Retroviruses (Chap. 15, p.308) and Gene Regulation (Chap. 14)

HIV (human immunodeficiency virus)

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

Retroviruses and Reverse Transcriptase

• 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)

regulatorygene

operatorpromotor

coding regions

regulatory region

mRNA

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)