Cell Cycle

Review of the Cell Cycle

Plant growth usually requires cell division

Plants need to be able to grow, and this process has two components: cell division and cell expansion. Sometimes growth (an irreversible change in size) is the result of just one or the other component. But often, as we shall see here, cell growth and cell expansion are coordinated in a process known as the cell cycle. Below are some critical points about growth of plants.

New cells are made by a multistep process

Cells that divide actively in plants are called meristematic. These cells divide repeatedly in what has been called the cell cycle. This process has two major parts: a long and rather less-interesting phase (called interphase) marked by typical biochemistry, and a short and quite dynamic phase (called mitosis) marked by nuclear changes and cellular changes culminating in cytokinesis.

The "typical" cell diagrams show cells in interphase. The nuclear envelope is intact, the chromatin is not condensed into chromosomes, but there may be one or more nucleoli visible in the nucleus. The usual biochemistry of cells occurs in this interphase. What might not be easily noticeable is that during the interphase the DNA in the nucleus is replicated. This replication process divides the interphase into three distinct time intervals: G1, S, and G2.

During the first Gap (G1) the cell is doing its usual biochemistry, but this also includes preparing materials needed for the S phase. In the synthesis (S) interval, the cell is replicating its DNA. To accomplish that, proteins locate origins of replication in the genome. These occur at 66kb intervals on dicot and 47kb intervals on monocot chromosomes. DNA polymerase and ligase enzymes replicate the chromosomes.

In the second Gap (G2) the cell returns to its usual biochemistry, but this also includes multiplication of the chloroplasts and mitochondria (by binary fission!) and production of materials needed for mitosis (a nuclear! event).

The mitosis itself involves the condensation and separation of the replicated chromosomes. Mitosis has been subdivided into the phases: prophase (condensation), metaphase (alignment and attachment to microtubules), anaphase (centromere separation and chromatid migration), and telophase (recovery of nuclear envelope and decondensation).

Diagrams of the mitotic phases are shown below for your review:

In the prophase of mitosis the spindle apparatus forms. In plants, this involves a microtubule-organizing center (MTOC) at each end of the cell. Precisely how that works is not clear. In animals, the MTOC is associated with centrosomes (centrioles) but precisely why that is so is not clear. It may be a coincidence that microtubules organize at a point in space occupied by the centrosome.

Here is a much finer view of the cytoskeleton during prophase and early metaphase of the mitotic cycle. Microtubules are shown by fluorescence microscopy in yellow and green. DNA is fluorescing in blue wavelengths.

The microtubule polarity has the - end of the tubule at the MTOC at the poles, and the + end of the tubule at the kinetochore of the condensed chromosomes. As you recall the + end of the tubule is where the tubulin polymerizes...as the microtubules grow from the poles, the + end extends to and binds to the kinetochore.

The kinetochore is a protein located at the centromere of each chromosome. After the nuclear envelope disintegrates, microtubules attach to the kinetochores. Microtubules extending from each end of the cell attach to the duplicated condensed chromosome at the centromere. The microtubules from one end attach to the kinetochore of one sister chromatid, those from the other end attach to the kinetochore of the other sister chromatid.

These growing microtubules from the ends push the chromosomes into the equatorial plane; this is sometimes called congressing the chromosomes as the cell enters metaphase. As the cell approaches anaphase, the condensing and adhesion proteins are digested by protease and topoisomerase activity resulting in the separation of the sister chromatids at the centromere.

Finally, the separated chromatids (now called chromosomes) are pulled toward the poles of the cell by the microtubules. Now how this occurs is still controversial. There are four competing hypotheses as to how this would happen: 1) a motor protein, dynein pulls the chromatids to the - end of the microtubules, 2) depolymerization of the microtubule at the kinetochore, 3) depolymerization of the microtubule at the pole, and 4) sliding of the microtubule with respect to other elements of the cytoskeleton. These hypotheses are shown below:

Of course, the true situation may involve a combination of these hypotheses...only further work on this will reveal the truth.

Cytokinesis also involves the cytoskeleton

The assembly of microtubules extending through the equatorial plane conduct vesicles to that plane. This assembly is known as the phragmoplast. These vesicles contain oligosaccharides that ultimately will be linked into polysaccharides for the new cell wall. Obviously, the arriving vesicles will have to include ones loaded with cell wall assembly and sculpting enzymes. These vesicles coalesce progressively into a fenestrated sheet in which the new walls polymerize. This coalescence and polymerization develops from the center of the cell toward the margins of the equatorial plane. This developing layer is called the cell plate. Cytokinesis by cell plate formation is the mechanism found in plants.

