Attracting Abundance

- 107 -

The Law

Belief - XIX

Our Cellular Biology - XVIII

All cells have the ability to detect the stimuli and respond to those stimuli in their environment. This ability, called cell signalling, not only promotes proper functioning of the cells but allows a cell to coordinate and communicate among groups of cells as in a teamwork. This teamwork is essential to make up communities called tissues. Because of cell signalling, the tissues discharge functions which otherwise will not be possible by an individual cell or groups of cells.

Note: Tissue is an organizational level between cells and complete organs. A tissue is a group of similar cells that together carry out a specific function. Organs are formed by grouping of tissues. There are four types of tissues in a human body:

• Epithelial tissues: which cover the organs thus creating a barrier between the organt and external environment. Example: skinE
• Connective tissues: which hold other tissues together. Therefore they have to strong. Example: bones ( made up of layers of tissues)
• Nervous tissues: which are parts of the nervous system. Example: brain, spinal cord
• Muscle tissues: are attached to the organs of the body and are responsible for movement.

To preserve proper tissue structure, adhesive molecules help maintain contact between nearby cells and and structures, and tiny tunnel like junctions allow the passage of ions and and small molecules between adjacent cells. Meanwhile, signaling molecules relay positional information among the cells in a tissue, as well as between cells and extracellular matrix. These signaling pathways are essential for maintaining the state of equilibrium known as homeostasis within a tissue.

Cell signaling also plays a crucial role in maintaining a balance between cell growth and death. The process of programmed cell death is called apoptosis. The orderly demise of certain proportion of cell is necessary for normal tissue turnover and maintenance of homeostasis.

Note: Cell death occurs either due to apoptosis or necrosis. Apoptosis is programmed and healthy. Cells that die by apoptosis are removed by white blood cells and do not harm the organism. Necrosis is the death of cells due to outside trauma or influence. Necrosis is harmful to organism, but does not necessarily cause death.

A variety of intracellular of extracellular signals can activate cell growth or apoptosis. When cells receive these signals, the carefully weigh the signals against each other before choosing a course of action. For example, the lack of nutrient or presence of toxins would likely cause apoptosis rather than cell growth. It was thought that damage to DNA also causes apoptosis, but the later day reserach has shown that this may not be true. Cells with DNA removed have lived their full cycle.

Cells court death cleanly and quickly during apoptosis during which a variety of catalysts (enzymes) disintegrate the proteins and nucleic acids.

Cells can replicate themselves. This ability of reproducing is one of the reasons of defining cells as living things. A cell cycle can be broadly categorized into three phases - interphase, mitosis and cytokinesis.

When a cell divides itself, it produces two daughter cells. This process is called mitosis. There are five distinct phases to mitosis.

• prophase: during this phase, the parent cell chromosomes - which were replicated during S phase -condense and become thousand of times more compact than they were during interphase. Because each duplicated chromosome consists of  two identical chromatids joined at a point, these structures now appear as X-shaped bodies when viewed under a microscope. The mitiotic spindle (a structure composed of microtubules which segregate chromosomes into daughter cells) also begins to develop during prophase. As the cell's two centrosomes (an organelle near the nucleus of a cell which contains the centrioles and from which the spindle fibres develop in a cell division) move towards opposite poles, microtubules gradually assemble between them, forming the network that will later pull the duplicated chromosomes apart.

Note:  Centrioles are a pair of minute cylindrical organelles near the nucleus, which are responsible for development of spindle fibres for cell division. Spindle fibres form a protein structure that divides the genetic material in the cell. The spindle fibre is necessary to equally divide the chromosome in the parental cell into two daughter cells.

• prometaphase: the nuclear membrane breaks down into numerous small vesicles. This exposes the DNA can now be directly accessed by the spindle microtubules. The microtubules grow outward from the centrosomes aand collapse backward looking for a chromosome. Eventually the microtubules find their target and connect to the chromsome.
• metaphase: DNA lines up in the middle. Centrioles connect to the DNA which is in condensed form called chromosome.
• anaphase: marks the beginning of cell division. During this phase, half of each chromosome is pulled towards the two cells that are beginning to form.
• telophase: is the end of the division portion of mitosis. the cell membranes closes around the new cells. Although each new cell is identical to the parental cell, they contain half of the DNA of the parental cell.

Interphase is the resting phase of mitosis. this is the time when the cell is doing its normal function and preparing for another round of replication. There is a point in the interphase that is important to cell's division. This phase is called the synthesis or S phase. The rest of the interphase are the growth (G) phases - the period before S phase is called G1 phase and the period after S is called G2 phase. During the S phase, DNA of the cell is replicated thus doubling the amount of chromosomes.

The process of sexual cell division is called meiosis. It is a type of cell division that results into four daughter cell.

Cytokinesis is the instant in which the cell actually divides. Basically, mitosis is like pulling the cell apart, and cytokinesis is the rip that gives you two cells which then go to interphase and grow.

So cell division cycle can be short written as interphase - mitosis - cytokinesis - interphase - mitosis - cytokinesis - interphase....... A typical human body cell divides itself approximately once every 24 hours. Interphase is the largest part of the cycle and comprises about 95% of cell cycle. 

See the animation video for mitosis:

In order to pass from one phase to the next, a cell must pass through numerous checkpoints. At each checkpoint, specialized proteins determine whether the necessary conditions exist. If so, then the cell is free to enter the next phase.. If not, progression through the cell cycle is halted. Errors in these checkpoints can have serious consequences including unplanned cell death of unrestrained growth of cells i.e. cancer. Each part of the cell cycle has its unique checkpoints. For example during G1, the cell passes through a critical checkpoint that ensures environmental conditions, including signals from other cells, are favorable for replication. If conditions are not favorable, then the cell may enter a resting phase. Another important checkpoint takes place just before the cell moves from G2 to mitosis. here a number of proteins scan scrutinize the DNA of the cell, making sure that it is structurally intact and properly replicated. The cell may pause at this point to allow time for DNA repair, if necessary. Yet another important checkpoint is in mid-mitosis. This check determines whether the chromosomes in the have attached properly to the spindle. This step decrease the possibility that the resulting daughter cells have unbalanced number of chromosomes.

The average cell will divide 50 to 70 times before the death of the cell. As the cell divides, the telomeres (regions of repetitive nucleotide sequences at each end of chromosomes which protect the chromosomes from deterioration or fusion with the neighboring chromosomes) at the end of chromosomes get smaller and before death of the cell the telomeres are exhausted.


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