556 MHR • Appendix 4
They also produce microtubule spindle fibres
that connect to the centromeres of the
chromosomes. The nuclear membrane
disintegrates as nuclear lamins in the membrane
become soluble. During metaphase,
chromosomes with clear chromatids line up at
the equatorial plane as spindle fibres pull each
chromosome (paired chromatids) toward
opposite centrosomes. During anaphase, the
chromatids separate and slide along the spindle
fibres to the centrosomes. Once the cell has
successfully divided the genome into two
identical daughter cells, telophase starts. The
nuclear membrane re-forms, first around each
new chromosome and later surrounding all of
the chromosomes together. This process ends
with cytokinesis when the cell cytoplasmdivides and the cell membrane pinches along
the equatorial plane to form separate cells.Part B: Germ Cells
Unlike the cell cycle described above, meiosis
is a linear process that produces terminally
differentiated gametes. Gametes do not divide
to produce other cells. During development as
an embryo, some cells, called germ cells, were
separated from the others. These germ cells
migrated by amoeboid movement through
abdominal cells to the gonads and divided to
form cells that could undergo meiosis. There
are two types of meiotic cells: spermatogonia
(which will produce sperm) and oogonia
(which will produce eggs). Germ cells also go
through G 1 , S, and G 2 before entering the Mtwo
daughter
cells are
formednuclear
membrane
reappearsdisappearing
nuclear
membranereplicated
chromosomespindle
fibrescentromerepolesister
chromatidscentrosomesnucleolusnuclear
membranenucleus
chromatinFigure A4.2 Mitosis in animal cellsInterphase
precedes mitosis.A Prophase
the chromatin coils to form
visible chromosomes.B
Metaphase
the chromosomes move
to the equator of the cell.C
Anaphase
the centromeres split and the sister
chromatids are pulled apart to
opposite poles of the cell.D
Telophase
two daughter cells are
formed. The cells divide as
the cell cycle proceeds into
the next interphase.