આ MCQ મોડ્યુલ આના પર આધારિત છે: Restriction Enzymes Gel Electrophoresis
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Restriction Enzymes Gel Electrophoresis
🎓 Class 12
Biology
CBSE
Theory
Ch 9 – Biotechnology: Principles and Processes
⏱ ~14 min
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આ મૂલ્યાંકન આના પર આધારિત હશે: Restriction Enzymes Gel Electrophoresis
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Tools of Recombinant DNA Technology - Restriction Enzymes and Gel Electrophoresis
Genetic engineering can be accomplished only if we have the key tools: restriction enzymes, polymerase enzymes, ligases, vectors and a host organism. This part takes the first of them apart — the enzymes that cut DNA at a chosen place, and the technique that lets you see what they have done.
How restriction enzymes were found
In the year 1963, two enzymes responsible for restricting the growth of bacteriophage in Escherichia coli were isolated. One of them added methyl groups to DNA; the other cut DNA. The second was called a restriction endonuclease.
The first of these to be understood properly was Hind II, characterised five years later. Its functioning depended on a specific DNA nucleotide sequence: Hind II always cut DNA molecules at a particular point, by recognising a specific sequence of six base pairs. That specific base sequence came to be called the recognition sequence for Hind II.
Besides Hind II, we today know more than 900 restriction enzymes, isolated from over 230 strains of bacteria, each of which recognises a different recognition sequence.
Why bacteria have them at all. Notice what the original experiment was about — restricting the growth of a bacteriophage. These enzymes are a bacterial defence: they chop up invading viral DNA. The methylating enzyme found alongside is the other half of the system, marking the cell's own DNA so that it is spared. Biotechnology did not invent the molecular scissors; it found them in use and borrowed them.
How they are named
The convention is precise and worth learning, because you will meet these names constantly.
- The first letter comes from the genus of the prokaryote the enzyme was isolated from.
- The next two letters come from the species.
- Any further letter refers to the strain.
- The Roman numeral that follows indicates the order in which the enzymes were isolated from that strain of bacteria.
So EcoRI comes from Escherichia coli RY 13: E from Escherichia, co from coli, and the letter R derived from the name of the strain. The I tells you it was the first to be isolated from that strain.
| Part of the name | What it stands for | In EcoRI | In Hind III |
|---|---|---|---|
| First letter | Genus | E — Escherichia | H — Haemophilus |
| Next two letters | Species | co — coli | in — influenzae |
| Following letter | Strain | R — strain RY 13 | d — strain Rd |
| Roman numeral | Order of isolation from that strain | I — the first | III — the third |
Nucleases: two ways to cut
Restriction enzymes belong to a larger class of enzymes called nucleases, and these are of two kinds.
Exonucleases remove nucleotides from the ends of the DNA.
Endonucleases make cuts at specific positions within the DNA.
Restriction enzymes are endonucleases — which is why they are useful. An enzyme that nibbles a molecule from its ends cannot open a gap in the middle for an insert to go into.
Recognition sequences and palindromes
Each restriction endonuclease functions by ‘inspecting’ the length of a DNA sequence. Once it finds its own specific recognition sequence, it binds to the DNA and cuts each of the two strands of the double helix at specific points in their sugar-phosphate backbones.
Every such enzyme recognises a specific palindromic nucleotide sequence in the DNA. Do you know what palindromes are? They are groups of letters that form the same words when read both forward and backward — for example MALAYALAM.
But there is a difference. In a word-palindrome, the same word reads the same in both directions. A palindrome in DNA is a sequence of base pairs that reads the same on the two strands when the orientation of reading is kept the same. For example, the following sequence reads the same on both strands in the 5′→3′ direction — and this is equally true if read in the 3′→5′ direction:
5′ —— G A A T T C —— 3′
3′ —— C T T A A G —— 5′
📐 Activity: make a palindrome yourself
Try to build a six-base-pair palindromic recognition site of your own, using only the base-pairing rules (A with T, G with C). Write your top strand 5′→3′, write the complementary bottom strand underneath, and then read the bottom strand 5′→3′ — that is, from right to left.
Predict: what relationship must the first three bases have to the last three, for the two readings to come out identical?
The rule. The last three bases must be the reverse complement of the first three. Choose any first half you like — say GGA — and the second half is forced: reverse it to AGG, complement each base, and you get TCC. The site is GGATCC, which is in fact the real recognition site of BamH I.
