WJEC Biology for A2: Student Bk

YOU SHOULD KNOW › › › ››› Viruses enter either the lytic or lysogenic cycle ››› Viruses are pathogenic through mechanisms including cell lysis, toxin production, cell transformation and immune suppression St u dy point Viruses do not have cell walls or metabolic pathways and so antibiotics do not affect them. Virus pathogenicity and reproduction Viruses have been described as ‘the ultimate parasite’ because outside living cells they are inert. They show none of the characteristics of life, except when inside a host cell, where they are replicated. ▪ In the lytic cycle, viruses immediately reproduce using the host’s metabolism to copy their own nucleic acid and synthesise new coat protein. Their release may be by: – Lysis of the host cell, e.g. the common cold virus. – Budding, in which case, they acquire an envelope from the host’s cell membrane, e.g. influenza virus. ▪ Some viruses are ‘ lysogenic ’. They integrate their nucleic acid into the host cell genome and may remain there for many cell generations with no clinical effect. They enter the lytic cycle at some later time, which is when they produce symptoms, e.g. herpes, HIV. KEY 1 Virion attaches to cell 2 Viral nucleic acid injected into cell leaving protein coat outside 3 Nucleic acid and capsid protein are synthesised using the host’s metabolism and they assemble to make mature virus particles 4 Cell lysis releases the viruses or 4a New virus particles bud from the cell surface 5 Viral nucleic acid integrates into a host cell chromosome Viruses can be pathogenic in various ways: ▪ Cell lysis: when bacteria are infected with bacteriophage, the pressure of new virus particles inside causes the bacteria to burst. In contrast, in virus-infected animal cells, it is the inflammation caused by T-lymphocytes or antibodies that brings about lysis. An example is rhinovirus, one of the approximately 200 viruses that cause the common cold, which lyses cells in the upper respiratory tract. ▪ Toxins: many viral components and their by-products are toxic. The mechanisms are not well understood, but the following have been observed: – Measles virus can cause chromosome fusion. – Herpes virus can cause cell fusion. – Viral proteins can inhibit RNA, DNA and protein synthesis. LYTIC CYCLE LYSOGENY 4a 3 4 1 2 5 Viral life cycles Option A: Immunology and disease 267