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Unit 6

6.5 Regulation of Gene Expression

2 min readnovember 29, 2021


Lani Himegarner

AP Biology 🧬

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Regulatory Sequences

There are sequences of DNA that have control over increasing or decreasing the expression of certain genes in the organism; essentially, they can turn a gene "on" or "off." These sequences are known as “regulatory sequences,” and they interact with proteins to do so. The genes that are expressed determine the phenotypes of the cell and/or organism. There are also different levels of expression that can affect this. 

Epigenetic Changes

DNA can be modified to change what will and will not be expressed as well. These changes are called “epigenetic” changes and are often reversible. Gene regulation occurs in both eukaryotes and prokaryotes, though the groups of genes that are regulated are different.


Prokaryotes utilize operons to regulate gene expression. An operon is a cluster of genes with a single promoter. Lac operons, which are present in E. coli cells, are a good example of inducible operons. Inducible operons are usually off and only turn on when the protein it creates is no longer needed. When the protein is needed again, it turns back off and continues to express the gene.

Image courtesy of LumenLearning.

It is regulated by a lac repressor and a catabolic activator protein (CAP). The lac repressor normally will block transcription, but when it senses the presence of lactose, it stops repressing so that the gene can be expressed. The CAP detects glucose, and activates transcription when glucose is low. Both the CAP and the lac repressor detect indirectly. The presence of cAMP lets the CAP know that glucose levels are low, whereas the lac repressor senses the lactose isomer allolactose. lacZ, lacY, and lacA are the genes that are expressed, and can be used for metabolism.

Repressible Operons

Trp operons are also found in E. Coli, but they are repressible operons. This means they only turn off when needed. When the repressor is bound to tryptophan, it blocks the expression of the operon. It is only turned off when the trp levels are low. When the repressor is not bound, the operon works very similarly to the lac operon described above. There is a promoter, operator, and various genes that are transcribed as a single mRNA.

Image courtesy of LumenLearning.

Check out the AP Bio Unit 6 Replays or watch the 2021 Unit 6 Cram


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