Restriction Enzymes

One cut on a circular plasmid makes it a linear molecule. This is a basic rule of plasmid digestion.

A restriction map shows positions of enzyme cut sites on DNA. It helps predict expected fragments.

In restriction digestion a restriction enzyme cuts DNA at its recognition sites. This is a common lab practical term.

In star activity an enzyme may cut outside its normal specificity. Wrong buffer or excess enzyme can cause it.

Many Type II restriction enzymes work properly in the presence of Mg ion. This is why reaction buffer is important.

Restriction enzymes are protein in nature and can become inactive by heat. Cold storage preserves activity.

Buffer gives the enzyme proper pH and ionic conditions. A wrong buffer can reduce or alter enzyme activity.

HaeIII is studied among examples producing blunt ends. EcoRI and BamHI are common sticky end examples.

EcoRI cuts its recognition sequence, GAATTC, in a staggered manner and produces short single-stranded 5′ overhangs called sticky ends. These ends help complementary DNA fragments join during cloning. In contrast, enzymes such as SmaI generally produce blunt ends. Exam tip: Remember EcoRI for sticky ends and SmaI for blunt ends.

EcoRI is a restriction endonuclease that recognises the palindromic DNA sequence 5′-GAATTC-3′ and cuts between G and A. Therefore, option A is correct. GGATCC is recognised by BamHI, AAGCTT by HindIII, and GGCC by HaeIII. Exam tip: learn common restriction enzymes along with their four- or six-base recognition sequences.

BamHI is a restriction endonuclease that recognizes the palindromic DNA sequence GGATCC and cuts DNA at this site. GAATTC is recognized by EcoRI, whereas AAGCTT is recognized by HindIII. Exam tip: memorise restriction enzymes along with their specific recognition sequences as pairs.

HindIII is a restriction endonuclease that recognizes the palindromic sequence 5′-AAGCTT-3′ and cuts within it. Therefore, option A is correct. GAATTC is the recognition sequence of EcoRI, while CCGG is recognized by enzymes such as HpaII/MspI. Exam tip: Memorize HindIII with AAGCTT and EcoRI with GAATTC as pairs.

The substrate of a restriction enzyme is DNA. It recognizes a specific recognition sequence in DNA and cuts it at or near that site. Starch, fat and chlorophyll are a carbohydrate, lipid and pigment respectively, so they are not substrates for restriction enzymes. Exam tip: remember restriction endonucleases as molecular scissors that cut DNA.

Restriction enzymes are proteins that recognize particular nucleotide sequences in DNA and cleave the DNA backbone at or near those sites. Because their cutting is sequence-specific, they are essential tools in recombinant-DNA work, where a vector and a desired DNA fragment must be cut in a controlled manner. They do not digest starch; starch digestion is mainly carried out by amylolytic enzymes such as amylase, which act on carbohydrate bonds. Therefore option D is the wrong statement. Options A, B, and C correctly describe the cutting activity, sequence recognition, and biotechnology use of restriction enzymes.

Bacteria defend themselves by cutting foreign DNA using restriction enzymes. This is their natural function.

A multiple cloning site provides many restriction enzyme sites. This increases options for insert cloning.

In double digestion two different restriction enzymes act on one DNA sample. It can be useful in directional cloning.

Two different ends help insert the DNA in the correct orientation. This is the main benefit of directional cloning.

Neoschizomers can recognize the same sequence but may cut at different positions. This is restriction enzyme terminology.

In partial digestion not every recognition site is cut. This may show unexpected larger bands in a gel.

In complete digestion the enzyme cuts all available recognition sites. This gives the expected fragment pattern.

In RFLP differences in lengths of cut DNA fragments are observed. Restriction enzyme helps create this pattern.

Restriction enzymes recognise specific DNA recognition sequences and cut the DNA at those sites. Because the positions of such sites can differ among individuals, fragments of different lengths may be produced, leading to a characteristic band pattern after further analysis. Joining fragments is done by DNA ligase, whereas size-based separation is performed by gel electrophoresis. Exam tip: remember restriction enzyme for specific DNA cutting and ligase for DNA joining.

Many restriction enzymes recognize palindromic DNA sequences. This is a common feature of recognition sites.

Direct answer: A, cutting may stop or decrease. A restriction enzyme recognizes a particular short DNA sequence and cuts it there. Methylation means that a methyl group has been added to a DNA base. In many bacteria, methylation marks their own DNA and prevents certain restriction enzymes from cutting it. Therefore, if the recognition site is methylated, the enzyme may not bind properly or may bind but fail to cut; the result is reduced or absent cutting, not a guaranteed change in DNA length. Option A is correct because it describes this inhibition. Option B is wrong: DNA does not always become short; it may remain uncut. Option C is wrong because an enzyme cannot turn into sugar. Option D is wrong because a gel is only a separation medium and does not close. Exam cue: methylation can protect DNA from restriction digestion, but the effect depends on the enzyme and methylation site.