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CHAPTER 3 Embryology - CHAPTER 3 EMBRYOLOGY Development...

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CHAPTER 3 EMBRYOLOGY Development accomplishes 2 major functions: differentiation (leads to the formation and organization of all the diverse cell types) and Reproduction I. OVERVIEW OF EMBRYOLOGY A. Embryology deals w the developmental events that occur during the prenatal period (the first 38 weeks that start w fertilization and end w birth) (3 parts to prenatal period): i. Pre-embryonic period —(2 wks), the zygote (single cell made by fertilization) becomes a multicellular blastocyst. Ends when the blastocyst implants in uterus lining. ii. Embryonic period —(3-8 wk), rudimentary versions of major organ systems. Called an embryo now. iii. Fetal period —(last 30 wks), fetus continues to grow in complexity and size. B. Note: pre-embryonic and embryonic periods together are called embryogenesis. (3 stages): i. Cleavage —zygote divides via mitosis to make a blastocyst (pre-embryonic) ii. Gastrulation —blastocyst cells make the primary germ layers (all body tissues arise from this) (embryonic) iii. Organogenesis —germ layers arrange to give rise to organs. (embryonic) II. GAMETOGENESIS A. Background i. Gametogensis —sex organs (ovaries, testes) make gametes (sex cells) ii. Human somatic cells have 23 pairs of chromosomes (22 autosomes, 1 pair of sex chromosomes), for a total of 46 chromosomes. 1. autosomes have info for most human characteristics 2. the sex chromosomes are XX or YX 3. A cell is diploid if it has 23 pairs (2n, 46 chromosomes) and haploid if it has just 23 chromosomes. (sex cells—oocyte or sperm cell—are haploid) iii. Gametogenesis starts with meiosis iv. Gametes/Sex cells made in the female are secondary oocytes, in male are sperm. B. Meiosis i. Sex cell division that starts w diploid parent cell and makes haploid daughter cells ii. Mitosis (somatic cell division) and Meiosis (sex cell division) differences: 1. Mitosis makes 2 daughter cells that are identical to the parent Meiosis makes 4 daughter cells that are different from the parent cell 2. Mitosis makes daughter cells that are diploid Meiosis makes daughter cells that are haploid 3. In Meiosis, crossing over occurs. (genes are exchanged b/w homologous chromosomes. Combines diff genes from both parents on one of the homologous chromosomes) iii. Before Meiosis comes Interphase, where the DNA is replicated to make sister chromatids. After interphase there’s 23 pairs of double stranded chromosomes.
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iv. Meiosis starts w a diploid parent cell in the gonad (testis or ovary). Parent cell has 23 pairs. In order for the organism to make its own sex cells, this parent cell must divide via meiosis. v. Meiosis I (separates maternal and paternal pairs of chromosomes) 1. Prophase I A. Synapsis —Homologous, double stranded chromosomes pair up to form a tetrad. B. Crossing over (gene from mom’s sister chromatid exchanges w dad’s to ensure genetic diversity) 2. Metaphase I A. homologous pairs line up n the middle, making a double line of chromosomes.
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