Download Chapter 12 Reproduction and Meiosis

Chapter 12
Reproduction and Meiosis
Part III
Organization of Cell Populations
Chapter 12
Reproduction and Meiosis
The earth is believed to house over 10 million species of organisms that have
prospered by reproducing individuals of the same species. Thus, one of the most
distinct characteristics of organisms is their creation of progeny while replicating
their own genomes; this mechanism is called reproduction. To perform
reproduction, organisms, including humans, have created and utilized sexes.
However, many organisms do not use sexes for reproduction. As an example,
unicellular organisms essentially multiply through somatic cell division, which
replicates the same cells. Among multicellular organisms, some animals multiply
by budding, and some plants multiply through regeneration from vegetative
organs (e.g., potatoes). What, then, is the purpose of reproduction using sexes?
The key to answering this question lies in meiosis and fertilization. This chapter
discusses the roles that sexes play by focusing on these two processes.
I. Sexual and Asexual Reproduction
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Reproduction is roughly classified into sexual and asexual types. Asexual
reproduction is a mechanism by which individuals produce multiple equivalent
progeny by division or other means, and is not accompanied by gene-level
changes. This mechanism is commonly found in bacteria (Fig. 12-1A) and
protista*1, but is also found in animals (jellyfish, hydras, etc.; Fig. 12-1B) and
plants (regeneration through vegetative organs (as seen in potatoes) or by cuttage;
Fig. 12-1C). Eukaryotic genomes are generally diploid, and have n pairs of
homologous chromosomes (n: the ploidy for each cell), generally expressed as 2n.
*1
Protista: The term “protista” is used to
collectively describe unicellular eukaryotes,
and includes flagellata and ciliates. Their
reproduction methods include binary fission,
multiple fission and budding.
In sexual reproduction, a haploid gamete derived from a paternal source (in
humans, a sperm) and one derived from a maternal source (in humans, an ovum)
fuse together to form a diploid zygote (in humans, a fertilized egg). In multicellular
organisms, this zygote performs repeated cell division to create new individual (Fig.
12-1D). Unicellular organisms such as yeast may also perform sexual reproduction
depending on their environment, and in such cases, a zygote creates new progeny.
Meiosis is the mechanism by which haploid cells are created from diploid cells.
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(A)
(B)
(C)
(D)
Figure 12-1
Sexual and asexual reproduction
II. Somatic Cell Division and Meiosis
In somatic cell division, a 2n parent cell doubles its DNA in accordance with the
cell cycle and distributes it to two daughter cells (Fig. 12-2A). In meiosis, on the
other hand, a 2n cell doubles its DNA and undergoes two successive divisions to
become four 1n cells (Fig. 12-2B). These two cell division types are different in
that, with meiosis, DNA distribution occurs after the first cell division without DNA
replication. However, this is not the only difference between the two types.
Let’s look at the meiotic process in detail (Fig. 12-3). A 2n cell has a pair of
paternal and maternal chromosomes known as homologous chromosomes. Each
chromosome is doubled by DNA replication and become two sister chromatids.
During the somatic cell division process, each homologous chromosome moves
independently, and two sister chromatids of the single chromosome are divided
into two cells during the fission process. In meiosis, on the other hand, homologous
chromosomes form pairs (synapsis). This pairing also occurs between sex
chromosomes (in humans, X and Y), and genetic crossover takes place between
paternal and maternal homologous chromosomes. Figure 12-4 shows formation
of multiple crossover points between homologous chromosomes and chromatids;
chromosome transfer takes place at crossover points, resulting in a change in
gene combinations in a process known as genetic recombination. This process
occurs randomly between homologous chromosomes, and involves the creation
of a variety of chromosomes in which paternal and maternal regions are mixed.
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Chapter 12
(A)
Reproduction and Meiosis
(B)
Figure 12-2 Somatic cell division and meiosis
(A) The somatic cell division cycle. A cell doubles its chromosomes in the S phase (the DNA synthesis phase) and
enters the M phase (the karyokinesis phase, which is followed by cytokinesis) via the G2 phase. In the M phase,
the doubled chromosomes are evenly distributed to two daughter cells, which then go through the G1 phase and
enter the S phase, thus repeating the cell cycle. (B) The meiosis cycle. A meiosis-induced cell performs one DNA
replication (premeiotic DNA synthesis) and then successively goes through the first and second meiotic divisions,
thereby creating 1n cells. In the case of an ovum, however, four equal ova are not necessarily created.
