Chapter 12 Reproduction and Meiosis - University of...

Chap t e r 12 R ep r od u c t i o n a nd Me i o s i s

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CSLS / THE UN IVERS ITY OF TOKYO 229

Part III

Organization of Cell Populations

Chapter 12Reproduction 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

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.

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.

*1Protista: 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.



I I . 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.

Figure 12-1 Sexual and asexual reproduction

(A) (B) (C) (D)


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Figure 12-3 Meiosis and somatic cell division processes

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.

(A) (B)



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

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 CrossoverEach chromatid (1 or 2) can cross with either sister chromatid (3 or 4).

Figure 12-5 Directions of kinetochoresIn the first meiosis, the kinetochores of the two chromatids face the same direction (left), but in somatic cell division they face opposite directions (right).

*2Kinetochores: 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.


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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

environment. By way of example, turtles tend to become male in low-

temperature 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

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.

I I I . 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

Column Sex Determination and Reversal

*3Sex 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.



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

Figure 12-6 Models explaining the consequence of meiosis

(A) (B)


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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.

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-genetic-

elements-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.

Figure 12-7 Molecular process of general recombination during the meiotic process



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

occurs after they become haploids. Sperms compensate for the disadvantage of

being haploids by forming a special structure called a syncytium*4. In other

*4Syncytium: a coenocyte created by the fusion of multiple cells that share the cytoplasm.

Figure 12-8 Molecular process of site-specific recombination by transpositionTransposase 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.


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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.

Figure 12-10 Life cycle of moss

(A)

(B)



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 mammalsA 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 mammalsProgeny 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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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.

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

Column Agrobacteria and Genetically Modified Plants



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

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. Ferti l ization

Preparations for Fertilization

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

Figure 12-13 Sperm structure

Table 12-1 Ovum sizes


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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.

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.

Figure 12-14 Pollen-tube guidance and ferti l ization in plants(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).(A) (B)

(C)



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

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

*5Asteroid body: Structure of microtubules growing radially from both poles of the mitotic spindle during cell division


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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. 12-16Formation of the asteroid body and fusion of the nuclei in the ovum (sea urchin)

Figure 12-15The process of ferti l ization involving a sperm and an ovum in mammalsThe 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).



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.

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).

Column Clone Animals


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Column Figure 12-1The cloning of a sheep



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.

Column Knockout Mice


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Column Figure 12-2 Method to select cells that have performed homologous recombinationCells 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.



• 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.

Summary Chapter 12


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[1]

Explain the mechanism that guarantees the species-specificity

observed during mammalian fertilization.

[2]

Describe the biological significance of meiosis.

[3]

Outline the advantages and disadvantages of both asexual

and sexual reproduction in terms of species survival and

propagation.

[4]

Determine whether the following statements are correct and,

if incorrect, provide the reasons.

A) The mechanism of producing haploid cells from diploid

cells is called meiosis.

B) In meiosis, only one copy of each chromosome type is

allocated to a germ cell.

C) Variations that occur during meiosis are not passed on to

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.

Problems

(Answers on p.259)

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