As to date there are two
types of cells that have been recognized duly, such as :
(
a) Eukaryotic cells, and
(
b) Prokaryotic cells.
Another third type, known as the
Urkaryotes, and are most probably the progenitor of the present
day
eukaryotes has now also been recognized duly.
The above
two types of cells* (a) and (b) shall now be discussed at length in the sections thatfollows :
Eukaryotic Cells
[‘eu’ = true ; ‘karyote’ = nut (refers to nucleus of cell)] ;
It has been observed that the
eukaryotic cells (Fig. 2.1) are explicitely characterized by the
presence of a
multiplicity of definite unit membrane systems that happen to be both structurally and
topologically distinct from the cytoplasmic membrane. Subsequently, these prevailing membrane systems
categorically enable the
segregation of various eukaryotic cytoplasmic functions directly into
specialized organelles.
** Endoplasmic reticulum (ER) represents the most complex internal membrane
system that essentially comprises of an irregular network of interconnected delimited channels
that invariably cover a larger segment of the interior portion of the cell. Besides, ER gets in direct
contact with
two other extremely vital components viz., nucleus and cytoplasmic ribosomes. The nucleus
membrane is duly formed by a portion of the
endoplasmic reticulum surrounding the nucleus ;
whereas, in other regions the surface of the membrane is particularly covered with the ribosomes wherever
synthesis of protein takes place. The proteins thus generated eventually pass
via the endoplasmicreticulum channels right to the various segments of the ensuing cell cytoplasm.
Nucleus.
The eukaryotic cell possesses the ‘genetic material’ duly stored in the chromosomes
i.e.,
very much within the nucleus. However, chloroplasts and mitochondria also comprise of characteristic
DNA
. The chromosomes are linear threads made of DNA (and proteins in eukaryotic cells) in thenucleus of a cell, which may stain deeply with basic dyes, and are found to be especially conspicuous
during mitosis. The DNA happens to be the genetic code of the cell ; and specific sequences of DNA
nucleotides are the genes for the cell’s particular proteins. However, the size and the number of the
chromosome vary widely with various organisms. Nevertheless, the nucleus invariably contains a
nucleolus
that is intimately associated with a
particular chromosomal segment termed as the ‘nucleolar
organizer’,
which is considered to be totally involved in ribosomal RNA (rRNA) synthesis.
Mitosis. Mitosis
refers to a type of cell division of somatic cells wherein each daughter cell
contains the same number of chromosomes as the parent cell.
Mitosis is the specific process by which
the body grows and dead somatic cells are replaced. In fact,
mitosis is a continuous process divided into
four
distinct phases, namely : prophase, metaphase, anaphase, and telophase.
A brief discussion of the aforesaid
four phases shall be given in the sections that follows along
with their illustrations in Fig. 2.2.
(
a) Prophase. In prophase, the chromatin granules of the nucleus usually stain more densely
and get organized into chromosomes. These first appear as long filaments, each comprising
of
two identical chromatids,* obtained as a result of DNA replication. As prophase progresses,
the chromosomes become shorter and more compact and stain densely. The nuclear membrane
and the nucleoli disappear. At the same time, the
centriole divides and the two daughter
centrioles,**
each surrounded by a centrosphere, move to opposite poles of the cell.
They are duly connected by fine protoplasmic fibrils, which eventually form an
achromaticspindle.
(
b) Metaphase. The metaphase refers to the chromosomes (paired chromatids) that arrange
themselves in an equatorial plane midway between the two
centrioles.
(
c) Anaphase. In anaphase, the chromatids (now known as daughter chromosomes) diverge
and move towards their respective
centrosomes. The end of their migration marks the beginning
of the next phase.
(
d) Telophase. In telophase, the chromosomes at each pole of the spindle undergo changes that
are the reverse of those in the prophase, each becoming a long loosely spiraled thread. The
nuclear membrane re-forms and nucleoli reappear. Outlines of chromosomes disappear, and
chromatin appears as granules scattered throughout the nucleus and connected by a highly
staining net. The cytoplasm gets separated into
two portions, ultimately resulting in two
complete cells. This is accomplished in animal cells by constriction in the equatorial region ; in
plant cells, a cell plate that produces the cell membrane forms in a similar position. The
period between two successive divisions is usually known as
interphase.
Mitosis
is of particular significance wherein the genes are distributed equally to each daughter
cell and a fixed number of chromosomes is maintained in all somatic cells of an organism.
Mitosis
are of two kinds, namely :
(
i) heterotypic mitosis : The first or reduction division in the maturation of germ cells, and(ii) homeotypic mitosis : The second or equational division in the maturation of germ cells.
