The Base Composition of DNA and RNA

In an early work on the compositions of DNA, Chargaff (1955) noted that the ratios of adenine to thymine and of guanine to cytosine are very close to unity in a very large number of DNA samples. This observation has been used to support the helical structure of DNA proposed by Watson and Crick (1953). From the base compositions of DNA and RNA given in the following tables, deduce the statistical significance for the statement that the base ratios (A/T(U) and G/C are unity for DNA but vary for RNA (Note Calculate the ratios first and then perform statistical analysis on the ratios). 

The five-carbon sugar in RNA is ribose, and the sugar in DNA is 2 -deox)Tribose. The only difference between these two sugars is the absence of an hydroxyl group on the 2 carbon of 2 -deoxyribose. The purines in both DNA and RNA are adenine and guanine. Both DNA and RNA contain the pyrimidine cytosine however, the fourth base is thymine in DNA and uracil in RNA. The chemical compositions of DNA and RNA are summarized in Table 24.1. 

RNA polymerase, like the DNA polymerases described earlier, takes instructions from a DNA template. The earliest evidence was the finding that the base composition of newly synthesized RNA is the complement of that of the DNA template strand, as exemplified by the RNA synthesized from a template of single-stranded ( ) XI74 DNA (Table 5 3). Hybridization experiments also revealed that RNA synthesized by RNA polymerase is complementary to its DNA template. In these experiments, DNA is melted and allowed to reassociate in the presence of mRNA. RNA-DNA hybrids will form if the RNA and DNA have complementary sequences. The strongest evidence for the fidelity of transcription came from base-sequence studies showing that the RNA sequence is the precise complement of the DNA template sequence (Figure 5.26). 

The base composition of a nucleic acid can be determined quite simply by digesting the DNA or RNA with non-spedfic phosphodiesterases and analysing the nudeotide or nucleoside products combined treatment with Serratia marcescens nuclease (i.e., Benzonase ) and PI nuclease yields nudeoside-5 -monophosphates. Further treatment with alkaline phosphatase leads to dephosphorylation and the formation of nucleosides that can be analysed by RP-HPLC. This procedure can also be used to detect and quantitate modified nucleotides and nucleosides. The base composition of double stranded nucleic acids follows the well known Chargaff rules ... 

Does the base composition of the RNA (the transcriptome) reflect that of the DNA genome, in prokaryotes and eukaryotes ... 

FIGURE 10.46 Melting point curves of DNA and RNA (approx). The exact shapes and positions of the curves are dependent upon base compositions. 

Analysis of the base composition of various fractions of the RNA revealed a high content of U and relatively low GC in the heavy species, a composition distinctly different from that of ribosomal RNA and similar to that of DNA. The rapidly labeled RNA could be hybridized with DNA in the presence of excess nonisotopic ribosomal RNA. Very similar findings were published simultaneously and independently by Attardi et al. (1966). 

Immediately after the Watson-Crick proposals were made in 1953, it was generally thought that ribosomal RNA (rRNA), which constitutes up to 90% of the total RNA of some cells, carried the genetic message from the nucleus to the cytoplasm. By 1960 it seemed unlikely. For one thing the size and composition of rRNA was similar for different bacteria, despite differences in base composition of the DNA (Chapter 5).27 It had been concluded that a relatively unstable, short-lived form of RNA must carry the message. Ribosomal RNA, however, was quite stable.28... 

RNA also differs from DNA in that there are not the same regularities in the overall composition of its bases and it usually consists of a single polynucleotide chain. There are different types of RNA, which fulfill different functions. About 80% of the RNA in a cell is located in the cytoplasm in clusters closely associated with proteins. These ribonucleoprotein particles specifically are called ribosomes, and the ribosomes are the sites of most of the protein synthesis in the cell. In addition to the ribosomal RNA (rRNA),... 

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