Botany 2104 - Plant Form and Function
Nuclear Divisions (Mitosis and Meiosis) and the Cell Cycle
terminology
cytokinesis = division of the cytoplasm
karyokinesis = division of the nucleus; two types: mitosis and meiosis
cell cycle = interphase + mitosis + cytokinesis
interphase = a period of cell growth and DNA replication that precedes either
mitosis or meiosis
Remember that the nucleus houses most of the genetic material of the cell
in the form of chromosomes. The chromosomes are made of a material
called chromatin. The chemical composition of chromatin is roughly
half DNA and half protein. The DNA, which is the portion of the chromatin
that actually specifies genetic characteristics, is a long, thin molecule
that wraps around the more globular protein portions of chromatin.
The proteins are called histones. So each individual chromosome consists
of a very long DNA molecule wrapped around histone beads.
chromatin
chromosome
chromatid
homologous chromosomes
ploidy: haploid, diploid
The Cell Cycle (a.k.a. cell division)
As a result of cell division, one cell is split in half to form two genetically
identical ells. Although the resulting
cells might differ functionally and morphologically at maturity, they contain
the same amount and type of DNA.
Interphase
Interphase lasts from the end of one nuclear division to the start of another.
Therefore, the majority of cells in an organism are in interphase.
Interphase itself can be divided into three steps: G1, S and G2.
During G1, or the first gap, the nucleus and cell are enlarging to
mature size. General metabolic activity is occurring. There is
a lot of RNA and protein synthesis going on. This is the step of the
cell cycle that most cells are arrested in. If this is a meristematic
cell, toward the end of G1 you will see synthesis of the enzymes
needed for DNA replication as well synthesis of proteins needed to form the
spindle apparatus.
During the S phase, the histones are synthesized and DNA is replicated (or
duplicated). This synthetic step causes each chromosome to go from a single
strand of chromatin to two connected strands of chromatin. The two
chromatin strands of one chromosome are called chromatids.
During G2, the second gap, RNA and protein synthesis continues as the cell
finished its preparations to divide the nucleus.
If you look at an interphase cell with a microscope, the
nucleus will be clearly visible and apparently just sitting there.
When the cell cycle was first studied, the only tool available was the light
microscope. As the intense activity of G1, S, and G2 cannot be seen
with a light microscope, researchers assumed the nucleus was resting, and
interphase quite mistakenly became known as the resting phase.
During mitosis, the nuclear membrane disperses and the
chromosomes become visible with a light microscope. Mitosis literally
means thread-like disease or condition. A cell in interphase, with
its nucleus intact, was viewed as the normal or healthy condition, while
a mitotic cell, with its nucleus disintegrated to release the thread-like
chromosomes, was abnormal. As a result, early researchers spent many
hours watching the events of mitosis and named four steps of this portion
of the cell cycle. These steps are prophase, metaphase, anaphase, and
telophase. Also, the cell was pictured as rather globe-like in appearance,
and so they also described the cell as having an equator and two poles.
Where can mitosis occur in a flowering plant? Meristems to produce
additional cells; ovules and pollen grains to produce gametes.
Prophase
During prophase the chromosomes become visible as the chromatin packs together
tighter and tighter. As the chromosomes become visible, you can now
see the two chromatids. They appear to be held together at a pinched
region of the chromosome. This "pinch" is the centromere. It
is not a discreet structure that attaches to the chromosome. It is
simply the region along the chromosome where the two chromatids are connected
to each other. There is, however, a structure that does attach to the
chromosome at the centromere region during prophase. This structure
is called a kinetochore. Two kinetochores attach to either side of
the centromere. At the end of prophase, the nuclear membrane disrupts,
and the chromosomes can now move within the cell.
Metaphase
In metaphase, the chromosomes move to the center of the cell and align themselves
with the centromeres along the equator of the cell and the chromatids of
each chromosome point toward opposite poles. Microtubules attach to
the kinetochores and extend to the poles. This arrangement of microtubules
forms what is called the spindle apparatus. The action of the microtubules
is such that the chromatids are separated as the microtubules pull at the
kinetochores.
Anaphase
The separation of the chromatids marks the start of anaphase. Once
the two chromatids have separated, they are now each a chromosome.
The chromosomes move to opposite poles as they are pulled away from the equator.
Telophase and Cytokinesis
Once all of the chromosomes have reached a pole, telophase begins.
