book

convincingly prove the role of nucleus for determination of characteristic
features of the cell or definite individual.
The investigations of Hämmerling are of great importance for
elucidating the nuclear-cytoplasmic interrelations. Acetabularia is a huge
unicellular plant, 3—5 cm in length. It possesses only one nucleus located
in the base of the pedicel, called rhisoid. The two Acetabularia species —
A. mediterranea and A. crenulata — differ morphologically in the umbels
formed on the top. The nucleus can separate from the cell by cutting off the
rhisoid, where it is located, and then can be implanted to another cell
cytoplasm. And always the nucleus of donor cell determines the shape of
umbel (Fig. 3–6).
Experiments with taking away and implanting of nuclei are carried out
with the filamentous green alga Spyrogyra, sea urchin eggs (Arbacia),
infusoria (Stentor), mollusca (Triton), etc. Some time enucleated cells have
continued to develop. That is the reason Harris (1973) to state that
“matrices for synthesis of specific proteins are kept in the protoplasm long
time after separating the nucleus, and the mechanisms controlling this
synthesis continue to function in it”.
The removal of the nucleus from the cell leads to serious disturbances
of its life processes and functions followed by its death. What is the exact
character of the alterations occurring in it as a result of enucliation is still
unclear. Undoubtedly they have a complex character. Most probably at
their root is the interrupted synthesis of RNA, ribosomes and proteins.
Naturally, enucleation does not influence equally on the synthetic
processes in different cells. In this respect a certain role is played by the
cytoplasm, closely connected with the nucleus in a united nuclear-
cytoplasmic complex. For that reason, in different cells these processes
fade away with different duration. In some cases that can occur only for
hours, while amoebae continue to live up to 2—3 weeks, and enucleated
fragments of Acetabularia — up to 2—3 months.

described it as areola usually more dark than the cell envelope, without
giving any picture. The term nucleus has been introduced by G. Valentin
(1836), who for the first time has described the nucleolus located in it. The
presence of nucleus in each cell has served as a leading idea for the
creating of cell theory by M. Schleiden and Th. Schwann.
At present, the nucleus is considered to be obligatory and irreversible
composite organelle of each eukaryotic cell. This concept has been
appreciated in view of the fact that a cell without nucleus cannot develop
normally and, what is most important, cannot reproduce. Cases are known,
where during a large part of the period of its existence the cells lack nuclei
(in cribriform pipes of higher plants and erythrocytes of mammals), but such
cells exist for relatively short time and are not capable to reproduce. They
have possessed nuclei before their differentiation and maturation.
Sharp discussions arise about the presence or absence of nucleus in
bacteria. Some authors accept its existence though it does not combine all
morphological peculiarities characteristic of nuclei in eukaryotic cells, and
others — reject it categorically. For that reason different names have been
assigned to it — “genophore”, “nucleoid”, “chromatin small bodies”, etc.
Usually cells have one nucleus from which, as a result of dividing, their
number increases. Multinuclear cells also exist, where the number of nuclei
reaches to several dozens or hundreds (in osteoclasts of bone tissue, the
symplasts in crossfurrowed muscle fibres, in tumour (HeLa) cells, etc.). The
reasons for the existence of such type of multinuclear cells are unclear.
Sometimes the presence of two nuclei in one cell is conditioned by a
certain differentiation of their functions. Such cases are observed in
Protozoa (Paramecium) where the one of the nuclei serves as a source of
genetic information, and the other one controls the metabolic processes of
the cell. This phenomenon characteristic of infusoria is known as nuclear
dualism. In their cells there are two types of nuclei: mitotically dividing,
transcriptionally weakly active diploid micronuclei (generative nuclei) and
such ones dividing by a mode resembling amitosis, rich in DNA,
transcriptionally highly active macronuclei (somatic nuclei). In the course of
the sexual process (conjugation or autogamy) the old macronucleus
degenerates and is replaced by a new, which develops from the
micronucleus (see Raikov, 1989).
It is clear from the above-mentioned that the well-formed nucleus or
nuclear equivalent (nucleoid) is an obligatory and irreversible composite
organelle of every living cell. This shows the enormous role performed by
the nucleus in the life activity and reproduction of the cell. According to
Kudo (1946) nuclear fragments of Protozoa with dimensions no more than
1/50 of the initial cell are capable to regenerate completely the lacking
mass of protoplasm. In this respect worthy of note are the experiments of
Hämmerling (1953, 1963) with the unicellular green alga Acetabularia. They

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Figure 3–5. Fertilization and division of cells in parasitic worms 
Ascaris (After van Beneden, 1883; From A. Mirsky. In: Molecules 
and Cells, vol. 4, 1969). 
1 — approach of the nuclei of spermatozoon and ovum; 2, 3, 4 — the 
nuclear envelope is break down and the chromosomes are clearly 
discernable; in every nucleus there are two chromosomes — a half of 
the normal diploid chromosomal set; 5, 6 — preliminarily, each 
chromosome is divided in two; 7 — the divided chromosomes separate; 
8, 9, 10, 11 — two centrioles appear, a dividsion spindle is formed 
between them; cell division begins; 12 — two cells are formed, there are 
four chromosomes in each nuclei. Two of them originate from the ovum, 
the other two — from the spermatozoon. 

