Meiosis & Sexual Reproduction

Cell division / Asexual reproduction  Mitosis  produce cells with same information  identical daughter cells  exact copies  clones  same number of chromosomes  same genetic information Aaaargh! I ’ m seeing double!

Asexual reproduction  Single-celled eukaryotes  yeast  Paramecium  Amoeba  Simple multicellular eukaryotes  Hydra  budding What are the disadvantages of asexual reproduction? What are the advantages?

How about the rest of us?  What if a complex multicellular organism (like us) wants to reproduce?  joining of egg + sperm  Do we make egg & sperm by mitosis? eggspermzygote What if we did, then…. Doesn’t work! No! 

Human female karyotype 46 chromosomes 23 pairs XX diploid = 2 copies 2n

46 chromosomes 23 pairs XY Human male karyotype diploid = 2 copies 2n

gametes How do we make sperm & eggs?  Must reduce 46 chromosomes  23  must half the number of chromosomes  haploid egg sperm 46 meiosis 46 fertilization 23 zygote

Meiosis makes sperm & eggs  46 chromosomes to 23 chromosomes  half the number of chromosomes egg sperm 46 meiosis haploiddiploid

Paired chromosomes  Homologous chromosomes  both chromosomes of a pair carry “matching” genes  control same inherited characters  homologous = same information diploid 2n 2n = 4 homologous chromosomes double stranded homologous chromosomes eye color (brown?) eye color (blue?)

Meiosis 1 overview  1st division of meiosis  4 chromosomes  diploid  2n  2 chromosomes  haploid  1n double stranded Copy DNALine Up 1 Divide 1 gamete prophase 1metaphase 1 telophase 1

 2nd division of meiosis  looks like mitosis Meiosis 2 overview  2 chromosomes  haploid  1n 4 gametes Line Up 2 Bye Bye 2 telophase 1 metaphase 2 telophase 2

Meiosis = reduction division  Meiosis  special cell division in sexually reproducing organisms  reduce number of chromosomes  2n  1n  diploid  haploid  half  makes gametes  sperm, eggs

Meiosis & mitosis  Meiosis to make gametes  sperm & egg  Mitosis to make copies of cells  growth  repair  development

Sexual reproduction lifecycle  1 copy  haploid  1n  2 copies  diploid  2n  1 copy  haploid  1n meiosisfertilization We ’ re mixing things up here! A good thing?

mitosis zygote Putting it all together… egg sperm 46 meiosis fertilization mitosis & development meiosis  fertilization  mitosis + development 46 gametes

The value of meiosis 1  Consistency over time  meiosis keeps chromosome number same from generation to generation from Mom from Dad offspring DadMom

The value of meiosis 2  Change over time  meiosis introduces genetic variation  gametes of offspring do not have same genes as gametes from parents  new combinations of traits from Mom from Dad new gametes made by offspring offspring We ’ re mixing things up here! variation

Trading pieces of DNA  Crossing over  during Prophase 1, sister chromatids intertwine  homologous pairs swap pieces of chromosome  DNA breaks & re-attaches tetrad synapsis prophase 1

Crossing over  3 steps  cross over  breakage of DNA  re-fusing of DNA  New combinations of traits What are the advantages of crossing over in sexual reproduction?

 Sexual reproduction introduces genetic variation  genetic recombination  independent assortment of chromosomes  random alignment of homologous chromosomes in Metaphase 1  crossing over  mixing of alleles across homologous chromosomes  random fertilization  which sperm fertilizes which egg?  Driving evolution  providing variation for natural selection The value of sexual reproduction metaphase1

Variation from genetic recombination  Independent assortment of chromosomes  meiosis introduces genetic variation  gametes of offspring do not have same combination of genes as gametes from parents  random assortment in humans produces 2 23 (8,388,608) different combinations in gametes from Dadfrom Mom offspring new gametes made by offspring

Variation from crossing over  Crossing over creates completely new combinations of traits on each chromosome  creates an infinite variety in gametes

Variation from random fertilization  Sperm + Egg = ?  any 2 parents will produce a zygote with over 70 trillion (2 23 x 2 23 ) possible diploid combinations

How does this explain: family resemblance & differences! Why are the kids so similar to the parents but not exact? Martin & Charlie Sheen, Emilio Estevez Michael & Kirk Douglas Baldwin brothers

Errors of Meiosis Chromosomal Abnormalities

Chromosomal abnormalities  Incorrect number of chromosomes  nondisjunction  chromosomes don’t separate properly during meiosis  breakage of chromosomes  deletion  duplication  inversion  translocation

Nondisjunction  Problems with meiotic spindle cause errors in daughter cells  homologous chromosomes do not separate properly during Meiosis 1  sister chromatids fail to separate during Meiosis 2  too many or too few chromosomes 2n n n n-1 n+1

Alteration of chromosome number all with incorrect number1/2 with incorrect number error in Meiosis 1 error in Meiosis 2

trisomy 2n+1 Nondisjunction  Baby has wrong chromosome number  trisomy  cells have 3 copies of a chromosome  monosomy  cells have only 1 copy of a chromosome n+1n monosomy 2n-1 n-1n

Human chromosome disorders  High frequency in humans  most embryos are spontaneously aborted  alterations are too disastrous  developmental problems result from biochemical imbalance  imbalance in regulatory molecules?  hormones?  transcription factors?  Certain conditions are tolerated  upset the balance less = survivable  but characteristic set of symptoms = syndrome

Down syndrome  Trisomy 21  3 copies of chromosome 21  1 in 700 children born in U.S.  Chromosome 21 is the smallest human chromosome  but still severe effects  Frequency of Down syndrome correlates with the age of the mother

Down syndrome & age of mother Mother’s age Incidence of Down Syndrome Under 30<1 in in in in in in in in in in in in in 12 Rate of miscarriage due to amniocentesis:  1970s data 0.5%, or 1 in 200 pregnancies  2006 data <0.1%, or 1 in 1600 pregnancies

Genetic testing  Amniocentesis in 2nd trimester  sample of embryo cells  stain & photograph chromosomes  Analysis of karyotype

Sex chromosomes abnormalities  Human development more tolerant of wrong numbers in sex chromosome  But produces a variety of distinct syndromes in humans  XXY = Klinefelter’s syndrome male  XXX = Trisomy X female  XYY = Jacob’s syndrome male  XO = Turner syndrome female

Any Questions?? What are the DISadvantages of asexual reproduction? What are the DISadvantages of sexual reproduction? What are the advantages of sexual reproduction? What are the advantages of asexual reproduction?