Home · A-Level Biology 9700 · Genetic technology · Recombinant DNA · Practice
A2 · Practice questions · Recombinant DNA

Get it, join it, prove it.

Six original Cambridge-style questions on making recombinant DNA and transferring a gene into an organism so that it is expressed.

Original questions All questions on this page are original work, written in the Cambridge AS & A Level style. They are not from past papers. They test the same concepts and skills the syllabus rewards.
Keep these straight

Source, vector, switch, signal.

01
Recall
[2 marks]

Define recombinant DNA and explain what is meant by genetic engineering.

  • Recombinant DNA is DNA made by joining DNA from two different sources (often different species). ✓
  • Genetic engineering is the deliberate manipulation of genetic material to modify specific characteristics of an organism, for example by transferring a gene so that it is expressed in another organism. ✓
02
Explain
[4 marks]

Explain how a gene for human insulin can be obtained from the mRNA of pancreatic β cells, and why this method is preferred to cutting the gene from human DNA when the gene is to be expressed in bacteria.

  • β cells make insulin, so they contain many copies of insulin mRNA, which is extracted. ✓
  • Reverse transcriptase uses the mRNA as a template to make a single strand of complementary DNA (cDNA). ✓
  • DNA polymerase makes the second strand, giving double-stranded DNA of the gene. ✓
  • mRNA has already had its introns removed, so the cDNA has no introns; bacteria cannot remove introns, so a gene cut from human DNA would not give a functional protein. ✓
03
Explain
[4 marks]

Describe the roles of restriction endonucleases and DNA ligase in inserting a gene into a plasmid.

  • A restriction endonuclease cuts DNA at a specific base sequence (restriction site), cutting the gene out of the donor DNA. ✓
  • The same restriction endonuclease cuts the plasmid open, so both have staggered cuts with complementary sticky ends. ✓
  • The sticky ends of the gene and the plasmid pair by complementary base pairing (hydrogen bonds). ✓
  • DNA ligase forms phosphodiester bonds, joining the sugar-phosphate backbones to make a recombinant plasmid. ✓
04
Explain
[2 marks]

Explain why a promoter may have to be transferred into an organism as well as the desired gene.

  • RNA polymerase must bind to a promoter next to the gene to start transcription. ✓
  • The promoter from the donor (eukaryotic) gene may not be recognised by the host's RNA polymerase (or may be absent from the inserted DNA), so without a suitable promoter the gene would not be expressed. ✓
05
Analysis
[3 marks]

Bacteria were treated with a recombinant plasmid carrying a human gene and a gene for green fluorescent protein (GFP). They were spread on an agar plate and 400 colonies grew. Under ultraviolet light, 12 colonies glowed green. Calculate the percentage of colonies expressing the recombinant plasmid and explain why the marker gene was needed.

  • 12 / 400 × 100 = 3 %. ✓
  • Only a small proportion of bacteria take up the plasmid, so most colonies do not contain the gene. ✓
  • Colonies that fluoresce are making GFP, so they contain and are expressing the recombinant DNA; these can be selected and the rest discarded. ✓
06
Recall
[3 marks]

Outline how a gene can be synthesised chemically, and give two advantages of this method.

  • The base sequence of the gene is known, or is worked out from the amino acid sequence of the protein (using the genetic code). ✓
  • Nucleotides are joined one at a time in the correct order by a machine; short sections are joined to make the whole gene. ✓
  • Advantages (any one, plus the above or a second): there are no introns; codons can be chosen to suit the host organism; no donor tissue or mRNA is needed; sequences such as restriction sites can be added at the ends. ✓

Mark this once you have attempted all six and checked your working. It records a Practiced badge on the topic and adds a one-time bonus. Revealing the solutions alone does not count.