Download Prophase II.

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.
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).
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
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chromosomes. Females have a pair called XX, both of them are alike' but
males are XY.
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.
(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.)
Pachytene. Pachytene begins when synapsis is
complete .
Crossing Over: Within the synaptonemal
complex, recombination is thought to be carried
out during pachytene. 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
non
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
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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. 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.
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
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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
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
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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.
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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