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Recombinant DNA
& Biotechnology

Master the tools of genetic engineering: restriction enzymes, cloning vectors, PCR, gel electrophoresis, and gene expression systems for the MCAT.

MCAT Biological Foundations

What is Recombinant DNA?

Recombinant DNA (rDNA) is a molecule formed by combining DNA from two or more different sources. This is achieved by cutting DNA with restriction endonucleases and joining fragments with DNA ligase, creating novel genetic combinations that do not exist in nature.

Restriction Enzymes & Sticky Ends

Before Cut (EcoRI recognition site) 5' ...G A A T T C ... 3' 3' ...C T T A A G ... 5'
Cut site
 After Cut: Sticky Ends 5' ...G 3' ...C T T A A
5' overhang
 A A T T C... 3' G... 5'
5' overhang
 Complementary: can re-anneal

Key Tools of rDNA Technology

Restriction Enzymes
Molecular scissors that cut DNA at specific palindromic recognition sequences
DNA Ligase
Joins DNA fragments by forming phosphodiester bonds between sticky or blunt ends
Vectors
Plasmids, phages, or BACs that carry foreign DNA into host cells for replication
Reverse Transcriptase
Synthesizes cDNA from mRNA template: produces intron-free copies for expression
DNA Polymerase
Taq polymerase amplifies DNA in PCR; thermostable at \( 72^\circ\text{C} \) extension temperature

Cloning Vectors: Plasmids

ori Origin of Replication
AmpR
Selectable Marker
 MCS Multiple Cloning Site Promoter Insert gene here

PCR: Amplifying DNA

Denature
\( 94-98^\circ\text{C} \)
 Strands separate Anneal
\( 50-65^\circ\text{C} \)
 Primers bind Extend
\( 72^\circ\text{C} \)
 Taq extends Repeat 25-35 cycles

Gel Electrophoresis: Separating DNA

Ladder S1 S2 S3 10 kb 5 kb 2 kb 1 kb 0.5 kb
(−)
(+)
 Migration

cDNA vs Genomic Libraries

cDNA Library Genomic Library
Source mRNA (via reverse transcriptase) Total genomic DNA
Introns No: only exons (processed mRNA) Yes: contains full gene structure
Regulatory Regions No: promoters/enhancers absent Yes: full upstream sequences
Tissue Specificity Yes: reflects expressed genes No: same for all cell types
Best For Protein expression in prokaryotes Studying gene organization

Gene Cloning Workflow

1. Cut DNA Restriction enzyme 2. Ligate Insert DNA ligase + vector 3. Transform Into host cell 4. Select Colonies Antibiotic resistance plate 5. Screen for Insert Blue-white / PCR / probe 6. Express & Analyze Protein production / sequencing Blue-white screening: Insert disrupts lacZ gene White colonies = insert present; Blue colonies = no insert (intact lacZ)

Blotting Techniques

Southern Blot
Detects specific DNA sequences. DNA is separated by gel electrophoresis, transferred to membrane, and probed with a labeled complementary sequence.
Northern Blot
Detects specific RNA sequences. Reveals gene expression levels and mRNA size in a given tissue or condition.
Western Blot
Detects specific proteins using antibodies. Proteins separated by SDS-PAGE, transferred, then probed with primary + secondary antibodies.

RFLP & DNA Fingerprinting

Restriction Fragment Length Polymorphisms (RFLPs) arise from differences in DNA sequence between individuals that create or abolish restriction enzyme recognition sites. These variations produce different-sized fragments when the same enzyme is used.

