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Asexual Reproduction
the creation of offspring without fusion of egg and sperm
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Sexual Reproduction
the creation of offspring by fusion of a male gamete (sperm) and female gamete (egg) to form a zygote
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4 types of asexual reproduction
- Fission: separation of a parent into 2 or more individuals of about the same size
- Fragmentation: breaking the body into pieces, which develop into adults
- Budding: new individuals arise from outgrowths of existing ones
- Parthenogenesis: the development of a new individual from an unfertilized egg
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Cost of sexual reproduction
less offspring
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Advantages of sexual reproduction (4)
- diversity is advantageous in catastrophic events
- increases in rate of adaptation
- higher reproductive success, offspring live longer
- shuffling of genes and elimination of harmful genes
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Hermaphroditism
each individual has male & female reproductive systems
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What does meiosis do?
- reduces the chromosome number in half
- produces gametes
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External vs. Internal Fertilization
- External fertilization: eggs shed by the female are fertilized by sperm in the external environment
- Internal fertilization: sperm are deposited in or near the female reproduction tract, fertilization occurs within the tract
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Ovulation
the release of mature eggs at the midpoint of female cycle
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Cortical Reaction
seconds after sperm binds to egg, vesicles beneath the plasma membrane release their contents and form a fertilization envelope
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How is the cortical reaction initiated?
by fusion of egg and sperm
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What is the role of the fertilization envelope?
To prevent polyspermy (entry of multiple sperm nuclei into the egg)
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What is a calcium wave? When does it occur?
- Ca2+ spreads across the egg
- correlates with the appearance of the fertilization envelope
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Capacitation
when secretions in the mammalian female reproductive tract alter sperm motility and structure
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Spermatogenesis vs. Oogenesis (3 differences)
- all 4 products of meiosis develop into sperm while only one of the 4 becomes an egg
- spermatogenesis occurs throughout adolescence and adulthood
- sperm are produce continuously unlike the prolonged interruption in oogenesis
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Ovarian Cycle vs. Menstrual Cycle
changes in the ovaries vs. changes in the uterus
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Events in ovarian cycle
- sequential release of FSH and LH stimulates follicle growth
- follicular phase ends at ovulation, and secondary oocyte is released
- after ovulation, the corpus luteum secretes progesterone and estradiol
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Events in the menstrual cycle
- Estradiol and progesterone promote endometrium to thick with blood vessels in preparation for embryo implantation
- If an embryo is not implants, the endometrium is shed (menstruation)
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Steps in Female Reproductive Cycle
- Ovulation
- Fertilization
- Cleavage
- Implantation
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What conditions are necessary for formation of sperm?
temperature must be lower than body temp.
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Oocyte
a partially developed egg
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Follicle
consists of an oocyte surrounded by support cells
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Oogenesis
process by which an oocyte develops into an ovum
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What phase is the oocyte stopped at before birth of the female?
primary oocyte paused at prophase of meiosis 1
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What happens to the oocyte when a female starts puberty?
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In vitro fertilization
eggs are mixed with sperm in culture dishes and the embryo is returned to the uterus at the eight-cell stage
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What are the two sex hormones? functions?
- testosterone & progesterone
- gamete production
- development of secondary sex characteristics
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Why does cortical reaction occur?
to prevent polyspermy
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Why are testes located outside of the body?
Because spermatogenesis cannot occur at body temperature
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Where does spermatogenesis take place?
in the seminiferous tubule
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When does spermatocyte go from 2n to n?
when it becomes a secondary spermatocyte
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What happens during meiosis II of spermatogenesis?
4 early spermatids are formed
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If thickening of endometrium is blocked, what else is blocked?
implantation of the fertilized embryo
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What do progesterone and estradiol do?
promote thickening of the endometrium
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When does ovulation occur in the reproductive cycle?
when LH is at its peak
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After ovulation occurs, what happens to the follicle tissue?
the leftover tissue forms the corpus luteum
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When progesterone and estradiol levels increase, what happens to the levels of FSH & LH? why?
- FSH & LH decreases
- this prevents another egg from maturing when a pregnancy may be under way
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Follicular Phase
growing follicle starts to secrete more and more estradiol
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Luteal Phase
left over corpus luteum starts to secrete progesterone and estradiol
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Proliferative Phase
estradiol signals follicle to grow, uterine tissue starts to thicken and rebuild itself
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Secretory Phase
maximal thickness of endometrium is achieved here
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Menstrual Phase
uterine lining disintegrates, release blood and endometrial tissue
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Maternal side of placenta comes from ________ while the fetal side of placenta comes from _______
endometrium, chorion
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Where does fertilization occur?
