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

Why must the genetic material be reduced? n (sperm) + n (egg) = 2n (zygote) from mom from dad child meiosis reduces genetic content Too many! Just right!

Meiosis occurs in 2 Stages: Meiosis I and Meiosis II

In Meiosis I homologous chromosome pairs separate

In Meiosis II sister chromatids separate

Homologous Chromosomes – * One inherited from each parent 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

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

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

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

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

Meiosis II Prophase II (haploid) Metaphase II Anaphase II Telophase II 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

Each cell is genetically unique 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 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 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 #2 Meiosis Video Link#3

Problems in Cell Division

Effects of Mistakes in Cell Division If a mutation occurs during mitosis the outcome will affect the organism the cell is in If a mutation occurs during meiosis the outcome will affect the organism’s offspring

Mistakes in Meiosis usually occurs during Anaphase Anaphase I - homologous chromosomes fail to separate or Anaphase II - sister chromatids fail to separate.

Problems in Meiosis Failure of chromosomes to separate –Nondisjunction Typically occurs during Anaphase I or II Results in too many or too few chromosomes in sex cells

Trisomy Monosomy (lethal) After the nondisjunction event, two gametes are produced, one with two copies of chromosome 21 and the other with none. The blue gamete is a “normal” gamete that can combine with the other (red) gametes to produce zygotes (purple) which are either trisomic or monosomic for chromosome 21. Trisomy Monosomy (lethal)

Nondisjunction can occur with either autosomes or sex chromosomes

Human Disorders due to Nondisjunction of Autosomes (#1-22) Trisomy 21 – Down’s Syndrome Trisomy 13 – Patau’s Syndrome Trisomy 18 – Edward’s Syndrome Trisomy = cell has 3 copies of a chromosome

Down’s Syndrome – Trisomy 21

Examples: Down’s syndrome – (Trisomy 21) 47 chromosomes, extra chromosome at pair #21

Patau Syndrome - Trisomy 13

Edward’s Syndrome Trisomy 18

Nondisjunction - Sex Chromsomes Turner’s Syndrome – X (only 1) Klinefelter’s Syndrome - XXY

Turner’s syndrome – X Only 45 chromosomes, missing a second X sex chromosome Girls affected – short, slow growth, heart problems

Klinefelter’s syndrome XXY – 47 chromosomes, extra X chromosomes Boys affected – low testosterone levels, underdeveloped muscles, sparse facial hair

Parental origin of meiotic error leading to aneuploidy

Problems in Cell Division

If a mutation occurs during mitosis the outcome will affect the organism the cell is in If a mutation occurs during meiosis the outcome will affect the organism’s offspring through sex cell inheritance

Usually the mistake occurs during anaphase I when either homologous chromosomes either fail to separate or during Anaphase II when the sister chromatids fail to separate.

Results in too many or too few chromosomes in offspring Problems in Meiosis Nondisjunction – Failure of chromosomes to separate during Anaphase I or II Results in too many or too few chromosomes in offspring

Meiotic Non-disjunction (Trisomy 21: 75% meiosis 1) After the nondisjunction event, two gametes are produced, one with two copies of chromosome 21 and the other with none. The blue gamete is a “normal” gamete that can combine with the other (red) gametes to produce zygotes (purple) which are either trisomic or monosomic for chromosome 21. Trisomy Monosomy (lethal)

Nondisjunction of Autosomes Trisomy 21 – Down’s Syndrome Trisomy 13 – Patau’s Syndrome

Down’s Syndrome – Trisomy 21

Examples: Down’s syndrome – (Trisomy 21) 47 chromosomes, extra chromosome at pair #21

Patau Syndrome - Trisomy 13

Nondisjunction - Sex Chromsomes Turner’s Syndrome – X Klinefelter’s Syndrome - XXY

Turner’s syndrome – only 45 chromosomes, missing a sex chromosome (X) Girls affected – short, slow growth, heart problems

Klinefelter’s syndrome – 47 chromosomes, extra X chromosomes (XXY) Boys affected – low testosterone levels, underdeveloped muscles, sparse facial hair

Parental origin of meiotic error leading to aneuploidy

Evertything past this point is just a duplicate – still double check

Results in too many or too few chromosomes in offspring Problems in Meiosis Nondisjunction – Failure of chromosomes to separate during Anaphase I or II Results in too many or too few chromosomes in offspring

Meiotic Non-disjunction (Trisomy 21: 75% meiosis 1) After the nondisjunction event, two gametes are produced, one with two copies of chromosome 21 and the other with none. The blue gamete is a “normal” gamete that can combine with the other (red) gametes to produce zygotes (purple) which are either trisomic or monosomic for chromosome 21. Trisomy Monosomy (lethal)

Nondisjunction of Autosomes Trisomy 21 – Down’s Syndrome Trisomy 13 – Patau’s Syndrome

Down’s Syndrome – Trisomy 21

Patau Syndrome - Trisomy 13

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 Meiosis Number of divisions 1 2 Number of daughter cells made through process 4 Are the new cells Genetically identical? Yes No # of Chromosomes compared to parent cell Same as parent Half of parent What cell go thru this division process Somatic cells Sex cells When do cells go thru this division process Throughout life At sexual maturity Role Growth and repair Sexual reproduction

MITOSIS MEIOSIS Occurs in somatic (body) cells Occurs 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

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

Parental origin of meiotic error leading to aneuploidy