Meiosis Stages of Meiosis Review Comparison of Meiosis and Mitosis https://www.youtube.com/watch?v=PICF-_yiAQ8

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

Point where chromatid connect http://www.emc.maricopa.edu/faculty/farabee/biobk/biobookmeiosis.html

Crossing Over Homologous Chromosomes Synapsis (pairing of homologous chromosomes) Chiasma form (Point where chromatid connect) Cross over at matching regions genetic recombination increases variation!!! Process itself varies(only certain part of DNA, forms bridge) Bacteria – asexual reproduction

Gametogenesis

Spermatogenesis continuous & prolific process Epididymis Testis germ cell (diploid) Coiled seminiferous tubules primary spermatocyte (diploid) MEIOSIS I secondary spermatocytes (haploid) MEIOSIS II Vas deferens spermatids (haploid) spermatozoa continuous & prolific process

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

Putting it all together… meiosis  fertilization  mitosis + development gametes 46 23 46 23 46 46 46 46 46 23 meiosis egg 46 46 46 46 23 zygote fertilization mitosis sperm development

An Introduction to Animal Development How a single cell—the fertilized egg—develops into a multicellular individual is one of the fundamental questions in modern biology. Gametes are haploid reproductive cells. In animals, male gametes are called sperm and female gametes are called eggs. Development proceeds in ordered phases through an animal’s life cycle. Gametogenesis Fertilization Cleavage Gastrulation Organogenesis

Egg Structure and Function Egg cells are relatively large and nonmotile. Their size is largely due to nutrient storage, required for early embryonic development. Quantity of nutrients varies across species. The relatively small mammalian egg only has to supply nutrients for early development, as embryos start to obtain nutrition through the placenta shortly following fertilization. Egg-laying species produce larger eggs; the yolk of the egg is the embryo’s sole source of nutrition prior to hatching.

Many conditions must be met before a zygote can form: Fertilization occurs when a haploid sperm and egg cells fuse, forming a diploid zygote (a fertilized egg). Many conditions must be met before a zygote can form: Gametes must be in the same place at the same time. Gametes must recognize and bind to each other. Gametes must fuse together. Fusion must trigger the onset of development. ***remember prezygotic – obstacle to mating or fertilization (behavioral or mechanical postzygotic – prevent hybrid offspring from developing

Why Does Only One Sperm Enter the Egg? Animals employ different mechanisms to avoid polyspermy, fertilization by more than one sperm. In sea urchins, fertilization stimulates the creation of a physical barrier. After fertilization, a Ca2+-based signal is rapidly induced and propagated throughout the egg, resulting in the formation of a fertilization envelope, which keeps away additional sperm. In mammals, the cortical granules release enzymes that modify egg cell receptors, preventing binding by additional sperm.

Cleavage Cleavage is the set of rapid cell divisions that take place in animal zygotes immediately after fertilization. Cleavage is the first step in embryogenesis, the process that makes a single-celled zygote into a multicellular embryo. Cleavage partitions the egg cytoplasm without any additional growth of the zygote. The cells created by cleavage divisions are called blastomeres. When cleavage is complete the embryo consists of a mass of blastomere cells called a blastula.

During gastrulation, extensive and highly organized cell movements radically rearrange the embryonic cells into a structure called the gastrula. Gastrulation results in the formation of embryonic tissue layers. A tissue is an integrated set of cells that function as a unit. Most early embryos have three primary tissue layers: ectoderm, mesoderm, and endoderm. These embryonic tissues are called germ layers because they give rise to adult tissues and organs.

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

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

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

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

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

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

Genetic testing Amniocentesis in 2nd trimester Analysis of karyotype 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 (infertility, less male hormone, possible learning disabilities) XXX = Trisomy X female(no physical difference) XYY = Jacob’s syndrome male(no physical diff) XO = Turner syndrome female (infertility, puffiness in hands and feet, heart and kidney Problems)