Question

A biochemist isolates, purifies, and combines in a test tube a variety of molecules needed for DNA replication. When she adds some DNA to the mixture, replication occurs, but each DNA molecule consists of a normal strand paired with numerous segments of DNA a few hundred nucleotides long. What has she probably left out of the mixture?

1. DNA polymerase
2. DNA ligase
3. Okazaki fragments
4. primase

Short answer

1. The option “DNA polymerase” is false.
2. The option “DNA ligase” is true.
3. The option “Okazaki fragments” is false.
4. The option “primase” is false.

Step-by-step solution

DNA replication

DNA replication is a process that involves the separation of the two DNA strands. After the separation process, the new strands are produced by using the existing strand as the template. DNA replication is an essential process in the cell division process.

Explanation of option (A)

DNA polymerase can produce the new strand from the older strand that elongates the DNA strand. There is another enzyme that participates in the DNA replication process.

In the given experiment, the mixture was added to the test solution from which the replication proceeded. It confirms the presence of the DNA polymerase enzyme. Without this enzyme, replication will not take place.

Therefore, the given statement is false.

Explanation of option (B) 

DNA ligase is the enzyme that joins the fragment of DNA that is replicated from the lagging strand. It can join the bases to fill the gap in the DNA.

In this experiment, the mixture added to the test solution containing DNA bases does not possess the DNA ligase enzyme. So, the DNA is segmented in this case.

Therefore, the given statement is true.

Explanation of option (C)

Okazaki fragments are the short sequences of DNA created in discontinuous fashion later united to form a continuous strand. DNA ligase enzyme plays a role in joining the discontinuous fragment.

Thus, the segmented portions of DNA found in the mixture are the Okazaki fragments which are not joined due to the absence of the DNA ligase.

Therefore, the given statement is false.

Explanation of option (D)

The enzyme that produces the short RNA sequence is known as primase. The short RNA segments are the primers. Primers act as the initiation site of the DNA replication that attaches complementary to ssDNA, and finally, it is removed.

In the given experiment, since the replication process has started, no lack of primase is seen.

Therefore, the given statement is false.