Chapter 4 Biochemistry6e

Chapter 4 Biochemistry6e - Chapter 4 Biochemistry...

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Chapter 4 Biochemistry Biochemical Evolution, 6 th Edition by Berg, Tymoczko and Stryer Transcription is the process whereby the DNA serves as the information by which cellular RNA polymerases make the RNAs of the cell, whether they are mRNA, tRNA or rRNA. Translation is the process whereby the information of mRNA is converted into the making of a polypeptide. In a cell, what normally occurs is: Transcription Translation DNA i RNA i Protein The Genetic Code is the informational content of the DNA by which the proper RNA and Protein are made. It is thus the blueprint that determines what an RNA is and what type of protein an mRNA will specify. One important difference to remember between prokaryotes and eukaryotes is that prokaryotes contain their genetic information in fully complete sections of their DNA which we term genes. Eukaryotes, in contrast, often have exons , or coding sequences of their genes interspersed between introns , or non-coding regions. Both introns and exons are transcribed, but only the exons are processed to result in a mature mRNA that will be transcribed into a protein. 4.1 Structure of Nucleic Acids DNA and RNA have structures that convey their genetic information. As stated previously, the difference between the two polymers is that DNA has thymine while RNA has uracil and that the sugar in DNA is deoxyribose while it is ribose in RNA (Figure 4.2) . Note that it is a 2' hydroxyl group which is removed in the deoxyribose. The lack of this hydroxyl renders the molecule much more stable with respect to hydroxyl ion cleavage than is ribose when the two are polymerized into DNA and RNA. Both molecules, however, are quite similar in a number of respects, including being linked by their sugar-phosphate backbones by which a phosphodiester linkages join the sugars of the molecules = 1
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respective skeletons (Figure 4.3) . Note also that the linkages are with the 5' and 3' carbons of the sugars and hence the 5' i 3' structure of the molecule. The chemical nature of the phosphodiester linkage is of interest (Figure 4.3 again) as it helps to stabilize the molecule to alkaline hydrolysis. This is because the negative charge of the backbone repels nucleophiles such as hydroxyls stabilizing them, especially when compared to other ester bonds, such as carboxylic acid esters. As noted above, DNA is even more stable as it lack the 2' hydroxyl group and probably this facet of DNA played a major role in its evolution as the cell = s repository of genetic information. The nitrogenous bases of DNA and RNA are either purines (2 heterocyclic rings) or pyrimidines (1 heterocyclic ring) (Figure 4.4) . Adenine and guanine are the purines while cytosine, thymine and uracil are the pyrimidines. Nucleosides and Nucleotides
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Chapter 4 Biochemistry6e - Chapter 4 Biochemistry...

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