Nature of science:

Obtaining evidence for scientific theories—Meselson and Stahl obtained evidence for the semi-conservative replication of DNA. (1.8)

• The replication of DNA is semi-conservative and depends on complementary base pairing.
• Helicase unwinds the double helix and separates the two strands by breaking hydrogen bonds.
• DNA polymerase links nucleotides together to form a new strand, using the pre-existing strand as a template.
• Transcription is the synthesis of mRNA copied from the DNA base sequences by RNA polymerase.
• Translation is the synthesis of polypeptides on ribosomes.
• The amino acid sequence of polypeptides is determined by mRNA according to the genetic code.
• Codons of three bases on mRNA correspond to one amino acid in a polypeptide.
• Translation depends on complementary base pairing between codons on mRNA and anticodons on tRNA.

Applications and skills:

• Application: Use of Taq DNA polymerase to produce multiple copies of DNA rapidly by the polymerase chain reaction (PCR).
• Application: Production of human insulin in bacteria as an example of the universality of the genetic code allowing gene transfer between species.
• Skill: Use a table of the genetic code to deduce which codon(s) corresponds to which amino acid.

• Syllabus and cross-curricular links:
• Biology
• Topic 3.5 Genetic modification and biotechnology
• Topic 7.2 Transcription and gene expression
• Topic 7.3 Translation

Aims:

• Aim 8: There are ethical implications in altering the genome of an organism in order to produce proteins for medical use in humans.

• Skill: Analysis of Meselson and Stahl’s results to obtain support for the theory of semi-conservative replication of DNA.
• Skill: Use a table of mRNA codons and their corresponding amino acids to deduce the sequence of amino acids coded by a short mRNA strand of known base sequence.
• Skill: Deducing the DNA base sequence for the mRNA strand.

Guidance:

• The different types of DNA polymerase do not need to be distinguished.