Nucleotides, DNA structure and replicationEdexcel A-Level Biology A: Subtopic test

1. 1
   A student extracts DNA and RNA from the same cells and analyses the nucleotides that each is made of.

   (a)

   Which component is found in the nucleotides of RNA but not in the nucleotides of DNA?

   [1 mark]

   • APhosphate
   • BRibose
   • CThymine
   • DAdenine

   (b)

   A sample of double-stranded DNA from the cells contains 30% adenine. What is the percentage of guanine in this sample?

   [1 mark]

   • A30%
   • B40%
   • C60%
   • D20%

   (c)

   Describe how two DNA nucleotides are joined together in a polynucleotide.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   Two samples of double-stranded DNA of the same length, sample X and sample Y, are heated slowly until the two strands of each separate. In sample X, 20% of the bases are adenine. In sample Y, 35% of the bases are adenine.

   (a)

   Which bonds are broken when the two strands of the DNA separate on heating?

   [1 mark]

   • APhosphodiester bonds between the nucleotides of each strand
   • BIonic bonds between phosphate groups
   • CHydrogen bonds between complementary bases
   • DPeptide bonds between bases

   (b)

   What is the percentage of guanine in sample X?

   [1 mark]

   • A30%
   • B20%
   • C60%
   • D40%

   (c)

   Explain why the strands of sample X separate at a higher temperature than those of sample Y.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   A section of one strand of a DNA molecule has the base sequence ATGCCGTA. This strand acts as a template during DNA replication in a dividing cell.

   (a)

   Describe how a new strand is formed on this template during DNA replication.

   [3 marks]

   (b)

   Deduce the base sequence of the new strand formed on this template, and explain why DNA replication is described as semi-conservative.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   In an experiment to test how DNA replicates, bacteria were grown for many generations in a medium containing only the heavy isotope ¹⁵N, so that all of their DNA contained ¹⁵N. They were then transferred to a medium containing only ¹⁴N and allowed to divide. DNA was extracted from samples taken after one generation and after two generations in the ¹⁴N medium, and centrifuged in a caesium chloride solution, in which DNA forms a band according to its density. After one generation there was a single band, between the positions of ¹⁵N DNA and ¹⁴N DNA. After two generations there were two bands of equal thickness, one at the intermediate position and one at the position of ¹⁴N DNA.

   (a)

   Explain how these results support the semi-conservative model of DNA replication, and why they are not consistent with the conservative model or the dispersive model.

   [6 marks]

   (b)

   Explain why the bacteria were grown for many generations in the ¹⁵N medium before being transferred. Predict, with reasoning, the results after three generations in the ¹⁴N medium.

   [6 marks]

   Total for question 4: 12 marks

End of questions