In the fallopian tubes
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In the blastocyst, the inner cell mass forms the _____ while the trophoblast forms the ______
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Low levels of estradiol inhibit what?
they inhibit secretion of LH and FSH, keeping them at low levels too
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What do LH and FSH stand for?
- Follicle Stimulating Hormone
- Leutinizing Hormone
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What does the hypothalamus secrete in order to signal the anterior pituitary to secrete FSH/LH ?
GnRH (Gonadotropin-Releasing Hormone)
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What inhibits the anterior pituitary from releasing FSH/LH?
low levels of estradiol
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What gland secretes FSH and LH?
anterior pituitary
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What stimulates/inhibits the hypothalamus from releasing GnRH?
- stimulated by high levels of estradiol
- inhibited by combination of estradiol/progesterone
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Homunculus theory
In the sperm there lives a little human and after fertilization the little human grows and grows
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Morphogenesis involves (2 processes)
- Gastrulation
- Organogenesis
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What is morphogenesis?
the process by which cells occupy their appropriate locations
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What is gastrulation?
cells of a blastula are rearranged into a three-layered embryo
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What is organogenesis?
the formation of organs
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What are the 3 embryonic germ layers & what do they form?
- Ectoderm- outer layer
- Endoderm- lines the digestive tract
- mesoderm- partly fills space in between endoderm and ectoderm
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Blastocyst
human equivalent of the blastula
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Inner Cell Mass
a cluster of cells at one end of the blastocyst
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Trophoblast
outer epithelial layer of the blastocyst
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Function of the trophoblast? What does it generate?
- initiates implantation
- generates the placenta
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Neurulation
the formation of primitive CNS structures
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Determination
process by which a cell or group of cells becomes committed to a particular fate
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Differentiation
refers to the resulting specialization in structure and function
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What are fate maps?
diagrams showing organs and other structures that arise from each region of an embryo
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How can we create a fate map?
- can take early stage embryo and label the different cell, map what they will become
- by imaging - using a green marker to label cells
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What happens if you ablate a cell before gastrulation?
will not affect it
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What are germ cells?
Specialized cells that give rise to sperm or eggs
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Conclusion about P. Granules?
they act as cytoplasmic determinants, fixing germ cell fate at the earliest stage of development
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What happens when blastocyst reaches the uterus?
- blastocyst implants itself into endometrium
- inner cell mass goes through gastrulation, produces three-layered embryo
- organogenesis starts
- neural plate curves inward and forms neural tube
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Totipotent Cell
can develop into all the possible cell types (fertilized egg)
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Pluripotent Cell
can develop into some cells (committed germ cell)
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The two ways a cell decides to become something else?
- #1: mother cell gives each daughter different materials, intracellular method (extreme case in c. elegant)
- #2: a group of cells tells other cells what to do
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How can cells influence other cells?
signaling molecules from the organizer induce the host cell to adopt different fates
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What kind of signaling is involved when cell sends signals to other cells to do something?
Paracrine signaling
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What does ZPA do? How many in a regular cell?
- only 1 in a regular cell
- produces an inductive signal that conveys positional info indicating “posterior”
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Is the neural tube formed only by grafted cells?
No, the graft also induces transformations in the host tissue and changes it as well
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What do AER and ZPA stand for?
- AER = apical ectodermal ridge
- ZPA = sone of polarizing activity
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What is the first stage in development after the embryo has been fertilized?
Cleavage
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What happens during cell Cleavage?
rapid cell division
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During Cleavage, __________ increases but ______ stays the same
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Where does Cleavage take place?
in the fallopian tube
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How are genetically identical twins made?
- clones from the same egg
- when embryo is split during early development
- early blastocyst splits into two
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How are fraternal twins made?
release and fertilization of two different eggs
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How do fraternal and identical twins each deal with the placenta?
- identical - share the same one, fight for nutrients
- fraternal- two placenta
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How are P. Granules distributed in a newly fertilized egg?
they all move to the posterior end before the first cleavage division
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Why can C. Elegans not have monozygotic twins?
because there is no grace period, the cells are committed early in development and P. Granules are not distributed equally
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Asymmetric Cell Division
- the two cells do not inherit the same things
- happens at the expense of becoming another tissue
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