Meiosis vs. Mitosis Animation Animation ▫1 division ▫daughter cells genetically identical to parent ▫produces 2 cells ▫2n  2n ▫produce cells for growth & repair ▫NO crossing over ▫2 divisions ▫daughter cells genetically different from parent ▫produce 4 cells ▫2n  1n ▫produces gametes ▫crossing over!! ▫division ▫Begins with interphase ▫PMAT

Meiosis Stages of Meiosis Review BioFlix Animation Unique Features of Meiosis

Crossing Over Homologous Chromosomes Synapsis Chiasma form Cross over at matching regions of nonsister chromatids genetic recombination increases variation!!! Process itself varies Bacteria – asexual reproduction

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

An Intro to Animal Development Fundamental biological question: How does a single cell—the fertilized egg—develop into a multicellular individual? Development proceeds in ordered phases through organism’s life cycle: ▫ Gametogenesis ▫ Fertilization ▫ Cleavage ▫ Gastrulation ▫ Organogenesis

Gametogenesis – sperm & egg production

Oogenesis MEIOSIS I MEIOSIS II first polar body second polar body ovum (haploid) secondary oocyte (haploid) primary oocyte (diploid) germinal cell (diploid) primary follicles mature follicle with secondary oocyte ruptured follicle (ovulation) corpus luteum developing follicle fertilization fallopian tube after fertilization

Egg Structure and Function Relatively large & nonmotile Size largely due to nutrient storage, required for early embryonic development Quantity of nutrients varies across species ▫ Relatively small mammalian egg supplies nutrients for early development ONLY  embryos start to obtain nutrition through placenta shortly after fertilization ▫ Egg-laying species form larger eggs  yolk of the egg is embryo’s SOLE nutritional source prior to hatching

Spermatogenesis Epididymis Testis Coiled seminiferous tubules Vas deferens spermatozoa spermatids (haploid) secondary spermatocytes (haploid) primary spermatocyte (diploid) germ cell (diploid) MEIOSIS II MEIOSIS I ▫ continuous & prolific process ▫ million sperm can be produced daily

Fertilization - haploid sperm and egg cells fuse to form diploid zygote (a fertilized egg). Certain conditions MUST be met before a zygote can form: ▫ Location: gametes in same place at same time. ▫ Recognition: recognize & bind to each other. ▫ Fusion! ▫ Trigger onset of development.

Successful fertilization leads to development – how do we become a person from 2 single cells? Do you think the process is the same for all living things? Explain and describe your vision of this process.

Why Does Only One Sperm Enter the Egg? Animals use different mechanisms to avoid polyspermy - fertilization by more than one sperm Ex: sea urchins - fertilization stimulates creation of a physical barrier ▫ Ca 2+ -based signal is rapidly induced & propagated throughout the egg, form a fertilization envelope keeps away additional sperm Ex: mammals - cortical granules release enzymes  modify egg cell receptors  prevent binding by additional sperm.

Cleavage Cleavage - set of rapid cell divisions in animal zygotes immediately after fertilization. ▫1 st step is embryogenesis - process that makes single-celled zygote into multicellular embryo. ▫partitions egg cytoplasm without additional growth of the zygote. ▫Cells created by cleavage divisions called blastomeres. ▫Completed cleavage results in embryo consisting of mass of blastomere cells called a blastula.

Gastrulation - extensive & highly organized cell movements radically rearrange embryonic cells into structure called the gastrula. ▫ results in formation of embryonic tissue layers (**tissue is integrated set of cells functioning as a unit) ▫ most embryos have 3 primary tissue layers: ectoderm, mesoderm, and endoderm.  these tissues are called germ layers because they give rise to adult tissues and organs. Gastrulation

Ectoderm - forms outer covering of adult body & nervous system Mesoderm - gives rise to muscle, most internal organs, & connective tissues like bone & cartilage Endoderm - produces lining of digestive tract or gut, along with some associated organs

What if something goes wrong? What happens then???

nondisjunction - chromosomes do not segregate correctly during meiosis Incorrect chromosome # Deletion Duplication Inversion Translocation ** Extra chromosomes often means survival** **Missing chromosomes often means DEATH**

Monosomy - One less chromosome due to missing chromosome in gamete Ex: Turner Syndrome KARYOTYPEKARYOTYPE

Trisomy - Gamete has an extra chromosome Ex: Trisomy 21 (Down Syndrome)

Polyploidy - C omplete EXTRA sets of chromosomes – almost ALWAYS lethal to animals – plants can be healthier & larger

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 chromosomes Results in variety of distinct syndromes ▫ XXY = Klinefelter’s syndrome male ▫ XXX = Trisomy X female ▫ XYY = Jacob’s syndrome male ▫ XO = Turner syndrome female