7. Nucleic acids

7. Nucleic acids

7.1 DNA structure and replication

Nature of science:

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

Understandings:

  • 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.

  • 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.

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

7.2 Transcription and gene expression

Nature of science:

  • Looking for patterns, trends and discrepancies - there is mounting evidence that the environment can trigger heritable changes in epigenetic factors.

Understandings:

  • 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.

Application 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

Nature of science:

  • 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.

Understandings:

  • 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.

Application 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.

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