Cloning

This summary provides a comprehensive and extensive overview of the molecular biology tools and techniques used in DNA cloning, as detailed in SCIE1106 Lecture 11 (Introduction to Cloning), building upon the foundational knowledge of DNA structure and molecular processes previously discussed.

SCIE1106 Lecture 13: Introduction to Cloning (Chapter 10)

The lecture introduces the foundational concepts and tools necessary for modern molecular cloning, including restriction enzymes, DNA ligation, plasmids, and separation techniques like agarose gel electrophoresis.

I. Restriction Enzymes: Origin, Function, and Specificity

Restriction enzymes are essential protein tools in molecular biology, recognized for enabling molecular cloning in the 1970s.

Origin and Bacterial Defence

Restriction enzymes are not a human invention; they are naturally occurring proteins found in bacteria. Their primary function is to serve as a bacterial defence system against foreign DNA, such as that injected by bacteriophages (viruses). The enzymes recognise the foreign DNA as non-native and cleave it apart by cutting.

Bacteria protect their own DNA from digestion using a second enzyme set called a methylase. When the bacterium replicates its DNA, the methylase chemically adds methyl groups to specific positions (usually an Adenosine, as in $Eco$R1) within the restriction recognition sequence. This methylation prevents the restriction enzyme from cutting the bacterial DNA.

Naming Convention

Restriction enzymes are named after the organism from which they were isolated:

  • The first letter comes from the Genus (capitalised) and the next two letters come from the Species (lowercase). This part of the name (e.g., Eco from Escherichia coli) is traditionally written in italics.

  • Other letters and numbers (like R1) indicate laboratory designation or which specific enzyme it is from that species (e.g., $Eco$R1, $Eco$R5).

Recognition Sites and Palindromes

Restriction enzymes recognise a very specific DNA sequence.

  • Palindromes: All relevant restriction enzyme recognition sequences are palindromic. This means the sequence reads the same backwards and forwards when comparing the 5' $\rightarrow$ 3' sequence of the top strand to the 5' $\rightarrow$ 3' sequence of the bottom (complementary) strand. Examples include GAATTC, madam, or nurses run.

  • Length: These recognition sites are usually between 4 and 8 base pairs long.

  • Cutting Frequency: The length of the recognition site determines how frequently the enzyme cuts in a large genome, which can be predicted statistically.

    • A 4 base pair (bp) recognition sequence cuts, on average, every $4^4 = 256\text{ bp}$.

    • A 6 bp recognition sequence cuts, on average, every $4^6 = 4,096\text{ bp}$.

Types of Cuts and DNA Ends

Restriction enzymes cut the DNA sequence in a very specific way. When a cut is made, the molecular bond between the 3'-hydroxyl ($\text{OH}$) and 5'-phosphate ($\text{P}$) groups (phosphodiester bonds) are cleaved. The resulting fragments always retain a 3'-hydroxyl and a 5'-phosphate end.

Restriction cuts produce three types of DNA ends:

  1. Blunt Ends (Blunt Cut): Created when the enzyme cuts both DNA strands at the exact same position. These ends are fully base-paired. (Example: HaeIII).

  2. Sticky Ends (Staggered or Cohesive Ends): Created when the enzyme cuts the strands at different positions, leaving a single-stranded overhang. These ends are "sticky" because the overhangs contain complementary bases that can base pair back together.

    • 5' Sticky End: The single-strand overhang runs in the 5' direction.

    • 3' Sticky End: The single-strand overhang runs in the 3' direction.

II. DNA Ligation and Recombinant DNA

DNA ligase is an enzyme that joins DNA fragments together, which is crucial for cloning.

  • Ligation Mechanism: DNA ligase forms a covalent bond between the free 3'-hydroxyl ($\text{OH}$) end of one fragment and the 5'-phosphate ($\text{P}$) end of another. This process requires energy, provided by ATP.

  • Efficiency: Ligation of sticky ends is much more efficient because the complementary overhangs stabilize the fragments by base pairing (hydrogen bonds), holding them in place for the ligase. Blunt ends can be ligated, but the efficiency is lower because they lack this base pairing stabilization.

  • Creating Blunt Ends: Restriction fragments with non-complementary sticky ends can be joined by first treating them with DNA polymerase and dNTPs (nucleotides) to fill in the staggered gaps, resulting in two blunt-ended fragments that can then be ligated.

