Q.Make a chart (with diagrammatic representation) showing a restriction enzyme, the substrate DNA on which it acts, the site at which it cuts DNA and the product it produces.

Restriction enzymes are molecular scissors that cut DNA at specific sequences. The most common type used in genetic engineering is a restriction endonuclease, which recognises a palindromic sequence — a sequence that reads the same forward and backward on both strands.

The substrate is a double-stranded DNA molecule. The enzyme scans the DNA for its specific recognition site. For example, the enzyme EcoRI recognises the sequence 5'—GAATTC—3' on one strand and its complement 3'—CTTAAG—5' on the other.

The cut is made at a specific point within this recognition site. Importantly, the cut is staggered — it is not a straight cut across both strands. EcoRI cuts between the G and the A on each strand, but on opposite sides of the helix. This produces short, single-stranded overhangs at each cut end.

The products are DNA fragments with sticky ends (also called cohesive ends). These overhangs are complementary to each other, which allows any two fragments cut by the same enzyme to be joined back together easily by DNA ligase.

Here is a diagrammatic representation of the action:

Substrate DNA (double-stranded):
5' — G A A T T C — 3'
3' — C T T A A G — 5'
         ↑   ↑
         |   |
    EcoRI cuts here

Products (two fragments with sticky ends):
Fragment 1:          Fragment 2:
5' — G               A A T T C — 3'
3' — C T T A A       G — 5'

Restriction enzymes, also called restriction endonucleases, are one of the most important tools in molecular biology and genetic engineering. They were discovered in bacteria, where they serve as a defence mechanism against invading viruses (bacteriophages). The bacterium protects its own DNA by methylating it at the same sites where the restriction enzyme would cut, so only foreign DNA gets cleaved.

The NCERT textbook explains that each restriction enzyme recognises a specific palindromic nucleotide sequence in the DNA. A palindromic sequence reads the same on both strands when read in the 5' to 3' direction. For example, the widely used enzyme EcoRI recognises the sequence 5'-GAATTC-3' on one strand and 3'-CTTAAG-5' on the complementary strand.

When the restriction enzyme binds to its recognition site, it cuts the DNA backbone at specific positions. The cut can happen in two ways. Some enzymes cut both DNA strands at the same point, producing blunt ends. Others make staggered cuts, cutting the two strands at different points, which leaves short, single-stranded overhangs called sticky ends or cohesive ends.

Let us take EcoRI as our example. The recognition sequence is:

5' - G A A T T C - 3'

3' - C T T A A G - 5'

EcoRI cuts between G and A on the top strand, and between A and G on the bottom strand. This produces fragments with sticky ends:

5' - G A A T T C - 3'

3' - C T T A A G - 5'

The single-stranded overhangs (AATT on one fragment and TTAA on the other) are complementary and can hydrogen-bond with each other.

Here is a diagrammatic representation showing the restriction enzyme, its substrate DNA, the cutting site, and the products:

RESTRICTION ENZYME (EcoRI)
        |
        v
    +-------+
    | EcoRI |
    +-------+
        |
        |  binds to recognition site
        v

SUBSTRATE DNA (double-stranded)
5' - - - - G A A T T C - - - - 3'
3' - - - - C T T A A G - - - - 5'
            ^     ^
            |     |
        cut here  cut here
            |     |
            v     v

PRODUCTS (two fragments with sticky ends)

Fragment 1:          Fragment 2:
5' - - - - G         3' - - - - C T T A A
3' - - - - C T T A A         5' - - - - G

Sticky ends:        Sticky ends:
5' overhang: G      3' overhang: C T T A A
3' overhang: C T T A A      5' overhang: G

The two fragments have complementary single-stranded ends. The overhang on fragment 1 (5'-G) can base-pair with the overhang on fragment 2 (3'-CTTAA) if the fragments are from different sources. This property is what makes restriction enzymes so valuable for creating recombinant DNA.

Alternative Approach: A Universal 4-Step Recipe for ANY Restriction Enzyme Chart

Rather than memorising the EcoRI example specifically, use this general 4-step recipe --

it works for drawing the chart for any restriction enzyme (BamHI, HindIII, PstI, etc.),

not just EcoRI.

Step 1: Write down the recognition sequence.

Every restriction enzyme recognises one specific palindromic sequence (reads the same

5'->3' on both strands). Look this up for the enzyme in question, e.g. BamHI recognises

5'-GGATCC-3'.

Step 2: Locate exactly where the enzyme cuts within that sequence.

This is enzyme-specific and must be looked up, not guessed -- e.g. BamHI cuts between

the two G's (G^GATCC), not in the centre of the sequence.

Step 3: Determine the end type produced.

If both strands are cut at the same position (symmetric cut), you get blunt ends.

If the two strands are cut at different positions (offset cut), you get sticky ends

with single-stranded overhangs.

Step 4: Draw the chart in four labelled panels -- (i) the enzyme name, (ii) the

substrate DNA with the recognition site marked, (iii) an arrow at the exact cut

position(s) on both strands, (iv) the resulting two fragments with their ends clearly

shown (overhang bases spelled out if sticky).

Key Takeaway:

The chart's four panels always answer the same four questions -- which enzyme, what sequence, where exactly does it cut, what does the product look like -- so this recipe

transfers directly to any other restriction enzyme you're asked about.