Nucleic acids

Nature of science:

Scientific method—the discovery of the structure of DNA is a good example of different approaches to solving the same problem. Scientists used models and diffraction experiments to develop the structure of DNA. (1.3) Developments in scientific research follow improvements in apparatus—double helix from X-ray diffraction provides explanation for known functions of DNA. (3.7)

• Nucleotides are the condensation products of a pentose sugar, phosphoric acid and a nitrogenous base—adenine (A), guanine (G), cytosine (C), thymine (T) or uracil (U).
• Polynucleotides form by condensation reactions.
• DNA is a double helix of two polynucleotide strands held together by hydrogen bonds.
• RNA is usually a single polynucleotide chain that contains uracil in place of thymine, and a sugar ribose in place of deoxyribose.
• The sequence of bases in DNA determines the primary structure of proteins synthesized by the cell using a triplet code, known as the genetic code, which is universal.
• Genetically modified organisms have genetic material that has been altered by genetic engineering techniques, involving transferring DNA between species.

Applications and skills:

• Explanation of the stability of DNA in terms of the interactions between its hydrophilic and hydrophobic components.
• Explanation of the origin of the negative charge on DNA and its association with basic proteins (histones) in chromosomes.
• Deduction of the nucleotide sequence in a complementary strand of DNA or a molecule of RNA from a given polynucleotide sequence.
• Explanation of how the complementary pairing between bases enables DNA to replicate itself exactly.
• Discussion of the benefits and concerns of using genetically modified foods.

• The Human Genome Project was an international research programme whose goal was to complete the mapping and sequencing of all the genes in the human genome.
• The policies on the labelling of genetically modified (GM) foods vary greatly in different countries.
• Most of the genetically modified organisms are protected by international patents. What effect does this have on the global economy and scientific community?

Theory of knowledge:

• DNA stores information but not knowledge.
• What are the differences between information and knowledge?
• The Nobel Prize in Physiology or Medicine 1962 was awarded jointly to Crick, Watson and Wilkins "for their discoveries concerning the molecular structure of nucleic acids and its significance for information transfer in living material". What is the role of collaboration in advancing knowledge?
• The existence of DNA databases opens up questions of individual privacy and the extent to which government has the right of access to personal information. Who has the right to access knowledge of an individual’s DNA?

• Structures of the nitrogenous bases and ribose and deoxyribose sugars are given in the data booklet in section 34.
• Knowledge of the different forms of RNA is not required.
• Details of the process of DNA replication are not required.
• Limit expression of DNA to the concept of a four-unit base code determining a twenty-unit amino acid sequence. Details of transcription and translation are not required.

• Knowledge of DNA sequencing has transformed several aspects of legal enquiry, including forensics and paternity cases. It is also widely used in studies of ancestry and human migration.
• DNA sequencing is an important aspect of the study of biochemical evolution.

Syllabus and cross-curricular links:
Topic 4.4—hydrogen bonding, intermolecular interactions
Topic 8.1—acid–base interactions
Biology topics 2.6 and 7.1—DNA and RNA structure
Aims:

• Aim 5: The story of the rivalry between the different teams involved in the elucidation of DNA structure in the 1950s is an example of a failure of effective collaboration and communication during scientific activities.
• Aim 6: Experiments could include DNA extraction from cells and investigation of its physical properties, and model building exercises of DNA structure, including the specific base pairings between a purine and a pyrimidine.
• Aim 7: Databases exist of genetic sequences from different organisms.
• Aim 8: Many ethical questions are raised by our knowledge of the human genome, including cloning, genetic engineering, gene therapy, and so on.