Traits of a new individual are determined by specific genes on chromosomes
inherited from the father and the mother. Humans have approximately 35,000
genes on 46 chromosomes. Genes on the same chromosome tend to be inherited
together and so are known as linked genes. In somatic cells, chromosomes
appear as 23 homologous pairs to form the diploid number of 46. There are
22 pairs of matching chromosomes, the autosomes, and one pair of sex chromosomes.
If the sex pair is XX, the individual is genetically female; if the pair is
XY, the individual is genetically male. One chromosome of each pair is derived
from the maternal gamete, the oocyte, and one from the paternal gamete, the
Chapter 1: Gametogenesis: Conversion of Germ Cells Into Male and Female Gametes 5
sperm. Thus each gamete contains a haploid number of 23 chromosomes, and
the union of the gametes at fertilization restores the diploid number of 46.
MITOSIS
Mitosis is the process whereby one cell divides, giving rise to two daughter
cells that are genetically identical to the parent cell (Fig. 1.2). Each daughter
cell receives the complete complement of 46 chromosomes. Before a cell enters
mitosis, each chromosome replicates its deoxyribonucleic acid (DNA). During
this replication phase the chromosomes are extremely long, they are spread
diffusely through the nucleus, and they cannot be recognized with the light microscope.
With the onset of mitosis the chromosomes begin to coil, contract,
and condense; these events mark the beginning of prophase. Each chromosome
now consists of two parallel subunits, chromatids, that are joined at a
narrow region common to both called the centromere. Throughout prophase
the chromosomes continue to condense, shorten, and thicken (Fig. 1.2A),
but only at prometaphase do the chromatids become distinguishable
(Fig. 1.2B). During metaphase the chromosomes line up in the equatorial plane,
Figure 1.2 Various stages of mitosis. In prophase, chromosomes are visible as slender
threads. Doubled chromatids become clearly visible as individual units during
metaphase. At no time during division do members of a chromosome pair unite. Blue,
paternal chromosomes; red, maternal chromosomes.
6 Part One: General Embryology
and their doubled structure is clearly visible (Fig. 1.2C ). Each is attached by
microtubules extending from the centromere to the centriole, forming the mitotic
spindle. Soon the centromere of each chromosome divides, marking the
beginning of anaphase, followed by migration of chromatids to opposite poles
of the spindle. Finally, during telophase, chromosomes uncoil and lengthen,
the nuclear envelope reforms, and the cytoplasm divides (Fig. 1.2, D and E ).
Each daughter cell receives half of all doubled chromosome material and thus
maintains the same number of chromosomes as the mother cell.
MEIOSIS
Meiosis is the cell division that takes place in the germ cells to generate male
and female gametes, sperm and egg cells, respectively. Meiosis requires two cell
divisions, meiosis I and meiosis II, to reduce the number of chromosomes to
the haploid number of 23 (Fig. 1.3). As in mitosis, male and female germ cells
(spermatocytes and primary oocytes) at the beginning of meiosis I replicate
their DNA so that each of the 46 chromosomes is duplicated into sister chromatids.
In contrast to mitosis, however, homologous chromosomes then align
themselves in pairs, a process called synapsis. The pairing is exact and point
for point except for the XY combination. Homologous pairs then separate into
two daughter cells. Shortly thereafter meiosis II separates sister chromatids.
Each gamete then contains 23 chromosomes.
Crossover
Crossovers, critical events in meiosis I, are the interchange of chromatid segments
between paired homologous chromosomes (Fig. 1.3C ). Segments of
chromatids break and are exchanged as homologous chromosomes separate.
As separation occurs, points of interchange are temporarily united and form an
X-like structure, a chiasma (Fig. 1.3C ). The approximately 30 to 40 crossovers
(one or two per chromosome) with each meiotic I division are most frequent
between genes that are far apart on a chromosome.
As a result of meiotic divisions, (a) genetic variability is enhanced through
crossover, which redistributes genetic material, and through random distribution
of homologous chromosomes to the daughter cells; and (b) each germ cell
contains a haploid number of chromosomes, so that at fertilization the diploid
number of 46 is restored.
Polar Bodies
Also during meiosis one primary oocyte gives rise to four daughter cells, each
with 22 plus 1 X chromosomes (Fig. 1.4A). However, only one of these develops
into a mature gamete, the oocyte; the other three, the polar bodies, receive
little cytoplasm and degenerate during subsequent development. Similarly, one
primary spermatocyte gives rise to four daughter cells, two with 22 plus 1
Figure 1.3 First and second meiotic divisions. A. Homologous chromosomes approach
each other. B. Homologous chromosomes pair, and each member of the pair consists of
two chromatids. C. Intimately paired homologous chromosomes interchange chromatid
fragments (crossover). Note the chiasma. D. Double-structured chromosomes pull apart.
E. Anaphase of the first meiotic division. F and G. During the second meiotic division,
the double-structured chromosomes split at the centromere. At completion of division,
chromosomes in each of the four daughter cells are different from each other.
X chromosomes and two with 22 plus 1 Y chromosomes (Fig. 1.4B). However,
in contrast to oocyte formation, all four develop into mature gametes.
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