Biology Class 12 NCERT Solutions: Chapter 8 Human Health and Part 4 (For CBSE, ICSE, IAS, NET, NRA 2023)

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Q. 11 What is the mechanism by which the AIDS virus causes deficiency of immune system of the infected person?


  • AIDS (Acquired Immune Deficiency Syndrome) is caused by the Human immunodeficiency virus (HIV) via sexual or blood-blood contact.
  • After entering the human body, the HIV virus attacks and enters the macrophages. Inside the macrophages, the RNA of the virus replicates with the help of enzyme reverse transcriptase and gives rise to viral DNA.
  • Then, this viral DNA incorporates into the host DNA and directs the synthesis of virus particles.
  • At the same time, HIV enters helper T-lymphocytes. It replicates and produces viral progeny there.
  • These newly formed progeny viruses get released into the blood, attacking other healthy helper Tlymphocytes in the body.
  • As a result, the number of T-lymphocytes in the body of an infected person decreases progressively, thereby decreasing the immunity of a person.

Q. 12 How is a cancerous cell different from a normal cell?


Table of Normal Cell V/S Cancerous Cell
Normal cellCancerous cell
1.Normal cells show the property of contact inhibition. Therefore, when these cells come into contact with other cells, they stop dividing.1.Cancerous cells lack the property of contact inhibition. Therefore, they continue to divide, thereby forming a mass of cells or tumor.
2.They undergo differentiation after attaining a specific growth.2.They do not undergo differentiation.
3.These cells remain confined at a particular location.3.These cells do not remain confined at a particular location. They move into

neighboring tissues and disturb

Q. 13 Explain what is meant by metastasis.


  • The property of metastasis is exhibited by malignant tumors. It is the pathological process of spreading cancerous cells to the different parts of the body.
  • These cells divide uncontrollably, forming a mass of cells called tumor.
  • From the tumor, some cells get sloughed off and enter into the blood stream.
  • From the blood stream, these cells reach distant parts of the body and therefore, initiate the formation of new tumors by dividing actively.