Viruses

Viruses are biological entities that rely on host cells to make more copies of themselves. They do not have cells or their own metabolism, but they can show signs of life when they infect a host. Their basic structure includes a nucleic acid core, made of either DNA or RNA, surrounded by a protein coat called a capsid.

Viruses are classified by the type of nucleic acid they contain and by their structure. Some viruses also have a lipid envelope that comes from the host cell membrane. Enveloped...

Viral genomes can be DNA or RNA, and their structure shapes how they replicate inside host cells. They may be linear or circular. They can also be single-stranded or double-stranded. These differences affect how a virus makes copies of itself and how it interacts with the host.

RNA viruses usually have smaller genomes than DNA viruses. That smaller size is linked to high mutation rates and fast evolution. RNA virus polymerases often lack proofreading, so mistakes are more common. This helps...

T4 phage infection of E. coli follows a lytic cycle that lets the virus make many copies fast. Bacteriophages, or phages, are viruses that infect bacteria, and T-even phages such as T4 are known for this well-studied process. The cycle ends with the destruction of the bacterial host.

The first step is attachment to the E. coli cell surface. Tail fibers on the phage bind to receptor molecules on the bacterial outer membrane, which helps the virus find the correct host. After binding, the...

The lysogenic cycle shows how a bacteriophage can stay inside a bacterial cell without killing it right away. Temperate phages, such as bacteriophage lambda (λ), use this strategy to persist in Escherichia coli while the host cell remains alive.

After attachment to the bacterial surface, the phage injects its genetic material into the cytoplasm. The linear phage genome then circularizes, which helps protect it from host nucleases. Next, phage-encoded integrase carries out recombination that...

DNA bacteriophages are viruses that infect bacteria and use bacterial cell machinery to make more copies of themselves. They carry either single-stranded DNA (ssDNA) or double-stranded DNA (dsDNA), and each group uses a different replication strategy. Their life cycles also shape how they interact with hosts and how scientists use them in biotechnology and medicine.

Some ssDNA phages keep their genomes very small and compact. The icosahedral phage ΦX174 shows this clearly because it uses...

Archaeal viruses infect archaea that live in extreme places such as acidic hot springs and hydrothermal vents. These hosts often belong to the phyla Euryarchaeota and Crenarchaeota. The viruses help shape host evolution and gene transfer, which adds genetic diversity to microbial communities in harsh environments.

Most archaeal viruses carry double-stranded DNA, or dsDNA, genomes. Some have single-stranded DNA, and some have RNA genomes. Their virions, or complete virus particles, show unusual...

RNA viruses use several replication strategies based on their genome type. They are grouped as positive-strand, negative-strand, or double-stranded RNA viruses. Some RNA viruses also use reverse transcription, which adds another layer to how they copy their genetic material.

Positive-strand RNA viruses have genomes that can act like messenger RNA, or mRNA, as soon as they enter a cell. This lets the virus begin making proteins right away. Bacteriophage MS2 infects Escherichia coli by attaching...

CRISPR is an adaptive immune system in bacteria and archaea that helps protect cells from viral infection. It works by identifying foreign genetic material, remembering it, and stopping the same invader later. In many cases, the main targets are bacteriophages, which are viruses that infect bacteria.

The first defense step begins during an initial infection. Cas proteins, which are CRISPR-associated proteins, detect foreign DNA by looking for specific sequences called Protospacer Adjacent...

CRISPR-Cas9 uses a bacterial defense system to change DNA in cells. The CRISPR-Cas system first evolved in prokaryotes to protect bacteria from invading genetic material such as viruses and plasmids. Scientists later adapted it into a genome-editing tool for plants, animals, and humans.

The key enzyme is Cas9, a DNA-cutting protein called an endonuclease from Streptococcus pyogenes. In the lab, a synthetic single guide RNA, or sgRNA, directs Cas9 to a chosen DNA site. The sgRNA matches the...

Viroids, prions, and satellites are subviral agents that can cause infection even though they are not complete viruses. They resemble viruses in spread and disease, but each lacks one or more key viral parts, such as a capsid or the machinery needed for replication.

Viroids are the simplest of these agents. They are circular, single-stranded RNA molecules with no protein coat. Viroids infect plants only and depend on host cell machinery to copy themselves. Host RNA polymerase replicates them...