RECOMBINATION

Recombination refers to the exchange of genes between 2 DNA molecules or between different parts of the same molecule. This results in new combinations of genes or genetic information on the chromosomes involved.

There are four ways in which recombination occurs in bacteria:

• 1. TRANSFORMATION - the transfer of "naked" DNA
• 2. CONJUGATION - the passage of plasmids thru direct physical contact between two bacteria
• 3. TRANSDUCTION- the transfer of genetic information thru virus (phage).
• 4. TRANSPOSITION- the movement of small pieces of DNA called transposons to different locations in the genome and between plasmids. These are sometimes called "jumping genes."

TRANSFORMATION - (See fig. 8.23 and 8.24, pgs. 213-214).

This was originally observed by Griffith in 1928 and the mechanism was worked out in experiments by Avery, MacLeod and McCarty in 1944.

These experiments showed that

• 1.) Genetic material can be transferred from dead cells to live cells.
• 2.) Genetic material is DNA

The experiments used Streptococcus pneumoniae. This organism exists in two forms:

• 1.) Smooth -- with a capsule (encapsulated) -- virulent
• 2.) Rough -- no capsule (unencapsulated) -- avirulent (non-virulent).

The experiment and results went as follows:

Bacteria injected into Mice	Mouse outcome	Bacteria isolated from Mice
Live Rough strain	Mice survive	a few Rough
Live Smooth	Mice die	Smooth
Dead Smooth	Mice survive	None
Dead Smooth with Live Rough	Mice die	both Smooth and Rough

Not all bacteria are as capable of Transformation. In the lab cells can be made competent by treatment with CaCl2. Gene guns have also been developed to shoot or inject DNA into cells. Electroporation involves treating cells with an electric field which helps them to take up DNA.

CONJUGATION (Fig. 8.26, pg. 215) -- The transfer of genetic information through direct Cell-to-Cell contact. This process is controlled by genes found on plasmids and "sex pili."

Define Plasmids -- extrachromosomal, circular pieces of DNA which can replicate independently

A plasmid called the F factor or the fertility factor is an example of how conjugation works:

E. coli mating types	Characteristics
F+	This is the donor cell. It has the F factor and can make the sex pili
F-	This is the recipient cell. It will become F+ if it gets an F factor from a F+ cell.
Hfr	This is also a donor cell. In this case the F factor has become integrated into the bacterial chromosome. When an Hfr cell mates it passes its chromosomal genes to the recipient cell.

click here to watch conjugation

There are many types of plasmids that can be passed through conjugation:

• Resistance plasmids (R factors) -- these plasmids contain genes for antibiotic resistance.
• Col plasmids -- these plasmids carry genes for bacteriocins, bacterial proteins which destroy other bacteria.
• Virulence plasmids -- these plasmids carry genes for toxins, adhesins, and other traits that allow the bacteria to establish infection and cause disease. An example would be the enterotoxin produced by enterotoxigenic E. coli (ETEC) the cause of traveller's diarrhea and infantile diarrhea.
• Metabolic plasmids -- some plasmids carry genes for unique metabolic properties of some bacteria. For instance nitrogen fixation by Rhizobium is a trait governed by a plasmid.

TRANSDUCTION

In transduction, bacterial DNA is transferred between bacteria through bacterial viruses. (See fig. 8.27 on page 216).

First a little Virology:

See pg. 333, Table 13.1 - A Comparison of Viruses to Bacteria.

See pg. 334, Fig. 13.1 - Comparison of Viruses to Bacteria and Human Cells.

Virus Structure: (See pictures on pgs. 335,336 and 337).

Note the genome, the capsid, the capsomeres, the envelope, the spikes or peplomeres. Nonenveloped viruses are often called naked viruses. Bacterial viruses are also called phage or bacteriophage. (See the example of the T-even phage on page 337.)

Virus Life Cycles:

Viruses are obligate intracellular parasites. There are viruses which infect bacterial cells as well as plant cells and animal cells. Each virus species however does have a very specific host range -- there are phage which are specific for E. coli or Staph. aureus as well as for particular strains of E. coli or Staph. aureus. Also, many human viruses are very specific and selective about which human tissues they infect -- for instance HIV infects human CD4+ cells (primarily helper T-cells) and hepatitis B virus infects hepatocytes.

There are two fundamental types of viral life cycles:

• 1.) The Lytic cycle (lytic viruses)
• 2.) The Lysogenic or temperate cycle (lysogenic viruses)

The Lytic Virus Life Cycle (See pg. 344, Fig. 13.14).

