new individuals arise from outgrowths of existing ones; yeast
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fission
separation of parent organism into two or more individuals of approximately equal size; anemone, flatworm
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fragmentation/regeneration
parent organism's body is broken into several pieces and each fragment develops into a new individual by regrowing lost body parts; starfish
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parthenogenesis
* egg develops without being fertilized * estrogen level determines ovulation * haploid or diploid offspring * XY system offspring are XX * ZW system offspring are ZZ, WW, or ZW * komodo dragon, bonnethead shark, water flea, bynoe's gecko, warramaba virgo grasshopper, rotifers
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ways to modify meiosis for parthogenesis
* mitotic division of primary oocyte * combination of first polar body with secondary oocyte * combination of egg with second polar body * haploid eggs divides by mitosis instead of fusing with sperm
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simultaneous hermaphroditism
* individuals have both male and female reproductive systems * any two individuals can mate together * common in stationary animals; barnacles, clams, tapeworms, slugs
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sequential hermaphroditism
* individual undergoes a sex-change in its lifetime * common in animals living in harems (many females one male) when male dies * protandrous: male first * protogynous: female first * only in external fertilization; bony fish, snails, marine worms
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aphids
* can alternate sexual and asexual reproduction- asexual in summer * sexual in fall
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sexual reproduction
* two morphologically distinct gametes fuse together * each gamete contains half the number of chromosomes of somatic cells * gametes are created by meiosis
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sex determination in birds and butterflies
* male: ZZ * female: ZW * rare female: WW
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choosing a mate
* pheromones * sexual selection: courtship ritual to determine species similarity * competition between males * caring for offspring
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steroid reproductive hormones
* mainly produced by gonads, some by adrenal cortex * invertebrates: ecdysteroids * vertebrates: androgens (testosterone), estrogens (estradiol), progestogens (progesterone)
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gonadotropins
* peptide hormones produced by anterior pituitary * control steroid hormone synthesis * follicle-stimulating hormone (FSH), luteinizing hormone (LH) * release controlled by hypothalamic gonadotropin-releasing hormone (GnRH)
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chorionic gonadotropin (hCG)
* produced by the trophoblast chorion (later placenta) * stimulates corpus luteum to keep producing progesterone during pregnancy
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gametogenesis
production of gametes in gonads; spermatogenesis or oogenesis
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spermathecae
* sacs in female reproductive system in which sperm can be kept alive for extended periods * common in insects
* secrete seminal fluid before ejaculation * mucus, buffers
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penis
* three cylinders of spongy erectile tissue * glans: sensitive head with thin outer layer * prepuce: foreskin surrounding glans
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seminiferous tubules cells
leydig cells:
* produce testosterone- located in surrounding connective tissue (interstitial)
sertoli cells:
* produce androgen-binding protein, inhibin, nutrients, and testicular fluid (sperm transport) * fill gaps between spermatogenic cells * dispose of excess cytoplasm from sperm * form blood-testis barrier through tight junctions
spermatogenic cells
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spermatogenesis
mitosis:
* stem cells in contact with epithelial basal lamina * spermatogonia divide: type A daughter maintains germ cell line, type B moves towards lumen
1. golgi apparatus packages acrosomal enzymes in vesicle 2. acrosome positioned at anterior end of nucleus and centrioles at the opposite end 3. microtubules form flagellum of tail 4. mitochondria multiplied and positioned around proximal portion of flagellum 5. removal of excess cytoplasm 6. sperm released from sustentacular (sertoli) cells into lumen
1. hypothalamus releases GnRH 2. GnRH stimulates anterior pituitary to secrete FSH and LH 3. FSH stimulates sertoli cells to secrete androgen-binding protein (ABP) 4. LH stimulates leydig cells to secrete testosterone 5. testosterone triggers spermatogenesis 6. testosterone and inhibin (sertoli cells; released when sperm count is high) inhibit hypothalamus and anterior pituitary
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capacitation
sperm undergo a change in the female reproductive tract that enables them to penetrate and fertilize an egg
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composition of seminal fluid (semen)
