Retroviruses (Chap. 15, p.308) and
Gene Regulation (Chap. 14)
HIV (human immunodeficiency virus)
Retroviruses and Reverse Transcriptase
Viruses that use the enzyme reverse transcriptase
to make sequences of single and double-stranded
DNA sequences from RNA
Example- AIDS
• the genome of HIV is made up of RNA, not DNA
• reverse transcriptase has a high error rate (up to
about 1 in 2,000 bases) when transcribing RNA into
DNA; this allows retroviruses to mutate rapidly
• when HIV enters lymphocyte cells, it makes DNA
from RNA using reverse transcriptase
• this DNA becomes inserted into the lymphocyte’s
chromosomes
• now when the lymphocyte replicates, the viral DNA
is replicated as well
• when the DNA is transcribed, it produces viral
mRNA, some that of which will eventually be
translated into viral proteins
• Drugs can "fool" the reverse transcriptase into
incorporating it into the growing DNA strand
instead of the HIV’s which then halts further
DNA synthesis.
• The reverse transcriptase of the virus (HIV)
prefers AZT triphosphate to the normal
nucleotide, deoxribose nucleoside triphosphate
that has no 3' hydroxyl to which to add the next
neucleotide, therefore chain elongation comes
to a halt and the virus cannot replicate.
The reverse transcriptase enzyme is very nonselective and recognizes
AZT as an Thymidine This drug gets incorporated into the growing
DNA chain and since there is no 3' hydroxyl to which to add the next
neucleotide, chain elongation comes to a halt and the virus cannot
replicate
Use of Reverse Transcriptase in Molecular
Biology
• Reverse transcriptase can be used to
make DNA from mature mRNA
– Example. The mature mRNA that codes for
human insulin can be made into DNA using
reverse transcriptase
– It is then spliced into host DNA such as E.
coli which reproduce, making more of the
DNA that codes for insulin.
Gene Regulation
• In all species, most of the genes, most of the
time, are shut down and only a very small
portion are transcribing/translating in order to
make proteins
• There must be a coordination between when a
gene is transcribed and cellular metabolism.
• Two basic “parts” of a gene:
– Coding region – codes for mRNA and
therefore, an eventual protein product
– Regulatory region – controls the
transcription of the coding region
Gene Regulation In Prokaryotes (p. 293-295
& Fig. 14.3 and 14.4 in text)
• operon – several protein coding regions
on a gene under the control of a
regulatory gene that controls a regulatory
region (consists of a promoter and an
operator)
mRNA
regulatory
gene
coding regions
promotor operator
regulatory region
Example: The lac Operon complex in E.
coli. Fig. 14.4 in text
• E. coli can utilize the sugar lactose when
glucose is not available.
• the enzymes necessary to do this are
normally not being produced by E. coli
• however, they can be produced when
lactose is present.
– therefore lactose is referred to as an inducer
molecule
Gene Regulation In Eukaryotes
• much less is known than in prokaryotes
• control systems utilizing regulatory
proteins that bind to DNA, much like
operon represors, also exist in eukaryotes
• some genes are activated by heat
• some genes are activated by the binding
of a steroid
• some genes can be regulated by the
uncoiling of nucleosomes
Eukaryotic Gene Regulation Animation (McGraw Hill)