Viral Replication

Outside the host cell, a virus is a lifeless particle with no control over its movements. It is spread by air, in water, in food, or by body fluids. Viruses infect both prokaryotes and eukaryotes.

Viruses first need to recognize a host cell before they can infect it. An enveloped virus can do so by a "lock-and-key" fit between certain envelope proteins and specific receptor molecules on the host cell. A viral infection begins when a virus enters the host cell. The viral genome takes over the metabolic machinery of the cell and makes new viruses. Viruses are obligate intracellular parasites—they replicate only by using host cell enzymes and organelles to make more viruses. DNA and RNA viruses differ in the way they replicate.

Replication in DNA Viruses

When the DNA of some DNA viruses enters a host cell, it makes mRNA, which is the template for making proteins during protein synthesis. The DNA of other DNA viruses inserts into the host cell's chromosome. This inserted viral DNA is known as a provirus. The host cell's enzymes then transcribe the provirus into mRNA and translate this RNA into viral proteins. DNA viruses also use the host cell's enzymes to make new viral DNA. The replicated viral DNA and proteins assemble to make new viral particles.

Replication in RNA Viruses

The genome of some RNA viruses enters host cells and serves directly as mRNA, which is translated into new viral proteins immediately after infection. The genome of other RNA viruses is first transcribed and thus serves as both a template for the synthesis of mRNA and as a template for the synthesis of more copies of the viral genome.

Word Roots and Origins retrovirus from the first two letters of the two words reverse transcriptase combined with the word virus

Viral nucleic acid

figure 24-3

This TEM and diagram show the structural complexity of a T phage. Scientists named the seven T phages— T1, T2, T3, T4, T5, T6, and T7—to match the order in which they were discovered. (TEM 138,600x)

figure 24-3

This TEM and diagram show the structural complexity of a T phage. Scientists named the seven T phages— T1, T2, T3, T4, T5, T6, and T7—to match the order in which they were discovered. (TEM 138,600x)

Word Roots and Origins lysis from the Greek lysis, meaning "loosening" or "dissolving"

Some RNA viruses, called retroviruses (RE-troh-viE-ruhs-uhz), contain the enzyme reverse transcriptase (tran-SKRIP-tays) in addition to RNA. Reverse transcriptase uses RNA as a template to make DNA, which then inserts into the host cell's genome. Reverse transcriptase reverses the normal process of transcription, in which DNA serves as a template for producing RNA. The host cell's enzymes transcribe the virus DNA, and cell ribosomes translate the RNA into proteins that become part of the new viruses. Human immunodeficiency virus (HIV) is a retrovirus.

Replication in Viruses That Infect Prokaryotes

Scientists have gained a better understanding of virus replication by studying bacteriophages (bak-TIR-ee-uh-FAYJ-uhz), viruses that infect bacteria. Bacteriophages, or phages, have complex capsids, shown in Figure 24-3. Phage capsids are made up of a hexagonal head filled with DNA. Attached to the head is a protein tail with one or more tail fibers. The tail fibers attach the virus to a cell. The tail helps the virus inject its genome into the host cell. The most commonly studied bacteriophages, T phages, infect Escherichia coli, a bacterium found in the digestive tract of many animals and humans. The TEM in Figure 24-3 shows an E. coli cell infected with many T phages. Research led to the discovery that many phages and other viruses can reproduce by one or both of two different processes: the lytic cycle or the lysogenic cycle.

Lytic Cycle

During the lytic cycle, a virus, such as a T4 phage, invades a host cell, produces new viruses, and ruptures (lyses) the host cell when releasing newly formed viruses. Viruses that reproduce only by the lytic cycle are called virulent. T phages are virulent viruses. Virulent viruses destroy the cells that they infect.

During the lytic cycle, a phage first attaches its tail fibers to specific receptor molecules on the cell surface of a susceptible bacterium, as shown in step Q of Figure 24-4. Recall that viruses cannot efficiently infect cells that do not have the specific protein receptors for that virus. The phage then injects its DNA into the cell but leaves its protein-containing head and tail outside the host cell. In step ©, the ends of the viral DNA attach to each other, forming a circle. The viral DNA remains separate from the host cell's DNA. In step ©, virulent viruses continue the lytic cycle. The viral DNA takes control of the host's protein-synthesis pathway, and the viral genome is copied. Enzymes transcribe mRNA from the viral DNA. Host ribosomes translate the mRNA into viral proteins, and enzymes replicate the viral DNA. In step Q, head proteins bind to the newly made phage genomes. Heads containing DNA bind to tails, and tails assemble into tail fibers. Finally, the phage enzyme called lysozyme digests the cell wall, and up to 200 new phage particles burst from the bacterial cell in a process called lysis (LIE-sis), shown in step ©.

Lysogenic Cycle

A lytic cycle directly bursts an infected cell, but an infection cycle called a lysogenic (LlE-soh-JEN-ik) cycle allows viruses to hide in their host cell for days, months, or years. A virus whose replication includes the lysogenic cycle is called a temperate virus.

As shown in step Q of Figure 24-4, temperate phages, such as phage lambda (X), enter bacteria in the same way that virulent phages do. In a lysogenic cycle, however, the phage DNA that enters the bacterial cell integrates into the host cell's chromosome, shown in step ©. Phage DNA that is integrated into a specific site of the host cell's chromosome is called a prophage (PROH-fayj). In step Q, the prophage is replicated when the host bacterium replicates its own DNA. Each daughter cell is therefore infected with a prophage. In this way, a single infected cell can give rise to a large population of infected cells. In step Q, the prophage can exit the bacterial chromosome and enter the lytic cycle. Radiation or certain chemicals can cause the phage DNA to leave the bacterial chromosome. Phage particles are replicated and assembled and are released as the host cell lyses.

figure 24-4

After entering the host cell, the DNA of a temperate virus can immediately start the production of new viruses in the lytic cycle. Alternatively, it can insert itself into the bacterial DNA in the lysogenic cycle. During lysogenic growth, the prophage does not harm the host cell.

Phage X

Bacterial chromosome

O In the lytic cycle, new viruses are made.

@ The cell lyses, which releases the viruses.

Phage X

Bacterial chromosome

@ The cell lyses, which releases the viruses.

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