Viruses with RNA Genomes Have Very Few Genes

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The smallest genomes are those of RNA-containing viruses. As explained in Ch. 13, "Mutations", this is related to their high mutation rates. RNA-based genomes have rates of mutation that are 1000-fold higher than DNA. Since each mutant gene product must continue to interact with other virus components, this limits the number of genes to no more than a dozen or so. The advantage of a high mutation rate to these viruses is that it allows them to change their proteins rapidly so evading recognition by host defense systems.The overall success of this strategy is shown by the fact that the majority of the best-known viral diseases such as flu are due to RNA viruses.

Given an RNA genome, there are three alternatives: double-stranded RNA, single-stranded positive RNA and single-stranded negative RNA. The terms positive and negative refer to the coding strand and the non-coding strand, respectively. Remember that when messenger RNA is made only one of the two DNA strands is used as the template (see Fig. 17.15). The mRNA will be complementary in sequence to the template strand and identical to the non-transcribed strand of the DNA (except for using U instead of T in RNA). The non-transcribed strand of DNA and the mRNA are both

The story of how Jenner invented the technique of vaccination against infectious diseases is well known. He observed that milkmaids who worked in close contact with cattle often caught cowpox, a mild disease. After recovering, they became immune to the much more severe disease, smallpox. So Jenner then tested this connection experimentally in 1796. After deliberately infecting patients with cowpox he found that they had indeed become immune to smallpox. Jenner called the material he used "vaccine" after the cows where it originated (vacca = cow in Latin) and the novel technique was named vaccination.

For a long time it was believed that cowpox virus and the vaccine virus were identical. However, in 1939 it was found that cowpox virus (re-isolated from cows) and the virus cultures maintained for use in vaccination were actually quite distinct. The vaccine cultures were then named Vaccinia to distinguish them from cowpox. It seems clear that Jenner himself did indeed vaccinate his patients with cowpox. Since viruses could not be cultured or purified in the laboratory until much later, those who followed Jenner constantly re-isolated new strains of what they thought was cowpox virus from cows and horses. At some time the original cowpox was replaced by a different virus, the present Vaccinia virus. No virus presently circulating in the wild corresponds to Vaccinia so its origin is unknown. Furthermore, the differences between Vaccinia and cowpox are too large for mutation to be responsible for cowpox evolving into Vaccinia. Presumably another poxvirus was circulating among cattle and horses during the nineteenth century and was eventually kept because it is even milder than cowpox.

Coding strand

Template strand of dsDNA

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FIGURE 17.15 Minus Strands

Plus and

Double stranded DNA has two strands, the template strand and the coding strand. The coding strand can also be referred to as the plus strand and the template strand as the minus strand. The sequence of messenger RNA is by definition the coding sequence and is identical to the plus strand and complementary to the minus strand.

mRNA is complementary to template and same sequence as coding strand

coding strands and are therefore positive or "plus" strands whereas the template strand is the negative or "minus" strand.

If a virus particle contains single-stranded RNA (ssRNA), there are two alternatives. The virus RNA can be either the plus strand or the minus strand. If the virus con-

negative or "minus" strand The non-coding strand of RNA or DNA positive or "plus" strand The coding strand of RNA or DNA

vaccination Immunization of a patient by introducing a milder or inactivated form of the disease-causing agent

Positive-Stranded RNA Viruses Make Polyproteins 469

Lysis gene Coat gene Replication gene

Translation Translation

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