Meiosis. Meiosis
refers to a specific process of two successive cell divisions, giving rise to cells,
egg or sperm, that essentially contain half the number of chromosomes in somatic cells. When fertilization
takes place, the nuclei of the sperm and ovum fuse and produce a zygote with the full chromosome
complement.
In other words, the phenomenon of
meiosis may be duly expatiated in sexually reproducing
organisms, wherein the prevailing cellular fusion followed by a reduction in the
‘chromosome number’
is an important and vital feature. The
two cells which actually participate in the sexual reproduction are
termed as
‘gametes’, which fuse to form a ‘zygote’. The above process is subsequently followed by a
nuclear fusion
and the resulting zygote nucleus contains two complete sets of genetic determinants
[2N]. In order to adequately maintain the original
haploid number in the succeeding generations, there
should be a particular stage at which a definite reduction in the chromosome number takes place. This
process that occurs after the fusion of
gametes is known as meiosis.
Fig. 2.3 illustrates the schematic representation of meiosis, and the various steps involved may
be explained sequentially as follows :
(1) Meiosis comprises of
two meiotic divisions viz., prophase I, and prophase II.
(2)
Prophase-I. It represents the first meiotic division, whereby the homologous chromosomes
become apparently visible as single strands that subsequently undergo pairing.
(3) Each chromosome renders visible as
two distinct chromatids and thus crossing over takes
place.
(4) It is immediately followed by
metaphase I, wherein the actual orientation of ‘paired chromosomes’
in an equatorial plane and the subsequent formation of a
‘spindle apparatus’
takes place.
(5) It is followed by Anaphase I, and the homologous centromeres gradually move to the opposite
poles of the spindle.
(6)
Telophase-I. It markedly represents the end of the first meiotic division, and formation of
two
nuclei takes place.
(7)
Interphase-II. Telophase-I is followed by Interphase-I during which the chromosomes get
elongated.
(8)
Prophase-II and Metaphase-II. In prophase-II and metaphase-II the division of centromere
and migration of the homologous
chromatids occurs, which is duly followed by anaphase-II,
and the desired second meiotic division resulting in the formation of
four haploid* cells.
Eukaryotic Protist.
It has been observed that in several eukaryotic protists belonging to higher
ploidy** (> 1) meiosis usually takes place after the formation of the
zygote and prior to spore formation.
In certain eukaryotes there may even be a critically pronounced alteration of
haploid and diploid generations
as in the case of the
yeast. Interestingly, in this particular instance, the diploid zygote produces
a
diploid individual that ultimately gives rise to haploid cells only after having undergone the phenomenon
of
meiosis. Consequently, the haploid cell may either multiply as a haploid or get fused withanother haploid of the ‘opposite mating type’ to generate again a diploid.
Special Points :
There are two cardinal points which, may be borne in mind with regard to the
Eukaryotic Protist
as stated under :
(
i) Despite of the fact that sexual reproduction could be the only way of reproduction in a large
segment of animals and plants ; it may not be an obligatory event in the life cycles of many
protists.
(
ii) In two glaring situations ; first, protists lacking a sexual stage in their respective life-cycle ;
and
secondly, such species wherein sexuality does exist : the sexual reproduction may be
quite
infrequent (i.e., not-so-common).
Important organelles in Eukaryotic Cells :
It has been amply proved and established that the
eukaryotic cells
invariably contain certain cytoplasmic organelles other than the nucleus. The important
organelles in eukaryotic cells usually comprise of
three components, namely : mitochondria, chloroplasts,
and the
Golgi apparatus, which shall now be described briefly in the sections that follows :
Mitochondria.
These are mostly found in the respiring eukaryotes and essentially contain an
internal membrane system having characteristic structure and function. The internal membrane of the
mitochondria (
cristae) possesses the necessary respiratory electron transport system. The exact number
of copies of mitochondria per cell solely depends upon the cultural parameters and varies from 1–20
mitochondria per cell. These are generated by the division of the preexisting organelles containing
ribosomes that usually resemble the bacterial ribosomes. However, the process of protein synthesis in
the mitochondria are very much akin to that in the
prokaryotic cells.
