Microbial models the genetics of viruses and bacteria answers

The viral genome duplicates and directs the host's protein synthesis machinery to synthesize capsomeres with free ribosomes and glycoproteins with bound ribosomes.

After the capsid and viral genome self-assemble, they bud from the host cell covered with an envelope derived from the host's plasma membrane, including viral glycoproteins. These enveloped viruses do not necessarily kill the host cell. Some viruses have envelopes that are not derived from plasma membrane. The envelope of the herpesvirus is derived from the nuclear envelope of the host. These double-stranded DNA viruses reproduce within the cell nucleus using viral and cellular enzymes to replicate and transcribe their DNA.

Herpesvirus DNA may become integrated into the cell's genome as a provirus, which remains latent within the nucleus until triggered by physical or emotional stress to leave the genome and initiate active viral production. This complementary strand is the template for the synthesis of additional copies of genome RNA. Retroviruses class VI have the most complicated life cycles. Some viruses damage or kill cells by triggering the release of hydrolytic enzymes from lysosomes.

Some viruses cause the infected cell to produce toxins that lead to disease symptoms. Others have molecular components, such as envelope proteins , that are toxic. In some cases, viral damage is easily repaired respiratory epithelium after a cold , but in others, infection causes permanent damage nerve cells after polio. Many of the temporary symptoms associated with a viral infection results from the body's own efforts at defending itself against infection.

Emergent viruses. Tumor viruses include retrovirus, papilloma virus, adenovirus, and herpesvirus types. Plant viruses Fig Genetic recombination. Plasmids , including the F plasmid , are small, circular, self-replicating DNA molecules. Episomes , are a genetic element that can exist as a plasmid or as part of a chromosome.

Episomes, like the F plasmid, can undergo reversible incorporation into the cell's chromosome. Temperate viruses also qualify as episomes. Plasmid genes are advantageous in stressful conditions. Transduction involves the use of a virus, a bacteriophage, to act as a conduit for shuttling bacteria genes from one cell to another, thus negating the necessity for cell-to-cell contact.

There are two different types of transduction: generalized transduction and specialized transduction. In generalized transduction, a bacterial host cell is infected with either a virulent or a temperate bacteriophage engaging in the lytic cycle of replication.

After the first three steps of replication absorption, penetration, and synthesis , the virus enters into the assembly stage, during which fully formed virions are made. During this stage, random pieces of bacterial DNA are mistakenly packaged into a phage head, resulting in the production of a transducing particle.

While these particles are not capable of infecting a cell in the conventional sense, they can bind to a new bacterial host cell and inject their DNA inside. Specialized transduction can only occur with temperate bacteriophage, since it involves the lysogenic cycle of replication. The bacteriophage randomly attaches to a bacterial host cell, injecting viral DNA inside.

The DNA integrates into the chromosome of the host cell, forming a prophage. At some point induction occurs, where the prophage is excised from the bacterial chrosomsome. In specialized transduction, the excision is incorrectly performed and a portion of bacterial genes immediately adjacent to the viral genes are excised too. Since this DNA is used as the template for the synthesis stage, all copies will be a hybrid of viral and bacterial DNA, and all resulting virions will contain both viral and bacterial DNA.

Once the cell is lysed, the virions are released to infect other bacterial host cells. Each virion will attach to the host cell and inject in the DNA hybrid, which could be incorporated into the host chromosome, if a prophage is formed.

In each of the cases of HGT, the process is only successful if the genes can be expressed by the altered cell. In conjugation, the genes are located on a plasmid, under the control of promoters on the plasmid. In transformation and transduction, where naked DNA is gaining access to the cell, the DNA could easily be broken down by the cell with no genetic expression occurring.

The most common mechanism of molecular recombination is homologous recombination , involving the RecA protein. In this process DNA from two sources are paired, based on similar nucleotide sequence in one area. An endonuclease nicks one strand, allowing RecA to pair up bases from different strands, a process known as strand invasion. Recombination can also occur using site-specific recombination , a process often used by viruses to insert their genome into the chromosome of their host.

This type of recombination is also used by transposable elements see next section. She demonstrated that transposable elements can be responsible for the activation or inactivation of genes within an organism. Transposable elements are relatively simple in structure, designed to move from one location to another within a DNA molecule by a process known as transposition. All transposable elements code for the enzyme transposase, the enzyme responsible allowing transposition to occur, and have short inverted repeats IRs at each end.

The simplest transposable element is an insertion sequence IS , which contains the transposase and IRs of varying lengths.

A transposon typically contains additional genes, with the exact type varying widely from transposon to transposon. A transposon can be removed from one location and relocated to another the cut-and-paste model , a process known as conservative transposition. Alternatively, it can be copied, with the copy being inserted at a second site, in a process known as replicative transposition.

Horizontal Gene Transfer HGT , conjugation, donor, recipient, conjugative plasmid, F-, F, transformation, naked DNA, competence, competent cell, DNA translocase, endonuclease, autolysin, RecA, transduction, generalized transduction, transducing particle, specialized transduction, molecular recombination, homologous recombination, resolvase, site-specific recombination, transposable elements, transposition, transposase, inverted repeats IR , insertion sequence IS , transposon, conservative transposition, replicative transposition.

Conjugation Conjugation is the process by which a donor bacterium transfers a copy of a plasmid to a recipient bacterium, through a pilus. Gene Acquisition via Transformation. Transduction Transduction involves the use of a virus, a bacteriophage, to act as a conduit for shuttling bacteria genes from one cell to another, thus negating the necessity for cell-to-cell contact. Generalized Transduction In generalized transduction, a bacterial host cell is infected with either a virulent or a temperate bacteriophage engaging in the lytic cycle of replication.

Generalized Transduction. Specialized Transduction Specialized transduction can only occur with temperate bacteriophage, since it involves the lysogenic cycle of replication.