Cytology
Dr.sarab
Meiosis:
Most reproduction occurs through sexual reproduction. Sexual
reproduction is the recombination or mixture of genes. This results in the
offspring having a combination of DNA from both parents. This will help
add to:
(l) the variation within a population or a species.
(2)this also creates unique individuals, which are not identical to the
parents.
For this to be possible, there has to be a special type of cell division. If
there was a union of two somatic cells, then the offspring would have
twice the number of chromosomes as the parents, and that would continue
to increase. This is not the case however, so there is a cell division that
reduced the number of chromosomes in half. This is important because
when we add the two parent cells together (fertilization) the offspring will
end up with the same number of chromosomes as the parents, but with a
combination of features.
Each species has a different number of chromosomes. For example,
humans have 46 chromosomes (23pairs) in every body cell. All cells
contain two complete sets of chromosomes( in humans each set has 23
making a total of 46) called the diploid (2n) number of chromosomes. The
exception to this is with sex cells, such as egg and sperm. Since these will
later combine they each only have one set of chromosomes , called haploid
(n) cells, and in humans this means they have 23 chromosomes total.
Haploid cells, or reproductive cells, are called gametes or sex cells. The
process which produces diploid cells was called mitosis. This occurs in all
somatic, or body cells. To obtain the reduced number of chromosomes for
haploid cells, they undergo what is called meiosis.
Meiosis is the formation of gametes, performed by reproductive cells
only. This will result in a reduction of the chromosomes number, forming
haploid cells meiosis produced 4 daughter cells, each with half the
number of chromosomes, that are not identical to each other. This is again
important because it, keeps the chromosome number constant over
generations, in the adult organism. When the egg (n) and the sperm (n)
combine (n + n) in fertilization, the offspring is a diploid cell
(2n).Different organisms have different numbers of chromosomes.
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when chromosomes pair, in preparation for meiosis, they arrange with
their homologous pair, otherwise known as homologs. This means that like
chromosomes pair with each other. Remember humans have 46
chromosomes, but23 pairs, each of the chromosomes in the pair are
exactly alike in size, gene location and location of the centromere. There is
one exception, the sex chromosomes. Females have a pair called XX, both
of them are alike' but males are XY.
The homologous Pairs of chromosomes ate
separated and end up in
4 daughter cells. This happens in two main
states, meiosis I and meiosis II. The Pairs
separate during meiosis I, and meiosis II is
when the sister chromatids
separate, much like in mitosis.
Meiosis I is a 4 step process. The start of
this is much like mitosis, the cells have gone
through the G1, S and G2 phases, thus the
DNA has been replicated and are in the form
of sister chromatids, connected at the
centromere. The main difference here is that the homologous pairs of
chromosomes pair up and then proceed as follows .
Prophase I is traditionally divided into five sequential
stages: leptotene, zygotene, pachytene, diplotene, and diakinesis.
Leptotene. Chromosomes condense tightly.
Zygotene. Pairing of homologous chromosomes. This is accomplished
with the help of the synaptonemal complex'' the synaptonemal complex
pulls together homologous regions of the DNA. While not identical,
homologous regions have a very high similarity.
(Synapsis The sites the homologues attach to are adjacent, so that the
members of each homologous pair of chromosomes are brought close
together. They then line up side by side, apparently guided by
heterochromatin sequences, in the process called synapsis . )
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Pachytene. Pachytene begins when synapsis is complete (just after the
synaptonemal complex forms; and lasts for days. This complex, about 100
nm across, holds the two replicated chromosomes in precise register,
keeping each gene directly across from its partner on the homologous
chromosome, like the teeth of a zipper. Within the synaptonemal complex,
the DNA duplexes unwind at certain sites, and single strands of DNA form
base-pairs with complementary strands on the other homologue. The
synaptonemal complex thus provides the structural framework that enables
crossing over between the homologous chromosomes.
