Operons: Inducible and Repressible Gene Expression

• Operons are gene switches that control gene expression in bacteria.
• There are two types of operons: inducible and repressible.

Inducible (lac) Operons

• Turn gene expression on.
• Analogous to inducing labor and producing milk.

Repressible (tryptophan) Operons

• Turn gene expression off.
• Always on unless a repressor is activated.
• When tryptophan is present, it functions as a repressor.
• The repressor restricts the RNA polymerase from producing RNA, halting transcription.

Inducible (lac) Operons

• Activated by lactose, a sugar (disaccharide) made of galactose.
• Three genes in a lac operon code for enzymes needed to break down galactose into glucose.
• Allolactose (an isomer of lactose) is an inducer that prevents the repressor from binding to the operator.
• Allolactose binds to and deactivates the repressor by changing its shape.
• For example, drinking milk introduces allolactose, which deactivates the repressor, allowing RNA polymerase to bind to DNA and transcribe lac genes, leading to lactose utilization for energy.
• In the lac operon, lactose functions as an inducer, making the repressor inactive. RNA polymerase then binds to the promoter.
• In the tryptophan (trp) operon, tryptophan acts as a repressor, preventing transcription.

The Human Genome

• Comprises 3 billion base pairs of DNA and roughly 24,000 genes.
• Merely 3% of the DNA encodes for proteins; the rest consists of regulatory and repeating sequences.
• Telomeres are composed of extended tandem (repetitive) repeats.

Human Gene Regulation

Chromatin Regulation

• Tighter winding decreases the likelihood of gene expression.

Methylation Regulation

• Adding methyl groups (CH3) to DNA silences it; removing methyl groups can activate genes.

Epigenetic Inheritance

• Modifications affect gene expression without altering the DNA itself.
• Environmental factors, like diet, stress, and prenatal nutrition, can alter gene expression leading to why identical twins can exhibit differences, with one developing a disease while the other does not, due to differences in gene expression.

Biotechnology

• Recombinant DNA involves combining DNA from two or more sources into one.
• Occurs naturally through viral transduction, bacterial transformation, conjugation, or transposons (jumping genes).
• Scientists manipulate DNA via recombinant DNA (biotech) or genetic engineering (GMOs).
• Example: gene therapy replaces an unhealthy gene, like hemophilia, with a healthy version.

Uses of Genetic Engineering

• Used for vaccine production.
• Increases growth rate in cows, fish, and pigs.
• Makes plants disease resistant.
• Increases milk production in cows.
• Creates pigs with less fat and leaner meat.
• Tadpoles can be modified with jellyfish fluorescence to detect pollution.
• Creating garden rice with vitamin A.
• Featherless chickens.
• Pest resistant cabbage containing scorpion tail poison.
• Bacteria that eats oil.
• Strawberries with antifreeze.

Why Clone Genes or Make Recombinant DNA?

• To produce proteins in large quantities affordably, like insulin, instead of replacing it with outside genes with riskier genes.
• Replaces nonfunctioning genes, an example of gene therapy.
• Replaces diseased genes
• Prepares multiple gene copies for research.
• Engineer bacteria to clean environment.
• Bacteria that eats toxic waste.

How to Clone a gene

• Isolate a gene of interest like, for example, insulin.
• Cut this out with restriction enzymes.
• Insert the gene into a plasmid (circular bacterial DNA).
• Undergoes binary fission.
• The gene is cloned when the bacteria reproduces.
• The bacteria are identified and the cloned gene is harvested

How to Cut DNA

• Restriction Enzymes
• Derived from bacteria and are named after their origin.
• Cuts DNA at specific sequences, e.g., GAATTC.
• Cuts are staggered, leaving sticky ends used to reseal specific places.
• For example, ECORI, after E. Coli Bacteria.

Gel Electrophoresis

• Separates DNA based on size (length) and charge.
• Smaller (negatively charged) DNA moves faster towards the positive side.
• Separates out proteins or amino acids after being cut out by restriction enzymes
• Used in crime scenes or DNA tests.

Operons are gene switches that control gene expression in bacteria. Inducible operons turn gene expression on, while repressible operons turn it off. Lactose activates inducible operons, while tryptophan acts as a repressor in repressible operons.