QUESTION 135 (5 marks)
Initially, both strands of one double-stranded DNA molecule carry a stable label. New nucleotides carry no label. Replication is semiconservative: each daughter molecule contains one template strand and one newly synthesised strand. DNA completes two rounds of replication with no loss or exchange of label.
a) Determine the number of double-stranded molecules and the number of labelled strands after two rounds. [2 marks]
b) Draw or describe the labelled/unlabelled strand composition of the four molecules. [2 marks]
c) Calculate the percentage of molecules containing any label. [1 mark]
Practice marking scheme
Answer
After two rounds there are four molecules: two with one labelled strand and two with no labelled strands. There are still two labelled strands, so 50% of molecules contain label.
Working
Each original labelled strand persists and templates an unlabelled partner in round 1. In round 2, each labelled strand again templates a new unlabelled partner, while each unlabelled round-1 strand templates another unlabelled strand. Thus two of four molecules are hybrids and none has two labelled strands.
Marking criteria
- States four double-stranded molecules. [1 mark]
- States two labelled strands. [1 mark]
- Shows/describes two molecules with one labelled and one unlabelled strand. [1 mark]
- Shows/describes two wholly unlabelled molecules and no wholly labelled molecule. [1 mark]
- Calculates 50%. [1 mark]
Practice question aligned to the current QCAA syllabus; review the worked solution and marking criteria.
View the QCAA syllabusCompare your working with the guide above.