LO: I can explain gene expression is controlled by operons.
Do Now: What is the purpose behind the 5’ cap - modified “start” of mRNA and poly-A tail - modified “end” of mRNA? Are they converted into proteins?
The 5’ cap protects the mRNA move through attacking exonucleases and helps the ribosome attach to the mRNA and start reading it to make a protein. The poly-A tail stabilizes the mRNA as it moves through the cytoplasm. After transporting the mRNA, they are degraded into free RNA nucleotides and recycled to new mRNA.
Every cell of the body contains all the DNA for the organism. Not all genes are necessary all the time. Cells must then have the ability to turn a gene on and off.
An operon consists of the following elements:
- Promoter - where RNA polymerase attaches, signaling the start of the gene
- Operator - where a repressor binds, stopping the transcription of that gene
- Structural Genes - genes coding for the enzymes, they are transcribed as a unit
Repressors or Inducers The trp operon is a repressible operon, if tryptophan is present, the operon is in the off state. The lac operon is an inducible operon, if lactose is present, the operon is turned on. The trp operon - exists in the “on” state and controls the production of tryptophan
The lac operon in bacteria. This region is normally in the “off” position, it turns on when lactose is present. In the absence of lactose, the lac repressor binds the operator, and transcription is blocked. In the presence of lactose, the lac repressor is released from the operator, and transcription proceeds at a slow rate.