Understanding Enzymes Acting on DNA Ends

The question asks to identify the enzyme that removes nucleotides specifically from the ends of a DNA fragment. Several enzymes work with DNA, but their actions differ based on where they cut or add nucleotides.

Analyzing Common DNA Enzymes

Let's examine the functions of the enzymes provided in the options:

• DNA ligase: This enzyme is responsible for joining DNA strands together by forming phosphodiester bonds. It is often used to seal nicks in DNA or to join two separate DNA fragments, such as inserting a gene into a plasmid. It works on DNA ends, but it joins them, it doesn't remove nucleotides.
• Restriction endonuclease: These enzymes are a type of nuclease that cleaves phosphodiester bonds within a polynucleotide chain. Restriction endonucleases are specific; they recognize and cut DNA at particular nucleotide sequences called restriction sites. These cuts occur internally within the DNA molecule, not from the ends.
• Restriction exonuclease: This is a type of nuclease that removes nucleotides from the ends of a DNA molecule. Exonucleases degrade nucleic acids from either the 5' or the 3' end. Restriction exonucleases are used in various molecular biology techniques to prepare DNA ends by removing terminal nucleotides. This action directly matches the description in the question.
• DNA polymerase: This enzyme synthesizes new DNA strands by adding nucleotides one by one to a growing DNA chain. It requires a template strand and a primer to start synthesis. Its function is to build, not remove, nucleotides from DNA.

Identifying the Correct Enzyme for Removing DNA Ends

Based on the functions described, the enzyme that performs the action of removing nucleotides from the ends of a DNA fragment is the restriction exonuclease.

Summary Table: Enzyme Actions on DNA

Revision Table: Understanding DNA Enzymes

Additional Information: Exonucleases in Detail

Exonucleases are a class of enzymes that hydrolyze (break down using water) phosphodiester bonds one at a time from the end of a polynucleotide chain. They can be specific for either the 5' end or the 3' end of a DNA or RNA molecule, or sometimes act on both ends. This directional removal of nucleotides from the terminal ends is their defining characteristic, contrasting with endonucleases which cut within the chain.

Restriction exonucleases are often used in laboratory techniques like DNA sequencing or preparing DNA for cloning by trimming unwanted overhangs or ends. For example, some exonucleases are used in primer removal during DNA replication or repair processes within cells.