The Chemical Nature Of Rna Differs From That Of

Ribonucleic acid (RNA) is a polymer of purine and pyrimidine ribonucleotides linked together by 3',5'-phosphodiester bridges analogous to those in DNA (Figure 35-6). Although sharing many features with DNA, RNA possesses several specific differences:

(1) In RNA, the sugar moiety to which the phosphates and purine and pyrimidine bases are attached is ribose rather than the 2'-deoxyribose of DNA.

(2) The pyrimidine components of RNA differ from those of DNA. Although RNA contains the ribonu-cleotides of adenine, guanine, and cytosine, it does not possess thymine except in the rare case mentioned below. Instead of thymine, RNA contains the ribonu-cleotide of uracil.

(3) RNA exists as a single strand, whereas DNA exists as a double-stranded helical molecule. However, given the proper complementary base sequence with opposite polarity, the single strand of RNA—as demonstrated in Figure 35-7—is capable of folding back on itself like a hairpin and thus acquiring double-stranded characteristics.

(4) Since the RNA molecule is a single strand complementary to only one of the two strands of a gene, its guanine content does not necessarily equal its cytosine content, nor does its adenine content necessarily equal its uracil content.

Figure 35-4. The double-stranded structure of DNA and the template function of each old strand (dark shading) on which a new (light shading) complementary strand is synthesized.

Figure 35-4. The double-stranded structure of DNA and the template function of each old strand (dark shading) on which a new (light shading) complementary strand is synthesized.

(5) RNA can be hydrolyzed by alkali to 2',3' cyclic diesters of the mononucleotides, compounds that cannot be formed from alkali-treated DNA because of the absence of a 2'-hydroxyl group. The alkali lability of RNA is useful both diagnostically and analytically.

Information within the single strand of RNA is contained in its sequence ("primary structure") of purine and pyrimidine nucleotides within the polymer. The sequence is complementary to the template strand of the gene from which it was transcribed. Because of this

Original parent molecule

Original parent molecule

First-generation daughter molecules

First-generation daughter molecules

Second-generation daughter molecules

Figure 35-5. DNA replication is semiconservative. During a round of replication, each of the two strands of DNA is used as a template for synthesis of a new, complementary strand.

Second-generation daughter molecules

Figure 35-5. DNA replication is semiconservative. During a round of replication, each of the two strands of DNA is used as a template for synthesis of a new, complementary strand.

complementarity, an RNA molecule can bind specifically via the base-pairing rules to its template DNA strand; it will not bind ("hybridize") with the other (coding) strand of its gene. The sequence of the RNA molecule (except for U replacing T) is the same as that of the coding strand of the gene (Figure 35-8).

Diabetes 2

Diabetes 2

Diabetes is a disease that affects the way your body uses food. Normally, your body converts sugars, starches and other foods into a form of sugar called glucose. Your body uses glucose for fuel. The cells receive the glucose through the bloodstream. They then use insulin a hormone made by the pancreas to absorb the glucose, convert it into energy, and either use it or store it for later use. Learn more...

Get My Free Ebook


Post a comment