• a.) Attachment - involves binding of a receptor on the virus to a complementary receptor on the host cell
• b.) Penetration - by variety of means (it differs from virus species to virus species and from bacteriophages to animal viruses) the viral nucleic acid gets into the host cell.
• c.) Biosynthesis - this involves (1) multiple replication of the viral genome and (2) transcription and translation of viral genes, ie., the making of viral components.
• d.) Maturation - Individual viral genomes get packaged into capsids, ie., virus particles are assembled.
• e.) Release or Lysis - The host cells bursts and releases the new virus particles.

 

Since the host cell genome may get chopped up and fragmented during this cycle, some of the host DNA may get packaged in some of the viral capsids. (See fig. 8.27, pg. 216). Thus bacterial genes can be moved around between bacteria via such "faulty" viruses. This is called transduction or generalized transduction.

 

The Lysogenic Viral Life Cycle (See pg. 346, Fig. 13.16)

These viruses may be (1) lytic or they (2) may infect the cell in a dormant manner. By dormant we understand that the viral genome recombines with the host genome. We say that the viral genome becomes integrated into the host genome. When integrated the viral genome is called a prophage or a provirus.

 

When in this state the virus is not actively replicating but the viral genome is replicated along with the cells chromosome. Thus an infected bacterial cell will give rise to a population of latently infected cells.

 

If the dormantly infected cell is "stressed" the prophage will pop out and a lytic cycle will begin -- leading to replication of the virus and lysis of the host cell.

 

Lysogenized bacteria may have new characteristics:

 

• 1.) Only Corynebacterium diptheriae cells lysogenized with phage can make diptheria toxin.
• 2.) The erythrogenic toxin made by Streptococcus pyogenes is only made by strains which are themselves infected with the lysogenic phage carrying the gene for the toxin.
• 3.) Similarly, it is a temperate phage that carries the gene for botulism toxin. Only Clostridium botulinum lysogenized with this phage will synthesize the toxin.

Sometimes when a prophage "pops out" of the host genome it may pull out some host genes with it. These will become part of the viral genome and get replicated with it and packaged with it. Thus lysogeny may result in the transfer of bacterial genes along with the viral genome to new host cells. This is called specialized transduction. (See pg 347, fig. 13.17).

Animal Viruses

Animal virus life cycles are very similar to the bacterial ones described above.

Many animal viruses which cause acute diseases with rapid onset, usually with dramatic symptoms and quick resolution (either for the good or the harm of the patient) are lytic viruses. Influenza virus and Ebola virus are two examples.

Many other animal viruses cause latent, and also persistant and chronic infections. In latent infections the virus is invisible -- no virus is made and infected cells look healthy. Persistently or chronically infected cells produce virus particles at a low rate and such cells may live for many years. These infections are very similar to the lysogenic infections discussed above.

Examples of these lysogenic animal virus infections include:

• The Herpes viruses
• Papilloma virus
• The Retroviruses

Herpes

• Herpes simplex type 1 -- HHV 1 -- cold sores,
  fever blisters
• Herpes simplex type 2 -- HHV 2 -- genital herpes
• Varicella-Zoster virus -- HHV 3 -- chicken pox, shingles
• Epstein-Barr virus -- HHV 4 -- mononucleosis, Burkitt's Lymphoma

  nasopharyngeal carcinoma

• Cytomegalovirus (CMV)- HHV 5 -- mononucleosis

  (See pg. 348, fig. 13.18; pg. 524, fig. 21.9)

Papilloma

• Human Papilloma Virus- HPV ---- warts, cervical and perianal cancer

Retrovirus (have reverse transcriptase)

• Human immunodeficiency virus -- HIV -- acquired immunodeficiency syndrome (AIDS)
• Human T-cell lymphotropic virus - HTLV-1 -- adult T-cell leukemia
  - HTLV-2 -- hairy cell leukemia

  (See pg 352, fig. 13.21; figs. and tables on pgs. 480-485)

CANCER

As you can see above, several of these lysogenic animal viruses have been associated with cancer. The development of cancer appears to take several steps -- at least a couple of different mutations; or in the case of the oncogenic (cancer causing) viruses above, a mutation plus a virus infection.

All animal cells have genes which control their growth and differentiation. Many of these genes are oncogenes (c-oncogenes) because if they are expressed abnormally they cause:

• 1.) uncontrolled growth
• 2.) dedifferentiation

These are two of the fundamental signs of transformed animal cells or cancer.

How do such genes come to be expressed abnormally?

Either through (1) mutation or (2) through infection with oncogenic viruses carrying mutated oncogenes (v-oncogenes).

Cells have mechanisms to protect themselves from damaged or foreign malignant DNA. In addition to the regular DNA repair enzymes, there are tumor suppressor genes and their protein products. p53 is such a gene (and protein) which examines the genome and detects damage. p53 stops cell division until the DNA is repaired. If the genome cannot be repaired, p53 triggers apoptosis (programmed cell death).

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