QUESTION 62 (5 marks)
One strand of a double-stranded DNA fragment is .
a) Write its complementary strand, showing direction at both ends. [2 marks]
b) Determine the percentage of adenine bases in the entire double-stranded fragment. [2 marks]
c) Explain why the number of guanine bases must equal the number of cytosine bases in the entire fragment. [1 marks]
Practice marking scheme
Answer
a) 3′-TTCAGGCT-5′. b) 25%. c) Each G pairs with a C.
Working
The antiparallel complement is . The given strand contains three A and one T, so the whole double-stranded fragment contains four A out of 16 bases: . Every G on either strand has a complementary C on the other, so total G equals total C.
Marking criteria
- Writes the correct complementary sequence. [1 mark]
- Shows antiparallel 3′ to 5′ orientation for the complement. [1 mark]
- Counts four adenines in the entire double-stranded fragment. [1 mark]
- Calculates 25%. [1 mark]
- Explains equality of G and C by complementary pairing. [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.