Explain why the precise length target DNA sequence doesn't get amplified until the third cycle. You may need to use additional paper and a drawing to explain your answer.

When you start the PCR amplification reaction, you have a template which has sequences where the primers will anneal. The forward primer anneals in the sense strand and the reverse one in the complementary sequence (see attached figure).

After the first PCR cycle, two types of fragments are obtained. First those which come from tha 5'-3' sequence, that are 3'-5' and will anneal the reverse primer in the following cycle. This fragments have the correct 3' end, but the 5' includes part of the template that doesn't correspond to our target (see attached figure)

Second, those fragments that come from the 3'-5' sequence, that are 5'-3' and will anneal the forward primer. This fragments also have the correct 3' end, but the 5' includes part of the template that doesn't correspond to our target (see attached figure).

Now, during the second cycle, are produced the first fragments that have the precise length of the sequence that we want to amplify (see attached figure).

So, it's not until the third cycle that the reaction starts amplifying exactly the sequence of desired length.

annyksl annyksl · Answer 2 · 2021-11-10T20:48:26+01:00

The DNA molecular is not amplified until the third cycle because it is in this cycle that the enzyme finds the beginning and end primers of the region to be amplified.

We can arrive at this answer because:

A PCR reaction takes place in three steps called Denaturation, Annealing, and Elogation.
In the Denaturation step, the DNA strands are unwound and separated from each other.
In the Annealing step, two primers are added, which delimit the region of the DNA strand that must be amplified.
It is because of these primers that the DNA is not amplified until the end of the Elongation step.
This is because, at this stage, the DNA Polymerase only amplifies the region between the reaction start and end primers.

You can get a better look at this in the image below.

More information:

https://brainly.com/question/14067232?referrer=searchResults