from Meiosis I prepare for another division
  • Metaphase II—chromosomes line up in the middle of the spindle
  • Anaphase II—sister chromatids are pulled apart
  • Telophase—the nuclear membranes reform and the cytoplasm is divided
gamete formation
Gamete Formation
  • Males produce 4 sperm cells each time meiosis takes place.
  • Females produce 1 egg cell and 3 polar bodies (non-functional cells) because the cytoplasm is not divided equally.
mitosis vs meiosis
Mitosis

Body cells

2 cells produced

Diploid (2 copies of each chromosome)

Genetically identical to each other AND to the original cell

Meiosis

Sex cells (sperm/egg)

4 cells produced

Haploid (1 copy of each chromosome)

Genetically different

Mitosis vs. Meiosis