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Figure 12-3 Meiosis and somatic cell division processes
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The point at which paternal and maternal chromosomes cross and attach is
called chiasma, and these paired chromosomes are lined up in the center of a
pair of mitotic spindles – the chromosome segregation apparatus. Then, following
the degradation of proteins that connect the homologous chromosomes, they are
segregated and distributed by the mitotic spindle to two cells (representing the
first division). The second division then occurs, in which the sister chromatids that
constitute the homologous chromosomes are segregated, and each is distributed
to one cell.
The microtubules that constitute the mitotic spindle bind to chromosomes with their
*2
Kinetochores: Kinetochores are regions found in
chromosomes. They contain highly repetitive
DNA sequences, and are bound to by many
proteins. During cell division, microtubules are
attached to these regions for chromosome
segregation (kinetochore). Kinetochores are
equivalent to the primary constriction sites of
chromosomes in higher eukaryote.
kinetochores*2, pushing and pulling them (see Chapter 6). The directions of
kinetochores are different in somatic cell division and in meiosis. In somatic cell
division (in which paired chromatids are carried in opposite directions),
kinetochores are positioned facing opposite directions, and in meiosis I (in which
paired chromatids are carried in the same direction), kinetochores are positioned
facing the same way (Fig. 12-5).
Figure 12-4 Genetic Crossover
Each chromatid (1 or 2) can cross with either
sister chromatid (3 or 4).
Figure 12-5 Directions of kinetochores
In the first meiosis, the kinetochores of the two chromatids
face the same direction (left), but in somatic cell division
they face opposite directions (right).
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Reproduction and Meiosis
Sex Determination and Reversal
The sex of mammals, including humans, is determined by the combination
of X and Y chromosomes. Males have one X and one Y chromosome, while
females have two X chromosomes. The SRY (i.e., the sex-determining region
of Y) of the Y chromosome plays an important role in forming male organs.
On the other hand, birds with heterozygous sex chromosomes become
females, and those with homozygous sex chromosomes become males. In
such cases, Z and W are used to express the sex chromosomes; females
have ZW chromosomes, and males have ZZ chromosomes. In addition,
there are many organisms, including fruitflies (Drosophila melanogaster), in
which the sex is determined by the ratio of sex chromosomes to autosomes*3.
Some plant species, such as the evening campion, use sex chromosomes to
determine their sex.
On the other hand, the sex of many organisms is changed by environmental
factors. The sex of some reptile species is determined by their thermal
*3
Sex chromosomes and autosomes:
Chromosomes that differ by sex and have
genes that are involved in sex
determination are called sex
chromosomes. Other chromosomes are
collectively called autosomes.
environment. By way of example, turtles tend to become male in lowtemperature conditions and female in high-temperature conditions.
Conversely, alligators tend to become female under low temperatures and
male under high temperatures. Sexual reversal is also found in fish; black
porgies change from male to female as they age, while giltheads change
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from female to male. Sex determination mechanisms are therefore diverse,
and are believed to have evolved as a survival strategy that enables
organisms to create progeny effectively in the natural environment.
III. The Purpose of Meiosis
Meiosis exists to provide gametes – the origins of next-generation progeny
– with diverse gene combinations by mixing paternal and maternal genes. In
this process, homologous chromosomes form pairs, which are distributed
independently to separate gametes (Fig. 12-6A). Gametes can therefore
have many combinations of homologous chromosomes. As an example, in
humans (which have 23 pairs of homologous chromosomes), the number of
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(A)
(B)
Figure 12-6 Models explaining the consequence of meiosis
the possible combinations is 223 (8.4 x 106). Genetic crossover also occurs
between paired homologous chromosomes, resulting in gene recombination.
Since crossover occurs independently in each sister chromatid, all four resultant
chromatids have different gene combinations (Fig. 12-6B). In this way, new
chromosomes with a mix of paternal and maternal chromosomes are created. In
chromosomal recombination, the greater the distance between two genes, the
more likely recombination is to occur between them. Recombination rarely takes
place if two genes are close to each other. The distance between genes can
therefore be estimated by measuring the gene recombination rate (known as
genetic mapping).
