13. A biochemist isolated and purified molecules needed for replication. When DNA was added, replication occurred but the DNA molecules were defective. Each consisted of a normal DNA strand with many DNA segments a few hundred nucleotides long. What has the biochemist left out of the mixture?

DNA replication is a process whereby a particular DNA makes a copy of or synthesizes itself. It consists of several steps with some important enzymes for successful, error free replication. The various steps are as follows;

Unwinding of the double helix structure of the DNA and formation of replication fork. The enzyme involved here is the DNA helicase.
Primer, a short piece of RNA becomes synthesized and binds to the 3' end of one of the 2 strands of the DNA, the leading strand. The enzyme involved is the DNA primase.
Replication of the leading strand then proceeds with the elongation of the primer through the addition of bases in the 5' to 3' direction leading to the formation of continuous strands.
The other strand of the DNA, known as the lagging strand starts its own replication by binding with multiple primers at different regions of the strand. Bases are then added to each primer leading to the formation of several, short discontinuous DNA strands known as Okazaki fragments. The enzyme involved in the elongation process is the DNA polymerase.
Next is the termination of the replication process after the formation of the continuous and discontinuous strands. Exonuclease enzyme removes primers from the synthesized strands. Primers are replaced by appropriate DNA bases and the Okazaki fragments are joined to form a unified DNA strand by an enzyme known as the DNA ligase.

The many, few nucleotides long DNA segments observed by the Biochemist are the Okazaki fragments that should have been joined together by the DNA ligase.

Hence, DNA ligase must have been the component left of the mixture.