  • Recombinant DNA: By cutting two different sources of DNA (e.g., human DNA and bacterial plasmid DNA) with the same restriction enzyme and then joining them with DNA ligase, a recombinant DNA molecule is created. This is defined as a molecule that has originated from two or more fragments that are not found together in nature.

III. Agarose Gel Electrophoresis

This technique is used to separate and analyse DNA fragments based on size.

  1. Gel Preparation: Agarose, a carbohydrate isolated from red algae, is dissolved by boiling in a buffer solution. This liquid is poured into a tray containing a comb. As the solution cools and solidifies, the comb creates hollow loading pockets.

  2. Separation: The gel is placed in an electrophoresis tank and overlaid with buffer. A voltage is applied across the gel.

    • DNA is negatively charged because of its phosphate backbone.

    • DNA fragments migrate away from the negative electrode towards the positive electrode.

    • The gel matrix acts as a sieve; smaller DNA fragments move fastest and furthest through the mesh of agarose particles.

  3. Analysis: A DNA size marker (fragments of known size) is run alongside the samples to determine the size of the cut DNA fragments.

  4. Visualization: DNA is visualized using a dye that fluoresces under UV light only when bound to DNA.

  5. Fragment Isolation: If a specific band of DNA is desired (e.g., a gene), the portion of the gel containing that band can be cut out using a scalpel, and the DNA can be eluted (extracted) from the gel slice for subsequent cloning.

IV. Plasmids and Cloning

The goal of cloning is the production of identical copies of a particular DNA molecule. To achieve this, the recombinant DNA is inserted into a vector (or carrier), typically a bacterial plasmid.

Plasmid Characteristics

Plasmids are small, circular, double-stranded, extrachromosomal DNA molecules found naturally in bacteria, separate from the much larger bacterial chromosome.

  • Genes: Plasmids carry genes beneficial for survival, such as those encoding antibiotic resistance. The transfer of these plasmids between bacteria is a mechanism contributing to multi-drug resistance.

  • Replication: Plasmids replicate independently from the chromosomal DNA. This allows a bacterium to maintain potentially hundreds of copies of the same plasmid, providing massive amplification for the cloned DNA.

Essential Plasmid Features (Vector Map)

A typical plasmid vector used in cloning contains key components:

  1. Origin of Replication (ori): The starting point for plasmid DNA replication, necessary for the cell to make copies.

  2. Antibiotic Resistance Gene: Acts as a selectable marker (e.g., Ampicillin resistance, $Amp^R$). Only bacteria that have successfully taken up the plasmid will survive when grown on medium containing that antibiotic.

  3. Multiple Cloning Site (MCS) or Polylinker: A short region containing many unique restriction enzyme sites (restriction sites that occur only once in the entire plasmid). This is where the DNA of interest is inserted.

  4. Promoter: A sequence often present that can turn on the expression of the inserted gene to produce the corresponding protein.

V. Introducing and Amplifying DNA (Transformation)

Transformation is the mechanism by which bacteria naturally take up DNA from their surroundings, although specific lab methods are used to increase efficiency.

In the laboratory, recombinant DNA is introduced into bacteria using methods that temporarily disrupt the bacterial membrane:

  • Heat Shock: The bacteria are exposed to a sudden temperature change (e.g., adding DNA and heating to 42°C). This loosens the plasma membrane, allowing the DNA to enter the cell.

  • Electroporation: The bacteria and DNA are subjected to a strong pulse of electrical current, which temporarily opens the membrane.

The bacteria that successfully take up the plasmid are then selected by culturing them in a liquid broth containing the relevant antibiotic. Only resistant bacteria (those carrying the plasmid) survive and reproduce, amplifying the recombinant DNA millions of times.

VI. Applications: Genomic Libraries

Cloning allows researchers to obtain sufficient quantities of DNA (amplification) for subsequent procedures like DNA sequencing or to produce specific proteins from a gene.

A significant application is the creation of a genomic library:

  • This involves cutting an entire genome (e.g., human genomic DNA) into millions of random fragments using restriction enzymes.

  • These fragments are randomly ligated into a population of plasmids and transformed into bacteria.

  • The resulting library consists of a set of bacteria, each carrying a different small fragment of the human DNA.

  • Genomic libraries were historically crucial for researchers sequencing the human and other complex genomes.