THE CELL AND CHROMOSOMES BEHAVIOUR
We can conceder the cell is the smallest unit in the life; starting from
unicellular animals to high living animals; in which we see unlike or
unreasonable cells like digestive system or nervous system cells, and even there
are differences between the same type of system cells as we see in circulatory
system cells.
Robert Hook 1665 is the first scientist who discovers the cell by using the
simple light microscope during the examination of some plant tissue, e.g. anion
cells.
After this period the studies continued till some authors put the theory of the
cell, Schleiden 1838 and Schwenn 1839, which explain that the cell is
instruction unit of the life.
Cell size and contents
Cell size varied according to the type and shape of the living animals, so we
see oval cell and semi-polygonal cells and there are some nerve cell may reach
their length to many meters, and In opposite to that some cells may have a
diameter may reach one micron, and also the hen egg conceder as a one cell.
In spite of the differences of the cells in the shape and size and content, but they
are have the same important structures, and one of this important structure is the
Nucleus, and this contain the chromosomes which conceder the genetic factors
carrier for the living species. Chromosomes looking like solitary tiny threads
and its diameter range 10 to 50 nanometer (n.m=1x10-9). This threats known as
Chromatin and it’s a complex compound consist of two parts, the first called
DNA and the other component is the Basic protein and its be either Histones or
Protamine. This chromatins threats and during cell division shrink over them
self and look to be thick shape known as chromosome (look clear under the
microscope), ex. Man 23 pair, horse 32 pair, donkey 31 pair and also for plants,
so in this case we use the term “2n” to discuss the duplicate shape for the all
number of chromosomes.
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In general chromosomes resemble each other, but there are some
feature we can use it for recognize and distinct chromosomes from each
others and the most of these are:
1- length of chromosomes
2- Centromer location on the chromosomes, and the relative length of
chromosomal arms
3- presents of satellites which separate from the chromosome by the secondary
constriction
Shape and structure of the chromosome
Sutton 1876-1916 was the first scientist who explains the relation between the
chromosome and genes in scientific form, and these genes carried by the
chromosomes, i.e. the gene is a part of chromosomes, and the most of his
sequences are:
1- The congenital characters carried on the sex cells (germ cell and ovum), and
this represent a bridge between one generation to another.
2- The genotype factors present inside the nucleolus of each germ cell and
ovum.
3- The genotype factors present inside the chromosomes and those factors
divided accurately during the two types of multiplication, mitosis and meiosis.
4- Chromosomes and mendelian factors appear in form of pairs.
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5-Chromosome division had been according to the type of cell division either
mitosis or meiosis.
Chromosomal morphology
In order to further understand chromosomes and their function we need to be
to discriminate among different chromosomes.
First, chromosomes differ greatly in size. Between organism the size
difference can be over 100-fold, while within a species some chromosomes are
often ten times as large as others. In a species karyotype, a pictorial or
photographic representation of all the different chromosomes in a cell of an
individual, chromosome are usually ordered by the size and numbered from
largest to smallest.(A pictorial or photographic representation of all the
chromosomes in a given individual.)
Second, chromosomes may differ in a position of the centromere, the place
on the chromosome where spindle fibers are attached during cell division. The
centromere is identified as a large constriction where the chromosome appears
to be pinched. In general if the centromere is near the middle, the chromosome
is metacentric; if the centromere is toward one end, the chromosome is
acrocentric (or submetacentric); and if the centromere is very near the end, the
chromosome is telocentric.
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Number of chromosome in some species
Individual
Man
Chimpanzee
Horse
Dog
Sheep
Cattle
Drosophila
diploid (2n)
46
48
64
78
54
60
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haploid (n)
23
24
32
39
27
30
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Cell Cycle and mitosis
Cell Cycle
Cellular reproduction is a cyclic process in which daughter cells are
produced through nuclear division (mitosis) and cellular division (cytokinesis).
Mitosis and cytokinesis are part of the growth-division cycle called cell cycle
(figure B1.2a). Mitosis which is divided into four stages (see figure B1.2b, is a
relatively small part of the total cell cycle; the majority of the time, a cell is in a
growth stage called interphase, Interphase is divided into three parts, G1 S, and
G2. The first gap phase, G1, follows mitosis and is a period of growth and
metabolic activity. The S phase follows G1 and is a period of DNA synthesis in
which the DNA is replicated. Another gap phase, G2, follows DNA synthesis
and precedes the next mitotic division. Certain mature cell type does not
continue to divide but remain in interphase (in G0).
