What Is an Add-On Factor?

An episome is an extrachromosomal genetic material that exists in the cytoplasm. Those who can be matched with chromosomes are called episomes, and those who cannot be matched are then called cytoplasms. These are all drug-resistant genetic materials and are important factors for the transfer of bacterial resistance. Episomes exist in cells in a free state, and may also be genetic material that is in a state of binding to chromosomes. The presence of episomes brings certain genetic traits to cells, but their deletion does not cause cell death. Examples are plasmids and mild phages.

Addenda are also called "free bodies". A group found in microorganisms such as bacteria

Well-transmitted plasmids, such as

F Episomal F factor

F factor: Donor cells contain a fertility factor called F factor. It plays an important role in the binding of bacteria.

Bacterial conjugation was first discovered in E. coli and later observed in other bacteria, mainly found in Gram-negative bacteria. Under the electron microscope, it can be observed that the bacteria are joined by the elongated pili. Whether bacteria can be used as gene transfer donors in conjugation depends on Fertility factor, also known as F factor. This is the first plasmid to be discovered. The F-factor encodes the production of pilus on the surface of bacteria. The characteristic of F factor is that it can promote the transfer of chromosomal DNA or plasmid from donor bacteria to recipient bacteria. The F factor determines that the encoded sexual pili can form a communication connection structure between the donor and the recipient bacteria, so that an intracellular cytoplasmic bridge can be formed between the two hybrid bacteria. F-factor can exist freely in the cytoplasm or integrate with bacterial chromosomes. If factor F is freely present in the cytoplasm, only F-factor DNA can enter the recipient bacteria through the cytoplasmic bridge during conjugation. However, the characteristic of factor F transfer is that from one starting point, only one DNA strand enters the recipient bacterium. In the future, the donor and the recipient bacterium use one DNA strand as a template and use the rolling circle to copy another complementary strand to form Complete double-stranded F-factor. This characteristic makes the F factor spread in the bacterial population like other bacterial plasmids that can be transmitted through conjugation, and similarly causes infection, that is, the original F + bacteria are still F +, and the F-receptor bacteria can become F + bacteria ^. .

Body-temperate phage

Mild phages are lysogenic phages: phages, such as E. coli phages, that do not proliferate and cause lysis of host cells after they adsorb and invade cells. After the phage infects the cell, it integrates (attaches) its nucleic acid to the host's nuclear DNA, and can replicate synchronously with the replication of the host DNA. Under normal circumstances, it does not cause host cell lysis. At this time, no morphologically visible phage was found in the cell. This phage was called a mild phage. Bacteria containing mild phage are called lysogenic bacteria ^[3] .

Episomal plasmid

A plasmid is a small DNA molecule (that is, a cell's epiphyllum and a cell's epithelium) originally attached to the non-cellular chromosomal or nuclear DNA of a cell and capable of autonomous replication. Although most of the plasmids have a circular configuration, they exist in many bacteria and yeasts, and even in chloroplasts and mitochondria of plants. However, in 1984, linear plasmids were found in actinomycetes such as Streptomyces coelicoler and prokaryotes such as Borrelia hermsii .

The DNA of natural plasmids ranges from thousands of base pairs to hundreds of thousands of base pairs. Plasmids naturally exist in these organisms. Sometimes a cell can have one or even several kinds of plasmids at the same time. The number of copies of a plasmid can vary from single to thousands in a cell. Sometimes some plasmids contain a drug resistance gene (for example, E. coli has a plasmid containing a tetracycline resistance gene). Some plasmids carry genes that can give cells extra physiological metabolism and even increase their pathogenicity in some bacteria. In general, the presence or absence of a plasmid has no decisive effect on host cell survival. ^[4] It is the most common carrier for genetic engineering.