A metabolic route is regulated. An excess of tryptophan in the route for tryptophan synthesis can both (a) block the activity of the first enzyme in the system (feedback inhibition), a quick reaction, and (b) suppress the expression of the genes encoding all subunits of the enzymes in the pathway, a slower response. TrpE and TrpD genes encode the two subunits of enzyme 1, whereas TrpB and TrpA genes encode the two subunits of enzyme 3.

(Before determining the order in which the genes functioned in the pathway, they were given names.) The sign “ - “represents inhibition.

Each step in the route is catalyzed by a different enzyme, and the five genes that code for the components of these enzymes are found on the same bacterial chromosome.

All five genes, which form a transcription unit, are served by a single promoter. (Recall that a promoter is a place on DNA where RNA polymerase may bind and start transcription; as shown in the image attached above, Figure 17.8.) Thus, transcription produces a single lengthy mRNA molecule that codes for the five polypeptides that comprise the enzymes of the tryptophan pathway (as shown in the image attached above, Figure 18.3a). Because the mRNA is interrupted by start and stop codons that signify where the coding sequence begins and ends, the cell can translate it into five distinct polypeptides.

• The term activator refers to the regulatory protein, called cAMP receptor protein (CRP), which is an activator, a protein that binds to DNA and stimulates transcription of a gene.

An animal virus with an envelope—a membrane outer layer—uses it to infiltrate the host cell.

Viral glycoproteins protrude from the envelope's outer surface and bind to particular receptor molecules on the surface of a host cell. The events in the replicative cycle of an enveloped virus with an RNA genome are depicted.

The protein portions of the envelope glycoproteins are made by ribosomes attached to the host cell's endoplasmic reticulum (ER), and the sugars are added by cellular enzymes in the ER and Golgi apparatus. The resultant viral glycoproteins are delivered to the cell surface, incorporated into the host cell membrane.

Some viruses have envelopes that aren't made of the plasma membrane. Herpesviruses, for example, are briefly coated in membrane generated from the host's nuclear envelope; they subsequently shed this membrane in the cytoplasm and gain a new envelope created from the Golgi apparatus membrane.

These viruses have a double-stranded DNA genome and reproduce within the nucleus of the host cell by employing a mix of viral and cellular enzymes to duplicate and transcribe their DNA. In the case of herpesviruses, copies of viral DNA can persist as mini-chromosomes in the nuclei of certain nerve cells. They stay dormant until some kind of physical or mental stress is activated.

Retroviruses are RNA animal viruses with the most complex replication cycles (class VI). These viruses feature a reverse transcriptase enzyme that transcribes an RNA template into DNA, resulting in an RNA S DNA information flow that is the opposite of the typical direction.

Retroviruses get their name from this unique occurrence (retro means "backward"). HIV (human immunodeficiency virus), the retrovirus shown, is of particular medical relevance since it causes AIDS (acquired immunodeficiency syndrome). HIV and other retroviruses are enclosed viruses that include two identical single-stranded RNA molecules and two reverse transcriptase molecules.

Viruses do not match our concept of a living entity. A virus that has been isolated is physiologically inactive, unable to reproduce its genes or regenerate its own ATP. It does, however, have a genetic design written in the universal language of life.

Viruses and prions are dangerous diseases in both animals and plants. Virus-related symptoms can be produced by either direct viral damage to cells or by the body's immunological response. Vaccines boost the immune system, allowing the host to fight itself against particular pathogens.

An epidemic, or widespread breakout of a disease, has the potential to become a pandemic or worldwide epidemic.

Outbreaks of developing viral illnesses in humans are often triggered by existing viruses that expand their host region. The H1N1 2009 flu virus was a novel mix of viral genes from pigs, humans, and birds that produced a pandemic.

The H5N1 avian flu virus has the potential to produce a flu pandemic with a high death rate. Viruses can also infiltrate plant tissues.

Bacteria use a variety of defense mechanisms against phage infections, including the CRISPR-Cas system.

Many animal viruses have a protective envelope. Retroviruses (such as HIV) employ the reverse transcriptase enzyme to convert their RNA genome into DNA, which may then be incorporated into the host genome as a provirus.

Because viruses can only multiply within cells, they most likely arose after the earliest cells, maybe as packed pieces of cellular nucleic acid.