Others you can check the same way: GAATTC (EcoR I), AAGCTT (Hind III), CTGCAG (Pst I), GTCGAC (Sal I). Write the complement of each and read it backwards; you will get the sequence you started with every time.
Why it matters. Because the site is symmetrical, the enzyme sees the same thing from either strand, and so cuts both strands in equivalent positions. The symmetry is not a curiosity — it is what makes a clean, reproducible double-strand cut possible.
Sticky ends, and why they matter
Restriction enzymes cut the strand of DNA a little away from the centre of the palindrome site, but between the same two bases on the opposite strands. This staggered cut leaves single-stranded portions at the ends — overhanging stretches called sticky ends on each strand.
They are named so because they form hydrogen bonds with their complementary cut counterparts. This stickiness of the ends is what facilitates the action of the enzyme DNA ligase.
Restriction endonucleases are used in genetic engineering to form ‘recombinant’ molecules of DNA, which are composed of DNA from different sources or genomes. When cut by the same restriction enzyme, the resultant DNA fragments have the same kind of sticky ends, and these can be joined together end-to-end using DNA ligases.
The one rule you must not forget. Normally, unless one cuts the vector and the source DNA with the same restriction enzyme, the recombinant vector molecule cannot be created. Different enzymes leave different overhangs, and different overhangs do not pair.
Separating and isolating the DNA fragments
Cutting DNA with restriction endonucleases gives you a collection of fragments. To use one of them, you must first separate them, and this is done by a technique known as gel electrophoresis.
The principle
Since DNA fragments are negatively charged molecules, they can be separated by forcing them to move towards the anode under an electric field, through a medium or matrix. The matrix most commonly used nowadays is agarose, a natural polymer extracted from sea weeds.
The DNA fragments separate — resolve — according to their size, through the sieving effect provided by the agarose gel. Hence the smaller the fragment size, the farther it moves. A gel therefore sorts an invisible mixture into an ordered ladder, largest nearest the well and smallest farthest away.
Seeing the result
The separated DNA fragments can be visualised only after staining the DNA with a compound known as ethidium bromide, followed by exposure to UV radiation. You cannot see pure DNA fragments in visible light and without staining. After staining and under UV, you can see bright orange coloured bands of DNA.
Getting the fragment out
The separated bands of DNA are then cut out from the agarose gel and extracted from the gel piece. This step is known as elution. The DNA fragments purified in this way are used in constructing recombinant DNA, by joining them with cloning vectors.
| Step | What is done | What it achieves |
|---|---|---|
| 1. Digestion | Incubate DNA with a restriction endonuclease at its optimal conditions | The DNA is cut into fragments at defined sequences |
| 2. Electrophoresis | Run the fragments through an agarose gel under an electric field | Fragments resolve by size; smaller ones move farther towards the anode |
| 3. Staining | Stain with ethidium bromide and expose to UV radiation | Bright orange bands become visible — without this the DNA cannot be seen |
| 4. Elution | Cut the band out of the gel and extract the DNA from the gel piece | A purified fragment, ready to be ligated into a vector |
🎯 Interactive: Read the Gel
Four lanes of the same plasmid, treated four different ways. Choose a lane and work out what the banding pattern is telling you.
🎯 Competency-Based Questions
Q1. A student cuts the gene of interest with EcoRI and the vector with BamH I, mixes them with a large excess of DNA ligase, and gets no recombinant molecules at all. The enzymes were active and the DNA was intact. Explain, and state what the student should have done.
Why it failed. Restriction enzymes cut a little away from the centre of their palindrome site, leaving overhanging single-stranded sticky ends. EcoRI leaves an AATT overhang; BamH I leaves GATC. Sticky ends join because they form hydrogen bonds with their complementary cut counterparts, and AATT is not complementary to GATC. With no base pairing to hold the ends together, ligase has nothing stable to seal, so no amount of enzyme helps.
What the student should have done. Cut both the vector and the source DNA with the same restriction enzyme. When cut by the same enzyme, the resultant fragments have the same kind of sticky ends and can be joined end-to-end using DNA ligase.
The principle behind it. Normally, unless one cuts the vector and the source DNA with the same restriction enzyme, the recombinant vector molecule cannot be created. The compatibility of ends, not the amount of ligase, is what decides whether ligation works.
Q2. Why must a restriction enzyme's recognition site be palindromic? Would an enzyme that recognised a non-palindromic six-base sequence be useful for making recombinant DNA?
