Chapter 27
The Reproductive System
1029
27
variability in the resulting gametes by scrambling genetic
characteristics of the two parents in different combinations.
Crossover also increases variability—during late prophase I, the
homologues break at crossover points and exchange chromo-
somal segments (Figure 27.7). (We will describe this process
in Chapter 29.) As a result, it is likely that no two gametes are
exactly alike, and all are different from the original mother cells.
Spermatogenesis: Summary of Events
in the Seminiferous Tubules
A histological section of an adult testis shows that most cells
making up the epithelial walls of the seminiferous tubules are
in various stages of cell division
(Figure 27.8a)
. Tese cells,
collectively called
spermatogenic cells
(
spermatogenic
5
sperm
forming), give rise to sperm in the following series of divisions
and cellular transformations (Figure 27.8b, c).
Mitosis of Spermatogonia: Forming Spermatocytes
Te
outermost tubule cells, which are in direct contact with the
epithelial basal lamina, are stem cells called
spermatogonia
(sper
0
mah-to-go
9
ne-ah; “sperm seed”). Te spermatogonia di-
vide more or less continuously by
mitosis
and, until puberty, all
their daughter cells become spermatogonia.
Spermatogenesis begins during puberty, and from then on,
each mitotic division of a spermatogonium results in two dis-
tinctive daughter cells—types A and B. Te
type A daughter
cell
remains at the basal lamina to maintain the pool of dividing
germ cells. Te
type B daughter cell
gets pushed toward the
lumen, where it becomes a
primary spermatocyte
destined to
produce four sperm. (±o keep these cell types straight, remem-
ber that just as the letter A is always at the beginning of our
alphabet, a type A cell is always at the tubule basal lamina ready
to begin a new generation of gametes.)
Meiosis: Spermatocytes to Spermatids
Each primary sper-
matocyte generated during the first phase undergoes meiosis I,
forming two smaller haploid cells called
secondary spermato-
cytes
(Figure 27.8b, c). Te secondary spermatocytes continue
on rapidly into meiosis II. Teir daughter cells, called
sperma-
tids
(sper
9
mah-tidz), are small round cells, with large spherical
nuclei, seen closer to the lumen of the tubule. Midway through
spermatogenesis, the developing sperm “turn off” nearly all
their genes and compact their DNA into dense pellets.
Spermiogenesis: Spermatids to Sperm
Each spermatid has
the correct chromosomal number for fertilization (
n
), but is
nonmotile. It still must undergo a streamlining process called
spermiogenesis
, during which it elongates, sheds its excess cy-
toplasmic baggage, and forms a tail. Follow the details of this
process in
Figure 27.9a
1
7
on p. 1032.
Each resulting sperm, or
spermatozoon
(sper
0
mah-to-
zo
9
on; “animal seed”), has a head, midpiece, and tail, which cor-
respond roughly to
genetic
,
metabolic
, and
locomotor regions
,
respectively (Figure 27.9a
7
). Sperm “pack” lightly.
Te
head
of a sperm consists almost entirely of its flattened
nucleus, which contains the compacted DNA. A helmetlike
acrosome
(ak
9
ro-sōm; “tip piece”) adheres to the top of the
Recall that prior to mitosis all the chromosomes are repli-
cated. Ten the identical copies remain together as
sister chro-
matids
connected by a centromere throughout prophase and
during metaphase. At anaphase, the centromeres split and the
sister chromatids separate from each other so that each daugh-
ter cell inherits a copy of
every
chromosome possessed by the
mother cell (Figure 27.6, le² side). Let’s look at how meiosis
differs (Figure 27.6, right side).
Meiosis I
Meiosis I is sometimes called the
reduction division
of meiosis
because it reduces the chromosome number from
2n
to
n
. As in mitosis, chromosomes replicate before meiosis
begins and in prophase the chromosomes coil and condense,
the nuclear envelope and nucleolus break down and disappear,
and a spindle forms.
But prophase I of meiosis includes an event never seen in
mitosis (nor in meiosis II for that matter): Te replicated chro-
mosomes seek out their homologous partners and pair up with
them. Tis alignment takes place at discrete spots along the en-
tire length of the homologues—more like buttoning together
than zipping. Tis process, called
synapsis
, forms little groups of
four chromatids called
tetrads
(Figure 27.6 and
Figure 27.7
).
During synapsis, a second unique event, called crossover,
occurs.
Crossovers
, also called
chiasmata
(singular: chiasma),
form within each tetrad as the free ends of one maternal and
one paternal chromatid wrap around each other at one or more
points. Crossover allows the paired maternal and paternal
chromosomes to exchange genetic material (see Figure 29.3,
p. 1098). Prophase I accounts for about 90% of the total period
of meiosis. By its end, the tetrads are attached to the spindle and
are moving toward the spindle equator.
During metaphase I, the tetrads line up
randomly
at the spin-
dle equator, so that either the paternal or maternal chromosome
can be on a given side of the equator (Figure 27.6). During ana-
phase I, the sister chromatids representing each homologue be-
have as a unit—almost as if replication had not occurred—and
the
homologous chromosomes
, each still composed of two joined
sister chromatids (a dyad), are distributed to opposite ends of
the cell.
As a result, when meiosis I ends, each daughter cell has:
Two
copies of one member of each homologous pair (either
the paternal or maternal) and none of the other
A
haploid
chromosomal number (because the still-united
sister chromatids are considered a single chromosome), but
twice the amount of DNA in each chromosome
Meiosis II
Te second meiotic division, meiosis II, mirrors mi-
tosis in every way, except that the chromosomes are
not
repli-
cated before it begins. Instead, the sister chromatids in the two
daughter cells of meiosis I are simply parceled out among four
cells. Meiosis II is sometimes called the
equational division of
meiosis
because the chromatids are distributed equally to the
daughter cells (as in mitosis) (Figure 27.7).
In short, meiosis accomplishes two important tasks: (1) It
reduces the chromosomal number by half, and (2) it introduces
genetic variability. Te random alignment of the homologous
pairs during meiosis I (see Figure 27.6) provides tremendous
(Text continues on p. 1032.)
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