juliana

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Figure 3–14. A scheme, illustrating some 
stages of the evolutionary development of 
mitotic spindle (After Alberts et al., 1986). 

Meiosis is a basic mechanism of forming generative, i.e. sexual cells and
provides great possibilities for hereditary mutability. As to the individual phases, it is realized
in the way of usual mitosis. These two processes are represented in Figure 3–15.
Endomitosis is a peculiar mode of replicating the chromosomes in the nucleus
through which the cells become polyploid, and their set of chromosomes can increase
several times. A special form of it is the polyteny, where as a result of several consecutive
replications the number of chromatids in each chromosome increases, at that the total
number of chromosomes remains unchanged. So-called giant chromosomes are formed
(see Chapter 2, Section 2. 6). The types of endomitosis are very varied and insufficiently
studied. One of the great “mysteries” of eukaryotic cell is the creation of
division spindle. The interest in it has stimulated the investigators to isolate it in a native state (Fig.
3–16) in order to be examined in more details. The question about the formation of division spindle in
prophase is still unclear. For the present, it is accepted that assembling the system of
microtubules is controlled by two mitotic centres, which form the poles. Usually, in animal cells the
mitotic centres are connected with the centrioles, for a long time considered to be organizers of

recombinations of the genetic material through exchange of segments
between homologous chromosomes (crossing-over).

In their review Dodge and Vickerman (1980) differentiate four types of
mitosis.

1. Closed mitosis with internal division spindle. In this case all
   the time the nuclear envelope remains intact, and the dividing apparatus
   is located within the nucleus. This type of mitosis is characteristic of a
   number of prokaryotes, unicellular algae, euglena, paramecia, etc. Up to
   now no fibres (microtubules), forming a division spindle, are observed in
   them.
2. Close mitosis with external division spindle. In this type of
   division, from the very beginning of the process fascicles of microtubes
   are formed on the one side of nucleus. After that, they through channels
   and tunnels penetrate in the nucleus, and the chromosomes attach to
   them. No centrioles or polar small bodies are observed. During
   anaphase the nucleus elongates and the chromosomes are separated,
   without any break down of the nuclear envelope. It is observed in
   different species of free-living and parasitic dinoflagellates.
3. Semi-open mitosis. In this case, the nuclear envelope remains
   substantially intact, but the gaps or polar fenestrae developed in it make
   possible the intervenrtion of cytoplasmic mitotic centres in the processes
   of nuclear division. Such type of mitosis is described in different species
   of green algae and fungi. In some of them one can already observe
   centrioles or polar small bodies and small kinetochores through which
   the chromosomes attach to the microtubules, and also shapely nucleoli.
   The very mitosis is realized with spiralizing the chromosomes, break
   down of the nucleolus, and inside the nucleus a structure resembling
   division spindle appears. During anaphase the chromosomes are
   distributed, the nucleus elongates, followed by dividing in two. The old
   nuclear envelope can breaks down and participate in the building nuclear
   envelopes of the newly obtained nuclei.
   • Open mitosis. This is the main type of division in higher
     eukaryotes. Naturally there are some differences in the individual plant
     and animal organisms, related to the organization of dividing structures.
     A peculiarity is, that in all cases the nuclear envelope break down
     already at the time of prometaphase and then the processes are realized
     openly in cytoplasm.
     Using the results of the investigations made on different prokaryotic
     and eukaryotic organisms described in the literature, Alberts et al. (1986)
     have made a scheme, illustrating some stages of the mitotic spindle (Fig.
     3–14).
     One variety of mitosis is meiosis called also reducing division. It is
     observed by forming the mature sexual cells in higher plant and animal
     organisms, normally having in their vegetative cells a diploid set of
     chromosomes obtained as a result of fertilization. Its biological essence
     lies in transforming diploid nucleus (2n) in haploid (1n) one, and

the two groups of daughter chromosomes. The spiral chromatin
despiralizes, the nucleolus appears and mitosis is accompliched.
Cytokinesis. The preparation of cytokinesis begins as early as during
the late anaphase or during telophase. A concavity appears on the cell
membrane, a furrow is formed, gradually it wedges into the cell and divides
the cytoplasm. As a result, two daughter cells are formed. During
cytokinesis the cell organelles of newly formed cells are also formed.
Cytological pictures of mitotic division in plant and animal cell are given
in Figures 3–12 and 3–13.