Mitosis
Mitosis is the period of time in the cell cycle in which the nucleus divides
and gives rise to two daughter cells that have chromosome numbers identical to
that of the original cell (figure B1.2b). Mitosis itself can be divided into four
different stages, listed in chronological sequences as: prophase, metaphase,
anaphase, and telophase. These stages actually merge one into another, but
each has definite characteristics, particularly in relation to chromosomal
behavior, that we can use to identify them. (Figure 4.7 and figure 1.2B).
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Prophase
At the beginning of mitosis (the end of G2 interphases). The chromosomes
coil and condense, thus becoming visible under the light microscope. The
chromosome have replicated and each has two sister chromatids at this stage
(figure 4.7a). The two sister chromatids are attached to each other at the
centromere. The chromatids are identical and are the result of DNA replication
in the earlier S phase of the cell cycle. Because the two sister chromatids are
attached at the centromeric region, they are considered one chromosome at this
stage.
Also during prophase, the nucleolus, the largest cell organelle besides the
nucleus, usually disappears and the nuclear membrane begins to break down
(figure 4.7b). The centriole, an organelle important in organizing cell division,
divides. Around it a new structure, the spindle apparatus, which is composed of
a series of spindle fibers that stretch to the poles of the cell, also appears
(figure4.8). Some spindle structure is fundamental to the proper movement of
the chromosome to the two poles in the latter part of mitosis.
(Prophase At the end of G2 interphase, the chromosomes have replicated, and each has two
sister chromatids attached to each other at the centromere. Prophase indicates the beginning
of mitosis, when the replicated chromosomes coil and condense. The centrioles divide and the
spindle apparatus also appears.)
Metaphase
The nuclear membrane completely disappears during metaphase, leaving
the chromosomes free in the cytoplasm (figure 4.7c). The spindle fibers attach
to the centromeres of the chromosomes, and the chromosomes gradually change
their orientation, moving to the metaphase (or equatorial) plane, an imaginary
region midway between the two poles. Generally, the chromosomes are most
condensed at this stage of mitosis, making it a good stage for preparation of
karyotypes. Because sister chromatids are attached at the centromeres, a
chromosome at metaphase when it is spread out in a karyotype appears either as
an X or s V, depending upon whether it is metacentric or telocentric,
respectively. The acrocentric is intermediate between these shapes.
(Metaphase. During metaphase, the spindle fibers from the spindle apparatus attach to the
centromeres of the chromosomes. The chromosomes gradually migrate to the middle of the
cell oriented between the two centrioles.)
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Anaphase
Anaphase, nearly always the shortest period of mitosis, begins with the
separation of the centromeres (figure 4.7d and figure B1.2). As a result, each
sister chromatid is now called a daughter chromosome. The spindle fibers
contract and cause the movement of the centromeres to the poles, with the two
identical (homologous) daughter chromosomes moving to opposite poles (figure
4.7e and figure B1.2). A chromosome appears at a V, J, or I at this stage,
depending upon whether it is metacentric, acrocentric or telocentric,
respectively.
(Anaphase. it begins with the separation of the centromeres and the contraction of the
spindle fibers pulling the centromeres toward the centrioles. As a result, the sister
chromatids, now called daughter chromosomes, separate, and homologous chromosome
move in opposite directions toward the centrioles.)
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Telophase
In telophase, the last stage of mitosis, the two sets of daughter chromosomes
have reached opposite poles (figure 4.7f and review box figure 1.2B) where they
begin to uncoil. The nuclear membrane reappears and encloses the
chromosomes, and the nucleoli are re-formed.
Next. In animals, the cell membrane develops a furrow; in plants, a cell plate
develops that divides the cell in two. This splits the two sets of daughter
chromosomes and the cytoplasm between the two daughter cells. The daughter
cells have chromosome numbers identical to the original cell.
(Telophase. The two sets of chromosomes reach opposites pole of the cell, where they begin
to uncoil. Telophase ends with cytokinesis (cellular division) and the formation of two
daughter cells.)
