Week 3 - Bacterial plasmids, gene transfer, and molecular tools (PCR, probes, libraries, microarrays)

Topic 1: Bacterial plasmids and transfer of genetic material

  • Overview

    • Understand differences in genome organization across viruses, parasites, and bacteria
    • Distinguish main genetic elements in bacteria: chromosome, plasmids, bacteriophages, mobile genetic elements
    • Learn natural and laboratory methods for moving genetic material between cells
    • Apply concepts to interpret gene transfer experiments
    • Key references: Clark (Molecular Biology, 2nd ed., 2010); figures from lecture materials

  • Sizes, genomes, and organization (core contrasts)

    • Viruses can have RNA or DNA genomes, single- or double-stranded; some have envelope, some do not
    • Bacteria typically have a single circular chromosome; some species have two chromosomes or even linear chromosomes (e.g., Borrelia burgdorferi)
    • Eukaryotic genomes are larger and more complex, with organelle genomes (mitochondria, chloroplasts) and vast non-coding regions
    • Parasitic eukaryotes can possess reduced genomes, yet giant viruses challenge simple parasite rules
    • Examples of genome sizes and structures across taxa (selected highlights):
    • Viral genomes range from very small to relatively large; virus and phage genomes can be circular or linear (DNA viruses often linear or circular; some giant viruses up to Mbp scale)
    • Bacterial genomes: typically 500 kbp to ~10 Mbp; some species have multiple chromosomes; plasmids range from ~1 kbp to ~1 Mbp
    • Eukaryotic genomes: human nuclear genome ~3.2imes1093.2 imes10^9 bp with ~20–22k protein-coding genes; organelle genomes (mitochondrial, chloroplast) are small but essential
    • Nucleotide and protein-coding content varies; introns and non-coding regions are common in higher eukaryotes but less so in many bacteria
    • Key numerical anchors (convert to consistent notation):
    • Human genome size: 3.2×109 bp3.2\times10^9\text{ bp} and genes: ~2×1042\times10^4–2.2×1042.2\times10^4 protein-coding genes
    • Bacterial genomes: typical chromosome sizes from ~5×1055\times10^5 to 1×1071\times10^7 bp; plasmids: 1 kb≤copy-number size≤1 Mb1\ \text{kb} \leq\text{copy-number size} \leq 1\ \text{Mb}
    • Giant viral genomes can reach up to roughly 2.5×1062.5\times10^6 bp (2.5 Mbp)
    • Viral particles range in size from ~100 nm to a few hundred nanometers; a human hair is ~7.5×104 nm7.5\times10^4\,\text{nm} in diameter, illustrating scale differences

  • Bacterial genome organization and elements

    • Main chromosome: contains essential genes; size varies by species; can be circular or linear
    • Plasmids: autonomously replicating, extrachromosomal DNA; typically smaller than the chromosome; copy number per cell varies widely (1–500 copies common)
    • Bacteriophages: viruses that infect bacteria; can mediate transduction
    • Transposable elements: mobile DNA (insertion sequences, composite transposons); can move within the genome or between elements
    • Plasmids carry functions beneficial under certain conditions (e.g., antibiotic resistance, virulence, metabolism, conjugation machinery)

  • Plasmids: properties and functions

    • Autonomously replicating dsDNA elements; exist independently of the chromosome
    • Size range: approximately 1 kbp to 1 Mbp
    • Copy number: typically 1–500 copies per cell; some higher-copy plasmids exist
    • Not usually essential for growth under all conditions, but may provide key advantages in particular niches
    • Copy-number control genes regulate per-cell copy number; replication and maintenance systems ensure stability
    • Plasmids encode diverse functions:
    • Copy number control, antibiotic and heavy-metal resistance, virulence factors, metabolic enzymes, antibacterial/antimicrobial production, mobility/conjugation factors

  • Plasmid copy number and maintenance (concepts to know)

    • Copy number is the average per-cell plasmid count; varies across plasmids and conditions
    • Regulation: plasmids possess genes that control their own replication to maintain a relatively constant copy number during cell proliferation
    • Consequence: genes on a plasmid are present in multiple copies per cell when copy-number is high, impacting expression levels
    • Visual concept (not a strict equation): genome replication competes with plasmid replication to set steady-state copy number

  • Example: plasmids altering bacterial phenotype

    • Shigella flexneri virulence often linked to a large 220-kb virulence plasmid
    • Virulence traits include attachment to and invasion of colonic epithelial cells and macrophage killing
    • Without the virulence plasmid, Shigella loses disease-causing capacity; these traits are plasmid-encoded, highlighting the impact of extrachromosomal DNA on pathogenicity

