4. DNA sequences can be cut into smaller fragments by A. gel electrophoresis B. DNA ligase C. restriction endonucleases D. DNA polymerase

The correct answer is C. Restriction endonucleases.

Here's why:

A. Gel electrophoresis is a technique used to separate DNA fragments according to their size. It doesn't cut DNA sequences.

B. DNA ligase is an enzyme that facilitates the joining of DNA strands together by forming a bond between the phosphate group of one strand and the deoxyribose group on another. It doesn't cut DNA sequences.

C. Restriction endonucleases, also known as restriction enzymes, are proteins that cut DNA at specific sequences. They are used in genetic engineering to cut DNA into smaller fragments that can be studied or manipulated.

D. DNA polymerase is an enzyme that synthesizes DNA molecules from their nucleotide building blocks. It doesn't cut DNA sequences.