Nucleic Acids Help
Deoxyribonucleic Acid (DNA)
Genes are made of the nucleic acid deoxyribonucleic acid (DNA). This molecule serves as the carrier of genetic information in all organisms other than some viruses. The double-helical structure of this long molecule is shown in Fig. 3-1.
The backbone of the helix is composed of two chains with alternating sugar (S)-phosphate (P) units. The sugar is a pentose (five-carbon molecule) called deoxy-ribose, differing from its close relative ribose by one oxygen atom in the 20 position (Fig. 3-2).
The phosphate group (PO4) connects adjacent sugars by a 3' to 5' phosphodiester linkage. In one chain the linkages are polarized 3' to 5'; in the other chain, read in the same direction, they are in the reverse order 5' to 3'. All nucleic acid chains pair in this antiparallel fashion, whether the pairing is DNA with DNA chains, DNA with RNA chains, or RNA with RNA chains. The units connecting one strand of DNA to its polarize complement consist of paired organic bases of four kinds: adenine, cytosine, guanine, and thymine (symbolized A, C, G, T, respectively), classified into two groups, the purines and the pyrimidines. Purines only pair with pyrimidines and vice versa, thus producing a symmetrical double helix. Adenine pairs with thymine by two hydrogen bonds; guanine and cytosine pair by three hydrogen bonds (Fig. 3-3).A hydrogen bond forms between a covalently bound donor hydrogen atom (e.g., an imino group, NH) with some positive charge and a negatively charged covalently bound acceptor atom (e.g., a keto group, CO) by sharing of a hydrogen atom. A base-sugar complex is called a nucleoside; a nucleoside plus a phosphate is called a nucleotide. Thus, DNA is a long polymer (i.e., a macromolecule composed of a number of similar or identical subunits, called monomers, covalently bonded) of thousands of nucleotide base pairs (bp). The "direction" of DNA strands can be indicated by whether the 5' phosphate or 3' hydroxyl is at the end (see Fig. 3-3).
EXAMPLE 3.1 Because A pairs with T, and G pairs with C, the ratio of A: T in one DNA strand is 1.0. The same is true for the ratio of G: C.
Ribonucleic Acid (RNA)
Another class of nucleic acids, called ribonucleic acid (RNA), is slightly different from DNA in the following respects:
- Cellular RNA is single-stranded; DNA is double-stranded. However, some viruses have a single-stranded DNA genome; a few viruses have a single-stranded RNA genome; very few have a double-stranded RNA genome.
- RNA contains ribose sugars instead of deoxyribose sugars.
- RNA contains the pyrimidine uracil (U) instead of thymine (T), and U pairs with A.
- RNA molecules are much shorter than DNA molecules.
RNA molecules have many important cellular functions. One of them is in protein synthesis, carrying information from the instructions coded into the DNA to the ribosomal sites of protein synthesis in the cell. This form of RNA is called messenger RNA (mRNA). Ribo somes contain a special type of RNA called ribosomal RNA (rRNA) that constitutes the bulk of cellular RNA. A third kind of RNA, called transfer RNA (tRNA), attaches to amino acids and during protein synthesis brings them into proper positioning with other amino acids using the mRNA-ribosome complex as a template. Some RNA molecules, called ribozymes, have enzymatic capabilities. All cellular RNA molecules are made from a DNA template. A single-stranded RNA chain may fold back upon itself and form localized "double-stranded" sections by complementary base pairing. A two-dimensional "cloverleaf" model of tRNA is shown in Fig. 3-8.
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