  • Mechanisms of gene transfer in microbes (vertical vs. horizontal)

    • Vertical transfer: chromosomal inheritance during cell division (DNA on chromosome, including mobile DNA elements and some bacteriophages)
    • Horizontal transfer (cell-to-cell):
    • Transformation: uptake of free DNA from the environment; DNA may integrate into the chromosome or replicate as a plasmid
    • Conjugation: direct cell-to-cell transfer of DNA; requires contact and specific transfer machinery; often plasmid-encoded (e.g., F plasmid) but can be chromosome-encoded
    • Transduction: DNA transfer via a bacteriophage intermediary; can occur even when cells are separated by a filter (phage particles pass through but bacteria do not)

  • Transformation (natural and lab) – key points

    • Definition: uptake of free DNA from the surroundings by a competent bacterium; DNA can integrate into the chromosome or replicate as a plasmid
    • Competence: a physiological state enabling DNA uptake; induced under certain growth conditions or in laboratory protocols
    • Laboratory transformation (common methods):
    • Chemical competence: calcium chloride (CaCl2) makes membranes permissive to DNA uptake
    • Electroporation: brief electrical pulses create pores in membranes to allow DNA entry
    • Plasmid DNA in transformation is advantageous because: plasmids replicate autonomously, can express genes without chromosomal integration, and are easy to select via markers

  • Conjugation – the F plasmid and beyond

    • Requires: donor cell with a sex pilus and the ability to form a DNA transfer bridge; contains oriT (origin of transfer)
    • Initiation steps: pilus tip binds receptor on recipient; pilus retraction brings cells into contact; a conjugative bridge forms
    • DNA transfer steps:
    • One DNA strand is nicked at oriT
    • The nicked strand is passed through the bridge to the recipient
    • Donor synthesizes the complementary strand to restore plasmid double-strandedness
    • Recipient uses incoming DNA as a template to synthesize its own strand
    • Outcome: both donor and recipient become capable of conjugal transfer (F+ status can spread)
    • Experimental demonstration: mixed strains A and B can exchange genes enabling growth on minimal medium, illustrating horizontal transfer of multiple loci

  • Transduction – phage-mediated gene transfer

    • Bacteriophages infect bacteria by attaching, injecting DNA, and either lysing to release progeny (lytic cycle) or integrating into the host genome (lysogenic cycle)
    • Generalized transduction: when a phage particle accidentally packages bacterial DNA instead of phage DNA; can transfer a fragment of the host genome to another bacterium
    • Key features:
    • Phages can pass through filters that separate bacteria, enabling transduction without direct cell contact
    • Transferred DNA can be stably maintained if it recombines into the host chromosome
    • Integration and survival: some transferred DNA contains integrases/recombinases enabling integration; others rely on homologous recombination with host DNA

  • Lifecycle basics and examples

    • Lytic phages: inject DNA, replicate, lyse host, and release progeny
    • Temperate phages (lysogenic): integrate into the host chromosome and replicate with it; later can induce lytic cycle under certain conditions
    • Generalized transduction emphasizes transfer of random host DNA segments due to packaging errors

  • Transformation vs. transfection (eukaryotic context)

    • Transformation (prokaryotes): uptake of naked DNA by bacteria
    • Transfection (animals and plants): introduction of foreign DNA into eukaryotic cells; outcomes include stable integration or transient expression
    • Methods in eukaryotes mirror bacterial approaches but use specialized reagents and devices (liposomes, chemical/physical methods, or gene guns)

  • Lab use and practical implications

    • Antibiotic resistance markers are widely used for selection of recombinant bacteria
    • Plasmids can express cloned genes for purification or functional studies
    • Conjugation, transformation, and transduction are foundational techniques in genetic engineering, microbial genetics, and biotechnology

  • Summary: key ideas to memorize

    • Genome organization varies across life forms; bacteria combine a chromosome with plasmids and mobile elements
    • Plasmids provide accessory functions and can be present in multiple copies; copy number is tightly regulated
    • DNA can move within and between organisms through transformation, conjugation, and transduction, both in nature and the lab
    • Knowledge of these mechanisms allows interpretation of gene transfer experiments and design of genetic engineering strategies

Topic 2: Probes, DNA amplification and genomic libraries

  • Learning outcomes for PCR, probes, libraries, and microarrays

    • Understand Polymerase Chain Reaction (PCR) and its applications in medicine and research
    • Describe uses of probes and PCR for detection, cloning, and diagnostics
    • Compare genomic and cDNA libraries and their utilities
    • Grasp principles of DNA microarrays and their role in gene expression analysis

