Obtain a gene for transfer
Compare extracting donor DNA, copying mRNA and chemical synthesis.
A Level; Cambridge International Biology 9700, 2028–2030. This is curriculum teaching, not medical advice or a claim that a treatment is suitable for an individual. Written responses are self-checked
Before you begin
- DNA stores genetic information; genes can be expressed to produce RNA and protein.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
Which enzyme can make cDNA from mRNA?
What you will learn
- Describe three routes for obtaining genes for transfer
- Relate each route to the sequence used in the construct
Build the explanation
A gene for transfer may be extracted from donor DNA, synthesised from donor mRNA using reverse transcriptase to make complementary DNA (cDNA), or synthesised chemically from nucleotides. The mRNA route can provide a coding sequence without eukaryotic introns, which is useful when a host cannot splice them.
Use the evidence carefully
A cDNA copy represents the processed transcript and may not include regulatory sequences. A chemically synthesised sequence depends on the required base sequence being known. The route chosen depends on the organism, gene and expression system.
Pause and explain
Why might processed mRNA be used to make a bacterial expression sequence?
Worked example
A human coding sequence is to be expressed in a bacterium that cannot splice introns. Which source route may be useful?
Show the worked solution
- Use processed mRNA as the template.
- Reverse transcriptase makes cDNA.
Answer Use donor mRNA and reverse transcriptase to make cDNA, which represents the expressed coding sequence without introns.
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Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
A known sequence is ordered and built from nucleotides. Which gene-source method is used?
Give me a hint
Identify the relevant material or technology and explain each step.
Compare my reasoning
- The sequence is assembled chemically.
- It is chemical synthesis.
Chemical synthesis from nucleotides.
Look for these in your work
- Method identified
- Steps linked to function
A donor DNA sample contains the gene sequence. What is the direct extraction route?
Give me a hint
Use the stated example to justify a conclusion.
Compare my reasoning
- Use the donor DNA as source material.
- Extract the required gene.
Extract the gene from donor DNA.
Look for these in your work
- Evidence interpreted
- Limits stated
Why might processed mRNA be used to make a bacterial expression sequence?
Give me a hint
Apply the idea to a new application or trade-off.
Compare my reasoning
- The mature transcript has had eukaryotic introns removed.
- Reverse transcriptase makes cDNA.
Reverse transcriptase can make cDNA from processed mRNA, producing a coding sequence that a bacterium does not need to splice.
Look for these in your work
- Application explained
- Claim appropriately bounded
Common mistakes
- Saying cDNA is copied from genomic DNA
- Assuming the mRNA copy includes a promoter
What can you explain now?
Which enzyme can make cDNA from mRNA?
Compare with the explanation
Reverse transcriptase
It synthesises a DNA copy using RNA as the template.
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Make it stick
What are the three specified ways to obtain a gene for transfer?
If you needed the solution, close it and solve the problem again from a blank page. A correct answer is stronger when you can explain the reason for each step.