The *lac* operon generally encodes for protein enzymes that are used to digest lactose. The expression of this operon is regulated by the binding of a *lac* repressor to its operator region.

In the absence of lactose, the *lac* repressor is normally bound to the *lac* operon. This prevents the operon from being expressed as there is no need to produce enzymes to digest lactose.

When lactose is present, the lactose molecules would bind to the *lac* repressor, which causes it to change its conformation. This would then cause the repressor to fall off the *lac* operon, which allows the operon to be expressed and produce enzymes to digest lactose.

Similar to prokaryotes, eukaryotic genes can also be regulated by activator and repressor proteins that bind to specific sequences in the DNA that promotes or inhibits its expression.

One example of a DNA sequence that promotes gene expression is an *enhancer* sequence. The binding of transcription factors also plays a role in the regulation of eukaryotic genes.

Additionally, the regulation of eukaryotic genes is far more complex than that of prokaryotic genes due to post-transcriptional and post-translational modifications such as RNA splicing. These processes can also influence the expression of eukaryotic genes.

Even though every cell contains the same genetic information that is encoded in their DNA, each cell can exhibit different structures and functions due to the process of *cell differentiation*.

The differentiation of cells during development is mediated by a series of genes known as **Hox genes**. These genes control the differentiation of cells and tissues in the embryo.

Furthermore, a mutation in any one of the Hox genes may cause an organism to develop organs at the wrong positions in the body. This may lead to deformities or even death of the organism.

Hox genes

A promoter is the region of DNA that indicates to an enzyme where to bind to make RNA.

4. A promoter is the region of DNA that indicates to an enzyme where to bind to make RNA.

In fruit flies, the hox genes are situated side-by-side in a cluster, while the hox genes found in the mice seem to be homologous to the flies. Their genes are clustered in an order that reflects a similarity from their body’s anterior part all the way to the posterior part. This indicates a similarity in the basic structure of their bodies. However, the difference between the fly and the mouse hox genes can be seen on the banks of the hox genes. In the mouse chromosomes, there are four banks of hox genes, whereas the chromosomes of the fly only contain one bar. This indicates that the chromosomes of the mouse and its morphology are more complex compared to the fly. Another difference is the blue color bar which represents the genes affecting the tail of the mouse. Since flies do not have a tail, their genes represented by the blue color bar looks different to the genes found in the mouse’s chromosomes.