Cell Cycle and Division

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What is the Cell Cycle?

Cell Cycle Definition: Cell cycle can be defined as the ordered sequence of events that occur in a cell in the preparation for cell division. Cells on the path to cell division process through a series of precisely timed and carefully regulated stages of growth, DNA replication, and cell division producing two genetically identical cells. The cell cycle phases consist of four stages in which the cell grows in mass, prepares for cell division and the under cell division and.  The four stages of cell cycle are:

  1. G1 phase

  2. G0 phase

  3. S phase

  4. G2 phase

All these stages come under the interphase stage of the cell cycle which takes place before the mitotic phase.  

Cell Cycle Interphase 

Interphase Definition/ What is Interphase? 

Interphase can be defined as the period between the ends of one mitotic cell division to the start of the next division. It is the longest part of the cell division. Specifically, it’s the phase between then end of telophase and the beginning of the next prophase. DNA synthesis does not take place throughout the entire interphase. It only occurs in a restricted part of the interphase called the S phase which is a period preceded and followed by two gap periods of interphase (G1 and G2 phase) where DNA synthesis does not take place. The interphase stages are divided into four main stages of the cell cycle. These are discussed below:

G1 Phase of Cell Cycle: It is the first phase of the interphase which starts at the end of the previous M phase and occurs till the beginning of DNA synthesis. The G1 phase is also called the growth phase as, during this stage, the biosynthetic activities of the cell is at a high. The cell increases its supply of proteins and the number of cell organelles increase. After these activities are completed, a cell has three options:

  • It can continue in the cell cycle and enter the S phase

  • It can stop in the cell cycle and enter the G0 phase for differentiation.

  • It can become arrested in the G1 phase 

The deciding point is called checkpoint or the restriction point and it is regulated G1/S cyclins which causes the transition from the G1 phase to the S phase. 

G0 Phase of Cell Cycle:  This is the resting phase of the cell cycle and the cells in the G0 phase of the cell cycle are called resting cells. Cells such as neurones leave the cell cycle to enter this phase and they never divide again. The absence of nutrients or growth factors causes cells to enter a resting state. 

G0 cells generally contain fewer ribosomes and RNA than the corresponding cycling G1 cells. They also synthesise protein less than half the G1rate. When a G0 cell is stimulated to grow by growth factors or by providing nutrients, there is a change in the rate of protein synthesis and it generally goes hand-in-hand with effect on the chromosome cycle.

S Phase of Interphase: This the intermediate phase that takes place between the G1 and the G2 phase and is the third stage in the sequence of cell cycle. It is a highly specialised phase of interphase and the word S stands for synthesis. DNA synthesis takes place in this phase. Before a cell can divide, it must produce a new copy of its chromosomes.

For making a new copy of the chromosome it needs both the replication of the long DNA molecule in each chromosome and the assembly of a new set of chromosomal proteins onto the DNA to form chromatin or chromatid.

By its end, each chromosome has been copied to two complete chromatids which remain joined together at their centromeres until the M phase that soon follows.

G2 Phase of Cell Cycle: The period from the end of the S phase until mitosis is called the G2 phase. It is usually the shortest part of interphase. In this phase intensive cellular synthesis occurs. Mitochondria and chloroplasts divide and energy stores increase.

Mitotic spindle begins to form. In the interphase, there are two control points such as G1/S and G2/M at which the cell takes a decision on whether to proceed or not to the next step. Two control points are also called checkpoints.

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Cell cycle diagram

M Phase of the Cell Cycle: It is a multiple-step process where duplicated chromosomes are aligned, separated, and move into two identical daughter cells. The first portion of the mitotic phase is called Karyokinesis and the second portion of the mitotic phase, called cytokinesis. Karyokinesis is divided into:

  1. Prophase

  • The chromosomes condense and they become visible

  • Spindle fibres emerge from centrosomes

  • Nuclear envelope breaks down 

  • The nucleolus disappears

  1. Prometaphase

  • Chromosomes continue to condense and the kinetochores appear at the centromeres

  • Centromeres move towards opposite poles

  1. Metaphase

  • The mitotic spindle is fully developed and the microtubules attach to kinetochores

  • The chromosomes orgamise themselves on the metaphase plate

  • Each sister chromatid attaches to a spindle fibre that originates from opposite poles

  1. Anaphase

  • Cohesin proteins that bind all the sister chromatids break down

  • The sister chromatids are then pulled towards opposite poles of the cell.

