All the living organisms, even the enormous ones, start their journey from a single cell. The process through which cells originate from existing living cells is called as cell division. Each cell have the ability to divide into two daughters cells (In mitosis), which again can grow and divide. This after many cycles (of growth and division), leads to formation of many cells required for formation of a fully functional organism (remember this all started from a single cell).
The process of cell division involves replication or duplication of DNA or genetic material and growth of the cell. This happens to be a very critical step for all living organisms and hence great care will be required to ensure proper division of cell, leading to the formation of two daughter cells with error free intact genomes and cytoplasmic organelles of their own.
There are two main types of cell division : the first one happens in somatic cells (vegetative division), wherein each cell divides into two identical daughter cells resembling parental cell, termed as mitosis.
The second one is meiosis that is found in germ cells or required to form gametes – egg and sperm. In meiosis the cell divides into four haploid daughter cells.
Definition of cell Cycle:
Cell Cycle is termed as an orderly and highly regulated sequence of events, where in a cell grows, replicates its DNA, and synthesizes the other important constituents (cytoplasm and its contents) required for cell division. The resulting daughter cells can again enter cell cycle independently to start a new cell cycle, based on the stimulatory and inhibitory messages they receive.
Eukaryotic cell cycle can be divided into two phases:
- M Phase (Mitosis and cytokinesis)
During the interphase, the cell growth and DNA replication occurs and in Mitotic phase two separate cells are formed. During the mitotic phase, the replicated DNA and cytoplasmic contents are distributed between two newly formed daughter cells.
- The interphase can be separated into three phases – G1, S, and G2.
- This division was first proposed in 1953 by Alma Howard and Stephen Pelc, based on their experiments done in plant meristem cells with regard to timings of DNA synthesis.
- According to the work done in bean (Vicia faba L.) roots, where they grew plants with a 32P radio isotope label and demonstrated that the radio isotope was found in the DNA present in nucleus only during interphase.
- By carefully analyzing the asynchronous cell populations and time frames with regard to presence of radio labelled is 32P incorporated into DNA, Howard and Pelc could assign time schedule to cellular life.
- They divided the events related to cellular reproduction into four phases: a cell division phase, the pre-S-phase (G1), the S-phase and period G2, leading to the birth of the concept of cell cycle.
G1 or Gap phase 1 :
- The first phase of interphase is G1 or called gap phase 1, as very little activity is observed through the lens of microscope in this stage.
- But this stage is considered to be very active at cellular level, especially after the considerable slowing of these biochemical activities in preceded mitotic phase.
- In G1 phase, the cell grows in size, increases the number of organelles (such as mitochondria, ribosomes), and prepares for huge task of replication of entire genome (DNA) in the next phase (S). This is done by accumulating molecules such as nucleotides, mRNAs, proteins, energy and more especially the centromere and the other components of the centrosomes are made.
- It is important to note that during this phase of cell cycle, the cells are fully functional and could carry out all the other normal functions as observed in a non dividing cell.
- G1 is the period or interval between mitosis and DNA synthesis in S phase. Depending upon the conditions and the kind of cell, interphase may last a day, weeks, or longer.
- In some cases (starvation) or in situations when a tissue has reached its required or target size, the cells will exit cell cycle and go in a least active G0 phase. The cells retain the capacity to re-enter the cell cycle at G1 should the need ever arise and some cells remain in G0 phase for eternity. eg: Nerve cells do not normally regenerate; they remain in G0.
S or DNA replication phase:
- The onset of S phase is determined by replication of genomic DNA.
- The replication of entire genomic DNA results in duplication or identical pairs of DNA molecules termed as sister chromatids. There sister chromatids remain firmly attached to the centromeric region. However it is important to remember that the DNA replicated in the S phase has not condensed into chromosomes yet.
- Along with the replication of genomic DNA in nucleus, S phase also is evident by duplication of centrosomes / centrioles in cytoplasm.
- Centrosome is a organelle present in the cytoplasm (very close to nucleus) and essential in the formation of mitotic spindle, assembly of microtubules, and regulation of cell cycle progression. Centrosome happens to be one of the best studied microtubule-organizing centers (or MTOCs) in animal cells.
- The microtubules emerge from centrosomes, These microtubules are organized, regulate their anchoring and release by centrosomes.
- Centrosome is made up of two perpendicular rod like centrioles and amorphous mass surrounding them called pericentriolar material.
- centrioles are made up of microtubules and plays a essential role during mitosis. During cell division, the two centrioles begin to move to both sides of the nucleus to initiate mitotic division.
- Later centrioles will release mitotic spindles which will pull the chromosome from both sides to start mitotic division.
- Centrioles are not present in the centrosomes of plants and most fungi.
|A centrosome with two centrioles is an organelle in cytoplasm involved in several processes||Centriole is rod like structure made up of tubulin. Two centrioles makes a centrosome, essential for cell division.||Centrosome is a structure within chromosome where two sister chromatids attach.|
G2 Phase (Gap2):
The S phase is followed by the second growth or gap phase. As we saw in the S phase the entire genome is duplicated, so there is a need for the cell to go through a quality control to check the integrity of the DNA synthesized.
The cell continues to grow and this phase also ensures everything required (proteins, energy, RNA etc.) for subsequent mitosis is available.
The microtubules that are used for the mobility of the chromosomes in Mitotic phase are assembled at G2.
Mitotic or M phase:
- M phase happens to be one of the dramatic phase of the cell cycle. which involves major reorganization of almost all cell components of the cell.
- Mitosis is also called equational division, is usually observed in diploid cells. It results is production of diploid daughter cells with identical genetic complement.
- Mitotic phase is a multi step process in which the duplicated chromosomes in S phase are aligned, separated, and sorted into two newly formed cells.
- This requires two steps : Karyokinesis (nuclear division) or also termed Mitosis – which is again simplified into four different phases for convenience – ( Prophase, metaphase, anaphase and telophase) and cytokinesis ( physical division of cytoplasm into daughter cells).
- Apart from division of nuclei (karyokinesis), Mitosis also accomplishes, the division of cytoplasm. This ensures physical separation of the cytoplasm and signals the end of successful cell division. As cytokinesis usually occurs in conjunction with mitosis, “mitosis” is often used interchangeably with “M phase”. But there are exceptions, as in some cells mitosis and cytokinesis occur separately. eg: early stage of Drosophila embryonic development.
- In animal cells, cytokinesis is achieved by appearance of the furrow in plasma membrane, which eventually deepens to physically separates the two newly formed daughter cells.
- In case of plant cells, a new cell wall must form between the daughter cells. The Golgi complex disintegrates into vesicles, which eventually fuse to form a structure called as cell plate at the metaphase plate. This will contribute to the parts of plasma membrane and divides the cell into two daughter cells.
- Enzymes use the glucose that has accumulated between the membrane layers to build a new cell wall.
- One of the significant contribution of mitosis is repair of cells. Cell division by mitosis contributes to replenish cells which are lost from the upper layer of epidermis , inner lining of the gut and blood cells.
- Most of the time when people refer to “cell division,” they mean mitosis. It is the kind of cell division required for growth of multicellular organisms.
In the next post few posts, we can look into another kind of cell division-Meiosis and also about regulation of eukaryotic cell cycle along with cell cycle checkpoints.
Images : Source openstax biology.