Viruses are tiny agents that cause infections in a wide range of hosts including animals, plants, bacteria and other viruses. In particular, viruses that infect bacteria are called bacteriophages, bacterio meaning "bacteria" in Greek and phage meaning "to eat". Bacteriophages are able to undergo lytic and lysogenic cycle to replicate; however, most undergo one or the other cycle to replicate. An example of a bacteriophage that is able to undergo both cycles is bacteriophage lambda (phage lambda). Bacteriophage lambda infects only the bacterium Escherichia coli strain k-12. Phage lambda is unique in its ability to turn replication genes on or off depending on the host's condition. When E. coli is infected with phage lambda and the cell dies due to an environmental factor, the phage will switch from the lysogenic to the lytic replication cycle.
Bacteriophage lambda was discovered by Esther Lederberg in 1950 while she was working in a laboratory with E. coli strain k-12. Lederberg is considered a pioneer of bacterial genetics; she was also an immunologist and microbiologist. She flourished academically, receiving a doctorate from the University of Wisconsin where she worked with many other pioneers of microbiology, genetics and immunology, including: Andre Lwoff, Edward Lawrie Tatum, George Wells Beadle, Frances Crick and James Watson. While at the University of Wisconsin, Lederberg was using ultraviolet light on E. coli strain k-12 to mutagenize that specific strain of the bacteria. After prolonged exposure to the ultraviolet light, the bacteria stopped growing and its condition slowly began to deteriorate. An hour and a half after the exposure to the ultraviolet light ceased, the bacteria began to lyse (burst). This led Lederberg to the discovery of bacteriophage lambda. The E. coli sample that Lederberg was using was infected with bacteriophage lambda. The phage was not detected because it was in the lysogenic cycle, which meant that the phage was a prophage, and thus that the phage genome was integrated within the bacterial genome. Bacteriophage lambda sensed that the bacteria was about to die, so it switched its replication genes on and converted to lytic replication, therefore causing the cell to lyse and release the phage into the environment. Lederberg is also accredited with the discovery of induction; the process of when the lysogenic cycle is terminated and the lytic cycle is activated due to adverse conditions caused by ultraviolet light. Lederberg, along with her team of researchers, was awarded the Pasteur award in 1956.
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Viruses have many different anatomical structures depending on what kind of cells they infect. The anatomical feature that is similar throughout all bacteriophage is the capsid. The capsid or head is a shell made out of protein that contains DNA or RNA, depending on the virus. The capsid also contains some internal proteins. The capsid can have many different configurations, from a polygon-shaped sphere, like an icosahedral, or a rod-shaped helix. The main functions of the capsid are that it allows the virion to attach to its host via special sites on the surface, contains the internal proteins that allows the virus to penetrate the host cell membrane, which enables it to inject the infectious DNA or RNA into the host cell's cytoplasm, and that it provides protection for the nucleic acid from the environment and digestion by enzymes. The capsid has structural subunits called capsomers that may contain one or many polypeptide chains. Some viruses have a secondary structure that protects the capsid itself, this is called an envelope. Not all viruses have an envelope; the envelope is made up of glyco-proteins and surrounds the entire capsid for optimum protection. The envelope has two lipid layers intermingled with protein molecules, a lipoprotein bi-layer, and also has a mixture of material that consist of the viral origin and some material from the membrane of the host cell. Besides a capsid, some viruses also contain a tail that is attached to the capsid which helps the virus penetrate the host cell's outer membrane and allows the virus to inject the DNA or RNA into the host cell. The tail consists of two main structures: the tail fibers and a tail sheath. The tail fibers are tiny leg like formations that help the phage attach on to the bacterial cell by clinging on to the surface receptors. The tail sheath is a tube like structure that runs from the capsid to the tail fibers; the tail sheath digs into the cell membrane of the host and the DNA or RNA travels down the sheath and into the cytoplasm of the host and the infectious cycle begins. For viruses without tails, specialized spikes are protruding directly from the capsid that play a similar role to that of tails; the spikes are made up of proteins and help the virus invade the host cell. Bacteriophage lambda has a capsid with an icosahedral configuration that is 55 nanometers in diameter that contains 350-575 capsomers or subunits of 37,000 Daltons; the capsomers are positioned in groups of 5 and 6 subunits or pentamers and hexamers. The tail is 180 micrometers long and contains a single tail fiber that is 25 nanometers long. Bacteriophage lambda does not possess an enveloped capsid.