As discussed above, intraspecies genetic diversity is increased during the meiotic
process through the formation of many homologous-chromosome combinations
and gene recombination by crossover. This diversity is believed to be
advantageous in creating progeny that can expand its habitat to a variety of
environments and adapt to rapidly changing circumstances.
IV. Genetic Recombination
We have discussed how DNA in chromosomes is rearranged via genetic
recombination during the meiotic process. However, genetic recombination occurs
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not only in meiosis but also in somatic cell division and viral infection. The resulting
DNA rearrangement brings about genetic diversity, allowing organisms to survive
in various environments. In this section, genetic recombination is more broadly
discussed, including general recombination and site-specific recombination.
General Recombination
General recombination occurs between homologous DNA regions, and includes
the type that takes place between homologous chromosomes during meiosis. In
recombination between such chromosomes, a double-strand break occurs in one of
the paired homologous DNA strands followed by the initiation of partial degradation
of the 5’ ends at the break point, thereby exposing the 3’ ends (Fig. 12-7). The 3’
ends recognize the similar DNA sequence in the paired DNA strand, and bind to it
through the action of proteins that mediate recombination. The partial synthesis of
complementary DNA then proceeds, and DNA recombination is finally completed
after the breaking of the DNA strands and the repair of the chromosomes.
Site-specific Recombination
Site-specific recombination is caused by specific short sequences. These sequences
are known as movable genetic elements, and move not only within the same
chromosome but also to other chromosomes, where they cause recombination.
Figure 12-8 shows the movement of a movable genetic element called transposon
and the rearrangement of chromosomes that accompanies it. In this figure, the
deletion of the movable genetic element and its insertion to a new site are shown.
Figure 12-7
Molecular process of general
recombination during the
meiotic process
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There are many types of movable genetic element, which are roughly classified
into those that move as DNA and those that move as RNA (despite being
incorporated into chromosomes as DNA). All organisms have large amounts of
movable genetic elements in their chromosomes, although element types vary by
organism. As an example, in the human genome, regions that encode proteins
account for less than 5% of the total, whereas the proportion of movable-geneticelements-like sequences is 45%. Although it is thought that many of the sequences
have been mutated and are no longer transposable, some are capable of
movement. On the other hand, as confirmed in a number of viruses, foreign genes
can enter host chromosomes using site-specific recombination.
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Figure 12-8
Molecular process of site-specific
recombination by transposition
Transposase recognizes and binds to short
repetitive sequences in transposons. Transposases
that bind to the repetitive sequences at both ends
are paired to form a complex consisting of a
dimer and a DNA loop. This DNA region is cut
out from donor chromosome A and is
incorporated randomly into target chromosome B.
In chromosome B, two DNA strands are nicked at
separate sites, forming a gap through which the
transposon enters. This gap is then repaired using
the complementary strand as a template, thus
creating short parallel repetitive sequences.
Chromosome A, broken by the removal of the
transposon, is reconnected.
V. Gametogenesis
Haploid cells created by meiosis do not immediately become gametes such as
ova and sperms. As an example, in angiosperms, a female haploid cell undergoes
three more divisions to create eight cells, one of which becomes an egg cell (Fig.
12-9A). Through a single division, a male haploid cell becomes a reproductive
cell (generative cell) and a vegetative cell to support a generative cell, and the
generative cell is further divided into two cells (Fig. 12-9B). The two resulting
sperm cells later fertilize an egg cell and a central cell of the female, respectively
(double fertilization). Interestingly, in lower plant forms such as moss, the
generation time of haploid cells is very long, while that of diploid cells is very
short (Fig. 12-10).
Compared with plants, higher animal forms such as mammals have a very short
haploid generation time. In mammals, although meiosis for oogenesis is initiated in
the early stages, the process is arrested at the primary oocyte stage in the prophase
of meiosis I (Fig. 12-11). In humans, the process then remains dormant for many
years. Once individuals mature and hormone secretion is initiated, meiosis is
resumed and ova are rapidly formed. Unfertilized ova are then promptly removed.
In mammalian males, spermatogenesis is initiated after sexual maturation. In
humans, it takes 24 days for a spermatocyte to complete meiosis and become
four spermatids, and approximately 9 weeks for a spermatid to become a mature
sperm (Fig. 12-12). Unlike ova, much of the differentiation process for sperms
*4
Syncytium: a coenocyte created by the fusion of
multiple cells that share the cytoplasm.
