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Meiosis. Learning the Basics…. Chromosomes contain genetic information Human somatic cells have 46 chromosomes Divided into 23 pairs of homologous chromosomes , or homologs ; one set of 23 from mother, one set of 23 from father. Specific traits are coded for in genes

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learning the basics
Learning the Basics…
  • Chromosomes contain genetic information
  • Human somatic cells have 46 chromosomes
  • Divided into 23 pairs of homologous chromosomes, or homologs;
    • one set of 23 from mother, one set of 23 from father.
  • Specific traits are coded for in genes
    • Example: eye colour, hair colour, etc
Each of your parents contribute to genes towards a specific trait, but they are different forms of the same gene, called alleles.
    • Mom’s gene codes for blue eyes
    • Dad’s gene codes for green eyes
    • Both are alleles for the “eye colour gene”
  • Mitosis involves cell division that ensures both daughter cells receive a full set of chromosomes (diploid cells)
  • Sexual reproduction involves the “merging” of two gametes (sex cells) together (eg. sperm and egg)

Twice the chromosomes = New species?!?

  • Division process that prevents this “doubling” of genetic material from occurring.
  • Creates gametes with half the number of chromosomes (haploid cells)
  • Fertilization is the fusion of 2 gametes (male + female) to get back to the diploid number
stages of meiosis
Stages of Meiosis
  • Prior to the start of meiosis, cell undergoes S Phase (chromosome replication)
  • Meiosis has two rounds of cell division:
    • Meiosis I
    • Meiosis II
  • Each round is divided into 4 sub phases:
    • Prophase, Metaphase, Anaphase, Telophase
prophase i
Prophase I

Tetrad of a homologous pair

  • Chromosomes condense & shorten, visible
  • Spindle fibresform
  • Centrioles moving towards poles of the cell
  • Homologous chromosomes form a tetrad made of 4 chromatids
    • Process of forming a tetrad is called synapsis
prophase i1
Prophase I
  • In the tetrad, chromatids often break at the end and swap places with the sister chromatid
  • This process is called recombination or crossing over and allows for genetic variation
  • The location of recombination is called the chiasma
recombination crossing over
Recombination/Crossing Over:

Homologous pair of chromosomes(tetrad)

metaphase i
Metaphase I
  • Tetrads line up along equator randomly

= Independent Assortment

  • Spindle fibres attach to the pair of sister chromatids
anaphase i
Anaphase I
  • The sister chromatids do NOT separate here (different than mitosis)
  • Instead, the pairs of chromosomes move apart to opposite poles
telophase i
Telophase I
  • Chromosomes condense slightly, nuclear membrane may form
  • Cytokinesis occurs forming two genetically different daughter cells
  • Each daughter cell has half the number of chromosomes as the parent cell (haploid - n)






Prophase I



Prophase I



Metaphase I


Anaphase I


Telophase I


Meiosis I

meiosis ii1
Meiosis II
  • Similar to mitosis, but no duplication of chromosomes during a very short interphase (no G1 or S phase)
  • Each chromosome (made of two "mixed" chromatids) lines up at equator, centromeres split, and each chromosome is pulled to opposite poles
  • End result – four haploid cells
prophase ii
Prophase II
  • Nuclear envelope begins to break down
  • Spindle fibres begin to form
  • Centrioles begin to move to poles
metaphase ii
Metaphase II
  • Chromosomes align along equator of cell.
  • Spindle fibres attach to centromeres of sister chromatids
anaphase ii
Anaphase II
  • Spindle fibres contract and pull sister chromatids apart
telophase ii
Telophase II
  • Nuclear envelope assembles
  • Chromosomes decondense
  • Spindle disappears
  • Cytokinesis divides each cell into two
results of meiosis
Results of Meiosis:
  • 4 haploid gamete cells
  • 1 copy of each chromosome
  • 1 allele of each gene
  • Different combinations of alleles for different genes along the chromosome




daughter cells

Prophase II


Metaphase II


Anaphase II


Telophase II


Meiosis II





Number of divisions



Number of daughter cells




Genetically identical?


Chromosome #

Same as parent

Half of parent


Somatic cells

Germline cells


Throughout life

At sexual maturity


Growth and repair

Sexual reproduction