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APPENDIX
4
Cell Division
The cells in a multicellular organism fit into one
of two categories: somatic cells and germ cells.
Somatic cells divide by mitosis to make up the
specialized cells of the body that the organism
relies on for life processes. Somatic cells also
include stem cells that retain the ability to
develop into required specialized forms. The
mature body has several different kinds of stem
cells, each kind capable of forming a limited
number of specialized cells. In addition to these
cells, most multicellular organisms also have
germ cells — those cells that are set aside to
produce the next generation of organisms. Germ
cells include the specialized gametes (eggs and
sperm) formed by meiosis and the unspecialized
cells that produce them.
some of these inhibitory factors can lead to
cancerous growth. Growth factors can induce
postmitotic or quiescent cells to move into G1
to continue in the cell cycle. This is important
to stimulate tissue cells required to repair a
wound or activate immune system cells that are
required for an immune response. During the S
phase, DNA replicates to double the genome of
the cell. The centrosome, where microtubules
are created and co-ordinated, also duplicates in
preparation for mitosis. During the G2 phase,
DNA replication stops and various protein
factors prepare the cell for mitosis, the M phase.
cell leaves the cell cycle
as a quiescent cell
G1
G0
growth and
repair
Part A: Somatic Cells
Mitosis
cyt
oki
ne
teloph sis
ase
anaphase
metaphase
hase
prop
G2
synthesis of
proteins
time
erphase
Int
Cells follow a cell cycle of growth and division.
Unspecialized cells, like those found in the
early embryo, can divide rapidly. As cells begin
to specialize, some will specialize to the point
that they are terminally differentiated, and
are no longer able to divide. Terminally
differentiated cells are fully specialized and
unable to undergo further change. All of these
cells follow the cell cycle as outlined in Figure
A4.1. The cell cycle can be divided into the
two main phases of mitosis (division) and
interphase (growth and metabolism).
Interphase can be further divided into three
discrete phases: G1 (Gap 1), S (Synthesis) and
G2 (Gap 2). Cell activity varies through these
different phases. Cells rely on a system of timed
interactions among cell structures, and proteins
that control these interactions control the
sequence of events that lead towards cell
division. Some cells leave the cell cycle for G0
(Gap 0) as quiescent postmitotic cells that are
metabolically active but do not grow or
continue along the cell cycle towards mitosis.
During the G1 phase, the cell grows and there
is an increased level of protein synthesis and
DNA repair. Certain protein molecules act as
inhibitory factors that stop further progress and
keep a cell in G1 . Mutations associated with
replication
of DNA
S
Figure A4.1 The cell cycle. Cells in the cell cycle go
through G1 (Gap 1), S (Synthesis), G2 (Gap 2), and
M (Mitosis). A specialized cell may leave the cell cycle
for G0 (Gap 0).
During the M phase, the cell goes through
visibly dramatic activity, as illustrated in
Figure A4.2 on page 556. During prophase,
the replicated DNA in the nucleus coils and
condenses to form distinct chromosomes.
Initially the chromosomes appear as single
threads, but later they appear as double threads
showing two chromatids. The centrosomes
separate to move away from each other and
form poles that will define the daughter cells.
Appendix 4 • MHR
555
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 cytoplasm
divides 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 G1 , S, and G2 before entering the M
A Interphase
precedes mitosis.
B Prophase
the chromatin coils to form
visible chromosomes.
centrosomes
nuclear
membrane
spindle
fibres
disappearing
nuclear
membrane
nucleolus
nucleus
chromatin
replicated
chromosome
nuclear
membrane
reappears
two
daughter
cells are
formed
pole
centromere
sister
chromatids
E Telophase
two daughter cells are
formed. The cells divide as
the cell cycle proceeds into
the next interphase.
Figure A4.2 Mitosis in animal cells
556
MHR • Appendix 4
C Metaphase
the chromosomes move
to the equator of the cell.