The nucleus or the Nuclear Equivalent (Nucleid) – Obligatory and Irreplaceable Organelle of Each Cell

Section 3.3. The discovery of cell nucleus is an important stage in understanding its
biological essence and development. According to literary data (see
Katznelson, 1939, 1963; History of Biology, 1972, 1975) the first who has
observed a nucleus in fish erythrocytes is Antony van Leeuwenhoek, but
his observations have not come to light. Fontana (1781) has depicted
nuclei in epithelial cells of snake epidermis. Then nuclei are observed by F.
Cavolini, J. Purkinje, Ch. Mirbel, F. Meyen, J. Müller, etc., but they have not
estimated their role and importance. At that time even the term “cell” have
not possessed a definite interpretation. Different terms have been used —
“grains”, “balls”, “pouchs”, etc. Recognizing the nucleus as a constituent of
plant cells is a merit of the English botanist Robert Brown (1833), who has

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Figure 3–4. Division of epithelial cells in salamander (After Flemming, 
1882; From A. Mirsky. In: Molecules and Cells, vol. 4, 1969). 
1 — nuclein containing chromosomes with a shape of disorderly located 
bands; formation of division spindle; 2 — separating the chromosomes in 
length; each of them is replicated, then they separate from each other and 
make for the opposite poles of the division spindle of the cell; 3 — formation 
of two equal chromosome sets; 4 — formation of two daughter cells.

image

Important contribution to understanding the essence of cell
division are the classical investigations of Flemming (1882 a, b) and
van Beneden (1883 a, b) already mentioned in Chapter 2, Section 2.
5. These studies shed light on the fusion of nuclei of sexual cells in
animal organisms and the behaviour of chromosomes in the process of
division, leading to the formation of two daughter cells (Figs. 3–4 and 3–
5). Gradually the idea arises that cell division is the only way of
forming new cells. The leading part in this process is assigned to the
nucleus. This problem will be considered again in the next Section 3. 3.

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Figure 3–3. Division of cells (erythrocytes) of chicken embryos
(After Remak, 1858; From Katznelson, 1963).

image

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Figure 3–1. Division of cells in filamentous algae of Conferva genus 
(After von Mohl, 1835; From Katznelson, 1963).

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Figure 3–2. Division of cells during pollen grain formation (After Nägeli, 
1842; From Katznelson, 1963). 
First pictures of dividing cells with constriction of protoplasm and forming 
barrier walls between the daughter cells. 

CELL DIVISION
CHAPTER 3

Biological Essence and Importance of the Cell Division

Section 3.1. Cell division is a unique biological mechanism, through which the
reproduction of living organisms is realized. Its biological essence and
importance find expression in redistribution of the genetic material in the
daughter cells and preservation of the hereditary information in the
following generations.
In unicellular organisms the cell division is a process of reproduction, since
from one mother cell two or more daughter cells are formed and each of them
participates independently in the following life cycle. In multicellular organisms,
through multiple divisions of the cells in particular tissues and organs the growth
and development of individuals is realized, that form specialized regenerative
cells — spores, gametes, eggs, spermatozoa, etc. The following generations are
reproduced from single regenerative cells or through their fusion and zygote
formation.
In the course of evolution different modes and types of cell division are
created, in which the basic predestination as a mechanism of reproduction
of the living organisms has been preserved. This question will be
considered once more further.

