Microbiology: Microbrial Gene Transfer

Specialized Transduction:

Similiar to generalized transduction, but only certain bacterial genes are transferred.

Generalized transduction, Step 4:

When the released phage particles infect a new population of bacteria at a later time, the bacterial genes will be transferred to to the newly infected recipient cells.

Two ways that conjugation differs from transformation

1.) Conjugation requires cell-to-cell contact 2.) Conjugation cells must be of opposite mating types; donors carry plasmid and recipients do not.

The purpose of microbial genetic material exchange

Allows for the sharing of genetic information that codes for proteins that may be needed for survival such as antibiotic resistance, exotoxins, enzymes, and other virulence factors.

Conjugation

Also called "bacterial sex." DNA is transferred DIRECTLY by cell-to-cell contact. The result is an extremely efficient exchange of genetic material. This exchange can occur between related or unrelated bacteria, it does not require competence. It is the number one mechanism for transfer of antibiotic resistance.

Transformation

Genes are transferred from one bacterium to another as "naked" DNA in solution. Works best when donor and recipient are closely related. a.) When recipient cell is in a physiological state that it can take up the donor DNA, it is said to be COMPETENT. COMPETENCE results from alterations in the cell wall that make it permeable to large DNA molecules. b.) Naked DNA fragments from one bacterium are released when the less lyses and these DNA fragments bind to the cell wall of another bacterium. c.) Ultimately, these fragments combine with the recipient bacterium's genome. (See Frederick Fiffith c. 1928)

Plasmid

In conjugation, a circular piece of DNA that replicated independently from the cell's chromosome. The genes a plasmid carries are not essential to cell growth.

Hfr cell

In conjugation, if a plasmid integrates into the recipient cell chromosome it is then called an Hfr cell (high frequency of recombination cell). If an Hfr cell passes a portion its chromosome onto an F-cel, an F-cell results. This is because the recipient cell may have received any piece of DNA , and not necessarily the piece of interest, the portion that conveys antibiotic resistance.

F+

In conjugation, the donor cell/fertility factor who carries the plasmid

F-

In conjugation, the recipient cell (who does not have the plasmid).

Sex Pilis

In conjugation, these are projections from the donor's cell surface that contact the recipient and help bring the two cells into direct contact; in gram-negative bacteria, the plasmid carries the genes that code for these (gram positive cells produce "sticky surface molecules" that pulls cells into contact with each other).

Generalized transduction, Step 1:

Infection by the virus. The phage attaches to the donor bacterial cell wall and inejcts its DNA into the bacterium.

Vertical Gene Transfer

Occurs when genes are passed from an organism to its offspring. What most eukaryotes do; bacteria are capable, but engage in horizontal gene transfer

Generalized transduction, Step 3:

Some pieces of the bacterial DNA are mistakenly packaged inside phage protein coats. The resulting phage particles now carry bacterial DNA instead of phage DNA.

Generalized transduction, Step 2:

The phage DNA acts as a template for the synthesis of new phage DNA and also directs the synthesis of phage protein coat. The bacterial coat is broken apart by phage enzymes.

Generalized transduction, Step 5:

Transduction can lead to recombination between DNA of the donor host cell and the DNA of the recipient host cell.

Types of horizontal gene sharing

Transformation, conjugation, transduction, transposition

Transposition:

Transposons (small segments of DNA that can move from one region of DNA molecule to another). Transposons can insert themselves into phages, plasmids, and bacterial chromosomes. They do not replicate independently but are copied during their host's DNA transcription. When they leave their host, there is a very good change that it is carrying new DNA away to another site. The importance of transposons is that they can carry genes for drug resistance and virulence factors.

(Generalized) Transduction

bacterial DNA is transferred from a donor cell to a recipient cell inside a virus that infects bacteria, called a bacteriophage, or phage. All genes contained within a bacterium infected by a generalized transducing phage are equally likely to be packaged in a phage coat and transferred.

Horizontal Gene Transfer

occurs when genes are passed from an organism of one generation to an organism of the same generation. This usually involves a DONOR cell who gives a portion of its DNA, and a RECIPIENT cell who accepts the DNA and incorporates it into its genome.

Genetic recombination

refers to the exchange of genes between two DNA molecules to form new combinations of genes on a chromosome.