Crossing Over: Within the synaptonemal
complex, recombination is thought to be
carried out during pachytene by very large
protein assemblies called recombination
nodules. A nodule’s diameter is about 90
nm, spanning the central element of the
synaptonemal complex. Spaced along the synaptonemal complex, these
recombination nodules act as large multienzyme “recombination
machines,” each nodule bringing about a recombination event. The details
of the crossing over process are not well understood, but involve a
complex series of events in which DNA segments are exchanged between
nonsister or sister chromatids. In humans, an average of two or three such
crossover events occur per chromosome pair.
Chiasma Evidence of crossing over can often be seen under the light
microscope as an X-shaped structure known as a chiasma (Greek, “cross”;
plural, chiasmata). The presence of a chiasma
indicates that two chromatids (one from
each homologue) have exchanged parts .
Like small rings moving down two strands of
rope, the chiasmata move to the end of the
chromosome arm as the homologous
chromosomes separate.
Diplotene. At the beginning of diplotene, the
protein lattice of the synaptonemal complex
disassembles. Diplotene is a period of intense
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cell growth. During this period the
chromosomes decondense and become very
active in transcription.
Diakinesis. At the beginning of diakinesis, the
transition into metaphase, transcription ceases
and the chromosomes recondense.
Metaphase I
Each joined pair of homologues then lines up on the metaphase plate.
The orientation of each pair on the spindle axis is random: either the
maternal or the paternal homologue may orient toward a given pole.
Anaphase I(The first division is called the Reductional Division, since it
is at this time that the chromosome count is reduced to one half In
anaphase I, the microtubules of the spindle fibers begin to shorten. As they
shorten, they break the chiasmata and pull the centromeres toward the
poles, dragging the chromosomes along with them. Because the
microtubules are attached to kinetochores on only one side of each
centromere, the individual centromeres are not pulled apart to form two
daughter centromeres, as they are in mitosis. Instead, the entire centromere
moves to one pole, taking both sister chromatids with it. When the spindle
fibers have fully contracted, each pole has a complete haploid set of
chromosomes consisting of one member of each homologous pair meiosis
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I results in the independent assortment of maternal and paternal
chromosomes into the gametes.
Telophase I
By the beginning of telophase I, the chromosomes have segregated into
two clusters, one at each pole of the cell. Now the nuclear membrane reforms around each daughter nucleus. Because each chromosome within a
daughter nucleus replicated before meiosis I began, each now contains
two sister chromatids attached by a common centromere.
Importantly, the sister chromatids are no longer identical, because of
the crossing over that occurred in prophase I . Cytokinesis may or may not
occur after telophase I. The second meiotic division, meiosis II, occurs
after an interval of variable length.
The Second Meiotic Division
After a typically brief interphase, in which no DNA synthesis occurs,
the second meiotic division begins. Meiosis II resembles a normal mitotic
division. Prophase II, metaphase II, anaphase II, and telophase II follow in
quick succession.
Prophase II. At the two poles of the cell the clusters of chromosomes
enter a brief prophase II, each nuclear envelope breaking down as a new
spindle forms.
Metaphase II. In metaphase II, spindle fibers bind to both sides of the
centromeres.
Anaphase II. The spindle fibers contract, splitting the centromeres and
moving the sister chromatids to opposite poles.
Telophase II. Finally, the nuclear envelope re-forms around the four sets
of daughter chromosomes. The final result of this division is four cells
containing haploid sets of chromosomes. No two are alike, because of the
crossing over in prophase I. Nuclear envelopes then form around each
haploid set of chromosomes. The cells that contain these haploid nuclei
may develop directly into gametes, as they do in animals.
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In female , during the meiotic cell division of female germ cells,
unequal division of the cytoplasm occurs which leads to the formation of :1- One primary Oocyte.
2- Three small polar bodies which will degenerate.
The primary Oocyte are formed early during the embryonic
development of female fetus. They remain in the prophase stage of
the first meiotic cell division for years until puberty is reached, then
the first meiotic cell division is completed.
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