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occurs after they become haploids. Sperms compensate for the disadvantage of
being haploids by forming a special structure called a syncytium*4. In other
Chapter 12
Reproduction and Meiosis
(A)
(B)
Figure 12-9
Gametogenesis in plants
(A) Female gametes. The figure shows a pattern
common to many angiosperms. A megaspore
mother cell is divided into four haploid cells by
meiosis I and II. Of these cells, only one matures
into a megaspore. Through three mitoses, this
megaspore becomes an embryo sac consisting of
eight cells. (B) Male gametes. A pollen mother cell
undergoes meiosis to become a pollen tetrad,
which becomes dissociated and produces four
microspores. The nucleus of each microspore moves
to the side wall before mitosis I. This mitosis has
unequal cell division, producing a large vegetative
cell and a small generative cell having a nucleus
with condensed chromatin structure. The generative
cell moves into the vegetative cell and divides into
two spermatids via mitosis II.
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Figure 12-10 Life cycle of moss
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words, a spermatogonia does not undergo cytokinesis during the first somatic cell
division and the subsequent meiosis, and the resulting cells continue to share the
cytoplasm. Haploid spermatids therefore inherit the cytoplasm from diploid cells,
and this cytoplasm controls the differentiation of sperms. The syncytium also
synchronously contributes to spermatogenesis.
Figure 12-11 Oogenesis in mammals
A primordial germ cell that moves into an ovary in early embryogenesis, becomes an oogonium. After performing several mitoses, the
oogonium starts meiosis I and becomes a primary oocyte. In mammals, primary oocytes are formed in the very early stages, and their
development is arrested in the early stage of the first division until the individual becomes sexually mature. Once this happens, a small
number of cells periodically mature under the influence of hormones, complete meiosis I to become secondary oocytes, and become
mature ova via meiosis II. During this process, two polar bodies are released. The stage at which ova are released from the ovary for
fertilization differs by species.
Figure 12-12 Spermatogenesis in mammals
Progeny cells derived from the same spermatogonium are connected through the cytoplasmic bridge until they are differentiated into
mature sperms. The structure is called syncytium. To aid understanding, the figure shows how two connected spermatogonia become
eight connected haploid spermatids through meiosis. The actual number of connected cells simultaneously differentiated through meiose is
much higher than shown in the figure.
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Reproduction and Meiosis
Agrobacteria and Genetically Modified Plants
It was discovered in 1974 that the agrobacteria-related swelling in plants is
caused by the circular DNA of bacteria. Subsequent studies showed that
part of this circular DNA is incorporated into the plant genomic DNA and is
replicated along with DNA replication. It was also found that the inserted
DNA contains plant hormone synthesis genes that promote the growth of
plant cells, as well as synthesis genes for special amino acids that bacteria
feed on. Indeed, bacteria cause the host plant to produce large amounts of
plant cells on which they feed. In other words, bacteria use the host plant as
a factory to produce their food. Based on these findings, this system was
proposed for use in artificially introducing various genes to plant cells. It is
currently common practice to introduce only target genes to plant cells via
agrobacteria by removing the genes that cause swelling. One somatic plant
cell can be directly differentiated to form a whole plant; this ability is known
as totipotency, and makes it easy to regenerate a plant from a plant cell with
introduced genes. The plants with artificially introduced genes are called
transgenic plants. Many transgenic plants have already been created, and
crops with a pest-resistance gene as well as pesticide-resistant plants are
widely cultivated. Since environmental destruction, including desertification,
is predicted to progress in the future, the creation of genetically modified
crops that can be grown under poor conditions is an urgent issue.
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VI. Specialization of Gametes
A particular characteristic of male gametes is that they exclude most organelles (other
than the nucleus) and the cytoplasm. The same applies to spermatids in the pollen
tube and mammalian sperms. This may be because the main task of male gametes
is to pass nuclear DNA on to ovum, and other intracellular components may adversely
affect embryogenesis in the female body. Indeed, it has been reported that male
mitochondrial DNA is actively destroyed in egg after fertilization.
