Lambda phage

Lambda phage is a bacteriophage that infects Escherichia coli (E. coli) bacteria. It is a temperate phage, meaning it can reproduce using both the lytic and the lysogenic cycles. The lambda phage has been extensively studied as a model organism in molecular biology and genetics since its discovery in the 1950s. Its genome is a linear, double-stranded DNA molecule, but it circularizes upon infection by joining its cohesive ends (cos sites).

Structure

Lambda phage has an icosahedral head structure, which contains the phage's DNA, and a long, flexible tail, which is used for attachment to the host cell. The head is about 60 nm in diameter, and the tail is approximately 150 nm long. The tail fibers at the end of the tail are responsible for recognizing and binding to specific receptor sites on the outer membrane of the E. coli cell.

Life Cycle

Lambda phage can undergo two different types of life cycles: the lytic cycle and the lysogenic cycle. The choice between these cycles is determined by environmental conditions and the health of the host cell.

Lytic Cycle

In the lytic cycle, the phage injects its DNA into the host cell, takes over the cell's machinery to produce new phage particles, and eventually causes the cell to lyse, releasing new phages. This cycle leads to the death of the host cell.

Lysogenic Cycle

In the lysogenic cycle, the phage DNA integrates into the bacterial chromosome and becomes a prophage. The integrated phage DNA is replicated along with the host cell's DNA and passed on to daughter cells. The prophage can remain dormant for many generations. Under certain conditions, such as stress, the prophage can be induced to exit the bacterial chromosome and enter the lytic cycle.

Genetics

The lambda phage genome is approximately 48.5 kilobases in length and encodes about 60 genes. The genes are organized into early, middle, and late operons based on their expression timing after infection. The regulation of these genes is a well-studied example of a genetic switch, involving both positive and negative control mechanisms.

Applications

Lambda phage has been used extensively in molecular biology research. It has served as a valuable tool for understanding gene regulation and genetic recombination. Additionally, lambda phage vectors have been developed for cloning and manipulating DNA, making it an important tool in genetic engineering and biotechnology.

See Also

• Bacteriophage
• Escherichia coli
• Molecular biology
• Genetic engineering