REGENERATION:
Regeneration is the process of renewal, restoration, and growth of lost body parts.

ABILITY OF REGENERATION:
Depends on :

• Differentiation and Complexity

The more complex an organism the more will its cells be differentiated and lesser the power of regeneration.

PROCESS:

Some organisms are capable of dedifferentiation, in which specialized cells become more basic.
This process is involved in the regeneration of limbs in animals which are capable of this feat, with the basic cells differentiating again to construct the needed tissues, bones, and other types of cells for the replacement.

A small molecule dubbed reversine, has been discovered that has proven to induce dedifferentiation in myotubes. These dedifferentiated cells could then redifferentiate into osteoblasts and adipocytes.

EXAMPLES:
SPONGES:small parts can regenerate whole colonies.
HYDRA OR STARFISH:chopped parts can form new animal.

ARTHROPODS:can regenerate lost appendages.
SALAMANDERS AND LIZARDS:can regenerate lost tails.

BIRDS AND MAMMALS:new cells can form at points of wounds.

CELL DIFFERENTIATION AND STEM CELLS:

The process during which young, immature (unspecialized) cells take on individual characteristics and reach their mature (specialized) form and function.

EXAMPLES:
Embryonic stem cells Fetal stem cells Cord blood stem cells and some adult stem cells are capable of performing this process.

Totipotent : Toti, meaning total, the zygote and the embryo during early stages of development have the potential to become any type of cell and can produce a whole human being.

Pluripotent: Cells which can differentiate into several different cell types,except placenta, are considered to be pluripotent

EMBRYONIC STEM CELLS:
Cells that form the inner mass of a blastocyst during embryological phase of growth (lasting 8 weeks)are called embryonic stem cells.Like all stem cells they can proliferate and give rise to specialized cells.

CULTURE OF EMBRYONIC STEM CELLS
Credit: The National Academy of Sciences has kindly allowed the images to be reprinted on the weird science blog. Check out the original link called Understanding Stem Cells, courtesy of the National Academies Press in Washington, D.C.

FETAL STEM CELLS:
The cells taken from a fetus (after 8 weeks of development an embryo is termed as such) are called fetal stem cells.

ADULT STEM CELLS:
These are undifferentiated cells found throughout the body that are involved in refurbishing the body with new cells to replace worn out ones.
Examples of stem cells in an adult:
Hematopoietic stem cells
Neural stem cells
intestinal stem cells

CORD BLOOD STEM CELLS:
The cells of the umbilical cord and placenta are unspecialized stem cells and can be cultured.

CAUSE:
Specific groups of genes are expressed in each cell type which leads to production of specific proteins (the other unneeded genes are turned off)
Proteins (coded for by the DNA) are the keys to differentiation in animal cells

The particular combination of genes that are turned on (expressed) or turned off (repressed) dictates cellular morphology (shape) and function. This process of gene expression is regulated by cues from both within and outside cells and lead to differentiation.

CHEMICAL COMPOSITION OF CHROMOSOMES:

The major chemical components of chromosomes are DNA, RNA, histone proteins and non-histone proteins. Calcium is also present in addition to these constituents.

DNA -Deoxyribonucleic Acid:
DNA is the most important of chemical components of chromatin, since it plays the central role of controlling heredity.

Histone proteins:
These are Lysine And Arginine rich, basic and stable proteins in chromosomes.
They are classified into five types called HI, H2A, H2B, H3 and H4. 

Non-histone proteins:
Non-histone proteins provide the scaffolding structure.

DNA PACKAGING:

The diploid human genome contains 6 billion base pairs of DNA per cell.This is enough to make some very lengthy journeys like, to the sun and back.300 times.In order for this massive amount of DNA to exist in a cell ,it has to be made very compact.In short, it needs to be packaged into a very small surface area to volume ratio.
Patterns of packaging are facilitated by associated proteins, which in eukaryotes, could be histones or non-histones.

The unit of compaction is nucleosome consisting of a segment of DNA wound in sequence around four histone protein cores.

