Deoxyribonucleic Acid

Nucleic acids include the macromolecules DNA and RNA which are critically important in genetic regulation, and the subunits from which these molecules are formed.These subunits are known as nucleotides.

Nucleotides are the subunits of nucleic acids, bonded together in dehydration synthesis reactions to form long polynucleotide chains. Each nucleotide, however, is itself composed of three smaller subunits: a five-carbon (pentose) sugar, a phosphate group attached to one end of the sugar, and a nitrogenous base attached to the other end of the sugar (fig. 2.29). The nitrogenous bases are nitrogen-containing molecules of two kinds: pyrimidines and purines. The pyrimidines contain a single ring of carbon and nitrogen, whereas the purines have two such rings.

Phosphate group

Phosphate group

Base

Five-carbon sugar

Base

Five-carbon sugar

NucIeotide

Nucieotide

Thymine

Cytosine

Adenine

NucIeotide

Bases

G J Guanine

Thymine

Cytosine

Adenine

■ Figure 2.29 The general structure of a nucleotide. A polymer of nucleotides, or polynucleotide, is shown below. This is formed by sugar-phosphate bonds between nucleotides.

The structure of DNA (deoxyribonucleic acid) serves as the basis for the genetic code. For this reason, it might seem logical that DNA should have an extremely complex structure. DNA is indeed larger than any other molecule in the cell, but its structure is actually simpler than that of most proteins. This simplicity of structure deceived some early investigators into believing that the protein content of chromosomes, rather than their DNA content, provided the basis for the genetic code.

Sugar molecules in the nucleotides of DNA are a type of pentose (five-carbon) sugar called deoxyribose. Each deoxyri-bose can be covalently bonded to one of four possible bases. These bases include the two purines (guanine and adenine) and the two pyrimidines (cytosine and thymine) (fig. 2.30). There are thus four different types of nucleotides that can be used to produce the long DNA chains. If you remember that there are twenty different amino acids used to produce proteins, you can now understand why many scientists were deceived into thinking that genes were composed of proteins rather than nucleic acids.

When nucleotides combined to form a chain, the phosphate group of one condenses with the deoxyribose sugar of another nu-cleotide. This forms a sugar-phosphate chain as water is removed in dehydration synthesis. Since the nitrogenous bases are attached to the sugar molecules, the sugar-phosphate chain looks like a "backbone" from which the bases project. Each of these bases can form hydrogen bonds with other bases, which are in turn joined to a different chain of nucleotides. Such hydrogen bonding between bases thus produces a double-stranded DNA molecule; the two strands are like a staircase, with the paired bases as steps (fig. 2.30).

Actually, the two chains of DNA twist about each other to form a double helix, so that the molecule resembles a spiral staircase (fig. 2.31). It has been shown that the number of purine bases in DNA is equal to the number of pyrimidine bases. The reason for this is explained by the law of complementary base pairing: adenine can pair only with thymine (through two hydrogen bonds), whereas guanine can pair only with cytosine (through three hydrogen bonds). With knowledge of this rule, we could predict the base sequence of one DNA strand if we knew the sequence of bases in the complementary strand.

Although we can be certain of which base is opposite a given base in DNA, we cannot predict which bases will be above or below that particular pair within a single polynucleotide chain. Although there are only four bases, the number of possible base sequences along a stretch of several thousand nucleotides (the length of most genes) is almost infinite. To gain perspective, it is useful to realize that the total human genome (all of the genes in a cell) consists of over 3 billion base pairs that would extend over a meter if the DNA molecules were unraveled and stretched out.

Yet, even with this amazing variety of possible base sequences, almost all of the billions of copies of a particular gene in a person are identical. The mechanisms by which identical DNA copies are made and distributed to the daughter cells when a cell divides will be described in chapter 3.

Chemical Composition of the Body

Deoxyribose H

Guanine Cytosine

Thymine

Chemical Composition Guanine
Adenine

■ Figure 2.30 The four nitrogenous bases in deoxyribonucleic acid (DNA). Notice that hydrogen bonds can form between guanine and cytosine and between thymine and adenine.

A

T

C

T

A

G

C

r_

A

T

C

Sugar-phosphate Complementary Sugar-phosphate backbone base pairing backbone

Sugar Phosphate Backbone

■ Figure 2.31 The double-helix structure of

DNA.

The two strands are held together by hydrogen bonds between complementary bases in each strand.

44 Chapter Two

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    What are the purine and pyrimidine bases of the nucleotide ? Explain the law of complementary base pairing ?
    8 years ago

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