  • Probes: detecting specific sequences

    • Probes enable identification of DNA sequences or transcripts in a sample (e.g., determining if a given mRNA originates from a specific gene)
    • Examples: labelled mRNA probes, chemically synthesized DNA oligonucleotide probes
    • Concept: hybridization of a labeled probe to a complementary target reveals presence/absence and localization of the sequence
    • Visualization: in situ detection in tissue sections or brain slices; probe binding intensity reflects relative expression

  • Labeling DNA/RNA and in situ analyses

    • Probes are labeled to enable detection (radioactive or fluorescent labels common in modern methods)
    • In situ hybridization provides spatial information about gene expression within tissues
    • Visual examples include brain atlas studies showing expression patterns of Rasd2 and Amigo2

  • Polymerase Chain Reaction (PCR): fundamentals

    • Core idea: DNA strands melt at high temperatures, allowing primers to anneal and a DNA polymerase to extend
    • Requirements: template DNA, DNA polymerase, primers, dNTPs, compatible buffer
    • Key history: PCR invented in 1985 by Kary B. Mullis; Nobel Prize in 1993
    • Temperature cycles: denaturation (>Tm), annealing (~Tm of primers), extension (optimal polymerase activity)
    • Thermostable polymerase (e.g., Taq) enables multiple cycles without enzyme inactivation
    • Tm concept: T_m = ext{temperature at which 50 ext{ }% of the DNA is denatured (lost helical structure)}

  • PCR mechanics and design principles

    • Primers must be designed to flank the target region; ends of the DNA template must be known
    • Typical PCR workflow: repeat cycles of denaturation, annealing, and extension to exponentially amplify the target sequence
    • End-point PCR applications: cloning, mutation introduction, diagnostics, and detection of the presence of specific sequences

  • Quantitative PCR (qPCR) and real-time PCR

    • qPCR monitors amplification in real time using fluorescent reporters
    • Applications: gene expression profiling, copy number variation, molecular diagnostics, genotyping
    • Data output: amplification curves used to quantify initial template quantity

  • PCR applications: broad uses across biology and medicine

    • Site-directed mutagenesis: introduce mutations via modified primers
    • Cloning: amplify and insert genes into vectors for expression
    • Diagnostic detection: identify pathogens or genetic variants
    • Genotyping and genetic testing: targeted detection of disease-associated variants (e.g., BRCA1/BRCA2)
    • Forensics and paternity testing: STR and VNTR markers amplified by PCR

  • Genomic and cDNA libraries

    • Genomic library
    • Contains the organism’s entire genome represented in clones (often in BACs for large fragments)
    • BAC vectors accommodate large inserts and are used for comprehensive genome representation
    • Pros: exhaustive representation including non-coding regions and introns; useful for sequencing and genome structure studies
    • Cons: not necessarily reflective of the transcriptome or gene expression in a given condition
    • cDNA library
    • Generated by reverse-transcribing mRNA to cDNA and cloning into plasmids
    • Contains only transcribed regions (the transcriptome); tissues produce distinct cDNA libraries reflecting gene expression in those conditions
    • Pros: direct representation of expressed genes; useful for expression studies and protein-coding gene identification
    • Cons: excludes non-coding regions and introns; may miss transcripts not present in sampled tissues

  • cDNA synthesis and library construction

    • mRNA is captured by its poly-A tail; poly-T primers initiate reverse transcription
    • RNase H creates nicks/gaps to facilitate DNA synthesis by DNA polymerase
    • Result: double-stranded cDNA that is cloneable and representational of the transcriptome

  • From libraries to practical uses

    • Genomic libraries: best for sequencing and studying genome structure; not ideal for direct expression studies
    • cDNA libraries: ideal for cloning expressed genes and analyzing transcript abundance across tissues
    • The choice depends on research goals: complete genome sequencing vs. transcriptome-focused analyses

  • Microarrays and genome-scale expression analyses

    • Microarray concept: attach or synthesize DNA probes on a solid surface; arrays can contain millions of probes
    • Two common formats:
    • Spotted arrays: probes prepared separately and spotted onto slides
    • Oligonucleotide arrays: probes synthesized in situ directly on the array
    • Genome chips may have >5 million probes and are tailored to disease, species, or genotyping needs
    • Expression analysis workflow:
    • Convert mRNA to labeled cDNA
    • Hybridize labeled cDNA to the microarray
    • Scan and quantify signal; use a reference to obtain relative expression levels

