Cells act as the building blocks of living organisms as they make up the whole framework of the body. Therefore, the cell as the basic unit of life is recognised. Both plants and animals possess cells as microscopic units. They are easily visible under a light microscope with 1 to 100 micrometres of dimensions. An electron microscope is necessary to study the detailed structure and other organelles present within it. The cell number and their shapes vary from species to species. However, the human body comprises 40 trillion cells and the alone brain consists of 80 billion of them. Robert Hooke was the first person to discover the cell in 1665. However, Matthias J. Schleiden and Theodor Schwann later gave the cell theory in 1839.
Cell Cycle
There are various phases and interphases of this process. Generally, after the division of cells that have nuclei present, they land into two phases, interphase and mitotic or meiotic, which further have some more processes to complete the cycle.
Phases of the Cell Cycle
There are different Phases of the Cell Cycle, they differ by organelles present in cells. As discussed, cells with nuclei have two phases: 1.) Interphase and 2.) Mitotic phase/ Meiotic phase.It generally takes about 24 hours for a cell to perform cell division or the cell cycle.
When a cell completes its cycle, it gets into the resting stage for some time before starting another cell division. This is the starting phase of the cell cycle.
Interphases
Interphase is the phase after the M phase that a cell has performed during its previous cell division. Interphase has three phases of cell division in itself, G1, S, G2 are the three phases, In interphase, the changes start taking place in the cell, its size and organelles increase in size and prepare for DNA replication in other stage and finally proceed to mitosis or cytokinesis after the last stage.
Gap1 (G1) phase
G1 or gap 1 is the start of interphase: cells start growing, and organelles also grow with cells. As G1 is the phase after the successful M phase of the previous cycle, the metabolic activities of the cell slow down during the M phase of cell division, which in the G1 phase again reach to high rate, G1 phase is also called the growth phase of the process. After the G1 phase, Cell has three options to proceed next, the cell enters the synthesis phase and continues cell division, to stop division and enter the G0 stage again to perform differentiation, or to start the cell division cycle again. The restrict point is the point which performs transitions and decides the next proceeding of the cell.
Synthesis (S) phase
The synthesis phase starts replicating DNA in itself and initiating itself to enter the G2 phase. In the S phase the cell partitions chromosomes and other organelles to get divided. When the DNA in the cell doubles and organelles like centrioles are also halved, then the cell is ready to enter the Gap 2 (G2) phase.
G2 phase
Cells continue to grow in the G2 phase, forming spindles. All macromolecules, proteins, organelles are replicated and multiplied in this phase before entering the mitotic phase and cytokinesis phase.
M (mitotic) phase
This phase is the chromosome division process; in this phase, a cell multiplies and aligns its chromosome to separate into two cells. The cell partitions its nuclei in two halves. This process is divided into four stages. 1.) Prophase 2.) Meta phase 3.) Anaphase 4.) Telophase.
Prophase: The nucleolus starts vanishing and the spindle fibres start developing. Chromosomes start condensation.
Metaphase: Centromeres reach the layers of cells from inside. The spindle gets fully evolved and the force to separate the cell from the spindle starts. Both nuclei move towards opposite poles inside a cell.
Anaphase: The sister chromatids separate and attach to opposite poles with chromosomes.
Telophase: The spindle is separated in this phase.
Cytokinesis
Cytokinesis instantly occurs after the M phase. Cytokinesis divides all cellular components or organelles of a cell into almost two halves and starts giving birth to two daughter cells, and lastly, cell division takes place and the cell gets divided into two daughter cells.
Meiotic phase- This process by which reduction in chromosome number takes place is called meiosis or reduction division. It’s complete in two steps.
- Meiotic I 2. Meiotic II
Meiotic I-(a) Prophase I: It is the longest and most complex phase of meiosis I. It is further divided into 5 sub-stages as:(i) Leptotene (ii) Zygotene(iii) Pachytene (iv) Diplotene (v) Diakinesis
(i) Leptotene : In this sub-stage, the chromosomes appear as thin, uncoiled thread-like structures.
(ii) Zygotene:Pairing or synapsis of homologous chromosomes takes place at this sub-stage.Synapsis or pairing leads to formation of bivalents.
(iii) Pachytene :Each chromosome of homologous pair is having 2 chromatids at this stage and thus bivalent or homologous pair of chromosomes is now having four chromatids (four stranded) which is called tetrad stage or tetravalent stage.Crossing over (Exchange of chromosomal segments between non-sister chromatids of homologous chromosomes) occurs at this stage
(iv) Diplotene:As forces of attraction between homologous chromosomes or bivalents reduce, so they begin to separate from each other and this separation of homologous chromosomes
(v) Diakinesis:Both nucleolus and nuclear membrane completely disappear at this sub-stage (or in early metaphase).
Metaphase, anaphase , telophase and cytokinesis have same process like as mitotic phase .After meiosis I phase ,cell enter into meiotic II(prophase, metaphase,anaphase,telophase and cytokinesis), which is also similar process like as mitotic phase and get four cell.
Conclusion:
We have learned the cell cycle’s importance, process, phases and various general facts. Cells are the essentials of our body; they perform a cycle called the cell cycle in which they replicate another cell through cell division. The cell cycle occurs in the cell itself: cells get halved into two daughter cells, which are DNA replications of the parent cell. To reproduce, cells divide their cytoplasm and other organelles into two parts and hence come into existence.