Chapter 20: Biotechnology – DNA Libraries, Gene Expression, and DNA Analysis

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Biotechnology: DNA Libraries and Gene Cloning

Genomic Libraries

Genomic libraries are essential tools in biotechnology, allowing researchers to store and access cloned genes from an organism's genome. These libraries are collections of cell clones, each carrying a specific DNA segment from a foreign genome integrated into a vector such as a plasmid, bacterial artificial chromosome (BAC), or bacteriophage.

Genomic Library: A complete set of clones containing DNA fragments that together represent the entire genome of an organism.
Vectors: Plasmids, BACs, and bacteriophages are used to carry and replicate foreign DNA segments.
Storage: Libraries are often stored in multiwell plates, with each well containing a unique clone.
BACs: Large plasmids capable of carrying large DNA inserts, useful for constructing libraries with large genomic fragments.

Example: A 384-well plate can store hundreds of unique clones, each representing a different segment of the genome.

Diagram of bacterial clones, recombinant plasmids, and BAC library construction Plasmid library construction with bacterial clones BAC library construction and storage in multiwell plates

cDNA Libraries

Complementary DNA (cDNA) libraries are constructed by cloning DNA synthesized in vitro from mRNA using reverse transcriptase. These libraries represent only the genes expressed in the original cells, not the entire genome.

cDNA Library: Contains DNA copies of mRNA, reflecting the subset of genes actively transcribed in a cell.
Reverse Transcriptase: Enzyme used to synthesize DNA from mRNA templates.
Applications: Useful for studying gene expression and producing proteins without introns.

Example: cDNA libraries are used to clone eukaryotic genes for expression in bacteria, avoiding issues with introns.

Reverse transcription of mRNA to cDNA Reverse transcriptase and DNA polymerase in cDNA synthesis Completion of cDNA synthesis Stepwise diagram of cDNA library construction

Screening DNA Libraries for Genes of Interest

Nucleic Acid Hybridization

To identify clones carrying a gene of interest, researchers use nucleic acid probes—short, labeled sequences complementary to the target gene. This process is called nucleic acid hybridization.

Probe: A labeled DNA or RNA sequence that binds specifically to the gene of interest.
Labeling: Probes are tagged with radioactive isotopes or fluorescent molecules for detection.
Screening: Clones are transferred to a membrane and probed to identify those containing the desired gene.

Example: Screening a library for the β-globin gene using a complementary probe.

Screening a DNA library with a nucleic acid probe

Expressing Cloned Genes

Bacterial Expression Systems

Cloned eukaryotic genes can be expressed in bacterial cells, but differences in gene regulation and the presence of introns pose challenges. Expression vectors with prokaryotic promoters are used, and cDNA (lacking introns) is preferred for bacterial expression.

Expression Vector: A vector with a strong prokaryotic promoter to drive gene expression in bacteria.
Introns: Eukaryotic genes often contain introns, which bacteria cannot process; cDNA is used to avoid this issue.

Example: Producing human insulin in bacteria using a cDNA clone.

Eukaryotic Expression Systems

Eukaryotic cells, such as yeast, are used to express eukaryotic genes, especially when post-translational modifications are required. Yeast cells are easy to grow and possess plasmids, making them suitable hosts.

Post-Translational Modification: Many eukaryotic proteins require modifications (e.g., glycosylation) not possible in bacteria.
Electroporation: A method to introduce DNA into eukaryotic cells by applying an electrical pulse.
Microinjection: DNA can also be injected directly into cells using fine needles.

Amplifying DNA: Polymerase Chain Reaction (PCR)

PCR Technique

The polymerase chain reaction (PCR) is a powerful method for amplifying specific DNA sequences in vitro. It involves repeated cycles of denaturation, annealing, and extension, resulting in exponential amplification of the target DNA.

Steps: Denaturation (heating), annealing (cooling and primer binding), extension (DNA synthesis).
Applications: Used for cloning, diagnostics, and forensic analysis.
Equation: Number of DNA molecules after n cycles:

Example: Amplifying a gene from a small DNA sample for further study.

PCR amplification cycles PCR cycle 2 yields 4 molecules PCR cycle 3 yields 8 molecules

Analyzing DNA: Gel Electrophoresis and Southern Blotting

Gel Electrophoresis

Gel electrophoresis separates DNA fragments by size using an electric field. DNA molecules migrate toward the positive pole, with shorter fragments moving faster through the gel matrix.

Principle: DNA is negatively charged and moves toward the anode.
Result: DNA fragments form distinct bands based on size.

Example: Comparing restriction fragments from different alleles.

Gel electrophoresis setup and results Gel electrophoresis technique Gel electrophoresis results

Restriction Fragment Analysis and RFLP

Restriction fragment length polymorphism (RFLP) analysis detects variations in DNA sequences by comparing the pattern of fragments produced by restriction enzyme digestion. RFLPs are useful for identifying genetic differences, such as those causing sickle-cell disease.

Polymorphism: Sequence variations that alter restriction sites.
RFLP: A change in fragment length due to a mutation at a restriction site.

Example: Detecting sickle-cell disease alleles using RFLP analysis.

Restriction fragment analysis of normal and sickle-cell alleles

Southern Blotting

Southern blotting combines gel electrophoresis and nucleic acid hybridization to detect specific DNA fragments. DNA is transferred to a membrane, probed, and visualized to identify fragments containing the gene of interest.

Steps: Restriction digestion, gel electrophoresis, transfer to membrane, hybridization with labeled probe.
Application: Used to detect disease alleles and study gene structure.

Example: Identifying β-globin gene fragments in sickle-cell disease.

Southern blotting technique and probe detection

DNA Sequencing

Dideoxy Chain Termination Method

DNA sequencing determines the order of nucleotides in a DNA fragment. The dideoxy chain termination method uses fluorescently labeled dideoxyribonucleotides (ddNTPs) to terminate DNA synthesis at specific bases, allowing the sequence to be read.

ddNTPs: Modified nucleotides that terminate DNA synthesis.
Fluorescent Labels: Each ddNTP is tagged for identification.
Detection: The sequence is determined by analyzing the labeled fragments.

Example: Sequencing a gene to identify mutations.

DNA sequencing with dideoxy chain termination DNA sequencing technique DNA sequencing results

Analyzing Gene Expression

Studying Expression of Single Genes

Gene expression can be analyzed using nucleic acid probes, Northern blotting, and reverse transcriptase-PCR (RT-PCR). These methods allow researchers to determine when and where genes are transcribed.

Northern Blotting: Gel electrophoresis of mRNA followed by hybridization with a probe.
RT-PCR: Reverse transcriptase synthesizes cDNA from mRNA, which is then amplified by PCR.
In Situ Hybridization: Uses fluorescent probes to visualize mRNA location in intact organisms.

Example: Studying β-globin gene expression during embryonic development.

In situ hybridization showing gene expression patterns

Technique	Main Purpose	Key Steps
Genomic Library	Store entire genome in clones	Fragmentation, cloning, storage
cDNA Library	Store expressed genes	Reverse transcription, cloning
PCR	Amplify DNA	Denaturation, annealing, extension
Gel Electrophoresis	Separate DNA by size	Electric field, gel matrix
Southern Blot	Detect specific DNA fragments	Electrophoresis, transfer, hybridization
DNA Sequencing	Determine nucleotide order	Chain termination, detection
Northern Blot	Analyze mRNA expression	Electrophoresis, hybridization
RT-PCR	Amplify mRNA as cDNA	Reverse transcription, PCR
In Situ Hybridization	Visualize mRNA location	Fluorescent probe, microscopy