Person A Cut Cut 120 bp 200 bp 120 bp Person B Cut ✕ SNP: no cut 120 bp 320 bp Gel: Person A shows 2 bands (120 & 200 bp); Person B shows 2 bands (120 & 320 bp)

Sanger Sequencing: Reading DNA

1
Template + primer + dNTPs + ddNTPs: The reaction mixture contains normal deoxynucleotides and a small amount of dideoxynucleotides (ddNTPs) that lack the 3'-OH group.
2
Chain termination: When a ddNTP is randomly incorporated, DNA synthesis terminates at that position because no phosphodiester bond can form without a 3'-OH.
3
Fragment separation: Fragments of every possible length are generated and separated by capillary electrophoresis (smallest fragments migrate fastest).
4
Fluorescent detection: Each ddNTP (A, T, C, G) carries a different fluorescent label. A laser reads the color of each fragment to determine the sequence.

Applications of Biotechnology

Gene Therapy
Introduce functional genes to replace defective ones in patients with genetic disorders
Transgenic Organisms
Organisms carrying foreign genes: Bt crops, GFP mice, human insulin in bacteria
Pharmacogenomics
Tailoring drug therapy based on individual genetic profiles using SNP analysis
Forensic Identification
STR analysis and DNA fingerprinting for criminal investigations and paternity testing

How to Solve rDNA Problems

1
Identify the goal: Are you cloning a gene, detecting a sequence, or determining fragment sizes?
2
Choose the enzyme: Match the restriction enzyme to the recognition site. Count cut sites to predict fragment numbers.
3
A linear DNA with \(n\) cut sites produces \( n + 1 \) fragments. A circular DNA with \(n\) cut sites produces \( n \) fragments.
4
Predict gel pattern: Smaller fragments migrate farther. Order fragments by size and map bands on the gel.
5
Check the probe: For Southern blots, only fragments containing the probe's complementary sequence will appear as bands.

Example: Restriction Digest

A linear DNA molecule of 10 kb is digested with EcoRI. The enzyme has recognition sites at positions 2 kb, 5 kb, and 8 kb. How many fragments result, and what are their sizes?

0 10 kb 2 kb 5 kb 8 kb 2 kb 3 kb 3 kb 2 kb 4 fragments total

Example: Circular Plasmid Digest

A circular plasmid of 6 kb has two EcoRI sites at positions 1 kb and 4 kb. How many fragments result?

1 kb 4 kb 3 kb 3 kb Circular: 2 cuts = 2 fragments

Common Mistakes

✕ Linear vs Circular Confusion
Using \( n + 1 \) fragments for circular DNA. Remember: circular DNA with \(n\) cuts gives \(n\) fragments, not \( n + 1 \).
✕ Genomic DNA in Prokaryotes
Trying to express eukaryotic genomic DNA (with introns) in bacteria. Prokaryotes cannot splice introns: use cDNA instead.
✕ Blotting Mix-up
Confusing Southern (DNA), Northern (RNA), and Western (protein). Remember SNoW DRoP mnemonic.
✕ Gel Migration Direction
Thinking larger fragments travel farther. DNA migrates toward (+) anode, and smaller fragments travel farther through the gel matrix.
✕ ddNTPs vs dNTPs
Forgetting that ddNTPs lack the 3'-OH (not the 2'-OH). The missing 3'-OH prevents phosphodiester bond formation.

Technique Summary

Technique Target Key Feature
PCR DNA Amplify: \( 2^n \) copies
Gel Electrophoresis DNA / RNA / Protein Separate by size
Southern Blot DNA Probe-based detection
Northern Blot RNA Gene expression level
Western Blot Protein Antibody detection
Sanger Sequencing DNA ddNTP chain termination
RFLP DNA Polymorphism detection

Question 1 of 5

A researcher wants to express a human protein in E. coli. Which DNA source should they clone into the expression vector?

Question 2 of 5

A circular plasmid of 8 kb is digested with a restriction enzyme that has 3 recognition sites. How many fragments are produced?

Question 3 of 5

In Sanger sequencing, dideoxynucleotides (ddNTPs) cause chain termination because they lack which critical group?

Question 4 of 5

Which blotting technique uses antibodies to detect a specific protein in a sample?

Question 5 of 5

After 20 cycles of PCR, approximately how many copies of the target DNA sequence are produced from a single template molecule?