(3) Fertilization: Beginning a New Organism Part 1 Flashcards
4 major events of fertilization
- contact and recognition between the sperm and the egg (ensures species specificity)
- regulation of sperm into the egg (only 1 sperm can penetrate)
- fusion of genetic material from sperm and egg
4, activation of egg metabolism to begin development
how does a sperm come about, and is it diploid or haploid?
sperm arise by meiotic cell division in testes; it is haploid (1 complete set of chromosomes; n)
function of the sperm
- deliver haploid nucleus to the egg
- activate egg’s metabolism so that development can proceed
sperm: form fits function! (explain)
- not much more than a highly motile nucleus
- it is comprised of a haploid nucleus, propulsion system, specialized plasma membrane to recognize egg of same species, and a sac of digestive enzymes to digest through protective layers of egg
spermiogenesis: plasma membrane
specialized to recognize and bind to egg of same species
spermiogenesis: acrosome
specialized vescile derived from golgi apparatus; contains digestive enzymes that help sperm penetrate egg
spermiogenesis: haploid nucleus
contains the paternally derived chromosomes
spermiogenesis: 2 centrioles
one becomes one of the mitotic poles in the first mitotic division of zygote; one becomes the organizing center for the outgrowth of microtubules within the flagellum
spermiogenesis: axoneme
the motor portion of the flagellum; comprised of a 9+2 configuration of microtubules coursing down the length of the flagellum and dynein motor proteins that hydrolyze ATP to cause sliding of microtubules along each other to whip the flagellum
spermiogenesis: mitochondria
surround the axoneme in midpiece to provide ATP for flagellar movement
describe the egg/ovum
arise by meiotic cell division in ovaries; haploid; much larger than sperm; contains organelles and other cytoplsmic constituents missing from sperm that are required for embryonic development
describe egg maturation
- meiosis produces 1 haploid ovum from 1 diploid primary oocyte; other 3 daughters are polar bodies, which degenerates
why is it advantageous to produce only 1 egg, sacrificing 2 polar bodies?
b/c we need the cytoplasm and organelles that sperm is missing
cytoplasmic constituents of the egg (name them)
- nutritive proteins
- ribosomes and tRNA
- maternal messenger RNAs
- morphogenetic factors
- protective chemicals
cytoplasmic constituents: nutritive proteins
source of energy and amino acids for developing embryo; usually in form of yolk proteins
cytoplasmic constituents: ribosomes and tRNA
protein synthesizing machinery; a burst of translation occurs soon after fertilization
cytoplasmic constituents: maternal messenger RNAs
stored but not translated until after fertilization
cytoplasmic constituents: morphogenetic factors
transcription factors or signaling molecules that direct early axis formation in embryo
cytoplasmic constituents: protective chemicals
found in species that develop externally; protect developing embryo from UV irradiation, microbes, predators
egg plasma membrane and external protective layers! (sea urchin egg)
jelly coat –> vitelline envelope –> egg cell membrane
(sperm has to penetrate through all three layers in order for fertilization to occur!)
describe the sea urchin egg (membrane and cortex)
- plasma membrane is microvilliated; they increase surface area of egg for binding of sperm
- cortex: 5 um layer of gel-like cytoplasm directly underneath the PM; contains actin microfilaments and cortical granules (needed for fertilization)
mammalian egg: zona pellucida (ZP)
layer of extracellular matrix (lots of fibers) outside the egg PM; equivalent of the sea urchin vitelline layer
mammalian egg: cumulus
a “cloud” of follicle cells that surround the ZP; equivalent of sea urchin jelly coat
how do sea urchins (phylum echinodermata) reproduce
they reproduce by broadcast spawning into sea water and external fertilization
5 steps of sperm/egg recognition in sea urchins
- chemoattraction of sperm to egg (species-specific)
- acrosomal reaction
- digestion of jelly layer by released acrosomal enzymes
- sperm binding to vitelline membrane (species-specific)
- fusion of egg and sperm plasma membranes
chemoattraction
secreted from egg attract sperm; chemotaxis
also activate sperm by increasing respiration and motility
species specific sperm activating peptides
resact
speract
acrosomal reaction: 2 components
- fusion of acrosomal vesicle with sperm plasma membrane and exocytosis of acrosomal enzymes that digest jelly coat
- protrusion of acrosomal process from sperm head that binds egg to vitelline layer and tethers sperm to the egg
species specific interaction between sperm and egg jelly coat triggers acrosomal reaction
- jelly coat of different species contains different sulfated polysacchardies
- sperm PM has proteins that bind only to the sulfated polysacchardies from the same species
- once boud to jelly coat, acrosomal reaction is triggered in sperm
digestion of jelly and binding of sperm to vitelline layer
- release of acrosomal enzymes into jelly coat (digestion of jelly coat)
- polymerization of actin to produce acrosomal process
- acrosomal process is studded by bindin proteins
- bindins bind with species-specificity to proteins in vitelline layer surrounding the egg
sperm binding to vitelline layer
bidnin binds to species-specific receptors on egg vitelline membrane
agglutination: in same species; can bind to each oher
ensuring species specificity of fertilization
- species-specific chemoattractants (correct egg to correct sperm)
- species-specific interation between sperm and sulfated polysaccharides of egg jelly coat
- species-specific interaction between sperm bindins and receptors of vitelline membrane
fusion of egg and sperm plasma membranes
- sperm and egg plasma membranes fuse, forming cytoplasmic bridge
- bridging of actin filaments in acrosomal process and egg to form a fertilization cone
- actin filaments widen cytoplasmic bridge
- sperm nucleus, centriole, and tail enter egg (sperm centriole will divide and form two poles of the mitotic spindle for first mitotic division of zygote (fertilized egg))