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Figure 3-12. Mitotic division of cells from the root tip of onion (Allium 
cepa). (After Dubinin, 1976). 
A — interphase: all chromosomes are despiralized; in phase S, i.e. 
synthesis of DNA, an autoreproduction of the chromosomes occurs. B, C, 
D, E — prophase of the mitosis: each chromosome represents two 
chromatids; a condensation of chromosomes occurs through their package 
and spirallization; the nuclear envelope disappear and the division spindle 
appears (E). F, G — metaphase: the chromosomes settle down on the 
equatorial plane; the two chromatids in each chromosome are still 
connected through centromeres. H, I — anaphase: separation of the new 
chromosomes (former chromatids) after dividing the centromeres. J — 
telophase: forming a cell barrier between the two daughter cells begins. K, L 
— formation of two daughter cells with equal sets of chromosomes. 

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Figure 3-13. Mitotic division in a typical animal cell (of fish). (Courtesy of 
J. D. Pickett-Heaps; From Alberts et al., 1986). 
(A) Interphase: the cell centre, where the centrioles are located, is small 
in size and separated from the nucleus. (B) Early prophase: the cell 
centre is doubled and approaches the nucleus; the number of 
microtubules coming out of it has increased. (C) Middle prophase: the 
two distant from each other stars settle down around the nuclear 
surface. (D) Prometaphase: the nuclear envelope is destroyed and the 
fibres of spindle already interact with the chromosomes. (E) Metaphase: 
the bipolar structure of the spindle is clearly seen; all chromosomes are 
settled down on its equatorial plane. (F) Anaphase: simultaneously (as 
“at the order”) chromatids move to the poles, carried by the fibres. (G) 
Early telophase: the chromatids are located on the poles, and the 
dividing furrow (shown by arrows) divides the spindle and shortens the 
distance between the chromatids. (H) Telophase: completed formation of 
the daughter nuclei, though they are still compact; almost completed 
cytokinesis; the residual small body is kept between the daughter nuclei. 

Sometimes mitosis is realized with a slow cytokinesis characterized by multiple
division of the nucleus and after undefinite time a dividing of the cell occurs. Thus
multinuclear cells are formed, as are the cases with symplasts of plant and animal
organisms, HeLa cells, etc. This question will be considered further.

The nucleolus or nucleoli (when they are more than one) begin to
destruct and gradually disappear. From cytoplasmic microtubules the
mitotic (division) spindle is formed, which is a bipolar fibrous structure. The
assembly of microtubules occurs out of the nucleus. In animal cells, where
there are centrioles, they appear like asterisks determining the direction
and plane where the spindle will lie. A bipolar division spindle is formed.
Prometaphase. The prometaphase begins with disruption of the nuclear
envelope (membrane) into small fragments, which morphologically do not
differ from those of the endoplasmic reticulum. The outside formed mitotic
spindle installs itself in the nuclear zone. From the centromeres of prometaphase
chromosomes specific structures called kinetochores are formed. During the
metaphase a special group of kinetochore microtubules are attached
to each kinetochore (Fig. 3–11). These microtubules radiate from the two
chromosome ends. They are opposed to each other and interact with the fibres of
bipolar spindle. By means of kinetochore microtubules the chromosomes start to
move. The mechanism of their motion is unclear. For the present, still unclear
remains the chemical composition of kinetochores.

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Figure 3–11. Schematic drawing of 
a metaphase chromosome showing 
its two sister chromatids attached 
to kinetochore microtubules (After 
Alberts et al., 1989). 

The number and size of
microtubules associated with kinetochores varies in the different kinds of
organisms. In some fungi the kinetochore is connected with only one pair of
microtubules, and in the cells of man they are from 20 to 40.
Metaphase. At a definite moment all chromosomes are positioned in
such a way, that their centromeres lie at equal distance from the poles on
the same flatness known as metaphase plate. Each chromosome on the
metaphase plate is kept by a pair of kinetochores and fascicles of
kinetochore microtubules, directed to the opposite poles of division spindle.
Anaphase. After completing the metaphase and dividing the
kinetochores in chromosomes, in conformity with intracellular signals each
chromatid begins to move slowly to the corresponding pole. All chromatids
move with equal speed. By their approaching the poles, the kinetochore
fibres are beginning to shorten. At that time mitotic spindle becomes longer
and the distance between the two poles increases. While metaphase is
more long process, anaphase can complete in a few minutes.
Telophase. When the daughter chromatids settle down on the poles,
the kinetochore fibres disappear. A new nuclear envelope is formed around

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Figure 3-10. A scheme illustrating the phases of mitosis (After Alberts et 
al.,1989). 