Mitosis, cell division that results in the production of two identical daughter
nuclei, is divided into four stages: prophase, metaphase, anaphase, and
telophase.
Meiosis
Before describing meiosis, we will summarize why it is fundamental for
maintaining the genetic state of an organism and for generating new genetic
types. First, meiosis allows the conservation of chromosome number in sexually
reproducing species. If not reduced via meiosis prior to fertilization, the
chromosome number would be doubled. Second, meiosis permits the different
maternal and paternal chromosomes to randomly assort into each gamete. And
finally (third), the presence of crossing over in meiosis I allows the exchange of
genetic material on homologous chromosomes and can result in new
combination of alleles at different genes on a chromosome. (See chapter 5 for a
discussion of crossing over.)
Meiosis produces cells with half the parental number of chromosome through
a process involving two cell division, meiosis I and meiosis II. Meiosis I is also
called reductional division, because the number of chromosomes is reduced to
the haploid number. In meiosis I, the sister chromatids remain attached to each
other while the homologous chromosomes separate. Meiosis II is called
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equational division and is much like mitosis in that the centromeres on sister
chromatids separate and the chromosome number remain unchanged (figure 4.9
and figure B1.3a).
Meiosis I
As with mitosis, both meiotic divisions can be divided into four stages, based
on the position of the chromosomes and other characteristics. The interphase
period before meiosis I is similar to that in mitosis, having an S period for DNA
synthesis.
Prophase I
The most complex phase of the whole process of meiosis, prophase I, is
itself broken down into five different parts (figure 4.9a-e and figure B1.3a). The
first stage, called the leptotene (thin-thread) stage, is marked by the appearance
of the chromosome as long threads when seen through a light microscope. This
stage is much like the start of prophase in mitosis.
However in the next stage, the zygotene (joined-thread) stage, unlike in
mitosis, homologous chromosomes pair side by side and gene by gene with
each other. The process of the lateral association of homologous, called
synapsis, occurs during zygotene and is a key difference between meiosis and
mitosis. When the two homologous chromosomes (the large red and blue pair or
the small red and blue pair), consisting of four chromatids, are paired, the
structure is called a bivalent. The next, or third, stage of prophase I, the
pachtene (thick-thread) stage, consists of a shortening and thickening of the
bivalents and is the stage during which synapsis is complete. During this stage,
portions of homologous chromosomes may be exchanged, a phenomenon called
crossing over or recombination. The structure that allows crossing over is the
synaptonemal complex, a structure made of protein and DNA that is found
between the synapsed homologous.
In the fourth stage, the diplotene (double-thread) stage, the homologous
begin to separate as the synaptonemal complex breaks down, particularly in the
region of either side of the centromere. The sister chromatids remain attached at
the centromeric region. In addition, the homologous chromosomes generally
have one or two areas, called chiasmata, in which they are still close or in
contact. Chiasmata are the physical evidence that recombination occurred
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earlier when the homologous were synapsed. Generally, there is at least one
chiasma per chromosome arm, but in long chromosomes, there may be several.
The last stage of prophase I, diakinesis, is characterized by shortened
chromosomes and the terminalization of the chiasmata; that is, the chiasmata
appear to move to the ends of the chromosomes. As in mitosis, at the end of
prophase, the nucleolus and the nuclear membrane disappear.
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Metaphase I
In metaphase I (figure 4.9f and figure B1.3a), the chromosomes have
aligned at the equatorial plane as homologous pairs and become attached to the
spindle fibers. The bivalents orient so that one centromere is one each side of
the metaphase plane, with the chiasmata lined up along it. As a result,
metaphase I is different from mitotic metaphase, in which homologous
chromosomes were not associated as bivalents.
Anaphase I
At the stage known as anaphase I (figure 4.9g and figure B1.3a), the two
sister centromeres in homologous chromosomes of each bivalent move apart, so
that one of each homologous chromosome pair goes toward each pole. As the
homologous move apart, the chiasmata completely terminalize. In anaphase I
unlike mitotic anaphase, the sister chromatids stay together, joined at the
centromere.
Telophase I
In most organisms, after the chromosomes have migrated to the poles, the
nuclear membrane forms around them and the cell divides into two daughter
cells, just as in mitosis. However, the exact cytological details of telophase I
(figure 4.9h and figure B1.3a) vary, particularly in plant.
