The Lac operon continued and other bacteria operons

We began discussing gene expression. Can you answer the questions below?

If every cell has the same DNA sequence, how do cells differ?

In Bacteria how is gene expression controlled?
ex. Lac operon - set of inducible genes involved in digestion of lactose. (can you explain the lac operon?)

Other Operons in bacteria

AraC Operon: Involved in the digestion of Arabinose.

When no arabinose present, the AraC repressor is bound to the operator and this prevents the RNA polymerase from binding therefore, no transcription of the "Arabinose Utilization Genes." When Arabinose is present, Arabinose binds to the AraC repressor which changes shape (but unlike the Lac Operon respressor), it stays bound, but moves out of the way which allows RNA Poly to come in and bind and transcribe the genes.

Trp Operon: Involved in the synthesis of Tryptophan (an amino acid)

When tryptophan is present, it is bound to the repressor (making the repressor "Active"). This keeps the repressor bound to the operon, thereby preventing transcription of the tryptophan synthesis genes. When there is no tryptophan in the environment, tryptophan is not bound to the repressor. The repressor changes shape and unbinds from the operon, allowing RNA polymerase to come in to transcribe the synthesis genes which will make tryptophan.


Can you answer the questions below about operons?
1. How are the 3 operons different?
2. How are they similar?
3. Why would bacteria not want to make proteins in certain conditions (ex. if there is no arabinose present in the environment?)
4. What does this have to do with our question of the term? (. how does the environment of the bacteria increase/decrease the transcription of certain genes which determines the 'trait'  ex. being able to digest arabinose).


Operon video

The Lac Operon

The Lac Operon is from E.Coli bacteria and is an example of an inducible set of genes, that can be turned 'on' and 'off' as needed (for example if lactose is present in the environment!)

The genes of the Lac Operon are responsible for breaking down the sugar Lactose from the environment the bacteria may be in and converting it into usable sugars for cell respiration.

Expression of the genes of the lac operon is determined by the presence or absence of Lactose.

Molecules involved:

Operator - part of the promoter. short sequence to which the repressor binds to inhibit transcription by RNA Polymerase. This binding is what stops the RNA Polymerase from binding to the rest of the promoter.

Promoter - Where RNA Polymerase binds to begin transcription after removal of the repressor protein

Repressor - protein that is bound to the operator in the absence of the sugar lactose. When lactose is present, it binds to the repressor and removes it from the operator. This allows the RNA Polymerase to bind to the promoter to begin transcription

Lactose - sugar involved in this operon. Binds to the repressor to remove it from the operon, to allow for transcription of the 3 genes that are involved in allowing the bacteria to use lactose for energy. 

Lac Z, Y, A - 3 genes involved in the process of importing and breaking down of lactose for energy. One gene is involved makes the transport protein to increase the amount of lactose within the cell (from it's ENVIRONMENT!). The other genes are involved in the breakdown of lactose into smaller sugars. 

This video (to 5:00) is a good explanation of the lac operon

So now can you answer the questions below?

1. What are the molecules involved in the lac operon?

2. Describe the steps of gene expression of the lac operon?

3. How does this relate to our question of the term?