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Eukaryotic Cell Cycle

Eukaryotic Cell Cycle
Eukaryotes are organisms having very complex cellular structure. This article deals with eukaryotic cell cycle along with its phases and regulation.
Ishani Chatterjee Shukla
Let's get to the roots of what eukaryotes are before we proceed upon discussing life cycle of a eukaryotic cell. Cells are the building blocks of life as a single cell, in itself, is a functional unit of life. Sometimes a single cell can compose an entire organism (lesser, unicellular life forms like bacteria) while on other occasions, a cluster, sometimes reaching a count of millions, of cells may combine together to take the shape of a higher organism like mammals, reptiles, etc. With regards to cell biology, based upon their structures and corresponding functions, cells can be classified under two broad categories - prokaryotic cells and eukaryotic cells. Let's take a brief look at each of these cell types and follow it up with a detailed study of the cycle of eukaryotic cells.
What is a Prokaryotic Cell?
A prokaryotic cell is composed of a cell envelop which consists of a cell wall and a plasma membrane beneath the cell wall. Enclosed within this cell envelop is the cytoplasmic area that houses the cell genome (DNA or RNA, as the case may be) along with ribosomes and various other material. Although there exists a nuclear region in the cytoplasmic are, prokaryotic cells do not contain a nucleus. This is the most significant aspect that differentiates prokaryotic cells from eukaryotic cells. Archaebacteria and eubacteria are the two chief prokaryote life forms.
What is a Eukaryotic Cell?
Eukaryotic cells are greater in dimension than prokaryotic cells. Eukaryotes have a much more complex cellular structure than prokaryotes. Each eukaryotic cell is divided into several membranous compartments or regions and each such region functions like a different work unit, performing specific metabolic activities and functions. The most distinctive feature of a eukaryotic cell structure is the presence of a nucleus. This nucleus holds the cell's genetic information and the eukaryotic genome is structured in the form of chromosomes. Both prokaryotes and eukaryotes have flagellum-like structures that protrude from the outer face of their cell walls.
Eukaryotic Cell Cycle Phases
Before discussing the phases of eukaryotic cell life cycle, let's first understand what a cell cycle is. A cell cycle is the process by which each cell divides into duplicate cells that are replicas of each other. All the structural forms and genetic information are replicated and both the resultant cells are absolutely identical to each other in terms of structure, functions, etc. Prokaryotic cells undergo binary fission to replicate into two separate cells whereas more complex stages are involved in the cell division cycle of eukaryotic cells. Let's take a look at the various phases of eukaryotic cell division to understand the entire process.
Phase I: Resting Phase
In this stage, a eukaryotic cell has already undergone the cell division cycle and has, at present, stopped this process. It is neither undergoing the cell cycle nor is it performing any synthesis. Hence, this phase is known as the resting phase.
Phase 2: Interphase
This phase is divided into three sub-phases - Gap 1, synthesis and Gap 2. In Gap 1 phase, the cell undergoes phenomenal growth by taking in all the nutrients it needs to undergo cell division and by synthesizing all necessary enzymes to prepare for the next stage. Synthesis is the next stage that follows Gap 1 and this phase is characterized by the performance of activities that result in DNA replication. Gap 2 phase marks the completion of DNA replication and the cell undergoes phenomenal growth as a way of preparing for Mitosis which is the final link in the eukaryotic cell life cycle.
Phase 3: Mitosis
This is the stage that occurs after the DNA and all other cellular structures have replicated themselves. At this stage, the eukaryotic cells begins dividing into two, each half containing a completely identical cellular structure and genetic information as the other.
Eukaryotic Cell Cycle Regulation
The eukaryotic cell life cycle is regulated by a number of processes that work together to detect and repair any damage to the genetic structure of the cells besides arresting abnormal cell division rate. There are two types of molecular regulators that are active controllers and supervisors of the eukaryotic cell life cycle - cyclins (a protein family that regulate the multiplication of cells during cell cycle) and cyclin-dependent kinases (protein kinase family that play a major role in regulating cellular genetic transcription, processing mRNA and differentiating nerve cells). The cyclins activate the cyclin dependent kinases and once activated, the latter take control of and regulate the genetic replication and transcription process. CKDs cannot be activated on their own and cyclins are needed to activate the catalytic function of the CKDs. Such regulation of cell cycles is necessary to avoid uncontrolled cellular division which can result in inflammatory or tumorous accumulation of unnecessary cells. If such an extra baggage turns malignant, it becomes a cancer.
That precisely sums up the entire eukaryotic cell life cycle, right from the resting phase to mitosis. Although prokaryotes and eukaryotes more or less include all life forms on earth, there are a group of organisms, the Archaea, that exhibit aspects of both categories of cellular characteristics. This organism has somewhat blurred certain areas on the line that separates prokaryotes from eukaryotes. This raises a few significant questions - Is this a sign of the ongoing evolutionary machinery that is working towards merging the two different cell-lines? Or does this mark the emergence of a third cellular category altogether? Only time will tell us.
Transformed eukaryotic cells
Prokaryotic cell illustration