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3 Information: genes and genomes

Figure 3.7 Hydrogen bonding in the Watson–Crick base pairs Source : Adapted from Snape, A. & Papachristodoulou, D. (2018). Biochemistry and Molecular Biology (6th ed.). Oxford University Press.

H

CH 3

N

H

0

H

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7

8

C C

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C H Thymine

Adenine

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1

Sugar phosphate

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1

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H

Sugar phosphate

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H

O

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C C

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H

C N Cytosine

Guanine

3

6

4

1

Sugar phosphate

2

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1

2

C

C

N

N

N

H

0

Sugar phosphate

H

In DNA, base pairing links two polynucleotide chains, creating a double-stranded molecule that coils to create the famous double helix illustrated in Figure 3.1. RNA molecules are usually single stranded, but they often contain intramolecular base pairs that give the molecules a specific three-dimensional structure that is impor tant for their function (Figure 3.8). As we discuss later in this chapter, base pairing in DNA is crucial for its function as the repository of genetic information, since it allows accurate replication of the DNA molecule. Base pairing between single-stranded DNA and RNA can also occur, and is of crucial importance in protein synthesis, as described in Chapter 5.

PAUSE AND THINK The percentage of adenine bases in the human genome is approximately 30. What are the approximate percentages of each of the other three bases?

Answer: Thymine 30%, guanine 20%, and cytosine 20%.

Structural differences between DNA and RNA reflect their functions

Before moving on to discuss the three-dimensional structure of DNA, it is worth noting two further points of interest. First, DNA

Figure 3.8 Double-stranded DNA and tRNA. Models of (a) a short section of double-stranded DNA (Protein Data Bank code 1BNA) and (b) a molecule of tRNA (Protein Data Bank code 4TNA). The sugar–phosphate backbones are shown with bases protruding from them. (a) The two strands of the DNA double helix, here coloured grey and pink, are linked together by base pairing. (b) The tRNA molecule consists of a single strand, which folds to create a complex three-dimensional shape stabilized by intramolecular base pairing. This shape includes regions of double helix, including that shown here in blue. We discuss the function of tRNA in Chapter 5.

(a)

(b)

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