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PCR, Cloning &
DNA Sequencing

Master the essential molecular biology techniques for amplifying, copying, and reading DNA: the foundation of modern biotechnology.

Biochemistry

What Is PCR?

Polymerase Chain Reaction (PCR) is a technique that amplifies a specific segment of DNA, producing millions of copies from a tiny starting sample. Invented by Kary Mullis in 1983, it revolutionized molecular biology, forensics, and medicine.

Essential Vocabulary

Template DNA
The original double-stranded DNA containing the target sequence to be amplified.
Primers
Short single-stranded DNA oligonucleotides (~18-25 nt) that flank the target region.
Taq Polymerase
A heat-stable DNA polymerase from Thermus aquaticus that synthesizes new DNA strands.
dNTPs
Deoxynucleotide triphosphates (dATP, dTTP, dCTP, dGTP): the building blocks for new DNA.
Vector
A DNA molecule (e.g., plasmid) used to carry foreign DNA into a host cell during cloning.

Why These Techniques Matter

PCR, cloning, and sequencing form the core toolkit of molecular biology. They enable everything from diagnosing genetic diseases to engineering new proteins and tracing evolutionary relationships.

1
Medical diagnostics: PCR detects pathogens (e.g., COVID-19 testing) and genetic mutations.
2
Gene therapy & drug production: Cloning allows mass production of therapeutic proteins like insulin.
3
Genomics & evolution: DNA sequencing reveals entire genomes, enabling comparative and personalized medicine.

The Three Steps of PCR

1. Denature 94-98 °C 2. Anneal 50-65 °C Primer Primer 3. Extend 72 °C Taq Polymerase

Exponential Amplification

DNA Copies Cycle Number 0 5 10 15 20 25 0 max Exponential phase Plateau

Molecular Cloning Overview

Molecular cloning inserts a gene of interest into a vector (typically a plasmid), which is then introduced into a host cell (usually E. coli). The host replicates the plasmid, producing many identical copies of the gene.

Plasmid Vector Cut site Restriction enzymes Gene insert Recombinant plasmid Transform Host Cell E. coli

PCR vs. Cloning

PCR Molecular Cloning
Goal Amplify DNA in vitro Propagate DNA in vivo
Speed Hours Days to weeks
Requires cells? No Yes (host organism)
Product Many copies of short DNA Recombinant DNA in cells
Size limit Typically < 10 kb Can clone very large inserts
Fidelity Depends on polymerase High (cellular proofreading)

Sanger Sequencing

Sanger sequencing (chain-termination method) uses modified nucleotides called ddNTPs (dideoxynucleotides) that lack the 3'-OH group needed for chain elongation. When a ddNTP is incorporated, the growing strand terminates.

Template: 3' - A T G C C T A G - 5' Fragments: T A C G G A T C Read (short → long): 5' - T A C G G A T C - 3'

Next-Generation Sequencing

Next-generation sequencing (NGS) performs massively parallel sequencing of millions of DNA fragments simultaneously. This dramatically reduces cost and time compared to Sanger sequencing.

Illumina
Sequencing by synthesis: fluorescent reversible terminators are imaged each cycle. Short reads (~150 bp), very high throughput.
Nanopore
DNA passes through a protein pore; current changes identify bases in real time. Ultra-long reads (> 100 kb).
PacBio
Single-molecule real-time (SMRT) sequencing. Long reads (~15 kb average), useful for structural variants and de novo assembly.

Designing a PCR Experiment

1
Choose your target: Identify the gene or DNA region you want to amplify and find its sequence in a database.
2
Design primers: Select 18-25 nt sequences flanking your target with ~50-60% GC content and a melting temperature (\(T_m\)) of 55-65 °C.
3
Prepare the reaction mix: Template DNA, forward & reverse primers, Taq polymerase, dNTPs, buffer, and \(\text{Mg}^{2+}\).
4
Set cycling conditions: Program the thermocycler for ~25-35 cycles of denature (94 °C), anneal (\(T_m - 5\) °C), and extend (72 °C).
5
Verify your product: Run gel electrophoresis to confirm a single band of the expected size.

Example: PCR Copy Number

Problem: You begin with 10 copies of a template DNA molecule. After 20 cycles of PCR, approximately how many copies do you have?

Example: Restriction Cloning

Scenario: You have a gene flanked by EcoRI sites and a plasmid with a single EcoRI site in its multiple cloning site (MCS). Walk through the cloning steps.

1
Digest both DNAs with EcoRI: this creates compatible sticky ends (5'-AATT overhangs) on both the insert and the vector.
2
Ligate the insert into the vector using DNA ligase, which forms phosphodiester bonds between the sticky ends.
3
Transform the recombinant plasmid into competent E. coli via heat shock or electroporation.
4
Select for transformants using an antibiotic resistance marker on the plasmid (e.g., ampicillin).
5
Screen colonies by colony PCR, restriction digest, or blue-white screening to confirm the insert.

Example: Reading a Chromatogram

Sanger sequencing output is a chromatogram where each peak corresponds to a nucleotide. Each base is assigned a color.

A T G C C A T G Chromatogram Trace A T G C

Common Mistakes

✕ Wrong annealing temperature
Too high: primers won't bind, no product. Too low: non-specific binding, extra bands on gel.
✕ Confusing ddNTPs with dNTPs
ddNTPs lack the 3'-OH and terminate chains. dNTPs are normal building blocks. Mixing them up ruins sequencing or PCR.
✕ Forgetting antibiotic selection
Without selection, you can't distinguish bacteria that took up the plasmid from those that didn't.
✕ Incompatible restriction sites
If the insert and vector are cut with different enzymes producing non-compatible ends, ligation will fail.
✕ Contamination in PCR
Even trace DNA contamination amplifies exponentially. Always include a negative (no-template) control.

Summary of Key Techniques

PCR Cloning Sanger Seq. NGS
Purpose Amplify DNA Propagate gene Read sequence Read many sequences
Key Reagent Taq polymerase Ligase, vector ddNTPs Platform-specific
Throughput One target One gene One read Millions of reads
Time 1-3 hours Days-weeks Hours Hours-days
Output \(2^n\) copies Clone library ~800 bp read Gb of data

Question 1 of 5

In PCR, the annealing step is performed at a lower temperature to allow:

Question 2 of 5

You start with 5 copies of a DNA template and run 25 cycles of PCR. Approximately how many copies will you have?

Question 3 of 5

What is the role of ddNTPs in Sanger sequencing?

Question 4 of 5

During molecular cloning, after ligating the insert into a plasmid vector and transforming E. coli, how do you select for bacteria that contain the plasmid?

Question 5 of 5

Which feature best distinguishes next-generation sequencing (NGS) from Sanger sequencing?