Ligate Sticky Ends via DNA Ligation

DNA Ligation of Sticky Ends

Ligation of Sticky Ends, Summary of DNA ligation

We have already discussed a high level view of gene cloning in our Molecular Cloning Guide blog post. However, in that blog post we didn’t delve very deep into how we can perform each of the individual steps. Today’s blog post is about ligation. Ligation is the process by which two pieces of DNA can be glued together to form one piece. So, to begin, let’s assume you’ve already decided on a gene product that you want to clone. You’ve also designed primers and completed PCR on the open reading frame in your donor DNA (this could be genomic or non genomic DNA). Your next steps are to digest the PCR product with restriction enzymes and generate sticky ends. You’ll also want to digest your “shuttle” plasmid to generate complimentary sticky ends which will allow your “insert” DNA to click into position into your vector. It’s like a puzzle piece!

Note: It might be useful to look at our RNA Extraction & Isolation guide if you’re planning on making cDNA related to your gene

The above summary is demonstrated here:
Ligate Sticky ends using Ligase

Sticky Ends Insert into a Shuttle Vector

Only some Restriction Enzymes Create Sticky Ends

As you can figure out, generating sticky ends and complimentary ends is extremely important to the above process. However, several different restriction enzymes are available and each of them has different locations where they cut. Also, the type of cuts that they introduce may be “sticky” or “blunt”. Depending on the cloning strategy you are using, you may mix and match different enzymes to achieve different end goals. Ligation of “sticky ends” is much more efficient than ligation of “blunt” ends. Typically 10-100 times more T4 Ligase is required for blunt ends.

Here’s an image with various restriction enzymes and the kinds of ends they produce. Depending on the type of ends, your DNA ligation will proceed very differently!

Restriction Enzymes for DNA Ligation

Ligate DNA via DNA Ligase

Once the restriction enzyme digestion is complete, you can proceed to the ligation step. But, before you digest anything, make sure you’ve planned everything properly! You need to make sure that the insert will be ligated in the proper direction in the shuttle vector. Only once you’ve vetted your overall strategy, should you proceed to ligation and transformation, etc.

There are several kinds of ligase enzymes but the enzyme produced by T4 bacteriophage-infected E. Coli is the most common one. This ligase is called T4 ligase. Whereas normal E. Coli produce DNA ligase that uses NADH as a cofactor, T4 infected E.Coli produce a ligase that uses ATP as a cofactor. This enzyme will find the 3′ Hydroxyl and 5′ Phosphate within your sticky ends and it will form a phosphodiester linkage. If this is confusing, check out the Polymerase chain reaction (PCR) guide for images on what DNA looks like. This is shown here:

Ligation Protocol for T4 Ligase

Phosphodiester Bond Formation during Ligation

Protocol for Ligation of Transgene Insert into Shuttle Vector

Ligation enables fragments of DNA to be combined, such as the cut ends of transgene inserts and plasmids during cloning. This protocol describes the directional cloning of a XbaI/SalI-digested transgene into a shuttle vector, pAdtrackCMV, via cohesive end ligation.

Materials for DNA Ligation

XbaI/SalI digested, gel-purified insert (approx. 1 kb) and pAdTrack-CMV shuttle vector (approx. 9.3 kb; Plasmid #16405, Addgene)
Quick Ligation Kit (contains DNA ligase and 2X Reaction Buffer; #M2200S, New England Biolabs)
Agarose plate containing ethidium bromide
DNA standards

Ligation Methodology

  1. Estimate the DNA concentration of purified insert and vector preparations by applying 1 µl to an agarose gel plate (+ethidium bromide) alongside a range of DNA standards and visualizing under UV light.
  2. Prepare the ligation mix as follows:

    XbaI/SalI digested pAdtrackCMV 50 ng

    XbaI/SalI digested insert 17 ng

    Add water up to 10 µl total volume.
  3. Add 10 µl of 2X Reaction Buffer and mix.
  4. Add 1 µl of DNA ligase and mix.
  5. Microcentrifuge briefly to settle liquid to the bottom of the tube and incubate at 25°C for 5 min.
  6. Place on ice* and transform into desired bacterial strain.

Tips and Tricks for DNA Ligation

  • This reaction setup is using a digested insert to vector DNA molar ratio of 3:1. Inserts of different sizes will require a different amount to be added. Important ligation control reactions to include are (1) digested vector only and (2) digested insert only.
  • Ligation reactions can be stored at -20°C for future use

Applications of Ligation on SciGine

Construction of PB42 Vectors Via Ligation
Plasmid Construction via PCR and Ligation
Plasmid Ligation and Transformation in Yeast
Construct with Human p275UTR
Different DNA ligation methods discussed

Video Tutorial About Sticky Ends and Ligation

References

He et al Proc Natl Acad Sci U S A. 1998 Mar 3. 95(5):2509-14.
Sticky Ends Explained Well
DNA Ligation Theory
Gaastra et al. Ligation of DNA via T4 Ligase
Tsuge et al. One Step Assembly of DNA fragments

