Nucleic acids
B.8 Nucleic acids (2 hours)
Pause for thought
It's not every day that you find a king buried under a public car park. Archaeologists from Leicester University have confirmed "beyond reasonable doubt" that a skeleton found beneath a car park in Leicester, U.K. is that of the Plantagenet king Richard III who died at the battle of Bosworth Field in 1485. It was known that his body had been buried hastily in the Church of Greyfriars in Leicester. However the church was destroyed in the Reformation in the sixteenth century and the site of the church was lost. So how can a body that has been in the ground for more than five hundred years be positively identified? There is no one definitive piece of evidence but many separate clues which all add up to a near certain confirmation. The skeleton was that of a male in his late twenties or early thirties and Richard III was 32 when he died. There were several wounds on the body including eight to the head, which suggests that the victim died in battle. Carbon dating showed the bones to be from between 1455 and 1540 and there was also evidence of curvature of the spine which fits in with Richard III being the 'hunchback' king. Of relevance to this sub-topic is the DNA profiling evidence. To be able to get DNA from a skeleton that is 500 years old is amazing in itself but by carrying out the profile it has been shown to match living descendants through the maternal line. The BBC has produced a good article detailing how DNA testing works.
Richard III was reburied in Leicester Cathedral in March 2015. For a controversial account read the article written by Polly Toynbee in the Guardian.
Nature of science
Scientists used models and diffraction experiments to determine the structure of DNA. This is a good example of the use of different approaches to solve the same problem.
The use of X-ray diffraction provided evidence for the double helix, which led to the explanation of how DNA functions.Learning outcomesAfter studying this topic students should be able to: Understand:
Apply their knowledge to:
| Clarification notesThe structures of the nitrogenous bases and ribose and deoxyribose sugars can be found in Section 34 in the data booklet. The different forms of RNA, nor the details of DNA replication are not required to be known. The expression of DNA should be limited to the concept of a four-unit base code determining a twenty-unit amino acid sequence. The details of transcription and translation are not required. International-mindednessThe goal of the Human Genome Project, an international research programme, l was to complete the mapping and sequencing of all the genes in the human genome. The policies for labelling genetically modified (GM) foods vary greatly from country to country. What effect does the protection of genetically modified organisms by international patents have on the global economy and scientific community? |
Teaching tipsThe secret to students grasping this topic well is to get them to understand how adenine forms two hydrogen bonds with thymine and how cytosine can form three hydrogen bonds with guanine. This is often asked on examination papers but also is the key to the genetic code as it helps to explain how DNA replicates. You will also need to explain that the phosphate groups in polynucleotides carry a negative charge so that effectively 'molecules' of DNA and RNA are negative polyions. These hydrophilic phosphate groups can form hydrogen bonds with water molecules and with basic proteins, called histones, found in the chromosomes in the nuclei of eukaryotic cells. The intermolecular bonds formed between the hydrophilic parts of the polynucleotide chains and between the hydrophobic parts (due to the nitrogenous bases) help to stabilise the double-helical shape. Use models and videos to show this this and use the structures given in Section 34 of the data booklet. Stress how messenger RNA is used in translation for direct protein synthesis using the genetic triplet code and how this has led to the complete sequence of bases in human DNA - the human genome project - in 2000. Explain how genetic engineering can be used to modify the DNA in foods to produce higher yields and crops that are more resistant to disease etc. Get students to debate the pros and cons of genetic engineering. | Study Guide
Page 163 QuestionsFor ten 'quiz' questions (for quick testing of knowledge and understanding with the answers explained) see MC test: Nucleic acids. For short-answer questions see Nucleic acids questions together with the worked answers on a separate page Nucleic acids answers. Vocabulary listnucleotide |
Teaching slides
Teachers may wish to share these slides with students for learning or for reviewing key concepts.
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Other resources
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A useful image to show the double helix and the hydrogen bonding that occurs between the bases. You can download a similar image of this from Brooklyn.
2. Information on how students can make their own model of DNA. If you are short of cash then there is an interesting article in School Science on how students can make a model using just recycled materials.
3. A TED talk by James Watson on the discovery of DNA and some of today's DNA mysteries.
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4. A useful podcast from the Royal Society of Chemistry on
5. For a reasonably balanced article on the advantages and disadvantages of GM food see WebMD.
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For those with a real interest in the subject it is well worth reading the personal account of James Watson in his acclaimed book The Double Helix.