What palindromic means here. A palindrome in DNA is a sequence of base pairs that reads the same on the two strands when the orientation of reading is kept the same. In 5′-GAATTC-3′ paired with 3′-CTTAAG-5′, both strands read GAATTC in the 5′→3′ direction.
Why it matters. The enzyme has one active site and one sequence preference. Because the site is symmetrical, the enzyme encounters an identical sequence whichever strand it approaches from, and can therefore cut both strands at equivalent positions. The cut is the same on each strand, which produces the neat, reproducible staggered break that leaves complementary overhangs.
A non-palindromic site. Such an enzyme could still cut, but the two strands would present different sequences to it, so the two cuts would not be symmetrical. The overhangs produced at the two ends of a fragment would not be complementary to each other, and fragments cut from different DNAs by that enzyme would not reliably pair. The value of a palindromic site is precisely that it guarantees every fragment from every genome cut by that enzyme carries the same, mutually compatible ends.
Q3. Two DNA samples are run on the same agarose gel. Sample A gives a band very close to the well; sample B gives a band near the far edge of the gel. What can you conclude, and which electrode is at the far edge?
Which electrode. The anode, the positive electrode. DNA fragments are negatively charged molecules, so they are separated by forcing them to move towards the anode under an electric field. The wells are therefore at the cathode end, and fragments travel away from them.
What the positions mean. The fragments resolve according to size through the sieving effect provided by the agarose gel, and the smaller the fragment, the farther it moves. So sample B contains much smaller fragments than sample A. A band sitting very close to the well, as in sample A, is characteristic of large or undigested DNA.
A caution worth adding. Distance moved measures size, not amount. The brightness of a band, not its position, indicates how much DNA is present.
Q4. Why are exonucleases of no use for inserting a gene into a vector, even though they too cut DNA? Give one task for which an exonuclease would in fact be the right choice.
Why exonucleases cannot do the job. Exonucleases remove nucleotides from the ends of the DNA. To insert a gene you must open a gap at a chosen point inside the vector, which is what endonucleases do — they make cuts at specific positions within the DNA. An exonuclease would simply shorten the molecule from its ends, progressively destroying it, and would leave no defined site for an insert.
A second reason. Exonuclease action is not sequence-specific in the way restriction endonuclease action is. Restriction enzymes inspect the DNA and cut only at their own recognition sequence, which is what makes the cut reproducible and the fragment defined.
Where an exonuclease is the right tool. When you deliberately want to trim the ends of a fragment — for instance to remove an unwanted overhang, to shorten a fragment from one end, or to degrade linear DNA in a mixture while leaving circular plasmid DNA intact, since a circle has no ends for the enzyme to start from.
Q5. Eukaryotic cells have plenty of DNA-cutting enzymes, yet they are not used as sources of restriction endonucleases for cloning. Given what these enzymes do for bacteria, explain why eukaryotes would have little use for them.
What the enzymes are for in bacteria. The two enzymes first isolated in 1963 were responsible for restricting the growth of bacteriophage in Escherichia coli. One added methyl groups to DNA and the other cut it. Together they form a defence: incoming phage DNA is cut, while the cell's own DNA is methylated and therefore spared.
Why a eukaryote has less need of it. A eukaryotic cell keeps its DNA inside a nucleus, behind a nuclear envelope, and faces viral threats in quite different ways — through interferons, immune surveillance in multicellular bodies, and RNA-based silencing. A restriction-methylation system would also be dangerous in a cell with a very large genome: any site left unmethylated would be cut, and the enzyme would have to be kept out of the nucleus or the genome would be attacked.
The practical answer. The restriction enzymes used in genetic engineering — more than 900 of them — have been isolated from over 230 strains of bacteria. That is where the sequence-specific, self-protecting system evolved, and that is where biotechnology goes shopping for its scissors.
🧠 Assertion–Reason Questions
For each pair, decide whether both statements are true and whether the reason correctly explains the assertion.
Assertion (A): Vector and foreign DNA are normally cut with the same restriction enzyme.
Reason (R): Fragments cut by the same enzyme carry the same kind of sticky ends and can be joined end-to-end by DNA ligase.
Both A and R are true, and R is the correct explanation of A.
Sticky ends join because they form hydrogen bonds with their complementary cut counterparts, and only the same enzyme guarantees complementary overhangs. Unless the vector and the source DNA are cut with the same restriction enzyme, the recombinant vector molecule normally cannot be created.
