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How is HIV transmitted? What is the disease caused by this virus?

HIV (human immunodeficiency virus) can be transmitted through blood, semen, vaginal secretions and maternal milk.

HIV is the virus that causes AIDS (acquired immune deficiency syndrome), a disease characterized by destruction of cells of the immune system making the body susceptible to many opportunistic and severe diseases.

Which type of virus is the HIV? What is the enzyme reverse transcriptase present in HIV?

HIV is a retrovirus, i.e., a RNA viral (its genetic material is RNA and not DNA).

Reverse transcriptase is a specific enzyme of the retrovirus responsible for the transcription of the viral RNA into DNA within the infected (host) cell. This DNA then commands the production of viral proteins and the viral replication.

What are CD4 lymphocytes? What is the relation between these cells and HIV? How does HIV replicate?

CD4 lymphocytes are T helper lymphocytes that present in their plasma membrane receptor proteins called CD4.

CD4 lymphocytes are the cells that HIV infects and within which the virus replicates. HIV has proteins in its capsule that bind to the CD4 receptors of lymphocytes. Through that bound the virus fuses with the cell membrane and its content (RNA, reverse transcriptase, protease, etc.) penetrates into the cytoplasm and the viral replication process begins.

HIV RNA is then converted into DNA by the reverse transcriptase. The new DNA is inserted into the genetic material of the lymphocyte with the aid of enzymes called integrases. By transcription and translation this DNA commands the synthesis of proteins necessary for the assemblage of new virus. Long polypeptides are thus produced and then fragmented into proteins and viral enzymes by the enzyme protease. So new HIV virus are assembled and break the cell membrane to gain the circulation.