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Pronuklei

Pronuklei, also known as pronuclei in English, are the haploid nuclei formed in a fertilized egg from the paternal and maternal genomes before they fuse to form the zygote’s genome. After sperm entry, the sperm nucleus decondenses and becomes the paternal pronucleus, while the oocyte’s nucleus completes meiosis II to form the maternal pronucleus. Each pronucleus remains enclosed in its own nuclear envelope within the zygote.

The pronuclei migrate toward the center of the cytoplasm and replicate their DNA. During this period they

Timing and details of pronuclear formation and fusion vary across species. In many animals, two distinct pronuclei

In summary, pronuklei represent the temporary, separate paternal and maternal nuclear genomes in the early zygote,

are
separate
but
apposed
within
the
cytoplasm.
The
envelopes
of
the
pronuclei
then
break
down
in
a
process
called
pronuclear
envelope
breakdown,
allowing
the
paternal
and
maternal
genomes
to
align
and
undergo
karyogamy,
the
fusion
of
the
two
pronuclei
to
reassemble
a
diploid
zygotic
nucleus.
This
marks
the
end
of
the
pronuclear
stage
and
the
beginning
of
the
first
mitotic
division.
are
visible
for
a
period
before
genome
fusion;
in
other
groups,
especially
some
plants
and
cases
of
double
fertilization,
the
dynamics
differ
and
additional
nuclei
may
be
involved.
The
pronuclear
stage
is
a
transient
yet
critical
phase
of
fertilization,
establishing
the
genetic
complement
of
the
zygote
and
initiating
embryonic
development.
whose
subsequent
fusion
sets
the
stage
for
the
first
cellular
divisions
of
development.