DNA

DNA (DeoxyriboNucleic Acid) is called the genetic material because it contains the genetic information for every cell and tissue in an organism. DNA is a component of the chromosomes (proteins are the other component). DNA is one of two types of nucleic acid. Ribonucleic acid (RNA) is the other. As such, DNA is a polymer of deoxyribonucleotides linked through phosphodiester bonds (Figure 4.1).

Deoxyribonucleotides contain three components - a phosphate group, a modified sugar called deoxyribose, and a nitrogenous base (see here) (adenine, guanine, cytosine, or thymine). Adenine and guanine are called purine bases, whereas cytosine and thymine are called pyrimidine bases.

Deoxyribonucleotides are also called deoxyribonucleoside mono-, di-, or triphosphates if they contain 1, 2, or 3 phosphates, respectively. Deoxyadenosine triphosphate, for example, is a nucleoside triphosphate.

As seen in Figure 4.1, DNA is a polymer of nucleoside monophosphates. The backbone of the chain consists of alternating units of phosphate and sugar. The asymmetry of the nucleoside monophosphate monomers of DNA gives the chain a "polarity". We describe DNA's polarity relative to the numbering of the carbons in deoxyribose. In Figure 4.1, for example, the chain is said to be oriented 5' to 3' as it goes from top to bottom. That is, the phosophate residue is attached to the hydroxyl on the 5' carbon of one sugar residue and the 3' hydroxyl of the next one.

In cells, DNA consists of two strands wound around each other in a double-helical structure such that the phosphate-deoxyribose backbone is on the outside and the bases are on the inside. (Figure 4.11 and Figure 4.15b). The polarity of the two strands in a DNA molecule is opposite; that is, the 5' end of one strand matches up to the 3' end of the other strand. Thus, the strands are said to be "antiparallel."

The arrangement of bases in double-stranded DNA is not random. Adenine on one strand is always arranged adjacent to a thymine on the other strand and vice-versa. Guanine is similarly paired with cytosine and vice-versa. Hydrogen bonds between the base pairs hold the two strands together (Figure 4.10a). As Watson and Crick (discoverers of the structure of DNA) noted, the complementary nature of the bases provides a reasonably simple means for the molecule to be replicated because all of the information for making double-stranded DNA is contained within one of the two strands.

DNA is found in three predominant forms in cells - called B-DNA, A-DNA, and Z-DNA (Figures 4.15 and 4.26). Of these, the B-DNA form predominates. Figure 4.15 shows that both B- and A-DNA have major and minor grooves due to the particular orientation of the paired bases. In B-DNA, however, the two grooves are quite distinguishable, whereas in the A-DNA they are more nearly equal in width. Both B- and A-DNA are right-handed helices, whereas Z-DNA is a left-handed helix. Table 4.3 compares some of the physical parameters of B-, A-, and Z-DNA. Within DNA is the information necessary for making proteins (the workhorses of the cell). Proteins are not made directly from DNA, however. Instead, a complementary copy of the relevant portion of the DNA is made in the form of messenger RNA (mRNA), which is translated on a particle called a ribosome, using the genetic code, to direct the synthesis of protein.

See also: Nucleic Acids, Nucleoside and Nucleotide Naming, Phosphodiester Bonds, RNA, Proteins

INTERNET LINKS:

> 1. DNA

> 2. DNA Database at NCBI