* sperm * mucus: lubricant * water: liquid medium * buffers: neutralize acidic environment in male urethra and vagina * nutrients: nourish sperm * coagulating enzymes: clot semen in vagina then liquefy it * zinc: possible association with fertility * prostaglandins: aid in semen transport; decrease viscosity of cervix mucus * antimicrobial proteins: protect against pathogens
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fibrinolysis
breakdown of a clot
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chemicals aiding sperm activity
* chemokinetic: stimulate sperm to swim faster * chemotaxic: induce sperm to swim towards high concentration of chemical
* 4 gametes per meiotic division * spermatogonium (stem cell) present at birth * puberty to old age * >1 trillion sperm * 74 days to produce one gamete * one sertoli cell sustains many spermatocytes
oogenesis
* 1 gamete per meiotic division * primary oocyte (prophase 1) present at birth * fetal development to menopause * 13-50 years to produce one gamete * many granulosa cells sustain one oocyte
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estrous vs. menstrual cycles
estrous
* reproductive behavior occurs at specific part of ovulatory cycle * minimal uterine tissue is expelled
menstrual cycle:
* sexual receptivity occurs at several phases of ovulatory cycle * substantial uterine tissue is expelled
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induced ovulation
* release of mature eggs from ovaries is dependent on stimuli generated by copulation * rabbits, cats, shrews, camelids
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spontaneous ovulators
* ovulation results from endogenous processes more or less independent of mating * humans, rodents, dogs, farms, cows
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types of ovulatory cycles
induced ovulators, spontaneous ovulators
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hormones childhood vs. puberty
childhood:
* ovaries secrete small amounts of estrogen * estrogen inhibits GnRH release
puberty:
* leptin from adipose tissue decreases estrogen inhibition * GnRH, FSH, and LH are released * adult cyclic pattern achieved in \~4 years
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ovarian cycle
follicular phase
1. hypothalamus secretes GnRH 2. GnRH stimulates anterior pituitary to release FSH and LH 3. LH stimulates theca cells to produce androgens 4. FSH stimulates granulosa cells to grow and convert androgens into estrogens (aromatase) 5. inhibin and estrogen (low levels) inhibit hypothalamus and anterior pituitary 6. steep rise in estrogen stimulates hypothalamus and anterior pituitary 7. LH surge stimulates production of secondary oocyte and rupturing of follicle
luteal phase:
1. LH stimulates ruptured follicle to form corpus lutem and secrete progesterone, estrogen, and inhibin 2. progesterone, estrogen, and inhibin inhibit hypothalamus and anterior pituitary
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uterine (menstrual cycle)
proliferative phase:
1. estrogen and progesterone secreted by corpus luteum stimulates growth of endometrium 2. growth of endometrial glands
secretory phase:
1. endometrial glands secrete nutrient fluid that can sustain an embryo before implantation
menstrual phase:
1. corpus luteum disintegrates 2. drop in estrogen and progesterone causes constriction of arteries in endometrium 3. uterine lining disintegrates and blood and tissue is released
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four essential developmental processes
* cell proliferation and apoptosis * cell movement or differential expansion * cell differentiation * cell-cell interactions
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cell movement or differential expansion (development)
* reorganization of cytoskeleton * convergent extension: sheet of cells becoming longer and narrower * cell adhesion molecules * ECM components: fibers direct migrating cells, glycoproteins promote migration by providing anchorage
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cell differentiation (development)
* cells are initially determined/committed to a specific pathway and are later differentiated * differential gene expression
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cell-cell interactions (development)
change patterns of gene expression and cell activity
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initial cell differences
* unequal distribution of cytoplasmic determinants * inductive signaling
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hox (homeotic genes)
* specify segments of the body along the head-tail axis * protein product is a transcription factor * DNA sequence known as homeobox * order of genes is same order of expression along anterior-posterior axis (collinearity)
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homeosis
mutation or misexpression of hox genes that causes body parts to grow in an abnormal place
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spatial collinearity
correspondence between ordering of Hox genes along the chromosome and their expression along the head-tail axis
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loss of forelimbs in snakes
* Hoxc6 is normally expressed without Hoxc8 and gives rise to forelimbs * Hoxc6 and Hoxc8 expressed together in snakes prevents forelimb formation
