Question

The primary molecule that viruses depend on to reproduce ___. This molecule is protected by____.

How do some viruses evade being degraded by restriction modification systems of bacteria?

Explain why antibiotics work against bacteria but not viruses?

Answer 1

Restriction–modification (RM) systems are composed of genes that encode a restriction enzyme and a modification methylase. RM systems sometimes behave as discrete units of life, like viruses and transposons. RM complexes attack invading DNA that has not been properly modified and thus may serve as a tool of defense for bacterial cells. However, any threat to their maintenance, such as a challenge by a competing genetic element can lead to cell death through restriction breakage in the genome. This post-segregational or post-disturbance cell killing may provide the RM complexes with a competitive advantage

Restriction-modification system evasion by virus:

The restriction-modification (RM) system is a defense mechanism present in over 90% of sequenced bacterial and archeal genomes. Its consists of a modification enzyme that methylates a specific DNA sequence in a genome and a restriction endonuclease that cleaves DNA lacking this methylation

Bacterial viruses have evolved different strategies to evade the restriction-modification system. Some viruses encode their own methyltransferase in order to protect their genome from host restriction enzymes. Instead, bacteriophage T7 encodes the OCR protein that blocks the active site of several restriction enzymes by mimicking the phosphate backbone of B-form DNA. Other bacterial viruses use unusual bases on their genome to avoid restriction. Bacteriophages SPO1, SP82, and 2C replace thymidine with 5-hydroxymethyluracil while phages PBS1 and PBS2 thymine is completely changed to uracil.

Virus	Strategy of RM evasion	Viral protein
Bacteriophage T1	Viral methyltransferase	Dmt methyltransferase
Bacteriophage T3	Viral SAMase	S-adenosyl-L-methionine hydrolase
Bacteriophage T4	Viral methyltransferase	Dam methyltransferase
Bacteriophage T2	Viral methyltransferase	Dam methyltransferase
Bacteriophage T7	Inhibition of type-I RM enzymes	OCR protein
Bacteriophage P1	Internal capsid protein	DarA, DarB

Antibiotics work against bacteria but not viruses:

Antibiotics cannot kill viruses because viruses have different structures and replicate in a different way than bacteria. Antibiotics work by targeting the growth machinery in bacteria (not viruses) to kill or inhibit those particular bacteria. When you think about it structurally, it makes sense that an antibiotic could not work to kill a virus with a completely different set of replicating “machinery”.

Viruses are structurally different from bacteria. Viruses live and replicate inside of a human cell, they cannot live outside of a human cell.

Viruses insert their genetic material into a human cell’s DNA in order to reproduce. Antibiotics cannot kill viruses because bacteria and viruses have different mechanisms and machinery to survive and replicate. The antibiotic has no “target” to attack in a virus. However, antiviral medications and vaccines are specific for viruses.

Viruses have few genes, sometimes less than 10, and thus few proteins that can be targeted. By contrast, bacteria have thousands of different genes and proteins. There are just more targets to shoot at with bacteria.

Viruses use a lot of host cell proteins for their replication and spread. Any drug that targets host cell proteins will be toxic to the host.

Viruses often hijack host genes, and thus their proteins can be closely related to host proteins. Making a drug that discriminates between viral and host versions of a protein is very difficult.

Viruses are far more diverse than bacteria. Whereas most antibiotics are effective against many different species of bacteria, antivirals usually are very specific to one type of virus.