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3 Information: genes and genomes

DNA strands form base pairs spontaneously in solution An important consequence of the relatively weak non-covalent hydrogen bonding between base pairs in DNA is that double stranded DNA in solution can be separated (denatured or melted) by heat and then renatured (or annealed) on cooling. The temperature at which a particular DNA molecule denatures is determined by the number of hydrogen bondsmit forms, which in turn is determined by its length in base pairs and its sequence: G:C base pairs require more energy (higher temperatures) to separate them than A:T base pairs because of the higher number of hydrogen bonds they contain. If a mixture of double-stranded DNA molecules of different base sequences is heated and then gradually cooled, the complementary sequences will ‘find’ each other in the mix and reform double stranded base-paired molecules. This characteristic has practical uses in molecular biology as we see in Chapter 8. DNA strands are also separated by enzyme action during DNA replication, the sub ject of Section 3.2. PAUSE AND THINK If a mixture of short double-stranded DNA molecules with the sequences shown below is gradually heated, in which order will the DNA molecules denature, and why? Molecule 1: 5 ATTTGAGATTC3 ¢ ¢ 3 5 ¢ ¢ TAAACTCTAAG Molecule2 5 GGCCATCGGG3 : ¢ ¢ 3 5 ¢ ¢ CCGGTAGCCC Molecule 3: 5 TCGGTTTAGA3 ¢ ¢ 3 5 ¢ ¢ AGCCAAATCT Antibiotics have saved countless lives since penicillin was first used to treat bacterial infections in the 1940s. Yet in the early 21st century we are in dan ger of a returning to a world where deadly diseases such as tuberculosis and bacterial sepsis cannot be cured. The reason is the spread of antibiotic resistance. Antibiotics work by inhibiting processes in bacteria that are essential for their life or growth, such as making cell walls or synthesizing proteins. Bacteria develop resistance through gaining new or altered genes, which may chemically inactivate the antibiotics or pump them out of the bacterial cells. Frequently, these resistance genes are carried on plasmids. Plasmids help to spread antibiotic resistance very rapidly, because copies of their DNA can be made and transferred from one bacterium to others in its vicinity—a process known as‘horizontal transfer’that occurs alongside‘ver tical transfer’ from one generation to the next. Widespread overuse of antibiotics has fuelled the spread of resistance at an alarming rate. Doctors are often under pressure to prescribe them, REAL WORLD VIEW 3.1

denature first (24 hydrogen bonds), followed by molecule 1 (25 hydrogen bonds), and then molecule 3 (28 hydrogen bonds). The more hydrogen bonds that need to be broken, the more energy required and hence the higher the temperature needed to break them.

Answer : Based on the number of hy drogen bonds, molecule 3 would

Check your understanding of the concepts covered in this section by answering the questions in the e-book.

3.2 Genome organization and packaging The genomes of living organisms consist of DNA molecules that are millions of base pairs in length. These are termed chromosomes . Eukaryotic chromosomes are linear double stranded DNA molecules, while prokaryotes mostly have a sin gle circular double-stranded DNA chromosome. Prokaryotic cells may also contain additional small DNA molecules called plasmids (generally circular) that replicate independently of the main genomic DNA. We discuss the role of plasmids in spreading antibiotic resistance in Real World View 3.1. Interestingly, mitochondria and chloroplasts (eukaryotic cellular organelles that are believed to have evolved from symbiotic bacte ria) contain their own genomes, which like those of prokaryotes consist of circular double-stranded DNA. However, each organelle contains multiple copies of its genome rather than a single copy as found in bacterial cells. The genomes of mitochondria and chloro plasts encode some of the specialized proteins they require to func tion, while other proteins they need are encoded by the nuclear genome. The organization of the genome in prokaryotic and eukaryotic cells is shown schematically in Figure 3.10. even for viral illnesses against which they are ineffective. They are also used widely in agriculture where they are fed routinely to livestock ani mals to promote growth and are sprayed on to crops. As the antibiotics kill bacteria, including those that are non-pathogenic, bacterial strains that are resistant grow rapidly with reduced competition and can pass on their resistance genes. Very few new antibiotics have been developed since the 1980s. The race is now on to discover new drug targets in bacteria and agents that may work against them. There is also a great deal that we can and should be doing to change our behaviour to stop the further spread of resistance to existing anti biotics. You can find out more on this topic from the website of Antibiotic Action (http://antibiotic-action.com/), a global initiative established by the British Society for Antimicrobial Chemotherapy in 2011.

Plasmids and the spread of antibiotic resistance

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