Unit 3: Gene Cloning

Recombinant DNA (rDNA) is a form of artificial DNA that is created by combining two or more sequences that would not normally occur together. It is the cornerstone of modern genetic engineering and gene cloning.

The General Process of Creating rDNA

1. Isolation: Extracting the target DNA (gene of interest) and the vector DNA.
2. Digestion: Cutting both the target and vector DNA with the same Restriction Endonuclease to create compatible "sticky" or "blunt" ends.
3. Ligation: Joining the two DNA molecules together using the enzyme DNA Ligase to form a single recombinant molecule.

Once the recombinant DNA is created, it must be introduced into a host cell, usually E. coli, to be copied (cloned).

Bacterial Transformation

This is the process by which bacteria take up foreign DNA from their environment. Cells are made "competent" through chemical treatment (e.g., Calcium Chloride) followed by a brief Heat Shock, which creates temporary pores in the cell membrane.

Selection of Recombinant Clones

Not every bacterial cell will take up a plasmid, and not every plasmid will contain the target gene. Selection is necessary to identify the successful clones.

• Antibiotic Resistance: Using antibiotic-containing media to kill cells that did not take up the plasmid (which carries resistance genes like ampR).
• Blue-White Screening: A technique where successful insertion of the target gene disrupts a reporter gene (like lacZ), causing successful recombinants to appear as white colonies instead of blue.

The Polymerase Chain Reaction (PCR) can be used to specifically amplify a gene of interest before it is inserted into a vector, making the cloning process much more efficient.

Steps in PCR

• Denaturation: Heating the DNA to ~94°C to separate the strands.
• Annealing: Cooling to ~55°C to allow primers to bind to specific sequences.
• Extension: Heating to ~72°C for Taq Polymerase to synthesize new DNA strands.

In cloning, primers are often designed to include restriction sites, allowing the amplified product to be easily digested and ligated into a vector.

A DNA library is a collection of cloned DNA fragments that together represent an entire genome or the expressed genes of a cell.

Genomic Library

• Source: Total genomic DNA.
• Content: Contains all DNA sequences, including exons, introns, and non-coding regions.
• Construction: Genomic DNA is digested and fragments are ligated into high-capacity vectors like BACs or YACs.

cDNA Library

• Source: Messenger RNA (mRNA) extracted from a specific tissue.
• Content: Represents only the genes being expressed (active) at that time; does not contain introns.
• Construction: mRNA is converted into double-stranded "complementary DNA" (cDNA) using the enzyme Reverse Transcriptase before being cloned.

Once a library is created, it must be searched (screened) to find the specific clone containing the gene of interest.

• Genetic Selection: Designing the experiment so that only cells containing the desired gene can survive (e.g., providing a gene that fixes a nutritional deficiency in the host).
• Colony Hybridization: Using a labeled DNA or RNA "probe" that is complementary to the target gene. The probe binds (hybridizes) to the target DNA in the specific colony that contains it.
• PCR Screening: Using gene-specific primers to identify which pool of clones contains the target sequence.

6. Exam Focus: Tips and FAQs

Common Pitfalls

• Mistake: Thinking PCR creates recombinant DNA. Correction: PCR only amplifies DNA; ligation is needed to make it recombinant.
• Mistake: Assuming a cDNA library contains all the organism's genes. Correction: It only contains genes that were being transcribed into mRNA at the time of collection.

Frequently Asked Questions

Q: What is a "probe" in colony hybridization?
A: A probe is a short, single-stranded DNA or RNA fragment that is labeled (with radioactivity or fluorescence) and has a sequence complementary to the gene being sought.

Q: Why is Reverse Transcriptase used for cDNA libraries?
A: Because libraries are made of DNA, but the starting material for a cDNA library is RNA. Reverse transcriptase is the only enzyme that can synthesize DNA using an RNA template.