In the sperm nucleus, nuclear proteins are replaced by different kinds of nuclear
proteins and DNA is condensed during the maturation process, which inactivates
the nucleus. In addition to the nucleus, sperm have a flagellum, mitochondria and
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an acrosomal vesicle. All of these are devices to introduce the male nucleus into
an ovum; the flagellum is a strong propellant device enabling the sperm to swim
to the ovum, mitochondria are devices that supply the energy for the swim, and
the acrosomal vesicle is a sac containing hydrolases that allow the sperm to
penetrate the oolemma (Fig. 12-13). However, these devices become
unnecessary once fertilization takes place.
Ova are specialized in a number of ways. First, they are large (Table 12-1),
since they contain the nutrients, organelles, protein synthesis devices, etc. needed
after fertilization for embryogenesis. Second, animal ova have a zona pellucida
Figure 12-13 Sperm structure
on the surface; this layer physically supports the bulk of the ovum from the outside,
protects it from physical damage, and functions as a species-specific barrier that
only particular sperms (i.e., those of the same species) can penetrate.
VII. Fertilization
Preparations for Fertilization
Table 12-1 Ovum sizes
The fusion of female and male gametes is called fertilization. For this process to
occur species-specifically without multiple fertilizations, close interaction between
male and female cells is necessary. Prior to fertilization, male gametes or
gametophytes need to be activated. In human sperms, this activation (known as
sperm capacitation) is caused by bicarbonate ions in the genital duct of females.
The bicarbonate ions enter the sperm and promote the production of cAMP,
which changes the composition of lipids and glycoproteins in the plasma
membrane, lowers membrane potential and increases intracellular metabolic
activity and motor activity, thereby making the sperm fertile.
Since spermatids (male gametes) in plants do not have motor ability, they are
carried by pollen tubes (male gametophytes) (Fig. 12-14A). The activation of
pollen tubes and their extension to the embryo sac (a female gametophyte) have
been studied in torenia, whose embryo sac is denuded, making experimental
procedures easy. Pollen tubes acquire the ability to extend toward an egg cell
by passing through the style – a female tissue (Fig. 12-14B). This alone, however,
does not guide pollen tubes correctly to the ovum; such tubes can send spermatids
to the embryo sac only in the presence of species-specific elements secreted from
synergids in the embryo sac (Fig. 12-14C). The pollen tube enters the embryo
sac via a synergid and sends one spermatid to the egg cell and the other
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spermatid to the diploid central cell, thus allowing fertilization (double fertilization).
Studies on species related to torenia have shown that the response of pollen tubes
to the elements secreted from synergids is species-specific, indicating that these
elements lead to the species-specificity of fertilization.
Figure 12-14
Pollen-tube guidance and
fertilization in plants
Fertilization Process
When activated animal sperms approach the ovum surface, they are blocked by
special matrices called the zona pellucida or the vitelline membrane. The zona
pellucida mainly consists of three glycoproteins, Z1, Z2 and Z3. The Z3 protein
species-specifically binds to Z3 receptors on the plasma membrane of sperms
(Fig. 12-15). This binding triggers the acrosome reaction (Fig. 12-15), in which
hydrolase stored in the acrosomal vesicle of a sperm is released to the zona
pellucida and degrades it, allowing the sperm to penetrate. Fusion of the ovum
plasma membrane and the sperm plasma membrane then occurs, and the sperm
nucleus enters the ovum.
(A)
(B)
(A) Pollens attached to the stigma (1) extend
pollen tubes (male gametophytes). These tubes
reach the embryo sac (a female gametophyte)
after passing through the style (2), the placenta
(3), the funiculus (4) and the micropyle (5). Since
pollen tubes sometimes stray in the regions circled
by the broken lines, it is believed that signals that
guide the tubes exist in these areas. (B) The
extension of pollen tubes is promoted by elements
contained in the style. Pollen tubes that do not
pass through the style therefore do not elongate
significantly. (C) A pollen tube that has reached
the embryo sac enters one synergid via the
filiform apparatus in response to inducers
secreted from the synergid. The pollen tube
breaks down in the synergid, and two spermatids
fuse with the central cell and the egg cell,
respectively (double fertilization).
