The cell cycle, also known as the cell division cycle, is a well-defined and coordinated series of stages which actively dividing eukaryotic cells progress through. Ultimately this process leadsto a cell producing two daughter cells genetically identical to the parent cell.
Cell division process - scientific illustration. Image Copyright: Mopic / Shutterstock
Importantly, there are distinct checkpoints throughout the cycle which are to ensure that a cell progresses through the stages in an appropriate manner. For example, if DNA damage has been detected in a cell then it would not normally proceed to the next phase until this has been addressed.
The cell cycle is broadly divided into two constituent processes – mitosis and interphase. These are detailed below:
This is the stage when a cell grows and ultimately DNA replication occurs. It is composed of three stages:
Gap 1 (G1)
This is the stage at which the cell prepares itself to support DNA replication. This involves the cell increasing its metabolic processes. It also builds up protein and the amounts of organelles.This stage contains a G1 checkpoint which a cell will only pass if it is considered ready for DNA replication. This involves checking if the cell has appropriate energy reserves amongst other factors.
Cell growth and the synthesis of some proteins still continue in this stage. The major change that occurs however is the replication of chromosomal DNA within the cell. Thus, this stage ends with a cell containing twice the number of chromosomes it had begun with.
Gap 2 (G2)
During this stage, he cell continues to grow and ready itself for mitosis. As is the case with the G1 phase, there is a G2 checkpoint here to assess the cellular state – most importantly, this checkpoint is to ensure accurate replication of the chromosomes have occurred.
This occurs following interphase and is divided into the stages below:
During this phase, chromatin (made of DNA and proteins)starts to condense and become visible as the rod-like chromosomes. There are two identical chromosomes in the nucleus which are called sister chromatids. They are joined through a DNA point called a centromere. The nucleolus, a structure in the nucleus containing protein and RNA, begins to break down and eventually disappears. The centrosome, an organelle made of two cylindrical protein centrioles, triggers the development of the microtubules responsible for forming the spindle fibres that later separate the chromosomes.
This is when the nuclear membrane disintegrates. The microtubules connect to the kinetochores which are structure on the chromatids, and the chromosomes start to move to new locations.
This is the stage when structures in the nucleus start to separate. By this stage, the chromosomes are at their most coiled and condensed state – spindle fibers align them on a central plane equidistant from the nucleus. This plane is called the metaphase plate.
At the end of this stage is the M checkpoint (spindle checkpoint) which is required to ensure correct alignment of the chromosomes on the spindle.
The paired chromosomes split up at the centromeres and move to opposite poles of the cell. This is enabled by the kinetochores on the chromatids which shorten, moving along the spindle microtubules and responding to the polar microtubules.
Here, the cell lengthens with the help of the expanding polar microtubules. A new nuclear envelope encases the newly-separated chromosomes to create a new nucleolus. There are now two nuclei with their own chromosomes.
This is the third part of the cell cycle process. Although the process will have been gradually happening during the mitosis stage, cell division is only considered to be complete after each daughter cell has its own nucleus.
In cytokinesis, a fiber ring composed of actin contracts in order to shapes the structure of the single, elongated cell into two new daughter cells, thus finishing the process.
Reviewed by Afsaneh Khetrapal, BSc (Hons)