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KEY CONCEPT Meiosis creates sex cells that are haploid. The Process of Meiosis. One parent cell produces 4 gametes /sex cells Sex cells have half the chromosome number (haploid) as the parent cell (diploid) Daughter cells are genetically different than parent cell.
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KEY CONCEPT Meiosis creates sex cells that are haploid
The Process of Meiosis • One parent cell produces 4 gametes/sex cells • Sex cells have half the chromosome number (haploid) as the parent cell (diploid) • Daughter cells are genetically different than parent cell
Meiosis is also known as Reduction Division because as the cell divides, its chromosome number is reduced
from mom from dad child Too many! meiosis reduces genetic content Why must the genetic material be reduced? n (sperm) + n (egg) = 2n (zygote) Just right!
Meiosis occurs in 2 Stages: • Meiosis I and Meiosis II
Diploid2n Haploidn : Meiosis I (ANAPHASE I ) – Homologous Chromosome Pairs separate .
Diploid2n Haploidn : Meiosis I (ANAPHASE I ) – Homologous Chromosome Pairs separate .
Diploid2n Haploid n Haploid n : Meiosis I Meiosis II Homologous chromosomes separate Sister chromatids separate .
WHAT’S THE DIFF???? • Homologous Chromosomes – * One inherited from each parent * Similar gene segments but NOT identical Sister Chromatids Sister Chromatids • Sister chromatids – * Made in Interphase “S” * Genetically identical
Interphase I Cell carries out job Chromosomes stringy chromatin Chromosomes duplicated in “S” apart .
Prophase I (Early) Chromosomes condense (coil) Nuclear membrane breaks apart Spindle fibers form .
Prophase I Late Homologous chromosomes synapse and exchange DNA segments – crossing over Increases genetic diversity cause each chromosome now has genes from both parents mixed together
Crossing Over – Increases genetic diversity This increases genetic diversity important for survival and evolution
Metaphase I Homologous chromosomes line up side by side on the equator and attach to the same spindle fiber
Anaphase I Homologous chromosomes separate and move to opposite poles. Each pole receives only 1 copy of each chromosome
Telophase I and Cytokinesis Nuclear envelopes reforms Spindle fibers disappear Cytokinesis divides the cytoplasm into two cells
End of Meiosis I • Results of Meiosis I • 2 Haploid Cells • No doubling of chromosomes occurs before Meiosis II
Four Non-identical haploid daughter cells Prophase II (haploid) Metaphase II (haploid) Anaphase II (haploid) Telophase II (haploid) Meiosis II
Prophase II Nuclear envelope breaks apart Spindle forms.
Metaphase II Chromosomes line up on cell equator
Anaphase II Sister chromatids separate and move to opposite poles.
Telophase II and Cytokinesis Nuclear envelope forms Spindle fibers disappear Chromosomes unravel. Cytokinesis divides the cell’s cytoplasm into two cells
End of Meiosis II 4 Cells with Haploid # Each cell is genetically unique
Gametogenesis – Process of Haploid cells developing into mature gametes In males meiosis occurs in the testicles and forms sperm In females meiosis occurs in the ovaries and forms eggs
SPERMATOGENESIS • Sperm get flagella and become swimmers. • Sperm mainly contribute DNA to an embryo.
OOGENESIS Eggs contribute DNA, cytoplasm, and organelles to an embryo. Cytoplasm divides unevenly - the egg gets most of the contents; the other cells form 3polar bodies which can’t be fertilized.
What Meiosis is About Meiosis allows the creation of unique individuals through sexual reproduction. Meiosis Video Link
Meiosis Video Link #1 Meiosis Video Link #2 Meiosis Video Link#3
Problems in Meiosis Nondisjunction – Failure of chromosomes to separate during Anaphase I or II Results in too many or too few chromosomes in offspring
MeioticNon-disjunction (Trisomy 21: 75% meiosis 1) Trisomy Monosomy (lethal)
Nondisjunction of Autosomes • Trisomy 21 – Down’s Syndrome • Trisomy 13 – Patau’s Syndrome
Nondisjunction - Sex Chromosomes • Turner’s Syndrome – X • Klinefelter’s Syndrome - XXY
Meiosis differs from mitosis in significant ways. • Meiosis has two cell divisions while mitosis has one.
Mitosis or Meiosis? • 2 diploid 4 haploid • Crossing over between nonsister chromatids • Reduction division • Increases genetic variability vs. clones • Meiosis 1 Homologous chrom separate • Meiosis 2 Sister chromatids separate
Mitosis or Meiosis? • 2 diploid 4 haploid • Crossing over between nonsister chromatids • Reduction division • Increases genetic variability vs. clones • Meiosis 1 Homologous chrom separate • Meiosis 2 Sister chromatids separate