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(C)
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In addition to introducing a paternal gene set to an ovum, the sperm also activates
the ovum. In mammals, when a sperm fuses with the surface of an ovum, there is
+
a local increase in the level of intracellular Ca2 , which is then transmitted to the
entire ovum in a wave-like motion (known as a calcium wave; see Figure 9-4A in
Chapter 9) and activates the ovum. In the activated ovum, hydrolase is secreted
from the cortical granules in the cell to the zona pellucida, thus changing its
+
nature to block the entry of other sperms. On the other hand, the Ca2 signal
drives the developmental program of the ovum, leading the fertilized egg to the
next stages of development. These sperm roles can be replaced with physical or
chemical treatment. As an example, a frog ovum can be activated by a pinprick.
In some organisms, the ovum is activated without a sperm and develops into a
complete organism. This type of development is called parthenogenesis.
When only the sperm involved in fertilization enters the ovum, the sperm nucleus
undergoes nuclear protein conversion and swells, thus forming the male pronucleus
(Fig. 12-16). The centrosome located at the base of the flagellum also plays a
pivotal role in creating the asteroid body*5 in the ovum. Using the microtubules
of the asteroid body as rails, the female pronucleus is then carried to the center of
the asteroid body. In sea urchins, the male and female pronuclei fuse together,
forming a 2n fertilized nucleus. In mammals, ovulation occurs in the metaphase of
meiosis II, leading to fertilization. After fertilization, meiosis II resumes, and the
male and female pronuclei approach each other using the same mechanism seen
in sea urchins. However, they do not fuse together, and chromosomes derived
*5
Asteroid body: Structure of microtubules growing
radially from both poles of the mitotic spindle
during cell division
from the two nuclei start to exhibit the same behavior following the breakdown of
the nuclear membrane during division. The cell that thus becomes the 2n type
again performs cleavage repeatedly and undergoes the developmental process.
VIII. Species and Sexes
This book started with the diversity and uniformity of organisms in Chapter 1,
and now ends with the passing of genetic information from parents to children in
this chapter. All the knowledge presented demonstrates that organisms are
amazing entities that, while sharing similar basic apparatus, have evolved in
hugely diverse ways. An important function of organisms is their self-replication.
However, if organisms that emerged in ancient times had simply continued to
replicate themselves, the diversity of organisms we see today would not have
developed. The biodiversity that currently exists on the earth is proof that
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organisms have not only replicated but have also modified their genes during the
course of their evolution. Organisms originating in the sea altered the global
environment and found their way onto land and to the sea bottom, where they
used various strategies to secure species-specific niches (or habitats). To expand
a niche in manifold environments, it is advantageous for organisms to increase
their intraspecies diversity, and one strategy for it is to acquire sexes and develop
sexual reproduction. The existence of sexes allows the mixing of genes among
many individuals of the same species, while sexual reproduction allows genetic
Figure 12-15
The process of fertilization
involving a sperm and an
ovum in mammals
The membrane receptors of a mammalian
sperm bind to the glycoprotein Z3, which
is located in the zona pellucida, after
which the acrosome reaction of the sperm
occurs in the order of (1) to (5).
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Figure. 12-16
Formation of the asteroid body
and fusion of the nuclei in the
ovum (sea urchin)
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progeny, in which the genes of both parents are mixed through many combinations
of homologous chromosomes and through genetic recombination by crossover.
Here in the 21st century, we are fortunate to have genetic blueprints created by
decoding the complete genome sequences of many organisms, and such genetic
information for many more species will be published in the future. By comparing
the blueprints of multiple organisms, strategies to transfer specific genes to
progeny using sexes may be decoded in the not-too-distant future.
Column
Clone Animals
Clones are groups of organisms with identical DNA sequences. It is easy to
produce clones in plants (as demonstrated in potatoes and by grafting), and
bees clone themselves by parthenogenesis. Is it then possible to clone
humans? An adult human consists of 60 trillion cells; these include somatic
cells, which last only for one generation, and germ-line cells, which survive
to the next generation by producing ova and sperms. Gametes are not
clones, since different gene sets are created through meiosis (as already
discussed). Is it possible to produce clones using somatic cells without going
through these specialized reproduction cells? It has long been known that
individual animals can be produced by transplanting the somatic nucleus to
a nucleus-removed, unfertilized ovum in frogs. In February 1997, the birth
of a cloned sheep named Dolly was widely publicized. In this case, the
nucleus of a mammary cell (a somatic cell) from a female sheep was
implanted into a nucleus-removed, unfertilized ovum, which was then
implanted into a surrogate female sheep, thus producing a cloned sheep
with a gene composition identical to that of its mother (Column Fig. 12-1).