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Although viruses are not considered living organisms, they do have genetic material that allows them to replicate with the aid of a host. Viruses can have a genome that is made up of either DNA or RNA and the nucleic acid can be single stranded or double stranded. Viruses can either have DNA as their nucleic acid or RNA, they cannot contain both. DNA viruses are commonly double stranded, but they can be single stranded, have a lower rate of mutation, are more stable and the DNA replication takes place in the nucleus of the host. In contrast, RNA viruses are usually single stranded, although some are double stranded, are very susceptible to mutation, and are less stable and the RNA replication takes place in the cytoplasm of the host cell instead of the nucleus. RNA viruses can come in two different varieties, they can either be positive sensed or negative sensed. Positive-sensed RNA viruses are infectious without any need for transcription; negative sensed RNA viruses are not infectious until they undergo transcription which will turn them into infectious positive sensed RNA viruses. The following are examples of viruses with double stranded DNA genome: adenoviruses, herpes simplex viruses, varicella-zoster viruses and bacteriophages T2, T4 and lambda. Bacteriophage Ï†X174 and adeno-associated viruses (AAV) are examples of single stranded DNA viruses. Some positive sensed RNA viruses are: polioviruses, rhinoviruses, corona viruses and tobacco mosaic virus. Negative sensed RNA viruses include: human metapneumovirus, parainfluenza viruses and respiratory syncytial viruses. Viruses also have a great deal of variability when is comes to the number of base pairs a within a genome. A virus can have as little as a couple thousand base pairs to over a million base pairs, as found in Acanthamoeba polyphaga mimivirus. Bacteriophage lambda has a linear, single stranded DNA composition that is housed within the icosahedral capsid. The genome of the phage contains 48,490 base pairs that make up the two strands of the cos site.
The mode of infection of bacteriophage lambda is very similar to other viruses but there are some differences as to which receptor the phage attaches to the host cell. The journey of the phages genome, from the phage itself to the genes being integrated within the host cells' chromosome, can be characterized in the following steps:
The tail fiber of bacteriophage lambda attaches to the E. coli receptor that is specifically meant for the sugar, maltose. E. coli has a gene product, called lam B, which is a maltose operon protein molecule. A protein, J protein, on bacteriophage lambda's tail fiber is able to intermingle with the lam B gene of the host which enables the phage to safely attach on to the host cell membrane. Since the phage attaches to a maltose receptor, the host does not see the phage as a threat but believes that the phage is just another sugar entering the membrane.
After the phage has attached to the host, the phage genome is injected into the outer membrane of E. coli; the phage genome then travels within sugar transport pathway, which that allow it to enter the inner membrane of the host cell.
Once the phage genome has entered the cytoplasm of the host cell, the phage genome converts from a linear configuration to a circular configuration by connecting the sticky ends of its genome, which are guanine and cytosine rich. The circular configuration protects the phage genome from being degraded or destroyed by nuclease enzymes from the host cell.
After the circular chromosome of the phage reaches the nucleus, the genome is unwound using helicase, where negative super-coils are integrated and the phage chromosome begins to unravel. The host gyrase relieves any strain that is caused by the unraveling of the phage chromosome. The unwound, linear phage chromosome is integrated within the host genome and phage replication begins.
After the phage genetic material has been injected into the cell, the viral genome travels to the nucleus to be replicated via lytic or lysogenic cycles. In the lysogenic cycle, bacteriophage lambda genome is integrated into the host cell's genome by an attachment site called attÎ». . AttP is a gene sequence that is found on the phage genome and the sequence on the host is attB. Using Holliday junction, the two sequences are swapped with the help of host cell's IHF protein and the phage Int protein. The two proteins form an intasome when they bind to attP; intasome is a recombination of the two genomes. The phage has now successfully incorporated its genome into the host's genome, allowing the phage genetic material to be replicated along with they host's genome, without the host realizing that it has been infected. The integrated phage is now referred to as a prophage; prophage is in a mutual relationship with the host, the phage is being replicated without spending any of its only energy and the host is immune to another infection from a similar bacteriophage. The phage will continue to replicate until induction causes it to convert to the lytic cycle. In the lytic cycle, the phage genome replaces the host genome, so only the phage genome is being replicated. Replication of genes in the lytic cycle is accomplished in two stages. In the first stage or early gene replication, transcription and translation of the phage DNA occurs and key enzymes, like helicase, primase and polymerase, are replicated. In the second stage or late replication, genes for the capsid and tail are replicated. After gene replication has been completed, taking about 60 minutes to complete, the early genes are taken up by the new capsid and the host cell beings to burst and release the progeny. Most viruses will either enter the lysogenic cycle and then the lytic cycle or they will enter the lytic cycle directly. What makes bacteriophage lambda so unique is its ability to decide whether the lytic cycle or lysogenic cycle is more energy efficient depending on the host's condition. If the host is nutritionally sound and has high protease activity, the phage will opt to select the lytic cycle, as it requires more energy from the host and yields progeny faster. On the other hand, if the host has lowered protease activity as well as depleted nutrition, the phage will go into the lysogenic cycle and replicate until the host is no longer able to support the phage.
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Bacteriophage lambda is detected in a similar manner as other phages, by the formation of plagues on a lawn of bacterial growth. The plagues will only form on a lawn of E. coli strain k-12, as that is the only strain that bacteriophage lambda is able to infect. Since it has the ability to select which replication cycle that will yield the most progeny, thy phage is an excellent cloning vector, which enables the phage to be grown in a test tube. Another reason that lambda phage makes a good cloning vector is that it has a large DNA sequence, which allows larger foreign DNA to be inserted into the phage genome.
Bacteriophage lambda was discovered unintentionally by Esther Lederberg. The phage has a single tail fiber and has an icosahedral capsid. It infects E. coli strain k-12 by binding to the maltose receptor. Once the phage has entered the nucleus of the host, it will determine whether the lytic or lysogenic replication cycle should be used depending on the nutrition value of the host. This ability makes bacteriophage lambda very dissimilar to other phages and it also makes the phage a superior model as a cloning vector.