Assertion (A): In agarose gel electrophoresis, smaller DNA fragments travel farther.
Reason (R): Smaller fragments carry a greater negative charge and are therefore pulled harder towards the anode.
A is true but R is false.
Smaller fragments do move farther, but not because of charge — a shorter fragment carries less total negative charge, not more. They move farther because of the sieving effect provided by the agarose gel: the matrix impedes large molecules more than small ones. Charge is what makes all the fragments move at all; size is what makes them separate.
Assertion (A): A gel must be stained with ethidium bromide before the DNA bands can be examined.
Reason (R): Pure DNA fragments cannot be seen in visible light and without staining.
Both A and R are true, and R is the correct explanation of A.
The separated DNA fragments can be visualised only after staining with ethidium bromide followed by exposure to UV radiation, which reveals bright orange coloured bands. The staining does not separate anything — electrophoresis has already done that — it only makes the result visible.
Frequently Asked Questions - Restriction Enzymes and Gel Electrophoresis
What are restriction enzymes and when were they discovered?
In 1963 two enzymes responsible for restricting the growth of bacteriophage in Escherichia coli were isolated. One added methyl groups to DNA while the other cut DNA, and the latter was called restriction endonuclease. The first restriction endonuclease whose functioning depended on a specific DNA nucleotide sequence, Hind II, was isolated and characterised five years later; it always cut DNA molecules at a particular point by recognising a specific sequence of six base pairs.
How many restriction enzymes are known today?
Besides Hind II, more than 900 restriction enzymes are known, isolated from over 230 strains of bacteria, each of which recognises a different recognition sequence.
How are restriction enzymes named, using EcoRI as the example?
The first letter of the name comes from the genus and the next two letters come from the species of the prokaryotic cell from which the enzyme was isolated. EcoRI comes from Escherichia coli RY 13, and in EcoRI the letter R is derived from the name of the strain. The Roman numbers following the name indicate the order in which the enzymes were isolated from that strain of bacteria.
What is the difference between an exonuclease and an endonuclease?
Both belong to the larger class of enzymes called nucleases. Exonucleases remove nucleotides from the ends of the DNA, whereas endonucleases make cuts at specific positions within the DNA. Restriction enzymes are endonucleases.
What is a palindromic sequence in DNA?
A palindrome in DNA is a sequence of base pairs that reads the same on the two strands when the orientation of reading is kept the same. For example, 5'-GAATTC-3' paired with 3'-CTTAAG-5' reads the same on both strands in the 5' to 3' direction, and this is also true if read in the 3' to 5' direction. This is unlike a word-palindrome such as MALAYALAM, where the same word is read in both directions.
What are sticky ends and why are they called sticky?
Restriction enzymes cut the strand of DNA a little away from the centre of the palindrome site, but between the same two bases on the opposite strands, which leaves single-stranded overhanging portions at the ends called sticky ends. They are named so because they form hydrogen bonds with their complementary cut counterparts, and this stickiness facilitates the action of the enzyme DNA ligase.
Why must the vector and the foreign DNA be cut with the same restriction enzyme?
Because when cut by the same restriction enzyme the resultant DNA fragments have the same kind of sticky ends, and these can be joined together end-to-end using DNA ligases. Normally, unless one cuts the vector and the source DNA with the same restriction enzyme, the recombinant vector molecule cannot be created.
What is gel electrophoresis and how does it separate DNA fragments?
Gel electrophoresis is the technique used to separate the DNA fragments produced by restriction digestion. Since DNA fragments are negatively charged molecules they can be separated by forcing them to move towards the anode under an electric field through a matrix, most commonly agarose, a natural polymer extracted from sea weeds. The fragments resolve according to their size through the sieving effect provided by the agarose gel, so the smaller the fragment size, the farther it moves.
Why is ethidium bromide used in gel electrophoresis?
The separated DNA fragments can be visualised only after staining the DNA with ethidium bromide followed by exposure to UV radiation, since pure DNA fragments cannot be seen in visible light and without staining. After staining, bright orange coloured bands of DNA are visible in the gel under UV light.
What is elution in recombinant DNA technology?
Elution is the step in which the separated bands of DNA are cut out from the agarose gel and extracted from the gel piece. The DNA fragments purified in this way are then used in constructing recombinant DNA by joining them with cloning vectors.
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