Bacterial Transformation Protocol with Competent Cells

Bacterial Transformation with Competent Cells

Bacterial Transformation using Competent Cells: Summary

Since we have already learned Calcium Phosphate Transfection with mammalian cells, let’s now focus on bacterial transformation of DNA with competent cells. In general, bacterial cells take up naked DNA molecules or plasmids via a process called transformation. Usually, this happens at a slow rate, but when bacterial cells die in close proximity to others, or when they are stressed, the transformation process occurs at a much higher rate. However, not all bacterial cells can be transformed, so biologists use ‘Competent Cells’ which are more inclined to take up DNA. The end goal of transformation is to get bacteria that have your genes of interest so that they will replicate your genes along with their own. If the bacteria contain your genes of interest, you can use them to mass produce proteins, or just store them for extended periods of time because bacteria are so hardy. A good way to test whether your genes of interest were transformed is to include antibiotic resistance in your plasmid. This way, you can be fairly certain that if your bacteria are resistant to antibiotics, they are also carrying genes of interest to you.

Take a look at how natural transformation works:
Transformation Protocol with DNA

Transformation Biology in Bacteria

For bacteria, survival is key and transformation is one of their survival mechanisms. As biologists, we can make use of this survival mechanism for our benefit as well. To do this, we first incubate our competent bacteria with our plasmid and calcium chloride. Bacterial membranes are permeable to chloride ions, but not to calcium. So, as chloride ions enter the cell, the bacteria tend to swell (because they also intake water with chloride ions). Then we heat the bacteria in a process called ‘heat shock’ such that they turn on their survival genes. This causes the bacteria to uptake the surrounding plasmids. With the right design, this plasmid will then be recognized by bacterial DNA polymerases (remember our PCR Guide ?) and it will be expressed/replicated along with the bacteria’s normal DNA.

Take a look below to understand how biologists transform cells:

What is transformation

Transformation Biology

Selecting Transformed Bacteria with Antibiotic resistance in a plasmid

Selecting for Transformed Bacteria with the Lac Z Operon

Once your target plasmid is inside the bacteria, you still need to separate transformed cells from those that are not transformed. Another key challenge is that the transformation process may lead to some DNA being recombined so that your gene of interest is no longer functional. How do you select for cells that only contain functional target DNA that hasn’t been recombined? The trick is to use both antibiotic resistance and a Lac Z operon. By cloning your plasmid along with a Lac Z operon, you give your cells the ability to make a galactosidase protein. If cells have the galactosidase and you feed them X-Gal, they turn blue; cells without this operon are white. So, you first transform all your cells. Then you feed them IPTG to activate the Lac Z operon and cause cells to produce the galactosidase. Then you add in X-Gal and just pick out the bacteria that have functional Lac Z because the useful cells will be a bright blue!

Check out the figure below:

Transformation in Bacteria with LacZ

Transformation leads to Competent cells with LacZ operon

Bacterial Transformation Protocol

Transformation describes the uptake and incorporation of plasmid DNA into bacteria. Antibiotic resistance genes carried on plasmids allow selection of transformants. This protocol describes the transformation of DH5α E. coli with pAdtrackCMV (a vector carrying kanamycin resistance).

Materials for Bacterial Transformation

Ligation mix (20 µl) – insert ligated into pAdTrack-CMV shuttle vector (Plasmid #16405, Addgene)
DH5α competent cells (includes pUC19 DNA control; #18265017, ThermoFisher Scientific)
LB broth (#10855-021, ThermoFisher Scientific)
LB Agar selective plates (prepare from #22700025, ThermoFisher Scientific) with 50 µg/ml kanamycin (#15160054, ThermoFisher Scientific)

Step by Step Transformation Protocol

  1. Thaw competent cells on ice. Aliquot 50 µl into cooled Eppendorf tubes for each transformation reaction.*
  2. Add 5 µl of ligation mix to each tube.*
  3. Incubate on ice for 30 min.
  4. Heat-shock the cells for 20 sec in a 42°C waterbath.
  5. Place on ice for 2 min.
  6. Add 950 µl of warm LB broth per tube.
  7. Allow cells to recover at 37°C for 1 hour with gentle shaking.
  8. Spread 200 µl and 20 µl of each transformation mix onto warm selective plates.*
  9. Incubate plates overnight at 37°C.
  10.  

Notes on this methodology

  • We will talk about “Ligation” in another future blog post
  • Step 1. Unused cells can be refrozen and stored at -80°C for future use.
  • Step 2. As a transformation control, add 1 µl of pUC19 plasmid to one aliquot of cells (pUC19 confers resistance to ampicillin so will need to be seeded onto different selective plates).
  • Step 8. Transformation mix can be stored at 4°C and plated the next day if required.

Bacterial Transformation Video Tutorial

Applications of DNA Transformation on Scigine

References

Excellent Book about Bacterial Transformation
Guide to Common terms in Transformation – Oklahoma University
Compilation of History of Transformation and related protocols
He et al Proc Natl Acad Sci U S A. 1998 Mar 3. 95(5):2509-14.