Nucleic acids

The third class of polymeric macromolecules are the nucleic acids. These are deoxyribonucleic acid (DNA) and ribonucleic acid (RNA), and both are polymers of smaller molecules called nucleotides. As we shall see, there are important differences both in the overall structures of RNA and DNA and in the nucleotides they contain, so we shall consider each of them in turn.

The structure of DNA

The composition of a DNA nucleotide is shown in Figure 2.20(a). It has three parts, a five-carbon sugar called deoxyribose, a phosphate group and a base. This base can be any one of four molecules; as can be seen in Figure 2.21, these are all based on a cyclic structure containing nitrogen. Two of the bases, cytosine and thymine, have a single ring and are called pyrimidines. The other two, guanine and adenine, have a double ring structure; these are the purines. The four bases are often referred to by their initial letter only, thus we have A, C, G and T.

One nucleotide differs from another by the identity of the base it contains; the rest of the molecule (sugar and phosphate) is identical. You will recall from the previous section that the properties of a protein depend on the order in which its constituent amino acids

Nucleotide

Nucleoside

Nucleoside

HOCH2

HOCH2 O

HOCH2 O

OH OH

OH OH

(a) Deoxyibose (b) Ribose

Figure 2.20 A nucleotide comprises a pentose sugar, a phosphate group and a nitrogenous base (see Figure 2.21). Note the difference between the sugars (a) deoxyribose (DNA) and (b) ribose (RNA)

are linked together; we have exactly the same situation with nucleic acids, except that instead of an 'alphabet' of 20 'letters', here we have one of only four. Nevertheless, because nucleic acid molecules are extremely long, and the bases can occur in almost any order, an astronomically large number of different sequences is possible.

The nucleotides join together by means of a phosphodiester bond. This links the phosphate group of one base to an -OH group on the 3-carbon of the deoxyribose sugar of another (Figure 2.22). The chain of nucleotides therefore has a free -OH group attached to the 3-carbon (the 3' end) and a free phosphate group attached to the 5-carbon (the 5' end). This remains the case however long the chain becomes.

The structure of DNA however is not just a single chain of linked nucleotides, but two chains wound around each other to give the double helix form made famous by the model of James Watson and Francis Crick in 1953 (Figure 2.23, see also Chapter 11). If we compare this to an open spiral staircase, alternate sugar and phosphate groups make up the 'skeleton' of the staircase, while the inward-facing bases pair up by hydrogen bonding to form the steps. Notice that each nu-cleotide pair always comprises three rings, resulting from a combination of one purine and one pyrimidine base. This means that the two strands of the helix are always evenly spaced. The way in which the bases pair is further governed by the phenomenon of complementary base pairing. A nucleotide containing thymine will only pair with one containing adenine, and likewise guanine always pairs with cytosine (Figure 2.24). Thus, the sequence of

Erwin Chargaff measured the proportions of the different nucleotides in a range of DNA samples. He found that T always = A and C always = G. Watson and Crick interpreted this as meaning that the bases always paired up in this way.

Figure 2.21 Bases belong to two classes. Nucleotides differ from each other in the identity of the nitrogenous base. (a) In DNA these are adenine (A), cytosine (C), guanine (G) or thymine (T). The purines (A and G) have a two-ring structure, while the pyrimidines (C and T) have only one ring. (b) In RNA, thymine is replaced by a similar molecule, uracil (U)

Figure 2.21 Bases belong to two classes. Nucleotides differ from each other in the identity of the nitrogenous base. (a) In DNA these are adenine (A), cytosine (C), guanine (G) or thymine (T). The purines (A and G) have a two-ring structure, while the pyrimidines (C and T) have only one ring. (b) In RNA, thymine is replaced by a similar molecule, uracil (U)

nucleotides on one strand of the double helix determines that of the other, as it has a complementary structure. Figure 2.23 shows how the two strands of the double helix are antiparallel, that is they run in opposite directions, one 5'^ 3' and the other 3' ^5'. In Chapter 12 we shall look at how this structure was used to propose a mechanism for the way in which DNA replicates and genetic material is copied.

Figure 2.22 The phosphodiester bond. A chain of DNA is made longer by the addition of nucleotides containing not one but three phosphate groups; on joining the chain, two of these phosphates are removed. Nucleotides are joined to each other by a phosphodiester bond, linking the phosphate group on the 5-carbon of one deoxyribose to the -OH group on the 3-carbon of another. (These carbons are known as 5' and 3' to distinguish them from the 5- and 3-carbon on the nitrogenous base). Note that the resulting chain, however many nucleotides it may comprise, always has a 5'(PO4) group at one end and a 3'(OH) group at the other

Figure 2.22 The phosphodiester bond. A chain of DNA is made longer by the addition of nucleotides containing not one but three phosphate groups; on joining the chain, two of these phosphates are removed. Nucleotides are joined to each other by a phosphodiester bond, linking the phosphate group on the 5-carbon of one deoxyribose to the -OH group on the 3-carbon of another. (These carbons are known as 5' and 3' to distinguish them from the 5- and 3-carbon on the nitrogenous base). Note that the resulting chain, however many nucleotides it may comprise, always has a 5'(PO4) group at one end and a 3'(OH) group at the other

The structure ofRNA

In view of the similarities in the structure of DNA and RNA, we shall confine ourselves here to a consideration of the major differences. There are two important differences in the composition of nucleotides of RNA and DNA. The central sugar molecule is not deoxyribose, but ribose; as shown in Figure 2.20, these differ only in the possession of an -H atom or an -OH group attached to carbon-2. Second, although RNA shares three of DNA's nitrogenous bases (A, C and G), instead of thymine it has uracil. Like thymine, this pairs specifically with adenine.

The final main difference between RNA and DNA is the fact that RNA generally comprises only a single polynucleotide chain, although this may be subject to secondary and tertiary folding as a result of complementary base pairing within the same strand. The roles of the three different forms of RNA will be discussed in Chapter 11.

Figure 2.23 The model of DNA proposed by Watson and Crick has two chains of nucleotides joined together by hydrogen-bonded base pairs pointing inwards towards the centre of the helix. The rules of complementary base pairing means that the sequence of one chain can be predicted from the sequence of the other. Note how the chains run in opposite directions (antiparallel)

Figure 2.23 The model of DNA proposed by Watson and Crick has two chains of nucleotides joined together by hydrogen-bonded base pairs pointing inwards towards the centre of the helix. The rules of complementary base pairing means that the sequence of one chain can be predicted from the sequence of the other. Note how the chains run in opposite directions (antiparallel)

Was this article helpful?

0 0
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