The gene regulation mechanisms in eukaryotes can be broadly classified into two groups namely (i) irreversible or long term regulation and (ii) reversible or short term regulation. The reversible type of system is similar to the prokaryote gene regulation systems, which are just like on and off system. They include various mechanisms by which a cell responds to the environmental situations and enzyme activity control systems according to the necessity of the cell. However the irreversible mechanisms include permanent changes that take palace in the cells as a response to the need of the cell. Besides a cell has to undergo cell differentiation as well as determination processes and for such processes during the development of the tissues it is necessary to trigger the activity of certain genes once for all.

Moreover the regulation of the genes in eukaryotes is carried out at different levels and based on that the gene regulation mechanisms can be divided into four categories as:

(i)     Regulation at DNA level:

The mechanisms of regulation of the genes by controlling the transcription at the genome level fall into this category. During the mitotic and the meiotic cell division phases the genome in the cells undergoes different types of arrangement and results in the stalling of transcription. Such situations of genetic phases are often found in the barr body formation during dosage compensation, mitotic phase when chromatin is highly condensed to form chromosomes, mature nucleated erythrocytes of amphibians, stalled transcriptional process in sperm cells etc. As the regulation occurs at the DNA level itself this type of regulation is termed as regulation at DNA level.

(ii)     Regulation at transcription level:

There are different factors that effect the transcription at transcription level. Those factors that regulate the transcription of a gene are termed as gene regulatory molecules. Besides, the gene regulatory molecules are in turn produced by some specific regulatory genes. Those factors are listed below:
 
a.      RNA polymerases: Shortage of these enzymes effects the transcription of the genes.

b.      Endonucleases: They introduce nicks in the DNA strands and thereby effects transcription.

c.      Helicases, topoisomerases and other DNA helix stabilizing proteins: The helicases and topoisomerases are       important factors for stabilizing the DNA during transcription because they induce negative coiling and removes stress during the transcription. If they are present in the concentration enough for the process it will make DNA less available for the transcription.

a.      DNA methylation: the enzyme DNA methylase makes the DNA less available for transcription.

b.       ATP: These molecules are the energy source for the processes like transcription. They may bring down the rate of transcription if they are not adequately supplied.

c.       Ions and small molecules: Ions such as calcium, magnesium and manganese that are essential for the transcription may effect the formation of chromatin, which modulates the gene activity.

Gene battery model of transcription regulation:

This model is proposed by britten and Davidson for the first time in 1969 and it was later modified in 1973. This is also popularly known as britten-davidson model. Besides, this model is similar to the operon model of the prokaryote gene regulation. Gene battery model consists of a set of four genes namely sensor, integrator, receptor and structural or producer genes. The structural gene codes for the enzyme and the receptor gene acts as a operator gene in prokaryotes. The integrator gene codes for the RNA activator protein and this gene is comparable to the regulator gene. The sensor site or gene of the gene cluster in turn acts as the regulator for the transcription of the integrator gene. The sensor site is recognized by the hormones and certain proteins, which might cause the transcription of the regulator gene. Since the clusters of genes are controlled by the sensor like a battery, this model is termed as gene battery model.   

(iii)     Regulation at translation level:

In eukaryotes the m-RNA is not directly translated until it receives a signal and these switch mechanisms at translational level falls into this category. The best example for this is the masked m-RNA or informosomes. The storage of large quantities of m-RNA in the form of masked m-RNA in unfertilized eggs, which will later need translation of this m-RNA in order to provide nourishment of the eggs, is commonly practiced in sea urchins. Such type of mechanism which can trigger the inactive m-RNA into active form and begin translation is termed as translational control mechanisms.

(iv)   Regulation at post translational level:

Some post translational modifications of the proteins bring about the regulation by inhibiting the enzymatic activity and hence this type of regulation is termed as post translational control. For instance, some proteins or enzymes become active only when a subunit or entirely different proteins bind to it. As in the production of heamoglobin, the protein subunits heam and globin forms a complex and at times there may be heamoglobin deficiency in cases of iron-dependent anemic people.