Lecture 21: Forward Genetics (end) and Reverse Genetics Part 1

Define reverse genetic analysis and explain the overall strategy. Compare to forward genetic analysis.

• Reverse Genetics Strategy: The process of specifically mutating a known gene sequence (via deletion, disruption, or replacement) to determine what phenotype will result without requiring prior knowledge of gene function. Mutated genes are introduced back into host organisms to generate transgenic organisms.

• Comparison: Forward genetics begins with an observed mutant phenotype and works backward to identify the responsible gene ("phenotype to gene"). Reverse genetics begins with a known gene sequence, mutates it, and observes the resulting phenotype ("gene to phenotype").

Identify the three generations of sequencing technology and identify circumstances where each would most likely be used.

1st Generation (Sanger / Dideoxy Sequencing):

• Mechanism: Uses DNA polymerase alongside fluorescently labeled dideoxynucleotides (ddNTP chain terminators) lacking a 3'-OH group to synthesize fragments of varying lengths, which are then separated by size.

• Best Use Case: Sequencing small DNA fragments, single genes, and validating whole-genome sequencing results.

2nd Generation (Next-Gen Sequencing / NGS & Whole Genome Shotgun):

• Mechanism: Fragments the genome into small pieces, sequences millions of reactions simultaneously using reversible tagged chain terminators, and uses paired-end reads to span across repetitive sequence regions.

• Best Use Case: High-throughput, rapid whole-genome sequencing and large-scale comparative genomics.

3rd Generation (Long-Read Single-Molecule Sequencing):

• Best Use Case: Direct single-molecule sequencing without prior amplification, ideal for resolving complex structural variants and long repetitive regions across large genomes.

Define bioinformatics and recognize the contributions of this field to reverse genetic analysis.

• Definition: An interdisciplinary field combining computer science, statistics, and biology to store, organize, share, and analyze genomic datasets.

• Contributions: Assembles short DNA sequence reads into whole contiguous sequences (contigs), predicts gene structures and open reading frames, predicts protein structures/functions, and enables genome-wide guide RNA and siRNA design.

Explain the difference between gene disruption and gene replacement.

• Gene Disruption: A targeted molecular modification introduced directly within a gene (e.g., small insertions or deletions) that impairs or knocks out its function.

• Gene Replacement: The introduction of an exogenous DNA sequence engineered in vitro that replaces the endogenous genomic sequence via targeted recombination (e.g., creating specific hypomorphic or conditional alleles).

Explain the importance of double strand breaks (DSBs) in DNA to reverse genetic analysis.

Double-strand breaks (DSBs) act as the physical trigger that stimulates cellular repair pathways, allowing researchers to introduce precise mutations or integrate donor DNA at targeted genomic loci.

Explain the primary mechanisms cells use to repair DSBs. Non-homologous end joining (NHEJ) and homologous recombination (HR)

• Non-Homologous End Joining (NHEJ):

  • Directly ligates broken DNA ends together without requiring sequence homology.

  • Nucleotide trimming during repair frequently causes insertions or deletions (indels), making it the primary method for creating gene knockouts / disruptions.

• Homologous Recombination (HR / HDR):

  • Uses a complementary donor template (sister chromatid or exogenous DNA template) to repair the break accurately.

  • Enables precise sequence modifications and gene replacements (knock-ins).

Define Crispr-Cas and explain how it creates targeted DSBs.

CRISPR-Cas System

• Mechanism: Derived from a bacterial adaptive immune mechanism. An engineered single guide RNA (sgRNA) directs the Cas9 endonuclease to a complementary genomic sequence adjacent to a PAM site (5'-NGG), where Cas9 introduces a targeted DSB. Repair via NHEJ leads to gene disruption, while HR with a donor vector achieves gene replacement.

Define RNA interference (RNAi) and explain the mechanisms that use siRNA and miRNA.

RNA Interference (RNAi)

• Mechanism: A post-transcriptional gene silencing process where double-stranded RNA (dsRNA) molecules degrade target messenger RNAs (mRNAs) in the cytoplasm.

siRNA vs. miRNA:

• siRNAs (Small Interfering RNAs): Exogenous or long dsRNAs processed by Dicer and loaded into the RISC complex to yield near-perfect mRNA cleavage and degradation.

• miRNAs (MicroRNAs): Endogenously encoded hairpin structures that typically bind with partial complementary match to repress mRNA translation or accelerate decay.

Define a genome wide mutant screen and compare to a traditional mutant screen.

Genome-Wide Screens vs. Traditional Mutant Screens

• Traditional Screens: Focus on mutating single genes or screening individual mutants one by one to link specific phenotypes to unknown genes.

• Genome-Wide Screens: Utilize comprehensive genomic sequence data to systematically design molecular reagents (such as libraries of CRISPR sgRNAs or RNAi dsRNAs) that knock down or mutate every identified gene across an entire organism or cell line in parallel.

Explain the use of CRISPR and RNAi in genome wide mutant screens.

• CRISPR Screens: Transfect cell libraries with vectors encoding Cas9 and distinct sgRNAs to generate permanent genomic DNA alterations (null knockouts or knock-ins).

• RNAi Screens: Deliver dsRNA libraries (e.g., via E. coli feeding in C. elegans or high-throughput cell plates) to systematically cause mRNA knockdown.

  • Limitations of RNAi: Produces non-heritable effects, acts as a temporary knockdown (hypomorphic effect) rather than a complete knockout, and can suffer from off-target cross-reactivity.