Prophase () is the first stage of cell division in both mitosis and meiosis. Beginning after interphase, DNA has already been replicated when the cell enters prophase. The main occurrences in prophase are the condensation of the chromatin reticulum and the disappearance of the nucleolus.
Staining and microscopy
Microscopy can be used to visualize condensed
as they move through
meiosis and
mitosis.
Various DNA Staining are used to treat cells such that condensing can be visualized as the move through prophase.
The Giemsa stain G banding technique is commonly used to identify , but utilizing the technology on was originally difficult due to the high degree of chromosome compaction in plant cells.
Fluorescent stains such as DAPI can be used in both live Plant cell and animal cells. These stains do not band , but instead allow for DNA probing of specific regions and . Use of fluorescent microscopy has vastly improved spatial resolution.
Mitotic prophase
Prophase is the first stage of
mitosis in animal cells, and the second stage of
mitosis in
.
At the start of prophase there are two identical copies of each
chromosome in the cell due to replication in
interphase. These copies are referred to as sister chromatids and are attached by
DNA element called the
centromere.
The main events of prophase are: the condensation of
, the movement of the
, the formation of the mitotic spindle, and the beginning of
Nucleolus break down.
Condensation of chromosomes
DNA that was
DNA replication in
interphase is condensed from DNA strands with lengths reaching 0.7 μm down to
0.2-0.3 μm.
This process employs the
condensin complex.
Condensed chromosomes consist of two sister chromatids joined at the
centromere.
Movement of centrosomes
During prophase in animal cells,
move far enough apart to be resolved using a
light microscope.
activity in each
centrosome is increased due to recruitment of γ-tubulin. Replicated
from
interphase move apart towards opposite poles of the cell, powered by
Motor protein.
Interdigitated interpolar
from each
centrosome interact with each other, helping to move the
to opposite poles.
Formation of the mitotic spindle
involved in the
interphase scaffolding break down as the replicated
separate.
The movement of
to opposite poles is accompanied in animal cells by the organization of individual radial
microtubule arrays (asters) by each centriole.
Interpolar
from both
interact, joining the sets of
and forming the basic structure of the mitotic spindle.
Plant cells do not have centrosomes and the
can
Nucleation microtubule assembly into the mitotic apparatus.
In
,
gather at opposite poles and begin to form the spindle apparatus at locations called foci.
The mitotic spindle is of great importance in the process of
mitosis and will eventually segregate the sister chromatids in
metaphase.
Beginning of nucleoli breakdown
The
Nucleolus begin to break down in prophase, resulting in the discontinuation of ribosome production.
This indicates a redirection of cellular energy from general cellular metabolism to
Cell division.
The
Nuclear membrane stays intact during this process.
Meiotic prophase
Meiosis involves two rounds of chromosome segregation and thus undergoes prophase twice, resulting in prophase I and prophase II.
Prophase I is the most complex phase in all of meiosis because homologous chromosomes must pair and exchange genetic information.
Prophase II is very similar to
Mitosis prophase.
Prophase I
Prophase I is divided into five phases: leptotene, zygotene, pachytene, diplotene, and diakinesis. In addition to the events that occur in
Mitosis prophase, several crucial events occur within these phases such as pairing of homologous chromosomes and the reciprocal exchange of genetic material between these homologous chromosomes. Prophase I occurs at different speeds dependent on
species and
sex. Many species arrest
meiosis in diplotene of prophase I until
ovulation.
In humans, decades can pass as
remain arrested in prophase I only to quickly complete meiosis I prior to
ovulation.
Leptotene
In the first stage of prophase I, leptotene (from the Greek for "delicate"),
begin to condense. Each chromosome is in a
Ploidy state and consists of two sister chromatids; however, the
chromatin of the sister chromatids is not yet condensed enough to be resolvable in
microscopy.
Homologous regions within homologous chromosome pairs begin to associate with each other.
Zygotene
In the second phase of prophase I, zygotene (from the Greek for "conjugation"), all maternally and paternally derived
have found their homologous partner.
The homologous pairs then undergo synapsis,a process by which the synaptonemal complex (a proteinaceous structure) aligns corresponding regions of genetic information on maternally and paternally derived non-sister
of homologous chromosome pairs.
The paired homologous chromosome bound by the synaptonemal complex are referred to as bivalents or tetrads.
do not fully synapse because only a small region of the chromosomes are homologous.
The nucleolus moves from a central to a peripheral position in the Cell nucleus.
Pachytene
The third phase of prophase I, pachytene (from the Greek for "thick"), begins at the completion of synapsis.
has condensed enough that
can now be resolved in
microscopy.
Structures called recombination nodules form on the synaptonemal complex of bivalents. These recombination nodules facilitate genetic exchange between the non-sister chromatids of the synaptonemal complex in an event known as crossing-over or genetic recombination.
Multiple recombination events can occur on each bivalent. In humans, an average of 2-3 events occur on each chromosome.
Diplotene
In the fourth phase of prophase I, diplotene (from the Greek for "twofold"), crossing-over is completed.
Homologous chromosomes retain a full set of genetic information; however, the homologous chromosomes are now of mixed maternal and paternal descent.
Visible junctions called chiasmata hold the homologous chromosomes together at locations where recombination occurred as the synaptonemal complex dissolves.
It is at this stage where meiotic arrest occurs in many
species.
Diakinesis
In the fifth and final phase of prophase I, diakinesis (from the Greek for "double movement"), full chromatin condensation has occurred and all four sister chromatids can be seen in bivalents with
microscopy. The rest of the phase resemble the early stages of mitotic
prometaphase, as the meiotic prophase ends with the spindle apparatus beginning to form, and the
nuclear membrane beginning to break down.
Prophase II
Prophase II of
meiosis is very similar to prophase of
mitosis. The most noticeable difference is that prophase II occurs with a
Ploidy number of
as opposed to the
Ploidy number in mitotic prophase.
In both animal and
chromosomes may de-condense during
telophase I requiring them to re-condense in prophase II.
If chromosomes do not need to re-condense, prophase II often proceeds very quickly as is seen in the
model organism Arabidopsis.
Prophase I arrest
Female mammals and birds are born possessing all the oocytes needed for future ovulations, and these
are arrested at the prophase I stage of
meiosis.
In humans, as an example, oocytes are formed between three and four months of
gestation within the fetus and are therefore present at birth. During this prophase I arrested stage (
dictyate), which may last for decades, four copies of the
genome are present in the oocytes. The adaptive significance of prophase I arrest is still not fully understood. However, it has been proposed that the arrest of oocytes at the four genome copy stage may provide the informational redundancy needed to
DNA repair of the
germline.
[ The repair process used appears to be homologous recombinational repair][ DNA repair capability appears to be a key quality control mechanism in the female germ line and a critical determinant of fertility.][
]
Differences in plant and animal cell prophase
The most notable difference between prophase in and animal cells occurs because plant cells lack . The organization of the spindle apparatus is associated instead with foci at opposite poles of the cell or is mediated by chromosomes. Another notable difference is preprophase, an additional step in plant mitosis that results in formation of the preprophase band, a structure composed of . In Mitosis prophase I of plants, this band disappears.
Cell checkpoints
Prophase I in meiosis is the most complex iteration of prophase that occurs in both and animal cells.
See also
External links
