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Posting ID: 49913
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reverse transcriptase
What is reverse transcriptase and why do we find multiple copies of it in the human
genome?
I responded below using HIV as an example of reverse transciptase for illustrative purposes. I hope
this helps and take care.
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Bid Credits: 2
Deadline: September 24, 2005, 3:11 am EDT
1. What is reverse transcriptase and why do we find multiple copies of it in the human
genome?
Reverse transcriptase is an enzyme used by all retroviruses that transcribes the genetic information from
the virus from RNA into DNA, which can integrate into the host genome. Eukaryotes with linear DNA uses a
variant of reverse transcriptase, called telomerase, with the RNA template contained in the enzyme itself. The
enzyme collectively referred to as reverse transcriptase generally includes an RNA-dependent DNA polymerase
and a DNA-dependent DNA polymerase, which work together to perform transcription in the reverse of the
standard direction. Usually, transcription only runs from DNA to RNA, catalyzed by RNA polymerase. In
addition to the transcription function, retroviral reverse transcriptase carries a RNase domain, which belongs to
the RNase H family.
An example of a reverse transcriptase is the reverse transcriptase from the human immunodeficiency virus
type 1 (PDB 1HMV, EC 2.7.7.49).
Reverse transcriptase is commonly used in the field of research to be able to apply the polymerase chain
reaction technique to RNA. The classical PCR technique can only be applied to DNA strands, but with the help
of reverse transcriptase, RNA can be transcribed into DNA making PCR analysis of RNA molecules possible. The
technique is collectively called: Reverse Transcriptase Polymerase Chain Reaction (RT-PCR). Reverse
transcriptase is also used to create cDNA libraries from mRNA. Since HIV use reverse transcriptase, together
with integrase, to infect human DNA with viral DNA, reverse transcriptase inhibitors are used to prevent this.
Why do we find multiple copies of it in the human genome?
Having multiple copies (mutation) of all of the chromosomes is known as polyploidy and the chemical that
makes this possible is reverse transcriptase. In other words, multiple copies of the enzyme reverse
transcriptase is the effect of retroviruses and is used in the spreading of the disease.
Retroviruses - As mentioned above in the definition, certain viruses have the ability to insert a copy of
themselves into the genome of a host. The chemical that makes this possible (reverse transcriptase) is
widely used in genetic engineering.
Effects of retroviruses: Usually this is a way for the virus to get the host to do the work of reproducing the
virus. Sometimes, however, the inserted gene mutates and becomes a permanent part of the host
organism's genome. Depending on the position of the viral DNA in the host genome, genes may be
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disrupted or their expression altered. When insertions occur in the germline of multicellular organisms, they
can be passed on vertically (http://www.talkorigins.org/faqs/mutations.html).
In other words, viruses are tricky. They use all sorts of unusual mechanisms during their attacks on cells. HIV
is no exception. It is a retrovirus, which means that it has the ability to insert its genetic material into the
genome of the cells that it infects. But, infectious HIV particles carry their genome in RNA strands. Somehow,
during infection, the virus needs to make a DNA copy of its RNA genome. This is very unusual, because all of
the normal cellular machinery is designed to make RNA copies from DNA, but not the reverse. DNA is normally
only created using other DNA strands as a template. This tricky reversal of synthesis is performed by the
enzyme reverse transcriptase, shown here from PDB entry 3hvt. Inside its large, claw-shaped active site, it
copies the HIV RNA and creates a double-stranded DNA version of the viral genome. This then integrates into
the cell's DNA, and later instructs the cell to make additional copies of the virus (i.e., multiple copies). This is
the nature of reverse transcriptase (http://nist.rcsb.org/pdb/molecules/pdb33_1.html).
Viruses are tiny. They only carry enough genetic material to encode a few proteins. Many viruses, such as
poliovirus and rhinovirus, carry the bare minimum--just enough to specify their own structure and get synthesis
started once they get inside cells. The genome of HIV, on the other hand, carries instructions for building a few
enzymes that are used in the reproduction of the virus. Reverse transcriptase is one of these enzymes. But,
space in the HIV genome is still at a premium, so reverse transcriptase is encoded in a tricky way. It is composed
of two different subunits, but both are encoded by the same gene. After the protein is made, one of the subunits is
clipped to a smaller size (shown in yellow here) so that it can form the proper mate with one full-sized subunit
(shown in red). http://nist.rcsb.org/pdb/molecules/pdb33_1.html).
In fact, reverse transcriptase performs a remarkable feat, reversing the normal flow of genetic information, but
it is rather sloppy in its job. The polymerases used to make DNA and RNA in cells are very accurate and make
very few mistakes. This is essential because they are the caretakers of our genetic information, and mistakes
may be passed on to our offspring. Reverse transcriptase, on the other hand, makes lots of mistakes, up to
about one in every 2,000 bases that it copies (if this same error rate occurred in the "Molecule of the Month,"
there would be two typos in this month's installment). You might think that this would cause severe problems.
But, in fact, this high error rate turns out to be an advantage for the virus in this era of drug treatment. The
errors allow HIV to mutate rapidly, finding drug resistant strains in a matter of weeks after treatment begins.
Fortunately, the recent development of treatments that combine several drugs are often effective in combating
this problem. Since the virus is simultaneously attacked by several different drugs, it cannot mutate to evade
all of them at the same time. http://nist.rcsb.org/pdb/molecules/pdb33_1.html).
FINAL COMMENTS I HOPE THIS HELPS AND TAKE CARE.
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