Question

6. In class we discussed the lytic and lysogenic cycles in phage. Draw out the cycles. What are some of the implications of lysogeny to human health?

Answer 1

LYSOGENIC CYCLE

1. The prokaryotic cell is shown with its DNA which is shown in green.

2. The bacteriophage attaches and releases its DNA, shown in red, into the prokaryotic cell.

3. The phage DNA then moves through the cell to the host’s DNA.

4. The phage DNA integrates itself into the host cell's DNA, creating prophage.

5. The prophage then remains dormant until the host cell divides.

6. After the host cell has duplicated, the phage DNA in the daughter cells activate, and the phage DNA begins to express itself. Some of the cells containing the prophage go on to create new phages which will move on to infect other cells.

LYTIC CYCLE

In the lytic cycle, the viral DNA exists as a separate free-floating molecule within the bacterial cell, and replicates separately from the host bacterial DNA, whereas in the lysogenic cycle, the viral DNA is located within the host DNA. This is the key difference between the lytic and lysogenic (bacteria)phage cycles. However, in both cases, the virus/phage replicates using the host DNA machinery.

The lytic cycle, which is also commonly referred to as the "reproductive cycle" of the bacteriophage, is a six-stage cycle. The six stages are: attachment, penetration, transcription, biosynthesis, maturation, and lysis.

1. Attachment – the phage attaches itself to the surface of the host cell in order to inject its DNA into the cell
2. Penetration – the phage injects its DNA into the host cell by penetrating through the cell membrane
3. Transcription – the host cell's DNA is degraded and the cell's metabolism is directed to initiate phage biosynthesis
4. Biosynthesis – the phage DNA replicates inside the cell, synthesizing new phage DNA and proteins
5. Maturation – the replicated material assembles into fully formed viral phages (each made up of a head, a tail and tail fibers)
6. Lysis – the newly formed phages are released from the infected cell (which is itself destroyed in the process) to seek out new host cells to infect

떼 LYTIC CYCLE LYSOGENIC CYCLE

IMPLICATION OF LYSOGENY IN HUMAN HEALTH

The gut microbiota consists of various microorganisms including bacteria (bacterial microbiota), bacteriophages and eukaryotic viruses (virobiota) and other microorganisms (fungi and archaea). Bacteriophages—bacterial viruses that attack specific bacteria—have been found in both animal and human organisms (oral, gastrointestinal and respiratory tracts, urine and serum) . Phages constitute an alternative to the use of antibiotics and are particularly applicable in the treatment of antibiotic-resistant bacterial infections . Bacteriophages replicate by two alternative cycles: lytic and lysogenic life cycles . In the lytic cycle phages infect bacteria leading to the production of new phage particles and degradation (lysis) of bacteria. A single phage particle adsorbs to a bacterial cell, then multiplies within. Progeny phages lyse the bacterial cell and release approximately 50–100 phage particles in one growth cycle . During the lysogenic cycle the genetic material of the phage may integrate into the bacterial genome, a process which does not lead to lysis of the bacteria . In recent years there has been observed growing interest in the human gut microbiota and its effects on health and disease of humans . It is suggested that gut microbiota may have an influence on the development of metabolic inflammation in obesity and diabetes . Detection of phages may be performed by traditional bacterial lysis methods . Bacteriophage abundance, diversity and morphology are investigated in transmission electron microscopy . In recent years, researchers have begun to characterize the diversity of viruses using metagenomic methods . A huge number of phages in the intestine have been investigated using electron microscopy, and their diversity has been investigated in metagenomic studies .