STRUCTURE AND FUNCTION OF RNA

RNA is structurally similar to DNA!

Both nucleic acids are sugar-phosphate polymers and both have nitrogen bases attached to the sugars of the backbone- but there are several important differences.

• They differ in composition:

1. The sugar in RNA is ribose, not the deoxyribose in DNA (as we previously learned).
2. The base uracil is present in RNA instead of thymine.
   

• They also differ in size and structure:

1. RNA molecules are smaller (shorter) than DNA molecules,
2. RNA is single-stranded, not double-stranded like DNA.
   

• Another difference between RNA and DNA is in function. DNA has only one function-STORING GENETIC INFORMATION in its sequence of nucleotide bases. But there are three main kinds of ribonucleic acid, each of which has a specific job to do.

1. Ribosomal RNAs-exist outside the nucleus in the cytoplasm of a cell in structures called ribosomes. Ribosomes are small, granular structures where protein synthesis takes place. Each ribosome is a complex consisting of about 60% ribosomal RNA (rRNA) and 40% protein.
2. Messenger RNAs-are the nucleic acids that "record" information from DNA in the cell nucleus and carry it to the ribosomes and are known as messenger RNAs (mRNA).
3. Transfer RNAs-The function of transfer RNAs (tRNA) is to deliver amino acids one by one to protein chains growing at ribosomes.

http://ncc.gmu.edu/dna/rna.htm

Evolution of DNA metabolism

Further information: RNA world hypothesis

DNA contains the genetic information that allows all modern living things to function, grow and reproduce. However, it is unclear how long in the 4-billion-year history of life DNA has performed this function, as it has been proposed that the earliest forms of life may have used RNA as their genetic material.^[81][93] RNA may have acted as the central part of early cell metabolism as it can both transmit genetic information and carry out catalysis as part of ribozymes.^[94] This ancient RNA world where nucleic acid would have been used for both catalysis and genetics may have influenced the evolution of the current genetic code based on four nucleotide bases. This would occur since the number of unique bases in such an organism is a trade-off between a small number of bases increasing replication accuracy and a large number of bases increasing the catalytic efficiency of ribosomes.^[95]

Unfortunately, there is no direct evidence of ancient genetic systems, as recovery of DNA from most fossils is impossible. This is because DNA will survive in the environment for less than one million years and slowly degrades into short fragments in solution.^[96] Claims for older DNA have been made, most notably a report of the isolation of a viable bacterium from a salt crystal 250-million years old,^[97] but these claims are controversial.^[98][99]

http://en.wikipedia.org/wiki/DNA