Hi and welcome back to Free Science Lessons. By the end of this video you should be able to describe the structure of the Human Immunodeficiency Virus, or HIV. You should then be able to describe how HIV replicates and how HIV causes the symptoms of AIDS. And finally, you should be able to describe why viruses cannot be treated using antibiotics. Okay, so this video is about the human immunodeficiency virus, or HIV.
HIV is the virus which causes the disease Acquired Immunodeficiency Syndrome, which is also known as AIDS. I'm showing you the structure of HIV here. The first thing you need to understand is that HIV is an RNA virus, containing two RNA strands. The RNA strands are found within a protein capsid.
Inside the capsid, We also find enzymes, including reverse transcriptase. And we look at the function of reverse transcriptase later. On the outside of HIV, we find a lipid envelope.
And on the envelope, we find attachment proteins. The attachment proteins allow HIV to attach to its host cell. Just inside the lipid envelope, we find matrix proteins, which help to maintain the structure of the virus. Okay, so let's take a look at how HIV replicates.
Once a person's infected, HIV particles circulate in the bloodstream. An infected person is said to be HIV positive. HIV has several target cells, but the main one are helper T cells, which we saw in a previous video.
The attachment protein on HIV attaches to a molecule on the surface of helper T cells called CD4. Once HIV attaches to CD4, the HIV lipid envelope fuses with the cell membrane. Now, the capsid and its contents pass into the cell. At this stage, the reverse transcriptase enzyme converts the viral RNA into double-stranded DNA.
The DNA now moves into the cell nucleus and integrates with the host DNA. Now at this stage, the viral DNA can remain dormant for several years. However, at some stage, the HIV DNA becomes active and produces RNA. This includes messenger RNA encoding the HIV viral proteins and the HIV RNA genome. The RNA now moves to the cytoplasm, and the messenger RNA is translated by ribosomes into viral proteins.
Next, the virus particles assemble and then bud from the helper T cell. As they leave, each virus is surrounded by cell membrane, forming the lipid envelope. Now a key stage in the replication of HIV involves the conversion of RNA into DNA by reverse transcriptase.
Because of this process, HIV is an example of a retrovirus. Now once HIV becomes active and replicates, this leads to the death of helper T cells. As the number of helper T cells decreases, the patient's immune system can no longer function effectively. Without enough helper T cells, both antibody production by B lymphocytes and the cell-mediated immune response become less effective. At this point, the patient is described as having acquired immunodeficiency syndrome or AIDS.
Now, without an effective immune system, the patient is much more vulnerable to infections such as tuberculosis. Scientists refer to these as opportunistic infections. A patient with AIDS is also at a higher risk of developing cancer. And in fact, these diseases are the cause of death in patients with AIDS.
Now a key idea you need to understand is that antibiotics cannot be used to treat viral infections. Antibiotics kill bacteria. by targeting the bacterial cell structures, or by targeting metabolic processes within bacterial cells.
For example, penicillin prevents the cross-linking of peptidoglycan molecules in the bacterial cell wall. This causes bacterial cells to burst when water enters by osmosis. Now, viruses do not have any cellular structure, and rather than having their own metabolism, Viruses take advantage of the metabolic processes of their host cell.
So for these reasons, viruses cannot be treated using antibiotics. Instead, we treat viral diseases using antiviral drugs. For example, the antiviral drugs against HIV.
inhibit key enzymes in the HIV replication cycle, such as reverse transcriptase. Okay, so hopefully now you can describe HIV and AIDS.