Bacteriophage Lambda Lysogenic Cycle Biology Essay

Bacteriophage Lambda Lysogenic Cycle Biology Essay 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 â€Å"bacteria† in Greek and phage meaning â€Å"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. c oli 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. 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 me mbrane 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 n anometers 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.