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anterior-posterior limb formation
* zone of polarizing activity (ZPA) derived from mesoderm at junction of young limb and body wall * ZPA secretes sonic hedgehog (Shh)
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proximal-distal limb formation
* apical ectodermal ridge (AER) derived from ectoderm overlying mesodermal limb bud * AER directs limb development in nearby mesoderm called progress zone (PZ) * AER secretes FGF-2, FGF-4, FGF-8
1. sperm contacts egg’s jelly coat 2. exocytosis of acrosome content 3. hydrolytic enzymes digest jelly coat 4. acrosomal process protrudes from sperm head and binds to bindin on surface of egg to ensure species sameness 5. membrane depolarization: Na+ channels open and flows into cell 6. bindin receptor blocked to prevent polyspermy 7. sperm nucleus enters egg 8. Ca2+ released from smooth endoplasmic reticulum 9. wave of Ca2+ spreads from site of sperm entry to rest of inner membrane side 10. cortical granules fuse with membrane and release content into perivitelline space 11. cortical enzymes destroy sperm receptors and detach other sperm 12. cortical molecules increase osmotic value in perivitelline space 13. water is drawn in by osmosis and the vitelline layer is pushed further away from membrane 14. vitelline layer hardens and becomes fertilization envelope
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polyspermy block internal fertilization (mammals)
1. sperm weaves past corona radiata using surface enzymes 2. sperm binds to molecules in zona pellucida triggering Ca2+ rise within sperm 3. acrosomal contents are released and enzymes digest zona pellucida 4. acrosomal process protrudes from sperm and binds to egg surface receptors 5. sperm and egg membranes fuse and sperm nucleus enters cell 6. cortical reaction triggered by sperm entry 7. zona pellucida hardens and forms fertilization envelope
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egg activation
* triggered by Ca2+ surge from cortical reaction * secondary oocyte completes meiosis II and casts out second polar body * increase in rates of cellular respiration and protein synthesis
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fertilization steps
1. recognition between sperm and egg 2. regulation of single sperm entry 3. activation of egg metabolism 4. fusion of genetic material
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corona radiata
layer of granulosa cells surrounding secondary oocyte
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cleavage
* rapid series of mitotic divisions that follows fertilization * involves little or no cell growth or gene expression * S/M cell cycle * pattern influenced by amount of yolk and mitotic spindle orientation
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order of development
zygote > morula > blastula/blastocyst > gastrula
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egg/zygote polarity
* vegetal pole has more yolk; animal pole has less * differential distribution of proteins and mRNAs
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body axis formation in amphibians
* animal pole = anterior; vegetal pole = posterior * cortical rotation - point of sperm entry determines dorsal side; rotation exposes a gray crescent opposite to this point * combination of anterior-posterior and dorsal-ventral axes define left-right axis
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how is cleavage influenced by yolk
* little or no yolk * similar cell sizes (sea urchin) * complete cleavage (also frogs) * larger yolk * asymmetrical cell sizes * frogs - vegetal hemisphere has fewer but larger cells than animal hemisphere * incomplete cleavage - furrows do not penetrate yolk * discoidal or superficial
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discoidal cleavage
* type of incomplete cleavage * embryo forms disc of cells called blastodisc on top of yolk * birds (chicken), reptiles, fish (zebrafish)
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superficial cleavage
* type of incomplete cleavage * yolk is in the center (insects) * mitosis in absence of cytokinesis results in multinucleated egg * nuclei migrate to periphery of egg * membrane grows inward and divides the nuclei into individual cells
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how is cleavage influenced by mitotic spindles
* determine cleavage planes and arrangements of daughter cells * right angle or parallel to animal-vegetal axis - radial cleavage pattern * echinoderms (sea urchin), amphibians * oblique angle to animal-vegetal axis - spiral cleavage pattern
* unique to mammals * first cell division is parallel to animal-vegetal axis; second occurs at right angles * slowest within animal kingdom * not synchronous (no 2,4,8 progression) * compaction - cells form tight junctions and compact at 8-cell stage * from 16 to 32-cell stage cells separate into two masses * inner cell mass develops into embryo * outer cell mass develops into trophoblast which becomes chorion then placenta * trophoblast cells secrete fluid which forms the blastocoel and the embryo is now a blastocyst
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blastocoel
fluid-filled cavity within a blastula/blastocyst