Discovery and Insight into the Essence of Cell Division

Section 3.2. The discovery of cell division and penetrating its essence is not a single act,
but a long process of researchs. A detailed study of the history on this
problem is given in the monographs by Katznelson (1939, 1963) and “History
of Biology” (1972, 1975). According to these literary sources, first of all cell
division in filamentous algae is observed in 1832 by the Belgian naturalist
Charles Dumortier. In 1835 Hugo von Mohl has described dividing cells in
algae of Conferva genus (Fig. 3–1), but he has not noticed cell nucleus and
has not known anything about it. Only several years after K. Nägeli (1842 b)
also has described division of cells during pollen grain formation (Fig. 3–2),
some of them with nuclear structures. In the second part of his publications
under the common title “Cell Nucleus, Cell Division and Growth of the Cells in
Plants” Nägeli (1844—46) arrives at the extremely important idea, that “the
mother cell gives rise to two or more daughter cells through the process of
division, observed for the first time by von Mohl”. At that time Remak (1858)
has also observed division of cells in tissues of animal organisms and
erythrocytes of chicken embryos (Fig. 3–3).

viruses from bacteria through so-called “regressive” evolution. Most
recognized is the third hypothesis that the viruses originate from cellular
components similar to plasmids. Once appeared they have evolved from
simple to more complexly organized forms, and some of them could have a
polyphiletic way of evolving thus creating the enormous “Vira Kingdom”.

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Figure 2–90. Electron micrograph of DNA T2 phage released from the 
phage head under the action of osmotic shock (After Kleinschmidt et al., 
1962).

The behaviour of viruses as “terminators” of the living cells and
organisms and the absence of own, independent metabolism, do not give
grounds to be considered cells, even less adequate ones. Their biological
role and the causes for their arising are to be elucidated.

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Figure 2–89. Reconstitution of viral particles (TMV) from isolated protein 
subunits and RNA-molecules (After Fraenkel-Konrat, 1962).

In some viruses (phage φX 174) the nucleotide composition of DNA
differs from the normal one. The rule A + G = T + C is violated and the
“normal” formation of A—T and G—C base pairs is impossible. T-even
phages contain 5-hydroxymethylcytosine (HMC) instead of cytosine. In this
respect there are many other differences considered in detail in the
literature on virology and molecular biology. In unfolded state, after an
osmotic shock (sudden diluting of the phage suspension in a concentrated
salt solution with distilled water) the molecule of DNA has “enormous”
dimensions, about 50 nm in length, exceeding several hundred times the
length of the head containing it before (Fig. 2–90).
Now, let us return to the question posed: are the viruses cells? First
of all the answer to this question should be looked for in elucidation of their
origin and evolution. There exist three main hypotheses about the origin of
viruses: 1) from primitive precellular forms of life; 2) product of a
“regressive” evolution of bacteria; 3) came off and acquired independence
cellular components.
Most unacceptable is the first hypothesis that viruses are precellular
forms of life. More recognized is the second hypothesis about the origin of

The cells in which viruses can penetrate, develop and reproduce are
called permissive, and the cells where it is impossible — non-permissive.
Also, there exist cells, which are permissive for some viruses and non-
permissive — for others. The reasons for that are also unclear.
The interaction of viruses with the cells is a complex biological
process. It is realized in several successive phases: attaching the virus to
the cell surface; penetrating of virus particle or its components into the cell;
disintegration of the virus particle; synthesis of the viral components in the
cell; formation of mature viral particles; leaving the cell by mature viral
particles (virions). The term virion is used as a synonym of mature viral
particle.
Coming into contact of the viral particles with the cell surface is
attended by their adsorption, which is a result of their interaction with
mucoproteins located on the cell-wall surface. While it is known that the
phages are attached to bacterial cells by the tails and their spikes, which
prepare the channel for their differential penetration, in plant and animal
cells this question is still unclear. It is supposed that viral particles
penetrate intact in them by using mechanisms resembling pinocytosis. This
mode of penetration is called viropexis.
A great part of the investigations on viruses and phages are devoted to
their biochemistry and reconstitution. In this respect a thorough review is
the monograph of Tikhonenko (1966). The initial impression and the formed
opinion that phages are simply organized living particles composed of
strictly distinguished and easily separated protein molecules and nucleic
acid (DNA or RNA) stimulated some investigators to try to reconstitute them
practically (Fig. 2–89). At present, according to the prevailing concept their
structural and functional organization is much more complex and resists
such manipulations.
The genome of viruses is characterized by an exceptional diversity of
types of structural and functional organization. It can be one linear molecule
of DNA or RNA (rhabdo- or paramyxoviruses) or fragmented (orthomyxo-
viruses, buniaviruses, arenaviruses). For example influenza virus consists of
eight RNA molecules, each of them carrying the information for a synthesis of
one virus-specific protein, and in some cases their number is reduced to
three long segments. Besides, genome RNA (TMV, potato X-virus, cucumber
mosaic, poliomyelitis, influenza, encephalomyocarditis, plague in birds, f2,
etc.) or DNA (herpes, papilloma, adenovirus type 2 and 5,
pseudohydrophobia, measles in animals, SV40, φ X 174, f1, etc.) can be as
single chain, as well as double chain.