1. 1
   A plant geneticist studying drought tolerance in wheat (Triticum aestivum) sequences a short section of one strand of a DNA molecule. The base sequence of this strand is ATGCCGTTAG. She then hydrolyses a separate sample of the wheat DNA completely into its smallest component molecules.

   (a)

   What is the base sequence of the complementary strand paired with this section?

   [1 mark]

   • ATACGGCAATC
   • BUACGGCAAUC
   • CATGCCGTTAG
   • DGCATTACCGA

   (b)

   Which set of molecules would be produced by complete hydrolysis of the wheat DNA?

   [1 mark]

   • ARibose, phosphate, adenine, cytosine, guanine and uracil
   • BGlucose, phosphate, adenine, cytosine, guanine and thymine
   • CDeoxyribose, amino acids, adenine, cytosine, guanine and thymine
   • DDeoxyribose, phosphate, adenine, cytosine, guanine and thymine

   (c)

   Calculate the number of different base sequences possible for a length of DNA of ten nucleotides, as in the strand sequenced, and comment on what this shows about DNA as an information store.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   A virologist isolates the nucleic acid from a newly discovered virus that infects tomato plants. Chemical analysis shows that it contains the sugar ribose and the base uracil. The base composition is adenine 31%, uracil 22%, guanine 26% and cytosine 21%.

   (a)

   What type of nucleic acid does this virus contain?

   [1 mark]

   • ADouble-stranded DNA
   • BSingle-stranded RNA
   • CDouble-stranded RNA
   • DSingle-stranded DNA

   (b)

   A student says this virus shows that DNA is not the genetic material of all living organisms. Which is the best response?

   [1 mark]

   • AThe student is correct, because the virus uses RNA as its genetic material
   • BThe student is not correct, because the virus must also contain thymine
   • CThe student is not correct, because viruses are not considered to be living organisms
   • DThe student is correct, because RNA is the genetic material of plants

   (c)

   Explain how RNA nucleotides are linked to form the RNA polymer.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   Scientists inserted the human gene for insulin into the bacterium Escherichia coli. The bacteria made a protein with exactly the same amino acid sequence as human insulin. Comparisons of many species show that each of the 64 codons codes for the same amino acid in almost all organisms. A few exceptions are known: in vertebrate mitochondria the codon UGA codes for the amino acid tryptophan instead of acting as a stop codon, and in some ciliate protists such as Tetrahymena the stop codons UAA and UAG code for glutamine.

   (a)

   Explain how the insulin result supports the idea that the genetic code is conserved across life forms.

   [3 marks]

   (b)

   Evaluate whether the exceptions described weaken the use of the genetic code as evidence for universal common ancestry.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   Researchers are developing DNA as a long-term data storage medium. Computer files are converted into sequences of the four bases, and DNA with those sequences is synthesised. It has been estimated that one gram of DNA could in theory store about 215 million gigabytes of data. DNA extracted from a horse bone about 700,000 years old, preserved in permafrost, was still sequenced, although it was broken into short fragments. At present, synthesising DNA to store one megabyte of data costs thousands of dollars, and reading it back by sequencing takes hours.

   (a)

   Explain how the structure of DNA allows genetic information to be stored and copied accurately.

   [6 marks]

   (b)

   Using the information provided, evaluate DNA as a medium for long-term data storage.

   [6 marks]

   Total for question 4: 12 marks

End of questions