Genomics: History and Principles of DNA Sequencing

Vocabulary practice flashcards covering the timeline of genome sequencing, Maxam-Gilbert chemical degradation, manual and automated Sanger chain-termination sequencing, pUC19 plasmid vector architecture, capillary electrophoresis, and Phred quality scores.

Bovine Insulin Sequence (1951)

The first protein sequence ever determined, solved in the year $1951$.

Yeast Alanine tRNA Sequencing (Holley et al., 1965)

The first direct sequencing of an RNA molecule, which took one year to determine the $77$ nucleotides of alanine transfer RNA in yeast.

Haemophilus influenzae Genome (1995)

The first cellular organism to have its complete genome fully sequenced, published in $1995$.

Genome Sequencing Milestones (1997–2001)

The chronological completion of major model organism and human genomes: Escherichia coli and Saccharomyces cerevisae ($1997$), Caenorhabditis elegans ($1998$), Drosophila melanogaster ($2000$), and the Human genome sequence ($2001$).

454 Sequencing (2005)

The technology platform introduced in $2005$ that marked the beginning of next generation sequencing (NGS).

Maxam and Gilbert Sequencing

A chemical sequencing technique published in $1977$ where DNA fragments are radioactively labeled at the $5'$ end using alkaline phosphatase, treated with base-specific cleavage chemicals, and resolved by size via gel electrophoresis.

Maxam-Gilbert Chemical Cleavage Treatments

The four distinct base-modification chemical reactions: dimethylsulfate followed by heat for $\text{G}$, dimethylsulfate with mild acid for $\text{A} + \text{G}$, hydrazine for $\text{C} + \text{T}$, and hydrazine in $2\text{M}\text{ NaCl}$ for $\text{C}$.

Dideoxynucleotide Chain Termination

A mechanism stopping DNA elongation because the incorporated $2', 3'$-dideoxyribonucleotide triphosphate ($\text{ddNTP}$) lacks a $3'\text{-OH}$ group, leaving no nucleophile to attack the incoming $5'$ phosphate of the next nucleotide.

Sanger Sequencing Method (1977)

An enzymatic sequencing technique exploiting DNA replication principles by utilizing sequence-specific primers, standard $\text{dNTPs}$, and chain-terminating $\text{ddNTPs}$ across four individual reactions.

Sanger Sequencing Gel Autoradiograph

A very thin polyacrylamide gel separation read from bottom to top ($+\text{ve}$ to $-\text{ve}$) to infer the $5' \rightarrow 3'$ DNA sequence, because the smallest fragments migrate fastest and represent the initial nucleotides incorporated after the primer.

pUC19 Plasmid

A small ($2686\text{ bp}$), high-copy-number Escherichia coli plasmid vector used to clone and amplify DNA fragments for sequencing.

pUC ori

The specific plasmid origin of replication region where DNA replication begins in the host bacteria.

Ampicillin Resistance Gene (Amp(R))

A selectable marker within pUC19 that allows only bacteria successfully transformed with the plasmid to survive and form colonies on ampicillin-containing media.

Multiple Cloning Site (MCS)

A short segment containing numerous unique restriction enzyme recognition sites (e.g., EcoRI, SacI, BamHI, HindIII) engineered within the lacZ gene to facilitate insertion of foreign DNA.

Blue-White Screening (lacZ Gene)

A colorimetric identification method where functional lacZ produces blue bacterial colonies when no insert is present, whereas successful foreign DNA insertion interrupts lacZ, producing non-functional beta-galactosidase and white colonies.

ABI Automated Sanger Sequencing

A modern modification of Sanger sequencing utilizing fluorescently tagged $\text{ddNTPs}$ (each base tagged with a distinct dye) running through capillary tubes to permit optical readout by a laser.

Capillary Electrophoresis Sequencing

A high-throughput separation setup containing $16$ gel-filled capillary lanes through which DNA fragments migrate by size and pass a multi-wavelength laser readout.

Chromatogram

A four-color electrophoretic trace generated by computer software translating detected fluorescent emission peaks into the inferred nucleotide sequence ($\text{A}$, $\text{T}$, $\text{G}$, $\text{C}$).

Purine Stacking

A sequencing artifact caused by the physical interaction of adjacent purine bases resulting in overlapping or poorly resolved chromatogram traces where $\text{A}$ and $\text{G}$ cannot be cleanly separated.

Phred Quality Score (Q)

A quality score quantifying base-calling accuracy derived from the probability of base-call error ($e$) via the formula $Q = -10 \times \text{log}_{10}(e)$.

Phred Score Q = 20

A base-call confidence value corresponding to an error probability of $e = 1.0\%$ ($99\%$ base accuracy).

Phred Score Q = 30

A high-accuracy base-call confidence value corresponding to an error probability of $e = 0.1\%$ ($99.9\%$ base accuracy).