Principles of gene technology
CIE A-Level BiologyΒ· 30 min read
1. Key Enzymes for Genetic Manipulationβ β ββββ± 15 min
Restriction Endonucleases
Bacterial enzymes that cut double-stranded DNA at specific palindromic recognition sequences called restriction sites. They can produce either sticky ends (overhanging single-stranded DNA) or blunt (flush-cut) ends.
Example:
EcoRI cuts at the 6-base pair sequence 5'-GAATTC-3'
DNA Ligase
Enzyme that catalyses the formation of phosphodiester bonds between adjacent DNA nucleotides, sealing nicks in the sugar-phosphate backbone of joined DNA fragments.
Sticky ends are the most useful for genetic engineering, as complementary overhangs spontaneously hydrogen bond (anneal) to each other, holding the target gene and vector together before ligation.
Explain why cutting DNA with a restriction enzyme that produces sticky ends is useful for inserting a target gene into a plasmid.
- 1
Write the double-stranded sequence of a restriction site cut after the first base:
- 2
5'-A | AGCTT-3' \n 3'-TTCGA | A-5'
- 3
Cutting leaves overhanging single-stranded sequences (sticky ends) on both the cut target gene and cut plasmid.
- 4
Complementary sticky ends on the gene and plasmid can anneal via hydrogen bonding between complementary base pairs, holding the two fragments together.
- 5
This makes ligation by DNA ligase far more efficient than joining blunt-ended fragments.
Exam tip:
Always specify that DNA ligase forms phosphodiester bonds, not hydrogen bonds between sticky ends. Hydrogen bonds form spontaneously.
2. Vectors and Recombinant DNA Formationβ β β βββ± 20 min
A vector is a DNA molecule used to carry a foreign target gene into a host cell, where it can be replicated and expressed. The most common vectors are bacterial plasmids (small circular DNA separate from the bacterial chromosome) and bacteriophages (bacterial viruses).
Recombinant DNA
A hybrid DNA molecule formed by joining DNA fragments from two different unrelated organisms.
Outline how recombinant DNA containing the human insulin gene is produced.
- 1
Cut both the human DNA containing the insulin gene and the plasmid vector with the same restriction endonuclease.
- 2
This produces complementary sticky ends on both the cut insulin gene and cut plasmid.
- 3
Mix the cut fragments together; complementary sticky ends anneal via hydrogen bonding.
- 4
Add DNA ligase to catalyse formation of phosphodiester bonds, sealing the backbone to form a recombinant plasmid containing the insulin gene.
3. In Vivo vs In Vitro Gene Cloningβ β β βββ± 15 min
Gene cloning is the process of producing many identical copies of a target gene of interest. There are two main approaches used in gene technology:
Key differences between the two methods are outlined below:
In vivo cloning
Copies of the target gene are made inside living host cells. The recombinant vector is taken up by host bacteria, which replicate to produce many copies of the gene.
+ Pros: Can produce large amounts of the gene product (e.g. insulin protein), can be used to modify whole organisms
β Cons: Slow, requires multiple preparation steps
In vitro cloning (PCR)
Copies of the target gene are amplified directly in a reaction tube, outside a living cell, via repeated cycles of DNA replication.
+ Pros: Extremely fast, can amplify tiny amounts of starting DNA
β Cons: Only produces DNA copies, not protein product, higher risk of mutation
Check your understanding:
Which method is used to amplify DNA from a tiny crime scene sample?
In vivo cloning
In vitro (PCR) cloning
Both
Neither
Reveal answer
1 βCorrect. PCR rapidly amplifies very small amounts of starting DNA, making it ideal for forensic analysis. In vivo cloning is too slow and requires much more starting material.
4. Requirements for Successful Gene Expressionβ β β βββ± 15 min
Inserting a target gene into a host cell is not enough for expression (transcription and translation to produce the protein). The gene must be accompanied by regulatory sequences that the host cell can recognise.
A promoter sequence is required for the host's RNA polymerase to bind and initiate transcription. A terminator sequence is needed to stop transcription at the correct point.
Explain why an unmodified human genomic gene often cannot be expressed correctly in a bacterial cell.
- 1
Human genomic DNA contains non-coding introns within genes.
- 2
Bacteria do not have the spliceosome machinery required to remove introns from pre-mRNA.
- 3
Additionally, the human gene's native promoter is not recognised by bacterial RNA polymerase, so transcription will not initiate.
- 4
To fix this, scientists use complementary DNA (cDNA) made from processed human mRNA (which has no introns) and link it to a bacterial promoter before insertion.
Exam tip:
Introns are the most common exam answer for why eukaryotic genes do not express in prokaryotes β always mention this in essays or structured questions.
5. Common Pitfalls
Wrong move:
Claiming DNA ligase forms hydrogen bonds between sticky ends.
Why:
Hydrogen bonds form spontaneously between complementary base pairs, DNA ligase has a different role.
Correct move:
State that sticky ends anneal via spontaneous hydrogen bonding, then DNA ligase seals the sugar-phosphate backbone by forming phosphodiester bonds.
Wrong move:
Using different restriction enzymes to cut the target gene and plasmid.
Why:
Different enzymes produce different non-complementary sticky ends that cannot anneal, so no recombinant DNA can form.
Correct move:
Use the same restriction enzyme to cut both the gene and plasmid to produce complementary sticky ends.
Wrong move:
Confusing gene cloning with whole organism cloning.
Why:
Most gene technology questions ask about cloning the gene of interest, not an entire organism, leading to off-topic answers.
Correct move:
When 'cloning' is mentioned in gene technology, refer to it as producing many identical copies of the target gene of interest.
Wrong move:
Stating bacteria can splice introns out of eukaryotic genomic DNA.
Why:
Prokaryotes lack spliceosome machinery and do not process introns naturally.
Correct move:
Explain that intron-free cDNA (made from processed mRNA) is used to express eukaryotic genes in prokaryotes.
6. Quick Reference Cheatsheet
Component | Key Function |
|---|---|
Restriction endonuclease | Cuts DNA at specific recognition sites |
DNA ligase | Forms phosphodiester bonds to join DNA fragments |
Plasmid vector | Carries foreign gene into host bacterial cell |
Promoter | RNA polymerase binding site for transcription initiation |
Sticky end | Complementary overhang for efficient DNA annealing |
In vivo cloning | Produces gene copies + protein product in host cells |
In vitro (PCR) | Rapid DNA amplification from small starting samples |
When this came up on past exams
AI-estimated based on syllabus patterns β cross-check with official past papers for accuracy. Use only as revision-focus signals.
- 2022 Β· 2
Role of enzymes in gene technology
- 2023 Β· 4
Recombinant DNA production outline
- 2021 Β· 1
In vivo vs in vitro cloning comparison
Going deeper
What's Next
The core principles of gene technology covered here form the foundation for all applied genetic engineering topics, which are heavily weighted in CIE A-Level Biology Paper 4. Mastering these concepts will help you work through longer structured questions that require you to outline the process of creating genetically modified organisms, or evaluate the ethics of genetic modification. These principles also underpin other common exam topics like DNA profiling for forensics and paternity testing.