Question

DNA ligase is needed in a cloning experiment a. to promote hydrogen bonding between sticky ends. b. to covalently link the backbone of DNA strands. c. to digest the chromosomal DNA into small pieces. d. to do only a and b. e. to do a, b, and c.

Short answer

The correct answer is b. DNA ligase covalently links the backbone of DNA strands.

Step-by-step solution

Understanding the Role of DNA Ligase

DNA ligase is an enzyme that facilitates the joining of DNA strands together by forming a phosphodiester bond. It plays a crucial role in replication and repair of DNA.

Analyzing the Options

Let’s analyze each option to check which one is correct.

Option a: Promote hydrogen bonding between sticky ends

This is not correct. DNA ligase doesn’t promote hydrogen bonding between sticky ends but rather links the phosphate backbone.

Option b: To covalently link the backbone of DNA strands

This is correct. DNA Ligase links the phosphate backbone which is a type of covalent bond.

Option c: To digest the chromosomal DNA into small pieces

This is not correct. DNA ligase does not digest DNA; it links DNA strands together.

Option d: To do only a and b

This is not correct. As we have established, option a is not a function of DNA ligase.

Option e: To do a, b, c

This is not correct. Since options a and c are not functions of DNA ligase, this option is also incorrect.

Key concepts

DNA Replication

DNA replication is a fundamental process in which a cell duplicates its DNA before cell division. This precise mechanism ensures that genetic information is accurately passed from one generation to the next. During replication, enzymes called helicases unwind the double-stranded DNA, leading to the formation of a 'replication fork.' Here, another enzyme, DNA polymerase, adds new nucleotides to each strand, creating two identical DNA molecules from the original.
DNA ligase enters the replication dance when DNA polymerase encounters a strand previously synthesized in small segments called Okazaki fragments. These fragments need to be joined to form a continuous strand, and this is where DNA ligase performs its essential role, sealing the nicks and completing the structure of the DNA molecule.
Understanding the intricacies of DNA replication not only helps in grasping the basics of genetic inheritance but also has practical implications in fields such as genetic engineering and biotechnology.

Enzymes in DNA Repair

Similar to its role in replication, DNA ligase is indispensable in DNA repair mechanisms. The integrity of DNA is challenged by numerous factors, including ultraviolet light, radiation, and harmful chemicals that can introduce breaks in the DNA strands. Enzymes in DNA repair, such as nucleases, DNA polymerase, and DNA ligase, work in a coordinated fashion to restore the damaged sections of DNA to their original state.
DNA ligase acts as the final critical step in the repair process. After damaged nucleotides are removed and replaced, DNA ligase seals the remaining gaps, re-establishing the phosphodiester backbone and ensuring the structural continuity of the DNA double helix. Highlighting the role of DNA ligase in repair illuminates one of the many sophisticated cellular defense mechanisms against genetic damage.

Phosphodiester Bond Formation

Phosphodiester bond formation is at the core of DNA ligase's action. A DNA molecule consists of nucleotides linked together through phosphodiester bonds, which join the phosphate group of one nucleotide to the hydroxyl group of another. During DNA replication or repair, any interruptions in this backbone must be repaired to maintain DNA integrity.
DNA ligase facilitates the formation of these bonds by catalyzing a reaction that connects the 3'-hydroxyl end of one nucleotide with the 5'-phosphate end of another, creating a continuous DNA strand. By understanding the chemical reaction DNA ligase catalyzes, students can better appreciate its critical function in cellular processes and its use in various biotechnological applications.

Genetic Cloning

Genetic cloning involves creating identical copies of an organism or specific segments of DNA. It relies on the ability to isolate and copy the genetic material of interest. DNA ligase plays a versatile role in molecular cloning, a laboratory method used to create recombinant DNA.
In cloning experiments, DNA segments of interest are often inserted into vectors such as plasmids, which are then introduced into host organisms. DNA ligase is crucial for connecting the inserted DNA with the vector's DNA, ensuring that the genetic material can be replicated and expressed within the host. By facilitating the creation of recombinant DNA, DNA ligase allows researchers to study genes, produce proteins for therapies, and engineer genetically modified organisms for agriculture and research.