Ligase Enzymes

Ligase enzymes join DNA fragments and form phosphodiester bonds. In Class 11 link them with recombinant DNA and replication.

DNA ligase joins DNA fragments by forming phosphodiester bonds in the sugar-phosphate backbone. Therefore, it is used in recombinant DNA technology to join a vector with the desired DNA fragment. Restriction endonucleases cut DNA at specific sequences, whereas helicase unwinds the DNA double helix. Exam tip: remember ligase as the “DNA glue” enzyme.

In ligation DNA ends are joined with the help of ligase. It is essential for making recombinant DNA.

DNA ligase joins broken DNA fragments by forming a phosphodiester bond between the 3′-OH group of one nucleotide and the 5′-phosphate group of the next in the sugar-phosphate backbone. Hydrogen bonds occur between complementary nitrogenous bases, so they do not seal DNA breaks. Exam tip: remember ligase as the enzyme that joins cut DNA ends.

A restriction enzyme recognizes specific DNA recognition sequences and cuts the DNA. DNA ligase joins two DNA fragments by forming phosphodiester bonds between them. Therefore, option A is correct. Option B reverses the functions of the two enzymes. Exam tip: remember that restriction enzymes cut, whereas ligase links DNA fragments.

DNA ligase can join vector and insert DNA to make a recombinant molecule. Link it with the joining step in cloning.

DNA ligase seals nicks between adjacent DNA fragments or strands by forming phosphodiester bonds. This joining action is why it is called “DNA glue.” Cutting DNA at specific sites, as in option A, is the function of restriction enzymes, not ligase. Exam tip: remember ligase = joins; restriction enzyme = cuts.

Direct answer: D, the lagging strand. DNA polymerase can add nucleotides only in the 5′ to 3′ direction. Because the two DNA template strands run antiparallel, one new strand is made continuously toward the replication fork; this is the leading strand. The other new strand must be made away from the fork in short pieces, each begun by an RNA primer. These short pieces are Okazaki fragments, so they form on the lagging strand. DNA ligase later seals the remaining nicks between them, making one continuous strand. Option D is correct. Option A is wrong because the leading strand is synthesized continuously, not as Okazaki fragments. Option B is wrong because RNA may provide primers but is not the strand being described. Option C is wrong because proteins are not DNA replication strands. Memory cue: lagging means delayed and discontinuous; Okazaki fragments belong to lagging.

The governing concept is phosphodiester-bond formation in nucleic-acid ligation. DNA ligase seals a nick between adjacent DNA fragments by joining the 3′-hydroxyl end of one nucleotide or strand segment to the 5′-phosphate end of the neighbouring segment. This creates the continuous sugar-phosphate backbone, so option B is correct. Amino groups belong to proteins and other molecules, not the characteristic pair of DNA termini used here. “Fat ends” and water molecules are unrelated distractors. The reaction also requires suitable end chemistry and an energy source appropriate to the ligase, but the essential pair asked in the question is 3′-OH and 5′-phosphate.

T4 DNA ligase uses ATP during formation of a phosphodiester bond between adjacent broken DNA ends. ATP activates the enzyme, enabling it to join the DNA strands. \(NAD^+\) is a cofactor for some bacterial DNA ligases, such as E. coli DNA ligase, but not for T4 DNA ligase. Exam tip: Remember T4 DNA ligase as ATP-dependent, while most bacterial DNA ligases are \(NAD^+\)-dependent.

E coli DNA ligase is generally considered NAD plus dependent. Remember it separately from T4 ligase.

DNA ligase forms phosphodiester bonds to permanently join cut ends. In exams link permanent joining with ligase.

Ligase is a protein enzyme and can be affected by reaction conditions. Proper temperature and buffer are important.

Ligase is a protein enzyme that joins broken DNA ends by forming phosphodiester bonds. Excess temperature can denature its three-dimensional structure, disturbing the active site and reducing enzymatic activity. Option B is incorrect because higher temperature does not always increase enzyme action; every enzyme has an optimum temperature range. Exam tip: for protein enzymes, excessive heat commonly causes denaturation.

A ligation buffer maintains the suitable pH and ionic environment required for DNA ligase activity. It often contains essential ions such as \(Mg^{2+}\) and cofactors such as ATP that support joining of DNA fragments. Burning DNA or forming a cell wall is not a function of a ligation buffer. Exam tip: In enzyme-based reactions, a buffer primarily maintains optimal pH and provides required ions.

Sticky ends have complementary overhangs that can pair temporarily first. Then ligase makes them permanent.

Blunt ends lack overhangs so initial base pairing support is absent. Therefore joining can be harder.

DNA ligase can join compatible sticky and blunt ends. Joining is usually easier with sticky ends.

In self-ligation the vector joins its own ends without insert. This can be an unwanted result in cloning.

Two different ends can reduce the chance of vector self-joining. This is useful in directional cloning.

Removing 5 prime phosphate from vector ends can reduce self-ligation. In exams link phosphate removal with ligation control.

Ligase forms a bond between 3 prime OH and 5 prime phosphate. Lack of 5 prime phosphate can reduce joining.

A phosphodiester bond forms between 3 prime OH and 5 prime phosphate. This completes the DNA backbone.

Ligase needs energy to form a new phosphodiester bond. Depending on enzyme type ATP or NAD plus can be used.

Polymerase extends a strand by adding nucleotides. Ligase seals nicks between pre-existing DNA ends.