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between surface proteins of the sperm
and egg. Polyspermy is bad because, in addition to the extra
set of chromosomes, a sea urchin sperm donates a centriole.
The presence of additional centrioles during the first cell
division will result in additional cleavage furrows and
incorrect partitioning of the chromosomes.
The
wave of Ca++ induces the fusion of the cortical
granuleswith the plasma
membrane. The contents of the
cortical granules, released into the space between the
vitelline membrane and the plasma membrane, include a
trypsin-like protease that clips protein "posts" anchoring
the vitelline membrane and removes the sperm-binding sites.
The cortical granules also release mucopolysaccharides
(carbohydrates) that absorb water and cause the vitelline
membrane to expand and become the fertilization
envelope; this is the slow
blockto
polyspermy.
The
sperm and egg plasma membranes also
fuseand the sperm nucleus
enters and fuses with the egg nucleus. The wave of Ca++ and
the rise in intracellular pH activate the
metabolismof the
fertilized egg.
D.
Embryogenesis
The union of sperm and eggs
creates single cell,
thezygote, which gives rise
to a new organism through the process of embryogenesis. The
formation of a new organism requires both
morphogenesis(the
creation of form) and cell
differentiation.
In most
animals, embryogenesis can be divided into three major
stages:
(1)
Cleavage, in which rapid cell divisions divide the
mass of the zygote into many
cells and establish the basic embryonic axis;
(2)
Gastrulation, in which the gut is formed and the
embryo becomes multilayered;
and
(3)
Organogenesis, in which the major organ systems are
formed.
1.
CleavgeLab Handout, #7
a. Rapid
Cleavage cell divisions are
generally rapid. The embryonic DNA is not
transcriptionally active,
so you have basically DNA synthesis- mitosis.
b.
Control of the plane of cell division
1. First 3 cell
divisions
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