In plants, as in all eukaryotic life-forms, the cell cycle comprises the processes of cell division, constituted by three preparatory phases (G1,S,and G2), followed by mitosis (nuclear division) and cytokinesis (cytoplasm division).
One of the fundamental characteristics of living organisms is their ability to grow and reproduce. At the cellular level, growth is accomplished by a gain of mass, followed by division into two daughter cells.
Being a eukaryotic cell, plant cell must reach a sufficient size to ensure that the daughter cells will be large enough to survive. The cell achieves this status by making needed molecules, using synthetic biochemical reactions, and monitoring the outside environment to ensure continued favorable conditions for reproduction. The cell replicate all required macromolecules, such as the deoxyribonucleic acid (DNA), as well as the organelles and distribute appropriately the cellular contents, including the genetic material, to the daughter cells. The cyclical repetition of these steps comprises the eukaryotic cell division cycle or, more simply, the cell cycle.
Cell cycle phases
Cells, like organisms, are governed by life cycles. The life cycle of a cell is called the cell cycle. Cells spend most of their time in interphase. Interphase is divided into three stages: first gap (G1), synthesis (S), and second gap (G2).
During G1, the cell performs its normal functions and often grows in size. During the S stage, DNA replicates in preparation for cell division.During the G2 stage, the cell makes materials needed to produce the mitotic apparatus and for division of the cytoplasmic components of the cell.
At the end of interphase, the cell is ready to divide. Although each chromosome now consists of two sister chromatids, this is not apparent when viewed through a microscope. This is because all the chromosomes are in a highly relaxed state and simply appear as a diffuse material called chromatin.
During the mitotic (M) phase, the DNA chromosomes condense and are divided into the two daughter.
Mitosis itself is composed of four phases:
- Prophase, in which chromosomes are condensed, homologous chromosomes are paired together, and the spindle apparatus made of microtubules forms.
- Metaphase, in which the paired chromosomes are lined up across the center of the cell on the metaphase plate.
- Anaphase, in which the homologous chromosomes are pulled to separate poles in the dividing cell by the attached spindle apparatus.
- Telophase, in which the daughter cell chromosomes are collected together at the poles.
Mitosis is followed by cytokinesis, or the process by which the two daughter cells are physically separated. Cytokinesis begins toward the middle or end of nuclear division and involves not just the division of the cytoplasm but also the organelles.
In plants, after nuclear division ends, a new cell wall must be formed between the daughter nuclei. The new cell wall begins when vesicles filled with cell wall material congregate where the metaphase plate was located, producing a structure called the cell plate. This cell plate grows outward between the two new nuclei. Once the cell plate reaches the walls of the dividing cell, it forms the cell wall that separates the two new cells and thus the cytokinesis is complete.
Following cytokinesis, the cell returns to interphase. Mitotic daughter cells enlarge, reproduce organelles, and resume regular activities.
The cell will die if it goes through any cell cycle phase out of order. Therefore, the cell has evolved sophisticated checkpoints to ensure that critical events, such as DNA replication and cell division, occur in the correct order and that each required step is completed prior to movement to the next phase.
Additionally, external conditions, such as a change in nutrient availability, can cause a cellular checkpoint response. There are two main points in the cell cycle which are regulated by checkpoints, the G1-to-S transition and the G2-to-M transition.
The G1-to-S transition is important because once the cell commits to divide and starts to copy its DNA, it must complete cell division or die. After DNA replication is complete, the G2-to-M transition is important for the cell to make sure that all of the DNA has been copied correctly, or the daughter cells will not have the full complement of genetic material.
CDKs and Cyclins
The cell cycle is controlled by the actions of kinases called cyclin-dependent kinases, or CDKs. Kinases are enzymes that place a chemical group, called a phosphate group, onto other proteins in the cell. Because of the nature of the phosphate chemical group, the addition of these groups causes conformational changes in the three-dimensional structure of the proteins that accept them. Each CDK has a partner called a cyclin, which is required for the kinase activity to place phosphate groups on target proteins.
The CDK/cyclin pairs perform kinase reactions, which activate proteins leading to completion of cell cycle phases. Opposing the action of the CDK and cyclin pairs are cyclin-dependent kinase inhibitors, or CKIs, which act as brakes to prevent CDK/cyclin activity until the cell is ready to go on to the next step in the cell cycle.
Arabidopsis is a genus that has been widely characterized and used as an experimental model for plant growth and differentiation in scientific laboratories worldwide. In plants such as Arabidopsis, two major groups of CDKs have been studied, the A-type and B-type CDKs. The A-type CDKs show kinase activity during the S, G2, and M phases of the cell cycle and regulate the transitions from G1-to-S and G2-to-M phases. The B-type CDKs are active during mitosis and regulate only the G -to-M transition in plants.