Ligase Enzymes

Polymerase builds a strand by adding nucleotides. Ligase seals nicks between pre-existing DNA fragments.

Ligase joins broken DNA ends or DNA fragments by forming phosphodiester bonds. Helicase uses ATP energy to separate the complementary strands of the DNA double helix. Restriction endonucleases, not ligase or helicase, cut DNA at specific sites; therefore A, C, and D are incorrect. Exam tip: remember ligase as the DNA “joining” enzyme and helicase as the DNA “unwinding” enzyme.

DNA ligase mainly joins broken or nicked DNA ends by forming phosphodiester bonds. Therefore, DNA is the correct answer. Enzymes that act on RNA or proteins have different functions. Exam tip: In recombinant DNA technology, ligase is the enzyme used to join DNA fragments.

DNA ligase forms a phosphodiester bond between 5 prime phosphate and 3 prime hydroxyl. Remember this as the chemical basis.

5 prime phosphate is needed for phosphodiester bond formation. Its absence can affect ligation.

Ligation needs active ligase proper DNA ends and suitable buffer. Photosynthesis pigment is not related to ligation.

T4 DNA ligase can join both sticky and blunt ends. In blunt-end ligation conditions are kept more carefully.

Complementary bases of sticky ends can first base pair. Ligase converts this pairing into a permanent bond.

Sticky ends first base pair. Ligase forms a phosphodiester bond to make stable recombinant DNA.

DNA ligase joins DNA ends by forming phosphodiester bonds. Incompatible DNA ends cannot align or base-pair properly, so ligation efficiency may decrease. In contrast, active ligase, a suitable buffer, and compatible ends support successful ligation. Exam tip: sticky ends generally ligate more easily, whereas incompatible ends make ligation difficult.

Ligase needs energy to form a phosphodiester bond. Without an energy source ligation can be weak.

T4 DNA ligase is an ATP-dependent enzyme. Before joining broken DNA ends, it uses ATP to activate the 5′-phosphate end, allowing formation of a phosphodiester bond. NAD⁺ is a cofactor for some bacterial DNA ligases, but not for T4 DNA ligase. Exam tip: Remember T4 DNA ligase as ATP-dependent, whereas E. coli DNA ligase is NAD⁺-dependent.

E coli DNA ligase is generally considered NAD plus dependent. This is a direct point different from T4 DNA ligase.

A recombinant plasmid has vector DNA and insert DNA joined together. Ligase makes this joining permanent.

Ligated recombinant DNA can be introduced into a host cell. This can be the transformation step.

In self-ligation the vector joins its own ends. This does not make the desired recombinant plasmid with insert.

Directional cloning uses different ends to control insert orientation. Ligase then joins matching ends.

A proper ratio of vector and insert can increase the chance of desired ligation. At easy level link it with reaction planning.

If insert is low the chance of desired joining with vector may decrease. Insert amount is an important factor in cloning.

The governing concept is maintaining the stability and catalytic activity of a protein enzyme during storage. Ligase can lose activity if exposed to excessive heat, repeated warming, unsuitable pH, or other damaging conditions. Cold storage, using the temperature and buffer specified by the manufacturer, slows denaturation and degradation and therefore helps preserve functional enzyme molecules. Option A is correct. Open sunlight may heat the sample and can expose it to damaging radiation; boiling is expected to denature most proteins; and dry soil is neither sterile nor a controlled storage medium. In practice, aliquoting and avoiding repeated freeze-thaw cycles may further protect activity, but the essential exam answer is cold storage.

DNA ligase acts on nicked DNA or the ends of DNA fragments. Using energy from ATP (or NAD⁺ in some bacteria), it forms a phosphodiester bond in the sugar-phosphate backbone and seals the break. Starch, free fatty acids, and chlorophyll are not substrates of DNA ligase. Exam tip: remember ligase as the enzyme that “joins” DNA fragments.

Cutting DNA at palindromic recognition sites is the role of restriction enzymes. Ligase is a joining enzyme.

DNA ligase seals a nick, a small break in the sugar-phosphate backbone of a DNA strand, by forming a phosphodiester bond. Thus, it restores strand continuity during DNA repair. Removing damaged bases is done by enzymes such as DNA glycosylases, not by ligase. Exam tip: Associate ligase with joining DNA fragments and sealing nicks.

DNA ligase joins cut DNA fragments by forming phosphodiester bonds between their ends. Therefore, it is called the molecular glue in recombinant DNA technology and cloning. A restriction endonuclease cuts DNA at specific sites, whereas ligase seals the cut ends together. Exam tip: restriction enzymes cut; ligases join.

Restriction enzymes are called molecular scissors because they cut DNA. DNA ligase performs the opposite joining role.