Mitosis
Mitosis consists
of prophase, metaphase, anaphase, and telophase, with distinct cellular
activities characterizing each phase. This completes the duplication of the
nucleus, and is followed by cytokinesis, in which the cell divides to produce
two daughter cells.
Mitosis
Mitosis
is a form of eukaryotic cell division that produces two daughter cells with the
same genetic component as the parent cell. Chromosomes replicated during the S
phase are divided in such a way as to ensure that each daughter cell receives a
copy of every chromosome. In actively dividing animal cells, the whole process
takes about one hour.
The
replicated chromosomes are attached to a 'mitotic apparatus' that aligns them
and then separates the sister chromatids to produce an even partitioning of the
genetic material. This separation of the genetic material in a mitotic nuclear
division (or karyokinesis) is followed by a separation of the
cell cytoplasm in a cellular division (or cytokinesis) to
produce two daughter cells.
In some
single-celled organisms mitosis forms the basis of asexual reproduction. In
diploid multicellular organisms sexual reproduction involves the fusion of two
haploid gametes to produce a diploid zygote. Mitotic divisions of the zygote
and daughter cells are then responsible for the subsequent growth and
development of the organism. In the adult organism, mitosis plays a role in
cell replacement, wound healing and tumour formation.
Mitosis,
although a continuous process, is conventionally divided into five stages:
prophase, prometaphase, metaphase, anaphase and telophase.
Prophase
Prophase
occupies over half of mitosis. The nuclear membrane breaks down to form a number
of small vesicles and the nucleolus disintegrates. A structure known as the centrosome
duplicates itself to form two daughter centrosomes that migrate to opposite
ends of the cell. The centrosomes organise the production of microtubules that
form the spindle fibres that constitute the mitotic spindle.
The chromosomes condense into compact structures. Each replicated chromosome
can now be seen to consist of two identical chromatids (or sister
chromatids) held together by a structure known as the centromere.
Prometaphase
The
chromosomes, led by their centromeres, migrate to the equatorial plane in the
mid-line of the cell - at right-angles to the axis formed by the centrosomes.
This region of the mitotic spindle is known as the metaphase plate.
The spindle fibres bind to a structure associated with the centromere of each
chromosome called a kinetochore. Individual spindle fibres bind to a kinetochore
structure on each side of the centromere. The chromosomes continue to condense.
Metaphase
The
chromosomes align themselves along the metaphase plate of the spindle
apparatus.
Anaphase
The
shortest stage of mitosis. The centromeres divide, and the sister chromatids of
each chromosome are pulled apart - or 'disjoin' - and move to the opposite ends
of the cell, pulled by spindle fibres attached to the kinetochore regions. The
separated sister chromatids are now referred to as daughter chromosomes.
(It is the alignment and separation in metaphase and anaphase that is important
in ensuring that each daughter cell receives a copy of every chromosome.)
Telophase
The final
stage of mitosis, and a reversal of many of the processes observed during
prophase. The nuclear membrane reforms around the chromosomes grouped at either
pole of the cell, the chromosomes uncoil and become diffuse, and the spindle
fibres disappear.
Cytokinesis
The final
cellular division to form two new cells. In plants a cell plate forms along the
line of the metaphase plate; in animals there is a constriction of the
cytoplasm. The cell then enters interphase - the interval between mitotic
divisions.
Meiosis
Meiosis
is the form of eukaryotic cell division that produces haploid
sex cells or gametes (which contain a single copy of each chromosome) from diploid
cells (which contain two copies of each chromosome). The process takes the form
of one DNA replication followed by two successive nuclear and cellular
divisions (Meiosis I and Meiosis II). As in mitosis, meiosis is preceded by a
process of DNA replication that converts each chromosome into two sister chromatids.
Meiosis I
Meiosis I
separates the pairs of homologous chromosomes.
In
Meiosis I a special cell division reduces the cell from diploid to haploid.
Prophase I
The
homologous chromosomes pair and exchange DNA to form recombinant chromosomes.
Prophase I is divided into five phases:
- Leptotene:
chromosomes start to condense.
- Zygotene:
homologous chromosomes become closely associated (synapsis) to form pairs
of chromosomes (bivalents) consisting of four chromatids (tetrads).
- Pachytene:
crossing over between pairs of homologous chromosomes to form chiasmata
(sing. chiasma).
- Diplotene:
homologous chromosomes start to separate but remain attached by chiasmata.
- Diakinesis:
homologous chromosomes continue to separate, and chiasmata move to the
ends of the chromosomes.
Spindle
apparatus formed, and chromosomes attached to spindle fibres by kinetochores.
Metaphase I
Homologous
pairs of chromosomes (bivalents) arranged as a double row along the metaphase
plate. The arrangement of the paired chromosomes with respect to the poles of
the spindle apparatus is random along the metaphase plate. (This is a source of
genetic variation through random assortment, as the paternal and maternal
chromosomes in a homologous pair are similar but not identical. The number of
possible arrangements is 2n, where n is the number of chromosomes in
a haploid set. Human beings have 23 different chromosomes, so the number of
possible combinations is 223, which is over 8 million.)
Anaphase I
The
homologous chromosomes in each bivalent are separated and move to the opposite
poles of the cell
Telophase I
The
chromosomes become diffuse and the nuclear membrane reforms.
Cytokinesis
The final
cellular division to form two new cells, followed by Meiosis II. Meiosis I is a
reduction division: the original diploid cell had two copies of each
chromosome; the newly formed haploid cells have one copy of each chromosome.
Meiosis II
Meiosis
II separates each chromosome into two chromatids.
The
events of Meiosis II are analogous to those of a mitotic division, although the
number of chromosomes involved has been halved.
Meiosis
generates genetic diversity through:
- the exchange of
genetic material between homologous chromosomes during Meiosis I
- the random alignment
of maternal and paternal chromosomes in Meiosis I
- the random alignment
of the sister chromatids at Meiosis II
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