  1. Telophase

  • The chromosomes arrive at opposite poles.

  • Nuclear envelope forms around chromosome sets.

  • The mitotic spindle breaks down. 

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  1. Cytokinesis

  • Cytokinesis in animal cells starts as a ring of actin filaments forms at the metaphase plate. The ring contracts which form a cleavage furrow and divides the cell in two. 

  • In plant cells, Golgi vesicles come together at the former metaphase plate, forming a phragmoplast. 

  • A cell plate formed by the fusion of the Golgi vesicles of the phragmoplast grows from the center toward the cell walls. The membranes of the vesicles fuse to form a plasma membrane dividing the cell in two.

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Cytokinesis diagram

And after that, the eukaryotic cell cycle is initiated again staring with the G1 phase of interphase.

Cell Cycle and It's Regulation

The presence of different regulators at different cell cycles phases can be determined by means of cell fusion in different stages of the cycle. Cell fusion makes a hybrid cell. Hybrid cells are heterokaryons that contain two different nuclei in a common cytoplasm. 

Interphase nuclei during the phases of the cell cycle can be fused by various combinations:

  • When the S phase cell is fused with a cell in G1, it reveals that both nuclei in the heterokaryon replicate DNA. This suggests that the cytoplasm of the S phase cell contains an activator or regulator of DNA replication. The regulator is called S phase activator. It could be a regulator whose activation is decided when cells in G1 are ready to enter into a cycle of replication.

  • When a cell in the S phase is fused with G2 cell, the S phase nucleus continues to replicate but the G2 nuclei do not replicate. It suggests that DNA that has been replicated once becomes recalcitrant due to the effects of the S phase activator.

  • When a cell in the mitotic phase or M phase is fused with a cell at either G1 or G2 stage of interphase, it brings about the interphase nucleus to enter pseudo mitosis which is characterised by premature chromosome condensation in the interphase nuclei. This suggests that a mitotic phase inducer is present in dividing cells.

  • When S phase nuclei are fused to mitotic cells a complex pattern is found in which the S phase chromosomes have a fragmented appearance.

FAQ (Frequently Asked Questions)

1. What is the Significance of Cell Cycle?

The following points explain the significance or the importance of the cell cycle:

  • The cell cycle consists of two phases: interphase and the mitotic phase.

  • During interphase, the cell grows and DNA is replicated whilst during the mitotic phase, the replicated DNA and cytoplasmic contents are separated and the cell divides.

  • All multicellular organisms use cell division for growth, repair and maintenance of cells and tissues. Single-celled organisms use cell division for reproduction.

  • Somatic cells divide regularly and they contain two copies of each of their chromosomes.

2. What is the Importance of Cell Division?

The importance of cell division can be explained by the following:

  • Cell division is a pre-requisite for the continuity of life and forms the general basis of evolution to various life forms.

  • In unicellular organisms, reproduction takes place by cell division which produces two or more new individuals from the mother cell.

  • In multi-cellular organisms, life starts from a single cell called a zygote that transforms into an adult that is composed of millions of cells formed by successive divisions.

  • Cell division repairs and regenerates old and worn out tissues.

3. Who Discovered Cell Division?

A 19th-century scientist, Walther Flemming was the first scientist to document the details of cellular division.

4. What is Cytokinesis?

Cytokinesis Definition: Cytokinesis is the final step of cell division in which the cytoplasm of a mother cell is partitioned into two daughter cells. This process begins with the ingression of the cleavage furrow after sister chromatid segregation and is completed much later when the narrow cytoplasmic bridge connecting the two daughter cells is severed. Successful cytokinesis requires membrane trafficking, coordination of cytoskeletal and cell cycle regulatory pathways.