• Histone proteins provide the spools about which DNA winds thus compressing 1.8 meters of dna into 90 mm.The negatively charged coil of polynucleotide (due to phosphate) is strongly attracted by the positively charged histone protein which helps in compaction.

• The nucleosomes are linked together by the long strand of DNA called linker DNA. 

• The beads-on-string structure in chromatin is packaged to form chromatin fibres which then fold together into large looped domain.that are further coiled and condensed at metaphase stage of cell division to form chromosomes.

• The packaging at higher level requires non-histone proteins. 

HETEROCHROMATIN AND EUCHROMATIN:

In a typical nucleus, some region of chromatin are loosely packed (and stains light) and are referred to as euchromatin.Euchromatin is said to be transcriptionally active
The chromatin that is more densely packed and stains dark is called as heterochromatin. Heterochromatin is transcriptionally inactive.

NERVE IMPULSE

Neurons send messages electrochemically i.e chemicals(ions) cause an electric impulse.Neurons and muscle cells are electrically excitable and can transmit electrical nerve impulses.

SODIUM-POTASSIUM PUMP & ION CHANNELS:

A transport protein embedded in the cell membrane acts as "Sodium/potassium/ATPase".

It works to release the Na+ ions outside and K+ ions inside using ATP.

To check the function of Sodium-Potassium pump,there are also Na+ K+ ion channels in the membrane which when closed still leak allowing sodium inside and potassium outside down their concentration gradient.

THE RESTING MEMBRANE POTENTIAL:

The combination of Sodium Potassium pump and ion channels causes a stable imbalance of ions across the membrane.This imbalance of ions creates a potential difference between the inside of the neuronal membrane and its surroundings called the RESTING MEMBRANE POTENTIAL.

It is always negative inside the cell membrane and positive outside due to the presence of -vely charged proteins and +vely charged Na+ respectively.

In humans it is -70 mV.

ACTION POTENTIAL:

"WHEN A STIMULUS IS APPLIED, A BRIEF REVERSAL OF THE MEMBRANE POTENTIAL LASTING FOR ABOUT A MILLISECOND OCCURS.THIS IS CALLED THE ACTION POTENTIAL" 

ALL OR NOTHING LAW:

The action potential only occurs if the stimulus causes enough sodium ions to cross the membrane to change the membrane potential to a certain threshold (-30 mV) in humans.At this point Na+ rush inside.If depolarization is not strong enough to reach the threshold then no action potential is generated.

DEPOLARIZATION:

When stimulated past threshold,sodium channels open and Na+ ions rush inside creating a region of positive charge within.This is called depolarization.

The region of positive charge causes the nearby voltage gated sodium channels to close.

REPOLARIZATION:

Just after the Na+ channels close,the K+ channels open wide and K+ ions enter the cell bringing the charge across the membrane back to normal.This is called REPOLARIZATION.

                                                                                                                 

PROPAGATION OF IMPULSE:

The process continues as a chain reaction along the axon membrane.The influx of Na+ depolarizes and outflow of K+ repolarizes the membrane.

REFRACTORY PERIOD:

The time period within which the sodium and potassium are returned to their rightful places on either side of the membrane through the ATP pump is called refractory period.

While the neuron is in this period ,it doesn't respond to any stimulus.

SYNAPSE:

The region where the impulse moves from one neuron to another is called synapse.

It consists of :

PRE-SYNAPTIC MEMBRANE:The axon terminals.

POST-SYNAPTIC MEMBRANE:The dendrite of the other neuron or in some cases the cell membrane of the muscle,this synapse is called motor-end plate.

TRANSMISSION OF IMPULSE:

• The pre-synaptic membrane,axon terminals contain many vesicles filled with neurotransmitters such as acetylcholine ,gab,serotonin,glycine ,nor-epinephrine etc.

• As the impulse is propagated to the axon terminals,calcium channels present there open and allow diffusion of Ca++ ions from synaptic cleft to synaptic membrane.

• This causes the vesicles to release the neurotransmitter into synaptic cleft.

• The neurotransmitters bind to specific receptors on the post-synaptic membrane to generate action potential in the cell.

• Neurotransmitters are then reabsorbed by the axon terminals.Ca++ ions move out.