6.2 Process of Meiosis 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

6.2 Process of Meiosis Meiosis is also known as Reduction Division because as the cell divides, its chromosome number is reduced

6.2 Process of Meiosis Why must the genetic material be reduced? n (sperm) + n (egg) = 2n (zygote) from momfrom dadchild meiosis reduces genetic content Too many! Just right!

6.2 Process of Meiosis Meiosis occurs in 2 Stages: Meiosis I and Meiosis II

In Meiosis I homologous chromosome pairs separate

In Meiosis II sister chromatids separate

6.2 Process of Meiosis : Meiosis I (ANAPHASE I ) – Homologous Chromosome Pairs separate. Haploid n Diploid 2n

6.2 Process of Meiosis : Meiosis I (ANAPHASE I ) – Homologous Chromosome Pairs separate. Haploid n Diploid 2n

6.2 Process of Meiosis : Meiosis I Meiosis II Homologous chromosomes separate. Haploid n Diploid 2n Haploid n Sister chromatids separate

Sister Chromatids Sister chromatids – * Made in Interphase “S” * Genetically identical Sister Chromatids Homologous Chromosomes – * One inherited from each parent * Similar gene segments but NOT identical WHAT’S THE DIFF????

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

C rossing Over – Increases genetic diversity C rossing 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

Meiosis II Prophase II (haploid) Metaphase II (haploid) Anaphase II (haploid) Telophase II (haploid) Four Non-identical haploid daughter cells

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

6.2 Process of Meiosis SPERMATOGENESI S Sperm get flagella and become swimmers. Sperm mainly contribute DNA to an embryo.

6.2 Process of Meiosis OOGENESIS Eggs contribute DNA, cytoplasm, and organelles to an embryo. Cytoplasm divides unevenly - the egg gets most of the contents; the other cells form 3 polar bodies which can’t be fertilized.

“Putting It All Together” - Fertilization

What Meiosis is About Meiosis allows the creation of unique individuals through sexual reproduction. Meiosis Video Link

 Meiosis Video Link #1 Meiosis Video Link #1 Meiosis Video Link #1  Meiosis Video Link #2 Meiosis Video Link #2 Meiosis Video Link #2  Meiosis Video Link#3 Meiosis Video Link#3 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 chromosomes in offspring

© 2009 NHS National Genetics Education and Development CentreGenetics and Genomics for Healthcare Meiotic Non-disjunction (Trisomy 21: 75% meiosis 1) Trisomy Monosomy (lethal)

© 2009 NHS National Genetics Education and Development CentreGenetics and Genomics for Healthcare Nondisjunction of Autosomes Trisomy 21 – Down’s Syndrome Trisomy 13 – Patau’s Syndrome

Down’s Syndrome – Trisomy 21

© 2009 NHS National Genetics Education and Development CentreGenetics and Genomics for Healthcare Patau Syndrome - Trisomy 13

© 2009 NHS National Genetics Education and Development CentreGenetics and Genomics for Healthcare Nondisjunction - Sex Chromosomes Turner’s Syndrome – X Klinefelter’s Syndrome - XXY

© 2009 NHS National Genetics Education and Development CentreGenetics and Genomics for Healthcare

© 2009 NHS National Genetics Education and Development CentreGenetics and Genomics for Healthcare

6.2 Process of Meiosis Meiosis differs from mitosis in significant ways. –Meiosis has two cell divisions while mitosis has one.

6.2 Process of Meiosis MitosisMeiosis Number of divisions12 Number of daughter cells made through process 24 Are the new cells Genetically identical? YesNo # of Chromosomes compared to parent cell Same as parentHalf of parent What cell go thru this division process Somatic cellsSex cells When do cells go thru this division process Throughout lifeAt sexual maturity RoleGrowth and repairSexual reproduction

6.2 Process of Meiosis MITOSISMEIOSIS Occurs in somatic (body) cellsOccurs only in gonads (sex organs: ovary/testes) Produces cells for repair, maintenance, growth, asexual reproduction Only produces gametes (sex cells: egg/sperm) Results in identical diploid (2n) daughter cells Reduction division results in haploid (n) cells

6.2 Process of Meiosis 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

6.2 Process of Meiosis 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

© 2009 NHS National Genetics Education and Development CentreGenetics and Genomics for Healthcare Parental origin of meiotic error leading to aneuploidy