Meiosis Formation of Gametes (Eggs

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MeiosisFormation of Gametes (Eggs Sperm)
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Important Terminology

• Diploid (2N) two sets a cell that has 1 set
  of chromosomes from its female parent and 1 set
  from its male parent (2 copies of each
  chromosome)
• Homologous each chromosome from one parent has
  a corresponding chromosome from the other parent.
• Haploid (N or 1N)
• the number of chromosomes in a gamete (sex cell
  egg or sperm)
• Only one copy of each chromosome

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Polypoidy

• What would this mean???
• More than diploid (more than 2 sets of
  chromosomes)
• Triploid (3N)
• Tetraploid (4N)
• Not viable (survivable) in humans
• Seedless watermelons are triploid, formed from a
  cross between diploid and tetraploid watermelons.
• Because the 3 copies of the chromosomes wont
  line up correctly during meiosis, meiosis is
  halted, so no gametes (seeds) are formed.

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Definition of Meiosis

• The process of reduction division in which the
  number of chromosomes per cell is cut in half
  (reduction) through the separation (division) of
  homologous chromosomes in a diploid cell
• Homologous chromosomes (homologs) matched
  chromosomes (not necessarily identical) both
  before and after replication!

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Facts About Meiosis

• Preceded by interphase which includes chromosome
  replication
• Two meiotic divisions --- Meiosis I and Meiosis
  II
• Called reduction division
• Original cell is diploid (2n)
• Four daughter cells produced are monoploid (1n)
  (AKA, haploid)

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Facts About Meiosis

• Daughter cells contain half the number of
  chromosomes as the original cell.
• Produces gametes (eggs sperm)
• Occurs in the
• testes in males (Spermatogenesis)
• ovaries in females (Oogenesis)

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More Meiosis Facts

• Human cells start with 46 double stranded
  chromosomes (2n).
• After 1st division - 23 double stranded
  chromosomes (n) in each of 2 cells
• After 2nd division - 23 single stranded
  chromosomes (n) in each of 4 cells
•   Occurs in our germ cells that produce gametes

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Why Do We Need Meiosis?

• It is the fundamental basis of sexual
  reproduction.
• Two haploid (1n) gametes are brought together
  through fertilization to form a diploid (2n)
  zygote.

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Fertilization Putting it all together
2n 6
1n 3
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Replication of Chromosomes

• Replication is the process of duplicating a
  chromosome
• Occurs prior to division (interphase)
• Replicated copies are called sister chromatids.
• Held together at centromere

Occurs in Interphase
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A Replicated Chromosome
Gene X
Homologs separate in meiosis I and therefore
different alleles separate.
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Meiosis Forms Haploid Gametes

• Meiosis must reduce the chromosome number by
  half.
• Fertilization then restores the 2n number.

The right number!
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Meiosis Two Part Cell Division
Sister chromatids separate
Homologs separate
Diploid
Diploid
Haploid