The fenestrations often persist in the end wall between the cells after cytokinesis. These openings are called plasmodesmata. Portions of endoplasmic reticulum, called desmotubules, often extend through the fenestration. These openings allow the cytosol of adjacent cells to be continuous and for solutes and particles of 1.5-2.0 nm to pass from cell to cell. Sometimes the connected cytosols are referred to as the symplast.

Regulation of the cell cycle

One topic of recent interest is the study of how cells regulate the cell cycle. While meristematic cells cycle continuously, it is critical that each phase be complete before the next phase begins. One can imagine the disaster that would happen if mitosis began before the last chromosome was replicated, or if cytokinesis began before the chromatids of the chromosomes were completely separated. below is a recent model of how cells regulate the cell cycle.

Cells use Cyclin-Dependent-Protein-Kinases (CDKs) to regulate the cell cycle

One protein that is involved in cell cycle control is the CDK. This CDK protein, by itself, is useless but after combining with another protein called a cyclin and ATP, it can permit a cell to pass into the next part of the cell cycle.

The cyclins are relatively labile proteins, and their availability is regulated in turn by other cell cycle control proteins. These cell cycle control proteins add a ubiquitin polymer to the cyclin, marking them for destruction by the 26S proteasome, a complex of proteases. By destroying the cyclins, the CDK is inactivated.

The CDK-cyclin complex also must combine with ATP to be active. This combination is accomplished by yet other cell-cycle control protein, a protein kinase. Protein kinases add a phosphate from ATP to a protein...phosphatases remove the phosphate from a protein. The relative balance of protein kinases to phosphatases also helps determine whether the CDK-cyclin complex is active or inactive.

There are different kinds of protein kinases at work in the cell cycle. Some put the phosphate into one threonine of the CDK, other protein kinases put the phosphate on a different threonine of the CDK. Which threonine gets the phosphate determines whether the CDK-cyclin complex is active or inactive.

Now that we understand how a CDK can be active or inactive, we can go back to our cell cycle control figure and understand the rest of it.

In this simplified model, the inactive CDK is shown at about 1 o'clock as green with an empty binding site. This inactive CDK is produced in G1. As the cytokinesis is complete, the specialized DNA synthesis cyclin is also made. Als their concentrations rise, the CDK and cyclin combine. The combination alters the conformation of the CDK so that an active site is formed for a secondary messenger. This messenger protein binds and becomes phosphorylated by ATP degradation. The cell is now competent to go from G1 to S. The cell begins replicating the DNA.

As the DNA is synthesized, a cyclin ubiquitinating protein marks the synthesis cyclin for destruction and the 26S proteasome destroys it. This prevents the cell from attempting to duplicate its DNA again before the cell cycle is completely back around to G1. The inactive cyclin dependent kinase is free for the next control point.

A new cyclin, the mitotic cyclin, is produced as the synthesis phase ends. This binds with the CDK, altering its conformation to make an active site for a different messenger. The messenger is produced in G2, binds at the active site, and becomes phosphorylated to signal the cell to make the transition from G2 into Mitosis.

Later, during mitosis, the Anaphase Promoting Complex (APC) is produced and it marks the mitotic cyclin by attaching the ubiquitin polymer, and then the 26S proteasome digests the marked mitotic cyclin. This of course prevents the cell from going into mitosis without completing the cycle through to G2. The APC also includes enzymes that degrade the cohesion (cohesins) and condensation (cohesins) proteins permitting the separation of the sister chromatids at the centromere.

These points of control in the cell cycle can be stimulated or inhibited by plant growth substances (hormones). Auxins and cytokinins supplied with adequate sucrose stimulate the production of the synthesis cyclin, cytokinins stimulate the mitotic cyclin. Abscisic acid interferes with the synthesis CDK-cyclin complex, preventing the cell from cycling. These "effectors" can be manipulated by scientists to grow tissues in vitro, they become ingredients in the recipes for tissue culture. Such as those we use in our tissue culture project in lab.

To conclude, the observation that the cytoskeleton components are conserved is also reflected in the control elements for the cell cycle. In spite of the fact that the common ancestor of plants and humans is perhaps 1 billion years in the past, the control elements in the cell cycle are shared! The protein kinases, cyclins, DNA replication enzymes, ubiquitin-tagging, proteasomes, and all of the substantive details of the mitosis event are shared. Thus, what we have only recently learned is in fact the product of some very ancient evolutionary events. Cell cycling is a fundamentally primitive process.