Topic 7 AHL - Nucleic Acids (9hs)

7.1 DNA Structure and Replication

Making careful observations—Rosalind Franklin’s X-ray diffraction provided crucial evidence that DNA is a double helix. (1.8)

Theory of Knowledge

Highly repetitive sequences were once classified as “junk DNA” showing a degree of confidence that it had no role. To what extent do the labels and categories used in the pursuit of knowledge affect the knowledge we obtain?


• Nucleosomes help to supercoil the DNA.
• DNA structure suggested a mechanism for DNA replication.
• DNA polymerases can only add nucleotides to the 3’ end of a primer.
• DNA replication is continuous on the leading strand and discontinuous on the lagging strand.
• DNA replication is carried out by a complex system of enzymes.
• Some regions of DNA do not code for proteins but have other important functions


Applications and Skills

• Application: Rosalind Franklin’s and Maurice Wilkins’ investigation of DNA structure by X-ray diffraction.

⇒ Rosalind Franklin’s X-ray Diffraction Pattern of DNA by Biotech

• Application: Use of nucleotides containing dideoxyribonucleic acid to stop DNA replication in preparation of samples for base sequencing.
• Application: Tandem repeats are used in DNA profiling.

• Skill: Analysis of results of the Hershey and Chase experiment providing evidence that DNA is the genetic material.

⇒Hershey-Chase link

• Skill: Utilization of molecular visualization software to analyse the association between protein and DNA within a nucleosome.

7.2 Transcription and Gene expression

Looking for patterns, trends and discrepancies—there is mounting evidence that the environment can trigger heritable changes in epigenetic factors. (3.1)

Theory of Knowledge

The nature versus nurture debate concerning the relative importance of an individual’s innate qualities versus those acquired through experiences is still under discussion. Is it important for science to attempt to answer this question?


• Transcription occurs in a 5’ to 3’ direction.
• Nucleosomes help to regulate transcription in eukaryotes.
• Eukaryotic cells modify mRNA after transcription.
• Splicing of mRNA increases the number of different proteins an organism can produce
• Gene expression is regulated by proteins that bind to specific base sequences in DNA
• The environment of a cell and of an organism has an impact on gene expression

Transcription and trasnlation by LearnGeneticsUtah

Applications and Skills

Application: The promoter as an example of non-coding DNA with a function.
Skill: Analysis of changes in the DNA methylation patterns.

7.3 Translation

“Developments in scientific research follow improvements in computing—the use of computers has enabled scientists to make advances in bioinformatics applications
such as locating genes within genomes and identifying conserved sequences. (3.7)”


Initiation of translation involves assembly of the components that carry out the process.
• Synthesis of the polypeptide involves a repeated cycle of events.
• Disassembly of the components follows termination of translation.
• Free ribosomes synthesize proteins for use primarily within the cell.
• Bound ribosomes synthesize proteins primarily for secretion or for use in lysosomes.
• Translation can occur immediately after transcription in prokaryotes due to the absence of a nuclear membrane.
• The sequence and number of amino acids in the polypeptide is the primary structure.
• The secondary structure is the formation of alpha helices and beta pleated sheets stabilized by hydrogen bonding.
• The tertiary structure is the further folding of the polypeptide stabilized by interactions between R groups.
• The quaternary structure exists in proteins with more than one polypeptide chain.

Applications and Skills

Application: tRNA-activating enzymes illustrate enzyme–substrate specificity and the role of phosphorylation.

Skill: Identification of polysomes in electron micrographs of prokaryotes and eukaryotes.

Skill: The use of molecular visualization software to analyse the structure of eukaryotic ribosomes and a tRNA molecule.

⇒JMol link

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