All living things reproduce thanks to the essential mechanism of the cell cycle: the cycle of duplication and division of the cell. The primary function of the cell cycle is to perform DNA replication — duplicate accurately the vast amount of DNA in the chromosomes of the mother cell and then segregate the DNA into genetically identical daughter cells in a way that they receive a complete copy of the entire genome (chromosome is a threadlike structure made of protein and a condensed single molecule of DNA). It is also necessary to duplicate the macromolecules and organelles and double the size before division.
Cell cycle lengths differ from organism to organism, between different kinds of cells, and at different stages of an organism's life. In bacteria and yeast, as examples of unicellular organisms, each cell division results in a completely new organism, unlike a multicellular organism that requires many rounds of cell division.
Types of cell division
In eukaryotes exist two types of cell division: mitosis, with the formation of two genetically identical daughter cells, and meiosis, where the number of chromosomes is reduced by two for the formation of the gametes, germ cells, central for sexual reproduction. Prokaryotes divide mainly via asexual binary fission, an event resulting in the reproduction of living prokaryotic cells by diving the cell into two parts, each with the potential to grow to the size of the original. The main difference from mitosis is that bacterial cell lacks a nucleus and division occurs without the formation of a mitotic spindle — crucial for eukaryotic cells' division structure.
Phases of cell life
In this section we will discuss events occurring in the rapidly dividing mammalian cells (pic.1). Cell cycle is comprised of 4 stages:
1. The "gap" phase called G1 during which the cell grows and monitors its internal and external environment for DNA synthesis.
2. S phase (S — synthesis) when the cell replicates its DNA.
3. The "gap" phase G2 during which the cell is preparing for the mitosis phase and division.
4. M phase (includes mitosis — separation of duplicated chromosomes in two daughter nuclei and cytokinesis — subsequent split of the cell cytoplasm in two).
On average mammalian cell M phase takes about an hour. The whole period between one M phase and the next is called interphase.
In the G1 phase, the cell undergoes growth and accumulation of macromolecules, a high amount of protein synthesis occurs, organelles are produced and cytoplasm volume increases. So it does prepare for the subsequent S (synthesis) phase, in which the DNA is duplicated. In another intermittent phase, the G2 phase, the cells undergo further growth in preparation for cell division. If all requirements are met, the cell will undergo mitosis to yield two genetically identical daughter cells in the G1 phase. During all of the interphase, a cell generally continues to transcribe genes, synthesize proteins, and grow in mass. The cell cycle characteristically lasts between 10 and 20 hours in rapidly proliferating adult cells, but it can be arrested for weeks or months in quiescent cells or for a lifetime in the neuron of the brain. The prolonged arrest of this type usually occurs during the G1 phase and is referred to as G0, in which any further cell growth, DNA synthesis, and cell divisions stop.
Regulation of the eukaryotic cell cycle
The cell cycle includes control mechanisms known as cell cycle checkpoints after each key step of the cycle. These checkpoints ensure the proper replication of cellular components and division as well as check whether the cell can progress to the next phase. It is tightly regulated and controlled to prevent any fatal errors in the DNA of the daughter cells, which could lead to cell death or cancerous transformation. There are many checkpoints in the cell cycle, but the crucial ones are:
1. The G1/S cell cycle checkpoint, also known as the Major checkpoint, activates upon DNA damage and insufficient cellular size;
2. The intra-S phase checkpoint is triggered by replicative DNA damage;
3. The G2/M checkpoint activates upon DNA damage and incompletely replicated DNA. The mitotic checkpoint is activated upon incomplete spindle formation. The cell cycle is arrested as long as the damage persists, if it can be repaired, it will resume cycling. If the damages are fatal, the cell will undergo apoptosis (programmed cell death).
4. Spindle checkpoint (at the transition of metaphase-to-anaphase mitosis stages), activates upon wrong attachment of the chromosome to the mitotic spindle.
The cell cycle is regulated biochemically. The cell cycle phases are tightly regulated by the activation of Cyclin-dependent kinase (CDK) protein families by the cyclins. After complexing with their corresponding Cyclin, CDKs need to be further activated by CDK-activating kinases (CAKs) to drive cell cycle progression.
Conclusion
Eukaryotes reproduce through the essential mechanism of the cell cycle: a tightly regulated cycle of cell growth, duplication of DNA, and finally division into two daughter cells, comprised of G1, S, G2, and M phases. To secure the proper replication of cellular components, there are control mechanisms of cell cycle checkpoints: G1; intra-S phase; G2/M checkpoints, and the spindle checkpoint.