Development Chapter Three

Why are we interested in Development? 1.Genes regulate every step of development 2.Understanding what is normal will help frame what is not 3.It affects every one of us here

How are genes inherited? Genes are carried in the DNA DNA is condensed into chromosomes Each individual has two copies of every chromosome Sex cells (sperm or eggs) each have one copy of every chromosome Mating leads to one copy of every chromosome coming from one parent and other copy coming from the other parent –Variances are mixed in offspring

Reproductive System Gametes are formed in the gonads –Diploid cells are divided into separated haploid gametes Males – form sperm in seminiferious tubules Females – form oocytes in ovaries Sexual reproduction allows mixing of two haploid cells into one diploid zygote

Sexual vs. asexual Reproduction Sexual: Two sexes Must come together to mate Only half population has offspring Introduce variety into population Asexual: One sex (gender) Doesn’t need another to reproduce All of population has offspring Every individual is genetically identical

Germline vs. Somatic Cells Germline: “Sex cells” –Sperm and oocytes Gametes Haploid (1N) Formed by Meiosis Somatic: Non sex cells –Hair, eyes, gut, etc Not gametes Diploid (2N) Formed by Mitosis

Meiosis Two Stages: 1.Meiosis I – dividing and reducing 2N becomes 1N 2 chromosomes become 1 chromosome 4 chromatid become 2 chromatid 2.Meiosis II – dividing Exactly the same as Mitosis 2 chromatid become 1 chromatid

Meiosis

Phases Prophase – chromosomes condense and become visible Metaphase – chromosomes align Anaphase – paired chromosomes or sister chromatids separate by centromeres Telophase – cytokinesis divides into two cells

Crossing Over Not the TV show Homologous Chromosomes actually carry different alleles of same genes Genotype – represents the two alleles carried In Metaphase I, crossing over can occur

Sex Introduces Variety 1.Crossing Over – changes which alleles are on which chromatid 2.Meiosis – which alleles will be inherited together 3.Sexual Reproduction – mixing of two parents’ alleles Evolution can act upon different alleles keeping “beneficial” and removing “detrimental” alleles

Spermatogenesis Insert figure 3.7

Oogenesis Insert figure 3.11

Important Differences: Male Meiosis: 1. Begins at puberty 2. Completes meiosis immediately Female Meiosis: 1. Begins before birth 2. Meiosis is arrested at Prophase I from birth until ovulation Arrested again at Metaphase II unless fertilized 3. Produces one ovum, and three polar bodies 3. Produces 4 equal sperm cells

Fertilization: Insert figure 3.13

After Fertilization Ovum’s nuclear membrane dissolves Chromosomes from sperm cell and chromosomes from ovum come together Forming “Pronucleus” Zygote – now has 23 pairs of chromosomes again (2N) Zygote begins mitosis or cleavage

Implantation Insert figure 3.14

The Embryo Develops: ZygoteMorulaBlastocyst Gastrula

Early Embryo Timeline Ovulation Fertilization Day 1 Day 2 Day 3 Day 4 Morula Day 5 Day 6 Day 7 Day 8 BlastocystGastrulation Implantation Day 9 Day 14

Gastrulation: Blastocyst begins to fold forming three layers: Ectoderm: Outer layer –Skin, nervous tissue Mesoderm: Middle layer –Muscle, bones Endoderm: Inner layer –Internal organs

Embryo Timeline Week 1 Week 2 Week 3Week4 Week5Week 6 Week 7Week 8Week 9 Fertilization Implantation GastrulationFetus Organogenesis: Embryo transforms from three simple layers to form all the structures that will be present at birth: Spinal cord and brain (3 - 4 weeks) Internal organs and limbs (3 - 7 weeks) Muscles and bones (7 - 8 weeks)

Placenta ~ 3 weeks after conception chorionic villi connect the developing embryo to the mother’s blood supply Chorionic villi becomes placenta ~ 10 weeks, the placenta is fully formed “3 rd Brain” –Secretes hormones –Send nutrients and O 2 to fetus

Twins Monozygotic – come from one zygote –MZ twins –Absolutely identical DNA (100 %) –May have different environments Dizygotic – arise from two eggs, forming two separate zygotes –DZ twins –Siblings that happen to share a womb –Share 50 % of DNA

Fetal Period Weeks 8 to 40 (until birth) Sex differences physically appear 6 to 12 weeks –Determined by SRY gene on Y chromosome Fetus develops: –Hair, nails, vocal cords, and digestive system –Lungs develop last (~ 37 weeks) Mostly this is a time for specialization and growth

Birth Defects 1.Genetic Abnormalities Inherit mutated version of a gene De novo mutation Chromosomal abnormalities 2.Environmental exposures Toxins, drugs Poor diet or lifestyle of mother 3.Viruses Rubella, HIV, etc

Next Class: Enjoy Labor Day Holiday! Read Chapter Four (Now we are going to start actually learning Genetics!) Homework – Chapter Three Problems; –Review: 1, 4, 5 –Applied: 3, 5, 6, 9, 11