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page 75 1 A mononucleotide is a pentose sugar (either ribose or deoxyribose) with an attached phosphate and an attached organic nitrogenous base (either adenine, thymine, guanine or cytosine in DNA, or adenine, uracil, guanine or cytosine in RNA). 2 The complementary base pairs in DNA are adenine and thymine, and guanine and cytosine. Hydrogen bonds link the bases in a complementary pair that are bonded to opposite strands. This makes the ‘rungs’ of the DNA helix. As there are many base pairs in a DNA molecule, and therefore many hydrogen bonds, the two strands are held firmly together in the helical structure by the hydrogen bonds. page 77 1 Chargaff’s data showed that the proportions of guanine and cytosine are the same in DNA, as are the proportions of adenine and thymine. This must be the case if guanine forms complementary base pairs with cytosine and if adenine pairs with thymine. As there is no other physical association between bases in DNA, you would expect no other relationship between the proportions of each base, as Chargaff found. Pauling’s results showed that hydrogen bonding can maintain a 3D helical structure. Knowing that the complementary base pairs are held together by hydrogen bonds helps to explain how the DNA normally forms a helical structure. 2 Development of ideas in science depends on the piecing together of many different pieces of information. In the case of DNA, before Crick and Watson could decide on the best model for the structure of DNA, they had to understand the experimental results from Chargaff, Pauling and Franklin that their model would have to explain. So communication of ideas between scientists is essential. page 79 1 Meselson and Stahl showed that after one replication in a medium containing light nitrogen the density of the DNA produced was half-way between that of DNA containing all heavy nitrogen and DNA containing all light nitrogen. This could only be explained by the DNA having one strand containing light nitrogen and one containing heavy nitrogen, which is what you get after semiconservative replication. If there had been conservative replication they would have found half the DNA all heavy and the other half all light. 2 The molecule of DNA is unzipped by DNA helicase which breaks the hydrogen bonds between complementary base pairs. Free DNA nucleotides form hydrogen bonds with matching exposed bases on the DNA molecules. Then DNA polymerase and DNA ligase cause bonds to form between the nucleotides and so build the new DNA strands. Unit 1 Topic 2: Chapter 2.1 110 Edexcel AS Biology Implementation and Assessment Guide for Teachers and Technicians ©Pearson Education Limited 2008 Answers to Student Book text questions Chapter 2.1(cont.) page 81 1 a The DNA contains the genetic code for the protein. b The DNA unwinds at the point where the gene for the protein occurs. The coding (antisense) strand of the gene attracts nucleotides to matching base pairs and these join to form a strand of messenger RNA. When it is released from the DNA, the mRNA molecules passes out of the nucleus and attaches to a ribosome. b Transfer RNA molecules bring matching amino acids to the ribosome and attach to the messenger RNA so that the amino acids are aligned correctly to form the protein chain.