RNA structure is similar to DNA but contains the sugar ribose instead of deoxyribose and the base uracil instead of thymine. RNA can form secondary structures through internal base pairing. Some viruses use RNA as their genetic material, including tobacco mosaic virus (TMV) which contains only RNA and protein. Experiments in the 1950s-60s showed that the RNA of TMV determines the properties of progeny viruses and carries the genetic information rather than the protein, proving that RNA acts as the genetic material for some viruses.

2. RNA Structure
1. RNA structure is very similar to that of DNA.
a.It is a polymer of ribonucleotides (the sugar is ribose rather than deoxyribose).
b. Three of its bases are the same (A, G, and C) while it contains U rather than T.
2. RNA is single-stranded, but internal base pairing can produce secondary structure
in the molecule.
3. Some viruses use RNA for their genomes. In some it is dsRNA, while in others it is
ssRNA. Double-stranded RNA is structurally very similar to dsDNA.

4. The Discovery of RNA as Viral Genetic Material
• All known cellular organisms have DNA as their genetic material. Some viruses,
however, use RNA instead.
• Tobacco mosaic virus (TMV) is composed of RNA and protein; it contains no DNA.
In 1956 A. Gierer and G. Schramm showed that when purified RNA from TMV is
applied directly to tobacco leaves, they develop mosaic disease. Pretreating the
purified RNA with RNase destroys its ability to cause TMV lesions.
• In 1957 Fraenkel-Conrat and Singer showed that in TMV infections with viruses
containing RNA from one strain and protein from another, the progeny viruses
were always of the type specified by the RNA, not by the protein.
Typical tobacco mosaic virus (TMV) particle

6. Demonstration that RNA is the genetic material in tobacco mosaic virus (TMV)

7. • This proved that specificity of virus proteins was determined by
RNA alone and that proteins carried no genetic information.
• Hence RNA carries genetic information not proteins.
• The genetic RNA is usually found to be single stranded but in
some it is double stranded as in reovirus, Wound tumor virus.

8. • Experiments in the 1960s showed that messenger RNA has the ability to
store genetic information, while transfer and ribosomal RNA have the
ability to translate genetic information into proteins.
• Experiments performed two decades later showed that some RNAs can
even act as an enzyme to self-edit their own genetic code!
• These results raised two questions:
• 1) Why does RNA play so many roles in the flow of genetic information?
• 2) Why bother storing genetic information in DNA, if RNA alone could do
the job?

9. • The mutation rate or RNA dependent RNA polymerases is
substantially higher that DNA dependent DNA polymerases.
• We don't have any dsRNA-based organisms other than a couple of
viruses to look at so this is speculation, but dsRNA is much less stable
than dsDNA.
• Proteins usually fit in the major groove because it is roomier. The
minor groove is less accessible.
• Uracil-adenine is also rather weak and likes to "wobble" and bind
uracil-guanine instead. Base mismatches cause big problems if you
are trying to copy your genome exactly and divide.

10. • RNA has great capability as a genetic molecule; it once had to carry on
hereditary processes on its own.
• It now seems certain that RNA was the first molecule of heredity, so it
evolved all the essential methods for storing and expressing genetic
information before DNA came onto the scene.
• However, single-stranded RNA is rather unstable and is easily damaged by
enzymes.
• By essentially doubling the existing RNA molecule, and using deoxyribose
sugar instead of ribose, DNA evolved as a much more stable form to pass
genetic information with accuracy.