During sexual intercourse, sperm
are released into the vagina near the cervix,
which is the entrance to the uterus. The sperm
travel through the cervix, into uterus and up
the fallopian tubes. After being release from
an ovary, the egg cell moves through the fallopian
tube by tiny cilia that line the tube’s
walls. The egg cell only survives for approximately
24 hours after ovulation. Of the millions of
sperm that are released into the naturally acid
environment of the woman’s reproductive
tract, relatively few will survive to encounter
the egg cell.
When one of the sperm cells finally
succeeds in breaking through the egg cell's outer
membrane, the egg cell forms a protective barrier
preventing other sperm cells from entering. This
ensures that only one sperm cell fertilizes the
egg cell. Next, the sperm cell releases its nucleus
containing the man’s chromosomes. After
several hours, it unites with the nucleus of
the egg cell, which contains the woman’s
chromosomes. When the two nuclei fuse, their
genetic material combines together to create
a fertilized egg cell which is called a zygote.
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