During telophase, two nuclei form, one at each pole. The nuclear membrane
forms, the spindle apparatus disappears, the chromosomes uncoil, nucleoli
reappear. This marks the end of mitosis.
As a result of mitosis, the cell now contains two identical nuclei. In
order to make two cells, cytokinesis must take place. Cytokinesis is the
division of the cytoplasm and generally starts during telophase of mitosis.
The visible indicator of the start of cytokinesis is the formation of the cell
plate along the equator of the dividing cell. Cell plate formation starts
when vesicles from the Golgi apparatus deposit material that will form the
middle lamella between the two new cells. The membranes surrounding these
vesicles will form the plasma membranes for the two new cells. ER channels
that become surrounded as the middle lamella forms will form plasmodesmata.
Finally, cellulose for the cell walls will be secreted by the two new cells.
This cellulose will be trapped between the plasma membranes and the common
middle lamella. Eventually, the cytoplasm is cut in two, and two cells are
formed. What happens to the cell that started this whole process? It
no longer exists. That one cell, as a result of the cell cycle, has become
two genetically identical cells. If that one cell had four chromosomes, each of the
two new cells has four chromosomes. The new cells can then differentiate
to become specialized cells or remain as meristematic cells. One final
word about cytokinesis. It does not always happen, nor does it have
to happen at telophase. For example, coconut milk, the liquid endosperm of
the coconut seed, results from mitosis without cytokinesis.
Meiosis
Mitosis can duplicate any nucleus, no matter what the ploidy is. Meiosis
requires an even number ploidy (with homologous chromosomes able to pair) to
occur correctly. The general description of the process focuses on a
diploid nucleus, one with two sets of chromosomes. The specific
chromosomes from each set that can match each other are called homologous chromosomes. The presence
of homologous pairs of chromosomes is critical to the process of meiosis.
If the starting cell had two sets of chromosomes in the nucleus (diploid),
by the end of meiosis there will be four haploid (1 set) cells. Each
of the four cells will be slightly different genetically from the other three
because the events of meiosis will shuffle the genetic information present
in the two starting sets of the original diploid cell.
During meiosis, there are two successive nuclear divisions (meiosis I and
meiosis II), each with its own prophase, metaphase, anaphase, and telophase.
Where does meiosis occur in flowering plants? In ovules and anthers
to produce spores.
Prophase I
Remember that at the end of interphase, a nucleus has chromosomes that consist
of two chromatids each. During prophase I, the homologous chromosomes
pair up and exchange segments with each other in a process called crossing
over. The chromosomes contract; the nucleoli and nuclear membrane disappear.
shows crossing over.
Metaphase I
The homologous pairs align at the equator of the cell. Each pair aligns
independently of the other pairs. So the pair of “A” chromosomes might
have chromosome A facing “north” and chromosome a facing “south,” while the pair
of “B” chromosomes might have chromosome b facing “north” and chromosome B
facing “south.”
Anaphase I
The homologous pairs separate and moved to opposite poles of the cell.
Telophase I
This step might not occur. If it does, the chromosomes uncoil a little
and the nuclear membrane reforms.
By the end of meiosis I, the ploidy of the cell has been cut in half.
Prophase II
This phase will only occur if telophase I took place. Basically,
if you made nuclei during telophase I, you need to undo them during prophase
II.
Metaphase II
The chromosomes line up in the center of the cell, generally at right angles
to the metaphase I alignment.
Anaphase II
The chromatids separate and move to opposite poles.
Telophase II
The chromosomes uncoil. The nuclear membrane forms. The nucleoli
reappear. Cytokinesis takes place. The original diploid
cell is gone. In its place are four haploid cells that are genetically
different from each other.
Meiosis accomplishes two things as a prelude to sexual reproduction:
1. Reduces the number of sets of chromosomes (ploidy) by half.
Why is this important? During fertilization, two cells fuse to form
a new individual, thus restoring the original number of chromosome sets.
If the ploidy had not been reduced, fertilization would result in an increase
in chromosome sets.
2. Genetic recombination. The crossover exchanges of prophase
I and the random assortment of homologous pairs during metaphase I result
in new allele combinations. This allows sexual reproduction to maintain
the genetic variability in the population. (Genetic uniformity
could be - and is - accomplished via the less energy intensive process
of asexual reproduction.)
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3 September 2013