These cell organelles (rod/oval shape 0.5
μm in diameter) may be seen by employing a phasecontrast
or
electron microscopy. They mostly contain the enzymes for the aerobic stages of cell respiration
and thus are the usual sites of most
ATP synthesis chloroplasts [or Chloroplastids] :
Chloroplasts
are found in the photosynthetic eukaryotic organisms. The internal membrane
of the chloroplasts is termed as the
‘thylakoid’ which essentially has the three important components :
(
a) photosynthetic pigments, (b) electron transport system, and (c) photochemical reaction centres. The
number of copies of the chloroplasts depends exclusively upon the cultural conditions and varies from
40 to 50 chloroplasts per cell. These are also produced by the division of the preexisting organelles.
Generally, chloroplasts are the sites of photosynthesis. They possess a stroma and contain
four
pigments :
chlorophyll a, chlorophyll b, carotene, and xanthophyll.
Golgi Apparatus :
The Golgi apparatus is a lamellar membranous organelle invariably found
in the eukaryotic cells and consists of thickly packed mass of flattened vessels and sacks of different
sizes. The major functions of the
Golgi apparatus are, namely :
• packaging of both proteinaceous and nonproteinaceous substances duly synthesized in the
endoplasmic reticulum, and
• their adequate transport to other segments of the cell.
Golgi apparatus
may be best viewed by the aid of electron microscopy. It contains curved
parallel series of flattened saccules that are often expanded at their ends. In secretory cells, the apparatus
concentrates and packages the secretory product. Its function in other cells, although apparently important,
is poorly understood.
Prokaryotic Cells
[‘pro’ = primitive ; ‘karyote’ = nut (refers to nucleus of cell)] :
Prokaryote :
is an organism of the kingdom Monera with a single circular chromosome, without
a nuclear membrane, or membrane bound organelles. Included in this classification are bacteria and
cyanobacteria (formerly the blue-green algae) [SYN :
prokaryote].
In fact, the prokaryotic cell is characterized by the absence of the
endoplasmic reticulum (ER)
and the
cytoplasmic membrane happens to be the only unit membrane of the cell. If has been observed
that the
cytoplasmic membrane may be occasionally unfolded deep into the cytoplasm. An exhaustive
electron microscopical studies would reveal that most
prokaryotes {i.e., prokaryotic cells) only two
distinct
internal regions, namely : (a) the cytoplasm ; and (b) the nucleoplasm, as shown in Fig. : 2.5.
Cytoplasm : Cytoplasm
refers to the protoplasm cell outside the nucleus. It is granular in appearance
and contains ribosomes that are specifically smaller in size in comparison to the corresponding
eukaryotic ribosomes
.
Nucleoplasm :
It refers to the protoplasm of a cell nucleus. It is fibrillar in character and contains
DNA.
With
mycoplasmas* as an exception, other prokaryotes invariably comprise of a defined and
rigid cell wall. It has been observed that neither the membranous structures very much identical to the
mitochondria
nor chloroplasts are present in the prokaryotes. Besides, the cytoplasmic membrane happensto be the site of the respiratory electron in the prokaryotes usually. Interestingly, in the photosynthetic micro-
organisms
(bacteria), the photosynthetic apparatus is strategically positioned in a particular series of
membranous, flattened structures quite similar in appearance to the
thylakoids ; however, these structures
are not organized into the respective
chloroplasts but are adequately dispersed in the cytoplasm.
Thus, the cytoplasmic membrane contains a plethora of specific sites for the DNA attachment, and also
plays a major role in the cell division. Here, the cell membrane unlike in the eukaryotic cell does not
generally contain
sterols and polyunsaturated fatty acids (PUFAs). Mostly the fatty acids present are ofthe saturated type e.g., palmitic acid, stearic acid etc.
Importantly, the ‘
genetic component’ present in the prokaryotic cells is strategically located in
the ‘nucleoplasm’ that essentially lacks a defined nuclear membrane. Nevertheless, it comprises of
double
helical DNA
without any associated basic proteins. In fact, the very site of the DNA in prokaryotic
protists
is much smaller in comparison to that present in eukaryotes. In addition, the prokaryotes do
contain extra-chromosomal DNA, that may replicate autonomously, termed as the
‘plasmids’. However,
these can be lost from the cell without impairment of the
‘cell viability’. The prokaryotic cells
usually exist in a haploid state and predominently get divided by a process quite identical to
mitosis
although distinct stages are not recognized so frequently.
A good number of
prokaryotes do possess a cell wall that is vastly different in composition from
that of
eukaryotes, and invariably contains a rather rigid and well-defined polymer termed as the
peptidoglycan.*
It has been observed that certain prokaryotes which essentially possess this aforesaid
rigid structure distinctly exhibit
‘active movement’ with the help of flagella. Some prokaryotes mayalso display a ‘gliding motility’ as could be seen in the ‘blue-green bacteria’ quite frequently.