Читать книгу Astrobiology - Charles S. Cockell - Страница 53

Hydrogen bonding is found in many molecular interactions in life. In the next chapter, we discuss in more detail the structure of the information storage molecule of life: DNA. However, here, one feature of DNA is worth exploring: the role of hydrogen bonding in holding the molecule together. DNA is made up of two complementary strands that bind together to form its characteristic double helix shape. These two strands are held together down their middle by hydrogen bonding.

In the diagram in Figure 3.16 you can see the structure of a DNA double helix that has been flattened into a two-dimensional depiction (from its normal three-dimensional helical structure). Along the two edges of the structure, you can see the pentagonal deoxyribose sugars that make up its backbone linked together with phosphate groups. In the middle are the base pairs. The bases are linked in pairs with dotted lines showing the hydrogen bonding between them. The sequence of individual bases along a DNA strand encodes the genetic information. There are four bases: adenine (A), thymine (T), guanine (G), and cytosine (C). Adenine can only bind with thymine, and cytosine can only bind with guanine. There are two hydrogen bonds for the pairing between adenine and thymine bases, and three hydrogen bonds between guanine and cytosine.

Figure 3.16 Hydrogen bonding in the molecule DNA. (a) The dotted lines in the middle of the flattened two-dimensional molecule on the left are the hydrogen bonds that hold the two strands together. On the right (b) is the three-dimensional double helix.

Source: Reproduced with permission of wikicommons, Michael Ströck.

The hydrogen bonding is just strong enough to hold the strands together so that DNA does not fall apart easily. However, the hydrogen bonds are just weak enough so that the two strands can be unzipped when they need to be pulled apart for replication of the DNA molecule when cells divide, without the requirement for a large amount of energy to be expended. You can imagine that if the two strands were linked at every base pair with covalent bonds, it would require much more energy to pull apart the two DNA strands. In Chapter 4, we investigate the structure and functions of DNA in more detail. For now, let us note how the evolutionary process tends to select bonding types at the atomic level that are optimized to do particular tasks. Hydrogen bonding is ideal for a situation where molecular stability is required, but where the regular opening of molecular bonds means that it is also optimal to have a bonding type where energy requirements to break the bonds are minimized. You might like to ask yourself: Could you imagine a genetic material where two complementary strands are held together using van der Waals interactions that might offer a similar compromise?