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HIV can lodge itself on membrane receptor of T lymphocyte and then injects viral RNA and viral enzymes in host cell.


Human immunodeficiency virus abbreviated as #HIV# is a host-specific retrovirus that targets the major components of human immune system i.e #T#-#lymphocytes# or #T#-#cells#.

Infection cycle of HIV:

  • Attachment:
    Firstly the HIV virus attaches to the #Cd4# receptor site on the T-cells. These fuse with the t-cell membrane and get entry into the cell's cytoplasm where they are uncoated.
  • Reverse transcription and Integration:
    HIV virus being a retrovirus has a special enzyme known as reverse trancriptase on the #RNA# #"genome"#. This enzyme uses the #"viral RNA"# as a template to make a strand of #DNA# and then uses the DNA strand as a template to make a DNA double helix. Original viral RNA gets degraded.
    The viral DNA integrates into the chromosomal DNA of T-cell of the host, now becoming a provirus.
  • Translation:
    The proviral DNA is transcribed into RNA which uses host cell's machinery to translate viral proteins to encase newly generated viral genome.
  • Assembly and Release:
    So, new capsids assemble around #"viral RNA"# and reverse transcriptase molecules. And they bud off from plasma membrane of T-cell as mature HIV viruses, ready to attack more T cells.

Increasing number of Cd4 cells start to die due to HIV infection.


Our body would be non functional and we would die if we had no other medical intervention that will keep us alive. The human body without bones is just like an semi-empty sack.


Our skeleton is a very rigid structure of bones which provides support for our muscles, skin and its task is also to protect our vital organs.Whithout the bone we would be unable to do anything, beacuse our nerves, blood flow, lungs, organs would be blocked and squeezeed.

My teacher used to say that our bones were created by the most inteligent architect. When a skeletal muscle contracts, bone attached to the muscle moves. So in absence of skeleton, entire skeletal muscle system will become defunct and will surely atrophy.

Even act of breathing is dependent on movements of different bony elements of the rib cage.


Hypophosphatasia is a really interestng disrorder which leads to loss of all bone mineralization/ formation during childhood. Janelly Martinez-Amador was diagnosed with such a rare disorder:

If you want more:


Energy currency of all living cells is ATP (adenosine triphosphate) but this can not be stored in large amount.


  • ATP molecules are generated by cellular respiration: this molecule releases energy by breaking into ADP and inorganic phosphate, the reaction is catalysed by ATPase enzyme.


  • ATP stored in muscles may last only 10 seconds of exercise. Also consider the fact that ATP is a heavy molecule: molar mass of ATP is more than 500g/mol. Storage of ATP would have abnormally increased our weight. So it is natural that ATP is not stored in large amount but is continuously generated.

  • In muscle tissues, a special molecule called phosphocreatine is stored which can change an ADP immediately in an ATP in presence of enzyme.

About 120 gram of phosphocreatine is stored in our muscles.


  • To generate ATP, our muscle cells can use glucose as raw material for cellular respiration. About 500 gram of glycogen is stored in skeletal muscles of an adult body. The glycogen readily supplies glucose for production of ATP (both aerobically and anaerobically).


  • Skeletal muscles store another molecule named myoglobin which can quickly supply oxygen for aerobic respiration when there is a decline in concentration of oxygen in muscle. Myoglobin has a higher affinity towards oxygen, hence it dissociates from oxygen only at a lower partial pressure of oxygen compared to haemoglobin.


On average, the human skin regenerates itself about every three weeks,


New skin cells (keratinocytes) are continually forming at the bottom of the epidermis.

As new cells form, they push the more mature cells toward the skin's surface, moving them farther away from the blood supply that keeps them alive.


By the time a skin cell reaches the surface about three weeks later, it's dead.

Thus, the skin on your body consists of about 30 layers of flattened, dead keratinocytes.

Friction gradually removes the dead cells from the skin surface.

The epidermis can be about 3 mm thick in the palms of a manual labourer, while in the eyelids the thickness is only 0.05 mm.

Thus, skin cell replacement is faster in the thinner regions of the epidermis.


If you are ticklish, you quickly feel uncomfortable when someone touches your skin to make you laugh.


Being ticklish is both a neurological response and a learned behaviour.

Tickling as a neurological response

Being tickled stimulates your hypothalamus which, in turn, activates your fight-or-flight and pain responses.

When someone tickles you, you may laugh, not because you are having fun, but because you are having an autonomic nervous response.

In fact, the body movements of a ticklish person are similar those of someone in pain.

The automatic response may have developed early in our evolution.

The most ticklish parts of the body (feet, chest, neck, armpits) are the most vulnerable during combat.


We laugh and squirm when we're touched there because it's an evolutionary mechanism for self-defence.

We automatically try to get the tickler's hands away from these zones,

Tickling as a learned behaviour

Babies learn to laugh in response to tickling during their first few months of life.


The one-on-one activity during tickling opens the door to other forms of social interaction.

Thus, tickling also becomes a mechanism for social bonding.


It helps forge relationships between family members, friends, and lovers.

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