Clones have now been successfully created in many animals including
cattle and pigs, demonstrating that cloning is possible in mammals and
indicating that there are few biological barriers to human cloning. Another
important conclusion from these results is that even somatic cells have all the
information necessary in their nucleus to produce a complete mammal. In
other words, the various cells that make up an organism have all the
information needed to produce the organism, but they express only part of
this information, and the expression of other genes is masked (or suppressed)
(see Chapter 4).
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Column Figure 12-1
The cloning of a sheep
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Column
Knockout Mice
A knockout mouse is one in which certain gene functions have been
genetically disrupted through a method called targeting. As shown in
Column Figure 12-2, using the characteristic of the recombination that tends
to occur between DNA regions with similar gene sequences (homologous
recombination), a normal gene in an ES cell (embryonic stem cell) is
replaced with a gene in the knockout vector (targeting vector) prepared in
advance. In the following cell selection, cells that did not perform homologous
recombination and those in which the vector was incorporated into the
wrong chromosome are removed, and only ES cells that had the gene
inserted correctly and underwent homologous recombination are obtained.
These cells are implanted into the early embryo, and it is then implanted into
the uterus of a pseudopregnant mouse, thereby producing chimeric mice
consisting of cells with and without the target gene. Homozygous knockout
mice are then produced by crossing these mice with other mice. The functions
of the removed gene are identified by observing the phenotypes (traits) of
the mice thus created.
Recently, a method known as conditional knockout has also been used, in
which gene functions are disrupted only during certain stages or in certain
organs. This technique enables the production of mice that cannot be
created using the constitutive knockout method (in other words, the target
gene participates in development) as well as the observation of the gene
functions in adult mice.
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Chapter 12
Reproduction and Meiosis
Column Figure 12-2
Method to select cells that have performed homologous recombination
Cells that have not performed recombination die as a result of adding the antibiotic neomycin. Cells in which the
targeting vector has entered the wrong chromosome also die from the addition of ganciclovir (gcv), which inhibits
DNA polymerase, due to the existence of the thymidine kinase (TK) gene in the vector. Only cells that performed
homologous recombination survive.
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Chapter 12
Reproduction and Meiosis
Summary
Chapter 12
• Reproduction is classified into asexual and sexual types. In asexual
reproduction, equivalent progeny is created by cytokinesis or other means,
and no change occurs at the gene level.
• In sexual reproduction, paternal and maternal haploid gametes fuse
together to form a diploid zygote. In multicellular organisms, this zygote
divides repeatedly and develops into a new individual.
• The mechanism of creating haploid cells from diploid cells is called meiosis.
• During the meiotic process, crossover occurs between homologous
paternal and maternal chromosomes. Gene recombination takes place at
the crossover points, which changes the combination of genes. Many
combinations of homologous chromosomes are distributed to gametes. As
a result, diverse gametes with varied mixtures of paternal and maternal
genes are produced.
• In higher organisms, gametes become increasingly specialized for fertilization.
As an example, mammalian male gametes differentiate into sperms with a
flagellum, spermatogenesis and well-developed mitochondria.
• Fertilization occurs through the fusion of specialized male and female
gametes.
• Fertilization is precisely controlled to avoid multiple fertilizations and to
make the process species-specific. As an example, for the fertilization of
a sperm and an ovum to occur in mammals, the glycoproteins in the zona
pellucida on the ovum surface must species-specifically bind to the
receptors on the sperm membrane.
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Chapter 12
Reproduction and Meiosis
Problems
[1]
[4]
Explain the mechanism that guarantees the species-specificity
Determine whether the following statements are correct and,
observed during mammalian fertilization.
if incorrect, provide the reasons.
A) The mechanism of producing haploid cells from diploid
[2]
cells is called meiosis.
Describe the biological significance of meiosis.
B) In meiosis, only one copy of each chromosome type is
allocated to a germ cell.
[3]
C) Variations that occur during meiosis are not passed on to
Outline the advantages and disadvantages of both asexual
and sexual reproduction in terms of species survival and
propagation.
the next generation.
D) Since a germinated pollen has two spermatids, two
embryos are produced by fertilization.
E) Multicellular organisms do not perform asexual reproduction
under natural conditions.
(Answers on p.259)
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