D Anaphase
the centromeres split and the sister
chromatids are pulled apart to
opposite poles of the cell.
phase, but here meiosis involves two division
steps. The first step, meiosis I, reduces the
number of chromosomes without separating
the chromatids. During the second division,
meiosis II, the chromatids are separated to
form one or more haploid gamete cells.
Meiosis
synapsis and
crossing-over
occur
Spermatogenesis
Spermatogonia are diploid germ cells that
produce haploid sperm cells. In mammals and
many other organisms, spermatogonia divide by
mitosis to produce a dormant cell (that replaces
the parent cell) and some cells that will actively
divide by meiosis. This process is shown in
Figure A4.3.
During meiotic prophase, homologous
chromosomes come together and overlap in
synapsis. At this time, enzyme complexes
(recombination nodules) promote crossing over,
the exchange of genetic material between
homologous chromosomes. These synaptic
pairs remain together until they are separated
in anaphase I. Synapsis reduces the chances of
an unequal distribution of chromosomes, which
is also called nondisjunction.
At metaphase I, spindle fibres line the paired
chromosomes at the equatorial plane and
separate the chromosomes to opposite poles.
The daughter cells then proceed through the
steps of mitosis to form four haploid gametes,
which then mature to form sperm cells.
homologues align
independently
homologues separate
daughter
cells form
Oogenesis
The egg cell, or oocyte, has a different function
from the sperm cell. The egg cell provides
the energy and materials necessary to support
the growth and division of the embryo until
additional materials are available. As a result, egg
cells are relatively large and meiosis is modified
to help this accumulation of cytoplasm by
producing one haploid egg and two or three
small cells called polar bodies. This process is
illustrated in Figure A4.4 on the next page.
In most mammals, all the oogonia begin
oogenesis before the organism is mature — even
before birth. There are no reserve germ cells,
and the oocytes are arrested during prophase
in meiosis I. In human females, the hormones
LH and FSH (discussed in Chapter 6) trigger
the completion of the process by way of the
ovarian cycle.
sister
chromatids
separate
daughter nuclei are not genetically
identical to parent cell
Figure A4.3 A brief overview of meiosis. Synapsis in
prophase I allows the exchange of genetic information
between homologous chromosomes.
During this arrested period, which may
last up to 50 years for humans, some DNA
sequences are transcribed to produce mRNA
and proteins that support several mitotic
divisions of the early embryo. Other proteins
Appendix 4 • MHR
557
are produced that help the early embryo orient
and develop as it grows. Once stimulated,
meiosis I continues and one small polar body
is produced. This polar body contains one set
of duplicated chromosomes and very little
cytoplasm. The oocyte continues through
meiosis II and stops at metaphase II until it is
fertilized. Fertilization by a sperm cell, and the
arrival of the sperm pronucleus, lead to an
increase of calcium inside the oocyte. This
stimulates the oocyte to complete meiosis,
producing one haploid pronucleus and a
second polar body at the other centrosome pole.
The zygote has two pronuclei that move
directly to the S phase. Since the egg stored
quantities of mRNA and key proteins, the early
embryo skips phases G1 and G2 and alternates
between the S phase and M phase during
mitotic cleavage. As the two pronuclei prepare
for their first M stage, their membranes
disintegrate and the chromosomes merge to
form the nucleus of the new individual.
growth of oocyte
preparation for
metaphase I
synapsis of
homologues
first polar body
hormones
contains one
copy of each
homologue
metaphase I
meiosis begins
preparation for
metaphase II
stasis until
fertilization
second
polar body
chromatids
separate
fertilization
male pronucleus
enters egg
pronuclei prepare
for mitosis
Figure A4.4 Oogenesis is a specialized form of meiosis. Cytoplasm is not evenly
distributed. Hormones initiate meiosis, which arrests in metaphase II. Meiosis is
completed after fertilization.
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MHR • Appendix 4