Microbiology and Immunology

Genetic regulation of prokaryotic cells WORLD OF MICROBIOLOGY AND IMMUNOLOGY

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The sister-chromatids are separated and joined to different

centromeres, while the microtubules forming the spindle are

attached to a region of the centromere termed kinetochore.

During anaphase there are spindles, running from each oppo-

site kinetochore, that pull each set of chromosomes to their

respective cell poles, thus ensuring that in the following phase

each new cell will ultimately receive an equal division of chro-

mosomes. During telophase, kinetochores and spindles disin-

tegrate, the reorganization of nucleus begins, chromatin

becomes less condensed, and the nucleus membrane start

forming again around each set of chromosomes. The

cytoskeleton is reorganized and the somatic cell has now dou-

bled its volume and presents two organized nucleus.

Cytokinesis usually begins during telophase, and is the

process of cytoplasmatic division. This process of division

varies among species but in somatic cells, it occurs through

the equal division of the cytoplasmatic content, with the

plasma membrane forming inwardly a deep cleft that ulti-

mately divides the parental cell in two new daughter cells.

The identification and detailed understanding of the

many molecules involved in the cell cycle controls and intra-

cellular signal transductionis presently under investigation by

several research groups around the world. This knowledge is

crucial to the development of new anti-cancer drugs as well as

to new treatments for other genetic diseases, in which a gene

over expression or deregulation may be causing either a

chronic or an acute disease, or the impairment of a vital organ

function. Scientists predict that the next two decades will be

dedicated to the identification of gene products and their

respective function in the cellular microenvironment. This

new field of research is termed proteomics.

See also Cell cycle (eukaryotic), genetic regulation of;

Genetic code; Genetic identification of microorganisms;

Genetic mapping

GENETIC REGULATION OF PROKARYOTIC

CELLSGenetic regulation of prokaryotic cells

The ability of a bacterium to regulate the expression of the

myriad of genes contained in the chromosome and plasmidsis

essential to the growth and survival of the microorganism.

While a bacterium has many genes that code for a variety of

proteins, these genes are not all expressed at the same time.

Some genes are active all the time, while others are active only

at specific times in the growth cycle of the bacterium or in

response to a certain environmental condition. The amounts of

proteins that are produced are not all the same. Moreover, the

triggers that stimulate the expression of one genecan be quite

different from the triggers for another gene. The ability of a

prokaryotic cells (bacterial are the prototypical system) to

orchestrate the expression of the repertoire of genes consti-

tutes genetic regulation.

The activity of genes in the manufacture of compounds

by the bacterium, such as in the biosynthetic pathways of the

microbe, is often under a type of control known as feedback

inhibition. In this type of genetic regulation, the object of the

regulation is the first enzyme that is unique to the pathway

(not to the gene that coeds for the enzyme). In biosynthetic

pathways, there are typically a number of compounds that can

be formed in the various enzymatic reactions within the path-

way. Feedback inhibition occurs when the final product

inhibits the first biosynthetic enzyme. Blocking the first enzy-

matic step prevent the remainder of the enzymesfrom having

any material on which to act.

Feedback inhibition is possible because the biosynthetic

enzymes have two binding regions. If both sites are occupied

by the end product, the three-dimensional structure of the

enzyme is changed such that it cannot bind any more of the

protein it is supposed to enzymatically alter. But, when the

amount of the end product decreases, one of the enzyme’s

binding sites is no longer occupied, and the enzyme can

resume its function. The function of the enzyme can also be

affected by modifying the structure of side groups that pro-

trude from the enzyme molecule. This alteration is also

reversible, when the concentration of the blocking end product

is lowered.

Feedback inhibition is a genetic regulatory mechanism

that allows a bacterium to rapidly respond to changes in con-

centration of a particular compound. The bacterium does not

have to manufacture protein, as the molecules already exist and

are primed to resume activity once conditions are favorable.

Regulation also operates directly at the level of the

genes. This type of genetic regulation is called induction

(when the gene is stimulated into action) or repression (when

the gene’s activity is reversible silenced). Regulating the activ-

ity of genes, rather than the activity of the proteins made by

the genes can save a bacterium the energy of manufacturing

the protein.

Induction and repression depend on the binding of a

molecule known as RNApolymerase to regions that signal the

beginning of a stretch of DNAthat code for proteins. The three-

dimensional shape of the polymerase-binding region influ-

ences the binding of the RNA polymerase. The binding of

molecules called effectors can in turn influence the shape of

this region. If an effector alters the shape of the polymerase-

binding region so that the polymerase is able to bind, the effect

is called induction. If the effector binding prevents the poly-

merase from binding, then the effect is known as repression.

Induction and repression tend to cycle back and forth, in

response to the level of effector, and so in response to what-

ever environmental or other condition the particular effector is

sensitive to. A visual analogy would the turning on and off of

room light under the control of a very sensitive light meter, as

clouds obscured the sunlight from one moment to the next.

Another genetic regulatory mechanism that operates

only in procaryotes is termed attenuation. Attenuation requires

a close coupling between the synthesis of ribonucleic acid

from a DNA template (transcription) and the use of the RNA

as another template to manufacture protein (translation).

These processes are very closely coupled in procaryotes, par-

ticularly in the activity of enzymes that participate in the mak-

ing of amino acids.

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