  • Comparative genomic hybridization (CGH) and SNP/allele arrays

    • CGH workflow: compare patient DNA to control DNA to detect copy-number variations across the genome
    • SNP/allele arrays: detect polymorphisms and disease-associated variants; genome-wide scans for association studies
    • Data interpretation relies on software to identify dosage changes or allelic differences

  • Applications and ethical considerations

    • Microarrays enable large-scale studies of gene expression, genotyping, ancestry, and disease susceptibility
    • Ethical considerations include privacy, potential discrimination based on genetic risk, and the clinical implications of testing results

  • Practical examples and visualizations

    • In situ brain expression: examples show differential labeling intensity correlating with mRNA levels (spatial resolution in tissue)
    • Brain atlas resources (e.g., Allen Brain Atlas) provide expression maps across tissues and developmental stages

  • Key takeaways from the lecture materials

    • Probes, PCR, genomic libraries, and microarrays are interconnected tools for detecting, amplifying, and analyzing genetic information
    • Genomic DNA libraries give a complete genomic representation, while cDNA libraries reflect the transcriptome and tissue-specific expression
    • Microarrays and CGH enable high-throughput analyses of gene expression and genomic variation, respectively
    • Understanding the strengths and limitations of each method is essential for experimental design and data interpretation

  • Connections to foundational principles and real-world relevance

    • DNA weaving through transformation, conjugation, and transduction demonstrates how genetic information flows in microbes and can be manipulated in the lab
    • PCR revolutionized molecular biology by enabling rapid, specific DNA amplification; it underpins diagnostics, forensics, cloning, and research
    • Genomic and cDNA libraries underpin genome sequencing, expression profiling, and discovery of gene function in health and disease
    • Microarrays contributed to systems biology approaches, enabling genome-wide assessments of expression and variation that inform medicine, agriculture, and evolutionary biology

  • Equations and units to remember (LaTeX notation)

    • Genome sizes and scale anchors:
    • Human nuclear genome: ≈3.2×109 bp\approx 3.2\times 10^9\ \text{bp}
    • Protein-coding gene count: ≈2×104 to 2.2×104 genes\approx 2\times 10^4 \text{ to } 2.2\times 10^4\text{ genes}
    • Giant virus genome example: 2.5×106 bp2.5\times 10^6\ \text{bp}
    • Plasmid size and copy number ranges:
    • Plasmid size: from 1 kbp1\ \text{kbp} to 1 Mbp1\ \text{Mbp}
    • Copy number per cell: 1≤copy number≤5×1021 \leq \text{copy number} \leq 5\times 10^2 (typical range)
    • Visualization scales:
    • 100 nm scale bar used in virus figures; human hair ~7.5×104 nm7.5\times 10^4\ \text{nm} in width
    • PCR primer and denaturation concepts (Tm terminology):
    • Primer annealing and denaturation temperatures are chosen with respect to primer Tm; the fundamental concept is that denaturation occurs above Tm and primers anneal near Tm
    • Tm definition for clarity: Tm=temperature at which 50% of the DNA duplex is denaturedT_m = \text{temperature at which 50\% of the DNA duplex is denatured}

  • Ethical, philosophical, and practical implications

    • DNA technologies raise questions about privacy and genetic data usage (e.g., ancestry testing, disease susceptibility)
    • Forensic and paternity testing have legal and societal implications; accuracy and interpretation can affect individuals and families
    • Equitable access to advanced molecular diagnostics is a practical concern in medicine

  • Quick-reference glossary (from content)

    • Competent: capable of taking up DNA in transformation
    • OriT: origin of transfer in conjugative DNA transfer
    • F+ / F-: donor/recipient statuses in conjugation
    • BAC: bacterial artificial chromosome, a large cloning vector used for genomic libraries
    • CGH: comparative genomic hybridization for detecting copy-number variations
    • VNTR/STR: variable/short tandem repeats used in forensic genetics and paternity testing

  • Connections to earlier and later lectures

    • Topic 1 focuses on genome organization, horizontal gene transfer, and plasmid biology, forming the basis for understanding gene expression and regulation discussed later
    • Topic 2 builds on this by detailing molecular tools (PCR, probes, libraries, microarrays) that enable detection, quantification, and functional analysis of genetic information, bridging to topics on gene expression regulation and disease mechanisms

  • End of notes for Topic 1 and Topic 2

Topic 3: (Optional continuation prompts)

  • If you have access to additional pages, we can extend these notes to cover more on viral replication strategies, transcriptional regulation in bacteria vs. eukaryotes, or the practical applications of microarray data in clinical genomics.