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how long do embryonic cells remain totipotent in mammals
until the 8-cell stage
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when does embryo implantation occur
6 days; after blastula/blastocyst has formed
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modes of monozygotic twinning
* early embryo is cleaved and the halves develop into separate embryos * inner cell mass of a blastocyst is split and two separate embryos form enclosed in the same trophoblast * conjoined twins - inner cell mass does not completely separate or portions rejoin after splitting
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where does fertilization occur
upper oviduct
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how is early implantation of an embryo prevented
zona pellucida
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gastrulation
a blastula transforms into an embryo with three germ layers, body axes, and a primitive gut
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gastrulation in sea urchins
1. cells around the vegetal hemisphere flatten and invaginate into the blastocoel 2. some cells migrate and become primary mesenchyme cells 3. invagination becomes archenteron and mesenchyme cells become mesoderm 4. secondary mesenchyme cells attached to top of archenteron send out extensions to overlying ectoderm 5. mesenchyme extensions contract and pull archenteron inward 6. deuterosome - anus forms from blasopore and mouth from other side of contact
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blastopore
the origin of invagination during primitive streak formation
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gastrulation in frogs
1. cells of dorsal lip originate in gray crescent and invaginate to create archenteron 2. cells move from the embryo surface into the embryo by involution and become endoderm and mesoderm 3. blastopore encircles a yolk plug 4. deuterosome
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gastrulation in mammals, birds, and reptiles (not humans)
1. embryo forms a blastoderm and sits on top of a large yolk 2. upper layer of blastoderm (epiblast) moves towards midline of blastoderm then into the embryo toward the yolk 3. midline thickens and is called the primitive streak 4. epiblast cells remaining on top layer become ectoderm 5. epiblast cells detaching and migrating to the middle become mesoderm 6. epiblast cells at bottom layer become endoderm 7. hypoblast cells form sac surrounding yolk and stalk connecting it to embryo
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archenteron
primitive gut
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gastrulation in humans
1. trophoblast secretes enzymes which break down endometrium 2. trophoblast extends projections which cause capillaries to spill out blood 3. inner cell mass forms flat disk with outer epiblast and inner hypoblast layers 4. epiblast cells move inward through a primitive streak and form mesoderm and endoderm 5. extraembryonic membranes form
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amniotes
* organisms that develop within a fluid-filled sac contained in a shell or the uterus known as the amnion * extraembryonic membranes surround the embryo
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protostome vs. deuterostome development
protostome
* mouth forms from blastopore * spiral and determinate cleavage * solid masses of mesoderm split and form coelom
\ deuterostome
* anus forms from blastopore; mouth forms at other side of contact * radial and indeterminate cleavage * folds of archenteron form coelom
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coelom
principal body cavity between intestinal canal and body wall
* derived from dorsal mesoderm * core of large cells with fluid-filled vacuoles * rigid but flexible * in vertebrates it is replaced by skeletal structures
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extraembryonic membranes
* amnion - prevents dehydration and cushions mechanical shock; derived from epiblasts * chorion - exchanges gases between the embryo and surrounding air, forms the placenta; derived from trophoblast * allantois - disposal sac for metabolic wastes, forms the umbilical cord; derived from hypoblast * yolk sac - blood vessels in membrane transport nutrients from yolk into embryo, other nutrients stored in albumen; derived from hypoblast
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parturition
labor/birth
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initiation of labor
1. placenta secretes HCG which stimulates theca cells in corpus luteum to keep producing progesterone 2. progesterone inhibits uterine contractions 3. fetus secretes cortisol which signals placenta to secrete estrogen 4. increased estrogen causes production of oxytocin receptors by myometrium 5. estrogen causes formation of gap junctions between uterine smooth muscle cells 6. fetus produces surfactant protein A (SP-A) which induces inflammatory response in cervix to soften it 7. uterine stretching by fully grown fetus and pressure on cervix by the head stimulates the release of oxytocin from posterior pituitary 8. oxytocin stimulates uterus to contract and placenta to make prostaglandnd 9. prostaglandins and mechanical stimuli induce more frequent and stronger contractions - positive feedback