Botany 2104 - Plant Form and Function
Nuclear Divisions (Mitosis and Meiosis) and the Cell Cycle
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.
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 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.
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.
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.
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.
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.
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.
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.”
The homologous pairs separate and moved to opposite poles of the cell.
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.
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.
The chromosomes line up in the center of the cell, generally at right angles to the metaphase I alignment.
The chromatids separate and move to opposite poles.
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