Question

What are two (2) mechanisms of defense that bacteria have against viruses? Describe how each of these works TT T Ariel 3 (12pt) T MIT

Answer 1

Bacteria are constantly subjected to bacteriophages and other viruses. In response to this constant exposure to viruses ( eg. bacteriophage) bacteria have evolved several diverse antiviral defense mechanisms. These mechanisms include adsorption blocking, uptake block, abortive infection, restriction modification and the CRISPR-Cas system.

Restriction Modification

Restriction modification (R-M) is a defense mechanism which is widely spread among bacteria. There are several types of R-M and all of these typically involve at least two enzymes, a restriction endonuclease (REase) and a methyltransferase (MTase). The REase is responsible for the cleavage of intruding double-stranded DNA, e.g. phage genomes4, through the recognition of the specific nucleotide restriction sites. Upon genome cleavage the phage is not able to finish its life cycle. Methylation of restriction sites by MTase protects the host cell genome from cleavage and REases are categorized as different restriction endonuclease types depending on their specific mode of activity.

CRISPR

Bacteria can defend themselves against infection by bacteriophages using an adaptive immune system called CRISPR-Cas. CRISPR-Cas works through the incorporation of small pieces of DNA (“spacers”), taken from the infecting phage, into a specific spot on the bacterial genome, the so-called CRISPR-locus. As a phage carrying the same sequence would be recognised by CRISPR-Cas and destroyed, this ensures that the bacterium now is immune to that phage through its CRISPR-Cas system.

Mechanism of CRISPR- Cas II mechanism

Exogenous Spacer PAM DNA 2000 vanaa + cas .cas leader repeaturepeat repeat -/--- pre-crRNA Cas9 CrRNA TUDO

Please thumbs up if this answer helped you in any way. Thanking you in anticipation.