Prophase. Prophase is a transition between G₂ and M phases. The
chromatin, which during interphase is diffusionally distributed, gradually
condensates in shapely chromosomal threads. During phase S each
chromosome has replicated and consists of two daughter chromatids
connected with centromeres in between.

Indirect division, also called karyokinesis or mitosis (Greek: mitos —
threads), is a basic mode of division in eukaryotic cells, which have well-
formed nucleus, surrounded by a nuclear envelope. It is observed by a lot of
investigators — R. Remak, L. Auerbach, A. Schneider, O. Bütschli, I.
Chistyakov, E. Strasburger, etc., but it wins recognition with the works of
Flemming (1882 a, b) and van Beneden (1883 a, b) mentioned in Section 3. 2.
Mitosis is realized in two mutually connected stages: a) division of the
nucleus (karyokinesis); b) division of the cell (cytokinesis). This is
represented in Figure 3–8.
In order to be divided, the cell passes its life way, called cell cycle or
generation time (Fig. 3–9). Before to start the division it remains in the
so-called interphase, which spans the period between two consecutive
mitoses. For a long interphase was considered “period of repose”. It proved
that “the repose” is not intrinsic to the life. During the interphase there run
intensive preparatory processes. To reach the division of nucleus (phase M —
mitosis), it is necessary to synthesize DNA (phase S — synthesis). The period
between phase M and the beginning of DNA synthesis is denoted as phase
G₁, and the period after the termination of DNA synthesis and phase M — as
G₂. Actually, interphase includes the phases G₁, S and G₂. Its duration varies
in broad limits for the different kinds of cells (from hours to days, months and
more). Then the division begins. In some lower eukaryotic organisms phase
G₁ is absent and after the end of division the cells enter in phase S
immediately.

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Figure 3–8. Scheme of mitotic division as a process.
Mitosis is divided in five phases — prophase, prometaphase, 
metaphase, anaphase and telophase. After that cytokinesis occurs. This 
process is represented in Figure 3–10.

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Figure 3-9. Scheme of cell cycle.

Modes and Types of Cell Division

Section 3.4.

As it was mentioned in Section 3. 1, evolution has created
different modes and types of cell division, preserving its basic
predestination as a mechanism of reproduction of living organisms. Their
variety is so great, that it is difficult to be ranged over and explained
thoroughly. Essentially they can be reduced to two basic modes:
1. Direct division or amitosis.
2. Indirect division or mitosis.
There are reasons to suppose, that in evolutionary aspect the direct division
has preceded mitosis. But because of the combination of historical
circumstances, indirect or mitotic division is better investigated and is more
widely recognized in cytology than the amitotic. One of the reasons for that is the
fact, that during mitosis well-expressed chromosomes and dividing structures
are formed. They are easily observable under light microscope and long ago
focus the attention of researchers, while during amitosis they are lacking or not
yet established. Amitosis is put aside, and its importance is not appreciated
properly.

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Figure 3–7. Electron micrograph of a 
 bacterial cell during the process of 
division (Courtesy of S. C. Holt; From  Swanson and Webster, 1980). 

Direct division or amitosis is a basic mode of division in prokaryotic cells, which is realized without
formation of well-expressed dividing structures (Fig. 3–7). For the first time amitotic division is described by
Remak (1858), and the term “amitosis” is proposed by Flemming (1882) in order to distinguish from the
indirect division or mitosis, already known at that time. Except in prokaryotes there are data on amitotic
division of nuclei in cells of different tissues and organs in higher organisms (see Hessin, 1967).
Some authors doubt the existence of amitosis, others consider it defective mode of division, still others
deny it completely considering it an alternated mitosis. Possibly these
greatly differing viewpoints have incited Bucher (1958) to entitle his article “Does the
amitosis exist?” in which he definitely accepts the possibility of adequate nuclear
division in this way. If accept that amitosis is a process of division without forming the
characteristic of eukaryotic cell mitotic chromosomes and division spindle, which serve
as instruments for distribution of the genetic material in daughter cells, it can be
considered a really existing mode of cell division in the prokaryotic cells, whose
mechanisms are not clearly explained.

