If prophage dna leaves the bacterial chromosome the

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If prophage DNA leaves the bacterial chromosome, the lytic cycle begins. An environmental signal, e.g., stress of the bacterial cell, can cause the prophage to leave the bacterial genome . The prophage reacts and goes into the lytic cycle because if the bacterium dies before the virus reproduces the virus dies too. Start
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Bacteria have a variety of methods of defending against invading viruses, otherwise they’d go extinct. Surface proteins: Some bacteria have evolved surface proteins (i.e., on their outside) that are no longer recognized as receptors by the phages (i.e., the phage does not recognize the bacterium). Restriction enzymes: Bacteria also have restriction enzymes that recognize certain viral DNA sequences and chop them up. (These enzymes “restrict” virus attacks.) The bacterial DNA is protected from being cut by these restriction enzymes by being methylated (i.e., having CH 3 attached at various parts). Some kinds of restriction enzymes are called restriction endonucleases , because they cut inside the DNA instead of chewing it up from the end, as done by exonucleases Examples of restriction sites cut by restriction enzymes Bacteria have evolved defenses against phages Restriction enzymes are harvested from bacteria by biotech companies and sold for use in genetic engineering by scientists and commercial companies.
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Figure 19.7 Infection by a phage triggers transcription of the CRISPR region of the bacterial DNA. BACTERIAL CELL DNA from previous infection by the same type of phage CRISPR region of DNA Phage DNA from invading phage DNA from previous infections by other phages Transcription Repeats RNA transcript The RNA transcript is processed into short RNA strands. Each short RNA strand binds to a Cas protein, forming a complex. Processing RNA transcribed from repeat Complementary RNA Cas protein RNA Active sites that cut DNA Complementary RNA binds to DNA. The Cas protein cuts the phage DNA. 5′ 5′ 3′ DNA from invading phage Complementary RNA Phage DNA can no longer replicate. 3′ 5′ Resulting cut in phage DNA Degraded phage DNA 1 2 3 4 5 CRISPR-Cas* system: Some bacteria “remember” previous attacks by phages by storing viral DNA sequences in their genomes and using them to recognize and chop up the viral DNA. When attacked, the bacteria use their stored phage DNA to make RNA complementary to the invading viral DNA. This RNA joins with the CRISPR associate protein and together they attach to the phage DNA and cut it. Once cut, the invading viral DNA is exposed to exonucleases in the bacterium, which rapidly break it down. [*Clustered Regularly Spaced Palindromic Repeats =CRISPR Cas = CRISPR associated protein] Cas protein Viral Viral Viral The CRISPR-Cas System of protection
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Animal Virus Attack Strategies Exocytosis Some viral envelopes are formed from the host cell’s plasma membrane as the viral capsids exit the host. Viral membranes, like host membranes, are generally made by the endoplasmic reticulum of the host cell. Herpes, which is a double stranded DNA virus lives in the cell nucleus as a mini chromosome . It uses uses the cell’s nuclear membrane for its original envelope and then the plasma membrane when it leaves the cell.
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