Meiosis II
Between meiosis I and meiosis II there is usually only a short interphase
called interkinesis. No DNA synthesis takes place during this period; at this
point, each cell contains only a haploid complement (n) of chromosomes. (Note
that each chromosome contains two sister chromatids.) The next stage is
prophase II, which unlike prophase I is quick, uncomplicated, and generally
similar to mitotic prophase. In metaphase II (figure 4.9i and figure B1.3a), the
centromeres attach to spindle fibers and migrate to the metaphase plate. At the
start of anaphase II (figure 4.9j), as in mitotic anaphase, the chromosomes first
separate at the centromeres when they divide. One centromere begins to move
with one daughter chromosome to one pole, while the other centromere and go
to the opposite pole. There are n chromosomes at each pole (each chromosome
is composed of a single chromatid) when the nuclear membrane forms around
them in telophase II.
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Before we continue, it is useful to compare the change in chromosome
number and amount of DNA that occur during the mitotic and meiotic cycles
(see figure B1.3a). First, as we have already discussed, homologous
chromosomes synapse in meiosis I, allowing recombination and precise
separation of homologues, but this does not occur in mitosis. Second, at the
beginning of both mitosis and meiosis, there are 2n chromosomes in each cell
(figure 4.10a). Each chromosome is composed of two sister chromatids, so that
the amount of DNA is four times the haploid amount (indicated as 4C in figure
4.10b). At the end of mitosis, there are still 2n chromosomes, but the amount of
DNA has been reduced to 2C per cell. At the end of meiosis I (indicated by A-I,
or anaphase I, in figure 4.10), the number of chromosomes has been reduced to
(n) and the amount of DNA to 2C. As in mitosis, meiosis II does not change the
number of chromosomes, but the amount of DNA is halved to C as the sister
chromatids separate.
Spermatogesis and Oogenesis
The sequences of nuclear events in nuclear meiosis that we have described
are similar in the production of both male and female gametes; however, there
are cytoplasmic differences (figure 4.11).
The production of male gametes in animals, termed spermatogenesis, takes
place in the testes, the male reproductive organs. The process begins with
growth of an undifferentiated diploid cell called a spermatogonium, which then
differentiates into a primary spermatocytes. This cell, in turn, undergoes the first
meiotic division (the reductional division), becoming two haploid secondary
spermatocytes. After meiosis II, these cells each divide into two haploid
spermatids. The last step is the differentiation of the spermatids into new sperm
cells with long tails (figure 4.12).
Oogenesis, the production of female gametes in animals, occurs in the
ovaries, the female reproductive organs. The process is parallel to that of
spermatogenesis in that it also begins with a diploid cell, or oogonium.
However, the cytoplasim is concentrated into only one of the products, one of
the secondary oocytes, while the other cell products (first and second polar
bodies) receive only a small amount of cytoplasm. Meiosis takes place near the
cell membrane, allowing the formation of polar bodies with little cytoplasm.
The polar bodies remain on the surface of the egg where they eventually
disintegrate. Therefore, only one mature haploid egg is produced in oogenesis,
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while four mature haploid sperm are produced in spermatogenesis. The
concentration of cytoplasm in one egg cell provides nourishment for the
developing embryo after fertilization.
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Spermatogenesis is continuous in adult animals that reproduce all year round,
such as human, and seasonal in animals that breed seasonally. The capacity for
sperm production is extremely high in most animals, with adult males producing
many millions of sperm in their lifetimes. On the other hand, egg production in
female is generally much lower; for example, a human female can produce only
around five hundred mature eggs in her lifetime. All these eggs develop into
primary oocytes before the female is born and are arrested in prophase I until
puberty, when they resume meiosis and begin maturing into eggs one by one
and are ovulated each month. Many other eggs never mature and reabsorbed.
In higher plants, the formation of male gametes is known as
microsporogenesis. It is similar to spermatogenesis in animals in that meiosis
gives rise to four haploid cells, all of an equal size and all potentially functional.
Megasporogenesis, the formation of eggs in plants, is also similar to the meiotic
process described for animals. However, the products vary considerably among
species and are complicated by double fertilization. (It is best to consult a
botany text for more detales.)
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