It is difficult to say when and at which stage of the evolutionary
development of cell the double nuclear membrane arises, which are the
reasons for that and the mode of realization. These questions are of vital
importance, since they are directly related to the elucidation in principle of the
ways of realizing the structural organization of cells — by the way of
compartmentalization or that of endosymbiosis (see Chapter 2, Section 2.10).
Already prevailing is the concept that the “energetic” organelles of
eukaryotic cells — chloroplasts and mitochondria, which also possess
double membrane and its own DNA, before have been independently
existed prokaryotes and have been integrated into a common biosystem.
On these grounds some authors suggest the idea of endosymbiotic origin
also of the cell nucleus. In his review Geyer (1980) analyzes 34 randomly
selected textbooks, monographs and scientific papers from 1893 to 1979.
In 6 of them the nucleus is classified as an organelle, in 8 it is not
considered to be organelle, in 14 it is shown as compartment and in 6 there
is a lack of definite classification. The author is of the opinion that cell
nucleus is a compartment without relying on any evidence. This idea is very
abstract, to a great extent unacceptable, but it deserves attention.
As it was already mentioned in every nucleus of eukaryotic cells there
are one, and in some cases two or more nucleoli. These, usually spherical
small bodies (see Fig. 2–59) are easily observed in light or electron
microscope, when the cells are not in mitosis. During metaphase the
nucleolus visibly disappear and appear again in telophase, and acquire its
real shape and dimensions in the newly formed daughter cells. Its
characteristic peculiarities are the compact consistence and a changeability
regarding morphology and chemical composition, which is related with the
functions performed in the cell.
Some details of nucleolus structure were elucidated by means of
electron microscopy. In contrast to the basic cell organelles the nucleolus
lacks double membrane. It is an intranuclear organoid considered to be the
place of synthesis and assembly of ribosomal subunits.
The most important component of the cell nucleus is the chromatin, in
which composition DNA also participates. During mitosis the chromatin is
organized as chromosomes. They form the genome of cell, where the
whole history of its evolution is written. According to Bernal (1969) “the
nucleus is of programming importance. It keeps the keys of the past and
the future, near and more distant. The cytoplasm, in contrast to the
nucleus, is related with the present. It provides what is needed at the
present moment and its reactions are immediate”.

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Figure 3–6. Influence of the nucleus on the formation of umbel in 
Acetabularia mediterranea (med.) and Acetabularia crenulata (cren.) 
(After Swanson and Webster, 1980). 
The nucleus of donor cell determines the shape of umbel (a, b). If unite 
rhisoids of the two species, the umbel combines the distinctive features 
of loose rays of A. crenulata and round shape of A. mediterranea (c). 

In case of successful transplantation the restoration processes in the
cells run even faster, sometimes for minutes. This confirms the concept of
first-rate role of this organelle in the life processes of cell as a whole.
The cell nucleus possesses a complex structural organization and
chemical composition, which are a subject of special literature on this
question. They are closely related with the functions performed by it as a
basic genetic and regulating organelle of the cell.
The process of evolution has also shown its great creative capacity on
the structural organization of the nucleus. While in prokaryotic cells the
nucleoid is “freely” located in the cytoplasm, in eukaryotic cells the nucleus
is separated from it through a double nuclear membrane. The latter isolates
the basic genetic processes in the nucleus, related with DNA replication
and transcription and mRNA synthesis, from the processes of protein
synthesis realizing in the cytoplasm (see Fig. 2–35). Such spatial
separation of these processes is a qualitatively new stage in evolutionary
development of the cell. Besides, eukaryotic DNA, in contrast to
prokaryotic, is much more closely connected with histone and non-histone
proteins, that ensure its compact package and participate in gene
expression as well as in controlling the metabolic processes in cells. The
chromatin arises and the division spindle is formed.